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A cookie for the first person to find my outstanding typo.
I don't want your cookie. It's probably what caused that "typo."
The reference frames of the photons in your description are not equivalent. They may lack time, and see the universe as a volumeless sheet, but they all see a different sheet. Their FoR's are not equivalent.
E = hf for a photon.
There's no minimum energy, E, nor minimum frequency, f. h is Plank's Constant and describes the proportional relationship between energy and frequency.
There is no quantization of allowed energies aside from the maximum wavelength that could fit in the Universe, stretching from end to end. So long as the universe is finite, that wavelength is non-0. If the universe can reach "infinite" length across it, then there can be photons which are infinite red-shifted... Stretched to non-existence.
It's interesting to speculate a quantum entangled ground state where all photons have been stretched to overlap... that's at least like what you're describing... though from a QM argument and not a GR argument.
More fibre needed.Quote:
I don't want your cookie. It's probably what caused that "typo."
I don't think this is right. I'm not trying to outsmart you or anything, I'm not waving my dick about. I just believe this contradicts Planck's discovery, where he assumed a minimum energy value to predict the black body spectrum, and his "hack" solved the problem of predictions failing to match observations, aka the Ultraviolet Catastrophe. The "hack" in question was to assume that energy is quantum, that it is found in integer values. Planck himself expected his "constant" to be zero, but instead it was a very small but non-zero number. This was the birth of quantum mechanics. Well, perhaps not the birth. The maturity. Planck's work was built on foundations laid during the 19th century, but his constant was confirmation that light comes in discrete packets - photons - and allowed physicists of the day to develop quantum theory.Quote:
There's no minimum energy, E, nor minimum frequency, f
https://www.youtube.com/watch?v=tQSbms5MDvY&t=662s
I could definitely be jumping to conclusions, failing to fully understand what Planck actually discovered, but it seems to me to be the first concrete evidence of the quantum nature of the universe, including space, time, length, and indeed energy. That is, a minimum integer value. It's so tiny that from the pov of the macro world, it appears infinitely divisible, but this doesn't seem to be the case.
By the way, I made a schoolboy error with my assumptions. Where I say "minimum frequency", I actually mean "maximum frequency", or "minimum wavelength". But of course, shorter wavelengths mean more energy, not less. So there would likely still be a "minimum frequency" associated with the lowest energy level possible for a photon. But that "minimum frequency" or "maximum wavelength" could be larger than the universe, frankly I can't even begin to guess at that.
The blackbody radiation spectrum describes the emission of photons by electric charges that are not at absolute 0. There is a conservation of angular momentum involved, in that the photons are spin-1 objects, and the object emitting it will have to be able to shed rotation (if you'll allow the sloppy language) to emit the photon. The rotational states are quantized, so the emitted photons have quantized energies.
More broadly and more generalized, particles in bound states have quantized "allowable" states. Among those parameters confined by the boundaries of the system is the energy of a particle.
The blackbody spectrum is emitted by accelerating particles in a bound system - an atom for instance, or an ion in a plasma. The "allowed" energy of those particles in the bound system are quantized, and thus the photons they emit when changing energy states are discrete in frequency, corresponding to the difference in energy between the final and initial state of the particle's transition.
In the Heat Death universe, there are no particles left to be bound to any system. Thus the only boundary left is mere existence. The photons exist "in" the universe. So if the universe has a finite size, then that size quantizes the minimum frequency that can resonate, but it's not quantized unless the size of the universe is quantized. I.e. so long as the boundaries of the universe expand "smoothly" and not like a step function, then there is nothing left to quantize the bound states.
I suspect you'll dither over the propagation of photons in the EM fields being observation by the EM fields, and without that, there are no photons at all... but if those photons propagate through EM fields, then the expansion of spacetime will inevitably stretch their wavelengths to longer and longer lengths, corresponding to lower frequency and thus energy.
***
There's an open question in physics about the Conservation of Energy and how it relates to the changing energy of photons as they propagate through spacetime. It is readily observable that photons decrease in energy as they travel through expanding spacetime, but it is not remotely clear where that energy goes.
Something I watched earlier seems to imply there is compelling evidence (though far from proof) that light from before the big bang survived into what we call our universe. This supports Penrose's CCC theory, indeed he even went as far as to predict such "rings" of light in the CMB. It does seem there was a "before", but I still have a huge problem with the concept of time, and therefore space, in a massless universe.
https://www.youtube.com/watch?v=9Iyc7ZdnHMs
It seems the theory predicts that the last "universe" ended after the last supermassive black holes evaporated... it's still leading me down the path that a massless universe is unstable and marks an important threshold that radically changes the nature of the universe. If the last universe really did end with Hawking radiation, why did the new universe begin? And should we not expect a heat death to have the same effect?
It kinda feels like we're stretching an elastic band. The energy is converted from kinetic energy into potential energy - tension. If expansion stops, all that tension is released. Maybe this is what causes the big bang... when there is no mass left in the universe, there is nothing left to experience time, and therefore space, expansion ceases, and all these "stretched" photons "snap" from maximum red shift to maximum blue shift. Obviously this is highly speculative, and the tension analogy is very classical and not remotely quantum, but it's appealing.Quote:
It is readily observable that photons decrease in energy as they travel through expanding spacetime, but it is not remotely clear where that energy goes.
Going back to the discussion about causality. In the very early universe, cosmic inflation was insanely fast... as in, much faster than the speed of light. It took a fraction of a second to go from near singularity to a massive universe. Furthermore, we observe spacetime expanding faster than c when we observe very distant galaxies.
If spacetime can expand faster than light, why should we assume it can't contract faster than light?
IDK.
We don't have a model for the cause of inflation or the cause of the accelerating expansion of the universe we currently observe.
We don't have any good reason to assert that something like cosmic inflation but in reverse could never happen because we don't even know how it happens forward. All we know is that we do not observe anything moving faster than c, no matter how hard we try to. And we have to accept that as a fact and find the consequences, which bear out more facts. Those predictions are observed, so we're probably on to something, but we can never know how far from "finished" physics is because physics describes what we observe and we can never prove that we have observed all there is to observe.
So ... yeah.
I do like the idea that once it's a universe of photons, the expansion of the universe keeps stretching those photons to lower and lower frequencies, longer and longer wavelengths, and (if there is space and time) that is asymptotic behavior to a ground state like a Bose-Einstein condensate... where all the bosons (photons are bosons) in the system share the ground state. 'Cause there's an opposite effect on Bosons to Fermions - where fermions express what manifests as a force pushing out another identical particle from being in the same state at the same time, bosons actually express what manifests as a force pulling identical particles into the same state at the same time.
And if everything is tending toward that ground state where literally everything in the universe is in the same state at the same time... then I think we have your model though QM arguments without denying the existence of space or time in the universe.
Oh, my idea doesn't work for a lot of reasons.
In an expanding universe, the energy density can only go down, and the effect on the photon's wavelengths is less than the effect on the space they traverse... in short... it stretches the photons, but never in a way that the photon's wavelengths are longer than the space expanded... so they will not expand to "fill" the space.
There something called the Schwinger limit that I need to look into that probably has something to do with this.
Me asking questions of a proper PhD in GR physics in another forum.
Me: I have a friend asking a lot of questions based on his uneducated (but nonetheless pretty darn good) understanding of GR. And he's got this notion that in a universe of only photons, it becomes meaningless to talk about inertial reference frames, and since all that's left is photons, the only things left to observe the universe see it with no volume or time.
ProbablyMagnets: I think he's mostly right about a universe of just photons, under some assumptions that theres no unification. It's true at least that to completely describe the hilbert space of that universe you dont need the whole thing, only a single lightcone.but in that context theres no analogy to measurement so you're bound to find some things that dont make sense
IDK exactly what he just said, but I'm pretty sure he said you're onto something, ong.
EDIT: Dafuq happened to the formatting when I copy/pastad?
That's definitely not easy to understand word salad, but I'm glad your colleague didn't just laugh and say "yeah uneducated lol".
I really don't know about "onto something", I still haven't resolved the paradox of a process that takes time in a universe with no time. The best I can do is to say the "timeless" era is instant and actually does not take time to transition from massless to big bang to mass. At the instant of the "final interaction", a new big bang. But that's still cause and effect, so there's something not right.
As for the copy/paste, hold down shift with ctrl/v and it'll "paste in plain text" instead of whatever font you copied it.
I'm pretty sure that "uneducated (but nonetheless pretty darn good) understanding of GR" is a good way to describe what you understand. Maybe it's harsh that someone with a PhD considers anyone without a PhD uneducated, but I have a BS and next to ProbablyMagnets, I just have my foot in my mouth the whole time. I wouldn't consider myself educated in GR, so don't take it like a casual comment that you're uneducated.
You 'tard.
His attitude on Conformal Cyclic Cosmology is pretty much a brick wall. He says it's too far fetched to bother with applying actual physics to it because there are much better things to spend time on, basically. That statement reflects a typical attitude among people elbows deep in their tiny corner of post doctoral research.
I think you sussed out part of what he meant with a measurement problem. If what he means is that distances are ill defined, then it is equivalent to saying time is ill defined, and thus we have problems with saying a "timeless, volumeless" universe, because those words are meaningless in that universe.
I am uneducated, it's definitely not a term I have a problem with. And actually it's kinda liberating, as I feel I have more freedom to speculate and to philosophise.
This really does come down to what time actually is. It's not really about measurement, which is why it's not physics. I can totally understand why someone educated would dismiss these kind of speculations, because like your friend says, it's a waste of time when there are more productive things to think about. I'm not living within that boundary.
I'll just leave this here:
(vrchat map of a rotating space station)
https://www.youtube.com/watch?v=5IRnbHggM2M
Would these need energy to keep spinning? I know conservation and all that, but every time you move weight up and down its radius you make it spin slightly faster/slower. There could be various losses there, right? I saw two guys fight about it on quora. seemed inconclusive.
I don't think so. If you move up and down, you're just converting one form of energy to another, and then converting it back. It might spin slightly faster as you move to the centre, and then slow down again as you return to the edge. The total mass, and therefore inertia, remains constant. If such a city were to rely on imported resources such as water, then its mass is increasing, and this would slow the rotation down as inertia increases.
That's my best guess anyway. I look forward to mojo's comments.
Space isn't a perfect vacuum, so there would be very slight friction, which converts rotation into heat, but the energy lost will be miniscule on short timescales. A boost every century or so should sort that problem out.
I'd be more worried about asteroids. I can't imagine that would end well, what with there being no atmosphere to burn up the approaching danger.
The total angular momentum of an isolated system never changes. So long as nothing is carrying angular momentum away from the ship, any changes in the rate of spinning due to changes of geometry will return to the same rate of spin for each geometry. (I specifically mean the geometry of mass distribution about the rotational axis, here).
I_total * {omega} = [constant]
Where I_total is the total moment of inertia for the object about its rotational axis, {omega} is the angular velocity of rotation about that axis, and [constant] is whatever number that is at one time is the same number at all times.
If the spaceship burns fuel or otherwise transfers angular momentum away from itself, that is an "external" angular momentum in my model, just for clarity. It's arbitrary where we draw our boundary, but if we don't care about the angular momentum of the spent fuel, then we should not consider it "internal" to our system. Even though the fuel leaves the ship in a straight line, IF that straight line does not intersect the ship's axis of rotation, THEN it is carrying angular momentum away from that ship/system.
Yes, moving mass toward or away from the axis changes the moment of inertia of that object about the ship's axis of rotation, and if the I_total increases, then the {omega} decreases and vise versa. Moving enough mass could well move the axis of rotation, relative to the ship. Like making the station "wobble." However, provided those masses are moved about inside the ship by forces inside the ship, then there is no transfer of angular momentum away from the ship, and so it's total is constant.
The ship can wobble if the masses are pushed out of line, and you could effectively deform the ship into a rotated copy of it's prior self with enough moving of mass in the right ways. So while the direction of the ships angular momentum cannot change, the ships orientation with respect to that axis can change. Assuming one goes moving masses around willy nilly with no care to the greater effect on the stations dynamical false gravity.
I mean... an nice circular space station spinning the way it should could end up spinning like a flipped coin, but about the same rotational axis, if the movement of masses isn't done carefully or with a reactive system to counteract the humans moving about in their "random" ways.
I did think when I watched it that it was a little unrealistic to go the the centre. The closer to the centre you are, the more centrifugal force you experience and the "gravity" is stronger. If the ship is too small, even at the edge the "gravity" your head feels is more than your feet, to the point you would notice. I would imagine this would be very noticeable in the centre.
You have it backwards. The "gravity" is really a centripetal acceleration in this rotating reference frame.
a_cent = -{v_perp}^2 / r
where a_cent is the centripetal acceleration felt by the object, v_perp is the speed of the object perpendicular to the vector r, which points perpendicularly from the axis of rotation to the object.
(bold indicating a vector)
Note the - sign indicates a_cent is in the opposite direction of r, as a_cent points toward the center and r points away from the center.
v_perp is proportional to |r| for an object in circular motion, and always perpendicular to r.
We can derive this from the definition of radians {arc length} = r {theta} if we take the time derivative of both sides.
v_perp = r {omega}
where omega is the angular velocity of the spaceship, and r is the distance from the axis of rotation.
Plugging this in to the first equation above, dropping the - sign and vector notation to talk about the magnitudes of these forces, now.
a_cent = (r {omega})^2 / r = r {omega}^2
We see that for a given omega, increasing r is increasing a_cent, which means we feel that gravity increases with increasing distance from the axis of rotation.
EDIT: we also see that when we move toward the center, we keep decreasing our distance from the axis of rotation. Decreasing r decreases the a_cent, i.e. the strength of the artificial gravity. At the axis of rotation, there is no a_cent, and the artificial gravity is 0 m/s^2 at the axis.
EDIT2: OH FFS!!! Me losing points for rookie mistakes!!! a force is not equal to an acceleration!!! Gah!
Shame on me!!
All those times I talk about F_cent I really meant to say a_cent, the centripetal acceleration.
I am so ashamed. I have corrected my errors.
So wrong inertial force and wrong vector. Oops!
80 second physics demonstration showing the relationship between moment of inertia, I, and angular speed {omega} being conserved.
This system is analogous to moving masses about on a rotating spaceship. In this model, all the masses move in a spherically symmetric way, but that's not a necessary part of the law of conservation of angular momentum.
https://www.youtube.com/watch?v=64t-dVtDwkQ
EDIT: the V-sauce video "Laws and Causes" gives a beautiful link between the conservation of angular momentum and forces. I can link that, too if you're interested.
I came across this post on youtube...
I replied with this...Quote:
Einstein Violated? Objects lower in altitude accelerate toward the ground at a greater rate than those higher up by the ratio of the squares of their distances from the center of the attracting mass. So wouldn’t someone in a very long windowless elevator falling toward the ground of an atmosphere-less planet see a free floating object that is closer to the floor of the elevator than the observer (and thus at an incrementally lesser altitude) see that object drift away from him toward the floor of the elevator since it is accelerating downward at a greater rate than he is? If so that would violate Einstein’s principle of equivalency because an observer in a window-less elevator free-floating in deep space far removed from any gravitational field would see such an object within its walls not moving at all.
Am I thinking along the right lines here? Or is there a simpler explanation?Quote:
This is actually a good thought experiment. I'm not sure of this, but my best guess is that time dilation (and therefore length contraction) becomes a factor. The person looking at the ball won't see it accelerate at all, it will appear the same distance away until it hits the ground, even though it is subject to greater gravitation. This is because the observer and the object are not experiencing time flowing at the same rate... the lower object is "slower" from the point of view of the person.
This is just my speculation though. I'm going to ask someone smarter than me their opinion on this. I'll update if I get a reply.
Einstein's thought experiment elevator is in a uniform gravitational field. The one described in by the poster is not a uniform field, it varies in magnitude.
In the Einstein elevator, the acceleration of gravity is the same everywhere, the elevator isn't falling "toward a planet," it's in a hypothetical universe where gravity points the same direction with the same strength everywhere... or a "hand of God" is pulling the elevator "upward" with a constant force - giving the illusion of a constant acceleration due to gravity pointing "down."
EDIT: this uniform gravitational field in Einstein's thought experiment is not caused by any mass. It's simply hypothesized to illustrate a point.
A mass distribution that would create this kind of uniform field would be an infinite flat Earth. It's analogous to an infinite capacitor plate, replacing electric charge with gravitational mass and the Electric field with a gravitational field.
So you're saying that the observer would see the object moving away from him?
Cool, thanks.
I thought the moon went around the Earth. What's this parabola shit?
https://twitter.com/amazing_nature0/...29863297003523
I mean... throw me a softball, why don't ya?
https://twitter.com/PicPedant/status...44063486091266
Literally just scrolled down the page to find the answer.
Also... parabolas only curve in one direction, no inflection points.
Not really a question, but did you see the recent PBS Spacetime? They *might* have detected the gravity wave background, basically the sum of all gravitational waves, by observing pulsars across the galaxy. Pulsars are so accurate at timekeeping that the slightest discrepancy tells us something about the fluctuating distance between it and us.
Here's some pulsar porn...
https://www.youtube.com/watch?v=x5BQV3WX80E
The last one is spinning at 1/7 the speed of light, which would obviously create immense centrifugal forces. The fact this doesn't cause the star to disintegrate shows just how fucking ridiculous the gravity is.
Not really going to answer what is not really a question, I suppose.
I wonder what centrifugal force corrections you need when the spacetime is being frame dragged along with the rotation.
Did anyone watch the video of the Mars rover Perseverance landing? I get all squishy inside when I see that. Like... humans had to pre-program a million things correctly and then push a button and step back and wait and hope that they didn't just waste billions of dollars. To see it all go off flawlessly under automated control to gently set down a nuclear powered SUV with a scout helicopter on another planet just gets me all excited. It's a nice counter to the political world of doom and gloom to see how truly amazing humans can be sometimes.
https://www.youtube.com/watch?v=4czjS9h4Fpg
At around the 1:05 mark there looks like a river valley.
It's very exciting. We're obviously a long way off putting humans on Mars, probably won't see it in my life, but this is really great to see. It's worth every single penny. Humans can't survive the end of the world if we're stuck on this planet, so these missions are extremely important for our future as a species.
Absolutely wonderful.
What is the muon discovery?
It's that there's been an experiment running at multiple locations to test a vibrational (rotational?) movement of muons in a magnetic field (I think). The Standard model predicts a behavior. The experimental behavior observed is not that. It's sped up. Something is making the muons move faster. Something is increasing the energy of the movement. It's easy to hypothesize that there's some not-as-of-yet described force acting on the muons.
They've not officially released the results, yet, out of an abundance of caution and the luxury in physics to demand a 5 sigma result before making any bold claims. That is, they've already shown that this phenomenon is almost certainly not a mistake in collecting data or data analysis. (There could be mistakes in the experimental setup or the interpretation of the data, but the data, if good, is almost certainly disproving the prediction of the Standard Model) They've already reproduced the results at a 2nd (presumably independent) lab. As much as I'm holding out for the official announcement, as of today, I'm expecting that announcement in the coming years.
What it means is that there is something going on that is not explained by the Standard Model. That hasn't happened in about 100 years. The Standard Model has been the most precise and powerful predictive model ever developed by humans. The fact that a spotlight has been shown on what seems to be a phenomenon not accurately described by the SM is exciting. People are wildly speculating a lot of stuff that could come from a new discovery being real and a new group of physicists digging in to that and explaining it. At this time, that is all pure speculation.
This is how the BBC presented it, but their article was super dumbed down. I watched PBS Spacetime last night and they didn't mention a "new" force at all.Quote:
It's easy to hypothesize that there's some not-as-of-yet described force acting on the muons.
What stuck me from PBS was that a muon is basically a "heavy" electron... it's identical to an electron in all ways except mass. I find that very interesting. My best guess is the muon interacts with the Higgs field differently to an electron. What makes a muon different to an electron? Why does the muon interact differently? Why is it otherwise identical?
I'm not going to pretend to have anywhere near as good an understanding of the SM than I do GR, so this is all way beyond my grasp. A new "force" seems a little unrealistic though. The BBC even referred to gravity as a fundamental force. But GR says gravity is an inertial force, aka a "fictitious" force. And that makes sense when you understand what GR says gravity is... spacetime curvature rather than particle interaction. So I'm not paying any attention to what the BBC say.
I'm not surprised we have found something that doesn't agree with the SM though. It's clear there is something missing, from both the SM and GR, because they don't work together. I hope we're moving in that direction... unification.
Here's an article by Fermilab - much less dumbed down, but still dumbed down.
https://news.fnal.gov/2020/06/physic...t-calculation/
The actual experiment is subtle and ingenious, IMO. They have some muons in a magnetic field, and the muons rotate in that magnetic field, such that their intrinsic magnetic moment is rotating between aligned and anti-aligned with the magnetic field. When muons decay, they do so via the weak interaction, which is not parity conserving - it ejects an electron when the muon's magnetic moment is aligned with the external field, but it ejects a positron when it is anti-aligned. By observing the emission of electrons and positrons, the precession frequency (rotation frequency of the muons) can be calculated. This is being used to determine what is called the g-factor of muons.
I don't really want to get into g-factor, because it's one of those relatively deep QM things that comes up where the theory was off from experiments by a factor - named with the variable g - which is predicted to be equal to 2 by the Dirac equation (the fully relativistic big brother of the Schroedinger equation).
Basically, we had to inject this factor of 2 to make the theory meet up with experiments until an equation came along that actually predicted the value of 2 - putting the g-factor = 2 on firm theoretical footing.
Now we have an experiment that is indicating that g is not equal to 2 for muons. So there's something amiss with the Dirac equation - subtle though it must be. Or something more than the Dirac equation at play. We don't know, yet. We just have an indication that a prediction is not bearing out to experiment thus far. The explanation is not yet known.
EDIT: Apparently the real talking point is that physicists have quantified the light-light interaction - by which a muon emits and immediately re-absorbs a photon. Until now, the error bars on the experimental data *could have been* wide enough to encompass the theoretical predicted result. While the light-light interaction plays a tiny role in the muons motion, it plays the dominant role in the uncertainty of a measurement. It has been worked out that the contribution to the uncertainty from this interaction is not enough to close the gap between experiment and theory.
Thanks for the article, bookmarked for later consumption.
This is super interesting. It kinda implies that natural processes result in 50% matter and 50% antimatter, which of course begs the question of why we only observe matter in our observable universe. Of course I understand the idea that a simple imbalance of 0.00000000000000000001% is enough over time to result in a universe of matter and photons, but that imbalance is still weird. This is a digression though.Quote:
it ejects an electron when the muon's magnetic moment is aligned with the external field, but it ejects a positron when it is anti-aligned.
Matt on PBS did go into this, at least scratching the surface, and I got it enough to understand why they're excited. They were talking about G-2, which would be zero if SM predictions held, but it was a small fraction above zero. This involved Feynman diagrams, which are really cool in their own right and because I have a little knowledge of what they are, I could follow this line of thought. I know the most simple Feynman diagrams represent the highest probability, and that there are potentially infinite amounts of different configurations, but with more complexity they become ever more unlikely. I get what's happening here mathematically... we're converging like an infinite fraction that doesn't blow up to infinity. So I could understand how they got to this G-2 number mathematically using Feynman diagrams.Quote:
I don't really want to get into g-factor
I'll read that article after dinner.
The weak interaction is the only hypothesis I know of that uses currently known physics to explain why there is an abundance of matter and not anti-matter in the universe. Basically, as bad-ass as you already imagine the Big Bang to be, it probably contained more than a billion times more energy than the mass energy left in the universe we observe. Nearly all of that annihilated in matter / anti-matter cancellation, but a tiny imbalance in the chaos of that event left one weak interaction process favored over its equal opposite. And there is matter left in the universe. All of this is pure speculation, AFAIK. I don't think I've seen a mathematical model that lays out that imbalance in the early universe.
Oh. Did PBS Spacetime already hit this? I'll check it out right away.
On the Feynmann diagrams... I know some people doing research into them and apparently the diverging pathways available in the diagrams begin to dominate the diminishing probabilities after about 20 iterations down. The total energy in the exponentiating pathways possible drops and drops out to about 20-ish steps... so the early speculation was that they'd keep diminishing and be convergent. But now we're finding that the total energy available in the really complicated pathways starts to rise again. So the initial assumption that the more complicated terms played ever less relevant roles seems to have some doubts attached to it.
However, this is one group of researchers, and they're all quite convincing, but time will tell.
broken link?
^ that's what the "broken link" was showing.
Not sure why it didn't work for you.
How many Planck mass sized black holes would it take to account for dark matter? This seems such a simple solution for dark matter that you have to wonder why it's 2021 and they're only just considering it. The hypothesis is that when a black hole evaporates, it leaves behind a naked singularity with enough mass to be non-zero, but too little mass to evaporate.
That's after a talk about why dark matter probably isn't larger black holes.
https://www.youtube.com/watch?v=qy8MdewY_TY
A Planck mass (assuming h-bar and not h is used as Planck's constant - an arbitrary choice), is 22 micrograms. Not that small, on a particle scale. I mean, unlike the Planck length, which happens to be quite small, other Planck units are not dazzlingly evocative of quantum woo woo weirdness - something the Planck length enthusiasts don't really talk about much, but I digress.
It would take an astronomical number of them to account for the mass in the galaxies that we cannot see, but whose effects we can see.
What is the proposed mechanism that prevents Hawking radiation from evaporating this micro black hole?
If there are naked singularities in the universe, they are almost certainly quite rare. The mechanisms by which a black hole can shed its event horizon require it to somehow acquire angular momentum at a rate great enough to compensate for its increasing mass. There isn't a single known observation of a naked singularity, and if they exist, they are lost in the uncertainty of the data. Which means that if they are *there*, their effect on the universe we see is not a dominant factor in the universe's structure. I.e. for any proposed naked singularity that is consistent with GR, we can rule out that those are the dark matter that we see the effects of, but not the cause of.
What says that a naked singularity will not interact with light at all.. i.e. be dark - a signature requirement of dark matter?
My gut says, if they are possible in the theory of GR, and the vastness of the universe is what it is, then they're out there somewhere.
No idea, I guess it comes down to the quantum nature of energy. QM tells us that energy is quantised, that is comes in integer values and not in between. Perhaps a black hole with one base unit of energy is unable to evaporate further. What mechanism causes Hawking radiation? I'm not sure if this was a simplification or a correct interpretation, but my assumption was that virtual particles forming at the event horizon emit a particle in one direction and an antiparticle in the other, resulting in a photon escaping and therefore slow decay of mass. In this way, the black hole can only lose half of its mass. If energy is quantised, then we have a problem when the black hole is one unit in mass.Quote:
What is the proposed mechanism that prevents Hawking radiation from evaporating this micro black hole?
I used the term "Planck mass" because I (probably naively) assumed that was one base unit of energy. But maybe it isn't.
The vid sums up by saying that if this hypothesis is true, it's a problem, because such black holes would be undetectable. We may have a situation arise where the most simple explanation for dark matter is the most difficult to prove, if not impossible. We might never be able to detect dark matter.Quote:
There isn't a single known observation of a naked singularity
Not sure. Gravitational lensing was used to rule out moon-mass type black holes as dark matter candidates though, so it's not something they overlooked in their hypothesis.Quote:
What says that a naked singularity will not interact with light at all.. i.e. be dark - a signature requirement of dark matter?
Dark matter doesn't directly influence light, but its gravity does. Dark matter still causes gravitational lensing.
My gut says that if a black hole can't completely evaporate, then there will be an enormous amount of these undetectable relics in the universe. It seems like a great candidate for dark matter to me.Quote:
My gut says, if they are possible in the theory of GR, and the vastness of the universe is what it is, then they're out there somewhere.
Only in bound systems, but the BH is a bound system.
I'm just pointing out that energy is not always quantized. It's the boundary conditions that create quantization of energy and other things.
A particle not confined in any way (meaning a bit of hand waving about it's existence "in" the universe) can have any energy at all. A photon can have any wavelength, and therefore any energy.
You had me until "half of its mass."
An anti-particle falls in to the black hole, while its non-anti/normal particle partner escaped. It doesn't matter which is anti. The BH ejected mass/energy, which is conserved. If nothing counters this process, the BH evaporates... smaller BH's evaporate faster, so it's a runaway event, albeit with a very slow run-up, even in universal time scales.
The quantization of mass/energy is a thing, but it's not messing with us, here. At least, not in any way that I understand.
Particles decay into smaller particles all the time. Elementary particles decay into photons. The mass of particles may be quantized, but the energy of the photons they emit is not quantized. They can emit multiple photons at once, so long as all the conservation laws are followed.
Mass and energy are different things. Apples and oranges, if you will.
FYI, 1 unit of Planck Energy is 2 GJ -> 2 billion Joules. It's certainly not an upper or lower limit on anything.
That sounds like no change from the current situation. There's something causing a gravitation-like force out there, and we don't see anything that would be the culprit.
We'll get there. The progress of science comes in bursts.
Yeah, I'm not claiming to understand all the nuance, here.
Yeah. I don't consider that light interacting with dark matter, personally. I consider that dark matter interacting with spacetime and photons propagating through that spacetime interact with it. Maybe that's not the best distinction, but its working for me pretty good so far.
IDK. There are plenty of not-yet-explained things out there in the universe. I was hearing about black holes that are simply too big to have formed in the 13.7 billion years the universe has been here. IDK how they determine that, but it just shows that there is a lot of stuff out there that we don't quite understand.
Forgive me for arguing with someone far more qualified about these matters than I am, but really? I was under the impression that there was a lower bound, ie the Planck length, further that the Planck length represents the base unit of length at that wavelength would go up in these integer values.Quote:
A photon can have any wavelength, and therefore any energy.
That was pretty sloppy language, reading it back. Obviously a large black hole isn't losing half its mass in a single quantum process.Quote:
You had me until "half of its mass."
This really does come down to the idea that energy is quantized. I guess I need to read more about this concept but for the sake of argument, in the hope you can see what I'm getting at, let's assume it is, let's assume there is a smallest possible particle, with a corresponding antiparticle, and each has a mass of P. Let's say we have a black hole that has a mass of 2P. A quantum event on the event horizon causes the spontaneous forming of a particle-antiparticle pair. This pair is equal to the entire mass of the black hole. The particle escapes, and the antiparticle falls into the singularity. Now the black hole has a mass of 1. There is no longer enough mass to form a particle-antiparticle pair. This is what I meant by "half its mass". The final act of evaporation is for the black hole to lose half its mass, not all of it. The runaway evaporation process is the transition from a ridiculously tiny fraction of its mass evaporating, to 50%.
Sure, but "multiple" photons will always be an integer value. Isn't this what Einstein means by "discreet" when he talks about packets of photons? You can't have half a photon.Quote:
They can emit multiple photons at once, so long as all the conservation laws are followed.
Not so sure. I might be wrong here, but I consider them to be related in the same way electricity and magnetism are. And while we might think they are different things, they're not, since it depends on your frame of reference. One person might observe an electric field while someone else observes a magnetic field.Quote:
Mass and energy are different things.
I have a better idea of what mass is than energy though. Mass is a measure of inertia and is a property of things that move slower than light. That's your rest mass. But if you're moving relative to what you want to measure, it has some relativistic mass. That relativistic mass only exists though in the frame of reference of someone moving at a different velocity. If you're moving at 0.999c, then something else moving at 0.999c in the same direction has no relativistic mass and only rest mass, from your FoR. So relativistic mass is not a measure of inertia, it's a measure of velocity. I just turned relativistic mass into kinetic energy. And rest mass is essentially potential energy. Saying mass isn't energy seems the same as saying kinetic energy isn't potential energy, but again that depends entirely on your frame of reference.
The shorter the wavelength, the higher the energy.
A photon with a wavelength of the Planck Length would have energy of the Planck Energy - ~2 GJ
There is no upper bound on the energy a photon can have. Any photon's energy can be arbitrarily increased in accordance with Special Relativity. Its wavelength can correspondingly be arbitrarily short, with no relation to or limit set by the Planck Length.
Man... there's some stuff about Planck Length and micro black holes not evaporating in this article:
https://en.wikipedia.org/wiki/Planck_length
I don't understand this quote:
"The entire mass of the black hole will "evaporate", except for that part of it, which is associated with the energy of zero, quantum vibrations of the black hole's substance."
Then:
"Such vibrations do not raise the temperature of the object and their energy cannot be radiated"
That sounds like an object that has a temperature of absolute 0, and I'm not comfortable at all.
Oh, I see what you mean. You're saying the 1P mass of the BH is not enough to spawn the spontaneous production of a particle/antiparticle pair with combined mass of 2P.
The energy for the pair production doesn't come from the mass of the BH.
It comes from the so-called "quantum foam." All of spacetime, but especially the region of spacetime immediately surrounding a particle is a wash of virtual particles, particles that cannot be directly observed by definition. When virtual particles become observable, they are no longer virtual, they are particles.
The quantum foam is causing these pairs to pop into and out of existence all the time. The pair move slightly apart and then back together. If one of them crosses an event horizon and the other doesn't, then they cannot come back together.
The production of virtual particle pairs is not caused by the BH.
An integer multiplied by a non-integer still may or may not be an integer. Photons can have any energy. An integer number of photons can have any energy.
Consider different observers observing the same photon. The observers are moving relative to each other. Relative motion is not quantized (position is not quantized, and time is not quantized, so velocity is not). So given one of them measures the photon to have a certain energy, the other could measure that same photon's energy to be literally anything.
Even if speed was quantized, the relative angle between the 2 observers is almost definitely not quantized. Directional invariance is the symmetry behind Conservation of Angular Momentum, a conservation law not known to be or even hinted to be violated in any prediction or experiment.
I mean - Emmy Noether taught us that for every conservation law, there is an associated symmetry. For angular momentum, the symmetry is the fact that physics is the same in inertial coordinate reference frames, even if the coordinate axes are not aligned with each other. If Conservation of Angular momentum is true, then this directional invariance is true.
So even if the energy of photons is quantized (not saying it is) the Conservation of Angular Momentum implies that relative velocity cannot be quantized.
If relative velocity cannot be quantized, then different observers may observe photons moving relative to them with any (unquantized) energy due to the relativistic doppler effect.
You were simply exchanging mass and energy as though there's no conversion factor between them, is all.
You said, "I used the term "Planck mass" because I (probably naively) assumed that was one base unit of energy. But maybe it isn't."
I was just pointing out that there is a Planck energy, and it is the energy associated with a photon of a Planck length.
It is just assuming the Planck Length is the wavelength of the photon and we know that E = hf, and that wave speed is c, so we tie all that together and ignore the difference between h and h_bar and call it the Planck energy, even though we used h_bar in our calculation and it's clearly E = hf, and not E = h_bar f, but ... what's a factor of 2 pi among physicists?
I mean... I'm talking a bit of trash at the whole notion that the Planck Length is more than a trick of juggling numbers with the whole 2 pi talk, but ... I'm not wrong. That factor of 2 pi is just ignored, because the method of juggling numbers with various units to get out another number with another unit - called the Buckingham Pi method - completely ignores any constant factors in the conversion.
IDK if I'd put mass and energy on the same tit for tat relationship as electric and magnetic fields, though. Energy takes a lot of forms and all.
This was recommended in my YouTube feed and it's amazing.
Over my head, really, so any questions you may have about this I probably can't answer, but give it a shot.
https://www.youtube.com/watch?v=asEtNJ9sRcQ
That's my dinner time entertainment sorted. Chicken pie and chips tonight.
Dinner was good. That video was a bit heavier than PBS. Questions I have none, other than ones you can't answer... such as, where's gravity?
I thought this would be about string theory tbh, To someone uneducated, that seems like the most promising theory when it comes to combining QM and GR. The SM is great except for the elephant in the room.
Dude. I got this. It's not in the Standard Model of Particle Physics.
It's in the Lambda-CDM model.
Lambda for dark energy, CDM for Cold, Dark Matter.
The Lambda-CDM model assumes Einstein's GR is correct and adds on some hypotheses about dark energy and dark matter.
The current state of physics is we use the both the Standard Model and Lambda-CDM Model as proves most useful.
The SM describes 3 of the 4 known forces and Lambda-CDM describes the other 1.
It certainly doesn't feel complete, or like we have full understanding, yet. We have come a long way in the past 200 years, though. A long way.
While there is a lot of work yet to be done, it's not as though there is no bridge at all between QM and relativity.
The Dirac Equation (the relativistic big brother of the Schroedinger Equation) takes relativistic effects into account for quantum wave functions. Applying this equation to gold atoms shows that the innermost electrons absorb light in the blue-purple band, explaining why gold is yellow-ish in color.
We have a piece of how QM describes mass in the Higgs field (7%) and a piece that describes mass in the spacetime field (93%). So we do use both theories together in many useful cases.
Not to downplay the work that needs to be done, of course. But I think there's a popular misconception that there is no link at all between QM and GR - that the 2 models are totally incompatible - which is not the case.
***
I can't help but point out that string hypotheses (they're not theories by scientific standards) have been "the most promising" lead for almost 50 years, now. I'm less and less enamored of string hypotheses every year that goes by.
I certainly don't think they're incompatible because they're both clearly right. I guess I underestimated the progress we've made though. That'll be the result of a fairly limited education, and by fairly limited I mean PBS Spacetime and the occasional bit of half arsed research.Quote:
Originally Posted by mojo
Well that's the point, isn't it? If the SM is missing gravity, it's incomplete.Quote:
Dude. I got this. It's not in the Standard Model of Particle Physics.
Found me something to half arsed research.Quote:
It's in the Lambda-CDM model.
Lambda for dark energy, CDM for Cold, Dark Matter.
The Lambda-CDM model assumes Einstein's GR is correct and adds on some hypotheses about dark energy and dark matter.
haha I've found your kryptonite.Quote:
I can't help but point out that string hypotheses
I got bad news for superman... The sun isn't yellow, it's white. It looks yellow because the sky is blue - in that when you remove blue light from white light, it looks yellow.
Kinda the same thing as what I described earlier for why gold is yellowish in color, but by different physics that splits the blue/purple out from white.
But yeah. I should remind myself that the people working on string hypotheses are, in fact, both smarter and more knowledgeable than I am. Grain of salt with my skepticism, but ... sheesh ... what "string theory" even is keeps getting redefined. strings, loops, branes, etc. They're not limiting anything to strings.
They kinda got me when they said the maths works but only if there's ten dimensions plus time.
I mean, I always considered the "strings" to be the best word we have to give us some real world sense of what we're talking about. Loops are as much mathematical constructs as physical realities, you can see a mathematical loop with i. The word "loop" means something, just like "string". But these words are 1d to us, in the stringy quantum world they're presumably 10d, so of course the words we use are flawed. I think the word "brane" is short for "membrane", which is essentially a quantum field, no?
https://www.youtube.com/watch?v=srVKjWn26AQ
:/
You may have been more right about these Planck Relic Black Holes than I was even aware of in our last conversation.
They still didn't propose any mechanism by which the BH cannot radiate away the final photon to totally evaporate. They only said that if there is some mechanism, then these Planck Relics are possible.
They also didn't give any strong reason to suspect the widespread formation of these in the Big Bang or shortly thereafter. They did say that some models make it plausible, though, and that's better than nothing.
I don't understand why these Planck Relics can't collide with each other, and become a BH of 2x mass, and radiate away photons back to be a single Planck Relic, rather than 2. Their gravity is exceedingly tiny, but not 0. I expect there to be collisions between these if they are so numerous. Why do they not combine and annihilate back to 1?
Yeah I saw this, and it was a previous episode of PBS-ST that gave me the ideas I was talking about previously. Here they just go into more detail. It's nice they give a fuller explanation for Hawking radiation, rather than the simplified particle-antiparticle formation on the event horizon.
I would imagine they don't collide because quantum mechanics forbids it. Something about them having to both share the same quantum state in order to actually collide. I'm guessing here but you'd have to imagine these things are moving at near light speed. Either quantum mechanics forbids it, or it's so ludicrously unlikely that they will collide that they basically don't.
I've always wondered this with regular stellar black holes, when they collide. Here we have two singularities presumably merging into one. I've always thought it was more likely they would orbit each other at a Plank distance or something like that, rather than actually merge. I never could get comfortable with something that occupies the smallest space it's possible to occupy, colliding with something else of equal size. There's quantum effects here that are well beyond my grasp. I'm not sure an actual collision ever takes place, though it would seem like it to an observer who can't make perfect measurements.
IDK how to think about singularities. They are the limit as t-> infinity for all infalling particles.
But t -> infinity is a long way off.
According to PBS ST, the space and time coordinate flip axes when you cross an event horizon (EH), so that means you are free to move about in time, but inexorably moving in 1 direction in space. IDK what that even means. But it makes me question if there is a sensible way to talk about distances inside an EH.
A black hole is an object whose diameter is incomprehensibly larger than its circumference. The extreme curvature of spacetime makes understanding distance a mind-bender for me.
All we know for certain about the singularities (or whatever is inside the EH's) is that when they merge, 2 EH's become 1 EH.
There is some confidence to be had that GR, which predicted EH's, can at least somewhat describe what is beyond an EH. But we cannot measure anything beyond an EH, so we cannot know for certain.
We've talked about the Planck scale before, I said I understood it as the smallest possible distance. Today PBS Spacetime covered this in some detail. Matt gives a pretty good explanation, using a lot of maths beyond my grasp, into why the Planck length is so important. There's a few angles from which to look at it from. One, which I think we've discussed before, is that to measure distances shorter than the Planck length, you'll need a photon with a wavelength shorter than a Planck length. The problem is that such a photon will have enough energy to create a black hole, the threshold is the Planck length. Heisenberg's UP plays an important role too, if we successfully measure a distance shorter than the Planck length, we have 100% uncertainty in momentum and energy. This gives rise to virtual particles pairs emerging from the quantum vacuum. Let's say we're trying to measure the precise location of an electron... if we compact the mass into a volume smaller than the Planck length cubed, electron/positron pairs emerge and collide, possibly with the original electron we're trying to locate. This gives rise to a fundamental uncertainty in the location of the electron... if we accurately measure it, it might disappear and reappear a Planck length or two away from where we just measured it.
idk if that even makes any sense. It's way beyond me.
https://www.youtube.com/watch?v=snp-GvNgUt4
I've seen it.
Of course, there are some significant ways there are problems with measurement of extremely small distances. The Heisenberg Uncertainty Principle is going to mess with us if we try to push it to extremes.
I still don't think the Plank Length is significant for the reasons you've been putting forward in the past, unless I've dramatically misunderstood your point(s).
In the video what they're saying is that it's not so much the Planck Length that's significant as it is that the curvature of spacetime is more and more extreme in the vicinity of photons of ever-increasing energy.
One significant thing Matt was saying is that this distortion of spacetime means that the concept of distance loses meaning. Like, a black hole is an object whose diameter is larger than its circumference... so knowing the apparent outside diameter of the black hole doesn't tell you the diameter measured within the black hole. I.e. distance has lost its colloquial meaning.
What he did not say is that distances that small do not exist. What he did not say is that the universe has a "minimum step" distance that anything can move. What he did not say is that position is limited to points on a grid.
These are the things I've thought you were claiming about the Planck Length.
What he said was position-momentum uncertainty yields obscene levels of uncertainty in momentum when position is constrained to ever-decreasing sized volumes. So much that if you want to confine an electron to too tiny a volume (on the order of a Planck Length cube) that we can no longer be certain that there's exactly 1 electron in the volume.
Note that he repeatedly says phrases like, "when this volume is close to the Planck volume" the energy uncertainty approaches the energy of the particle in question - indicating that the exact volume is a function of each particle, I think. I mean that the energy uncertainty is a function of volume, but it takes different amounts of energy to create different particles, so the volume for each particle would be different.
Also note he does say it's just a trick of mathematics to get units of length out of other known constants.
Also note that he uses h_bar in his Planck Length, instead of h. This is an arbitrary choice and leaves us a factor of 2pi discrepancy between 2 possible choices for the value of the Planck Length. Which is more physical h or h_bar? It's an empty question, they differ by a factor of 2pi. We don't tend to consider constant factors to be more or less physical. It the relationship shown by the factor that matters, not the value of the number, as such. So... why choose h_bar over h? Why choose h over h_bar? There's a difference of a factor of over 6.25 between them. If we're going to tie physical meaning to the Planck Length, then we should have a solid argument for which of those (h or h_bar) is correct.
This was not done in the video. He didn't even mention that an arbitrary choice in there means we don't even know which value of the Planck Length is "the" value. All we are really saying is there's QM and GR weirdness when trying to discuss extremely tiny distances - distances on the order of a Planck Length. The Planck Length isn't really that significant, so much as there's a soft limit at the bottom where the universe simply refuses to define such small distances.
I've probably not used great language but I believe I was basically arguing that spacetime is quantised and the Planck is the base unit. That could of course be completely wrong.Quote:
I still don't think the Plank Length is significant for the reasons you've been putting forward in the past, unless I've dramatically misunderstood your point(s).
I mean, distance is weird already. It's FoR dependent and therefore not an absolute quantity. Distance changes just by accelerating. But does the Planck length change with acceleration? Or is this another "constant to all observers" thing? I kinda feel like it's the latter, like the speed of light. And in fact, I don't think that comparison is something to dismiss. Just like the speed of light is critical to General Relativity, to the macro, the Planck is critical to QM, the micro. There's an important relationship between the Planck and c that we haven't yet figured out, understanding that relationship is the key to quantum gravity. Planck is space, speed of light is time... space and time... spacetime.
If they're discussing if space is quantised, then we're talking about if distances that small exist or not. Obviously he didn't say there are no smaller distances, but he is saying that the Planck seems to point us in this direction of thinking. The title of the video is exactly this question... can space be infinitely divided?Quote:
What he did not say is that distances that small do not exist. What he did not say is that the universe has a "minimum step" distance that anything can move. What he did not say is that position is limited to points on a grid.
These are the things I've thought you were claiming about the Planck Length.
It's interesting that this factor is 2pi. I mean, earlier you were talking about the diameter and circumference of a black hole. It seems to me that the "arbitrary" choice of h or h-bar depends if you're applying the value to a straight line or a flat plane.Quote:
This is an arbitrary choice and leaves us a factor of 2pi discrepancy between 2 possible choices for the value of the Planck Length.
I mean, I really don't know why there's two different values, and which one is "right". But the fact the ratio between the two is 2pi means it's very probably got something to do with geometry. They're both right in different contexts.
Fun fact - protons taste sour.
The takeaway I got from the video is not that the Planck Length is a minimum distance something can move or anything about position existing on a grid of any kind. What he said was that photons that could elucidate such tiny distinctions in position would be so high in energy that they would distort spacetime to the point where "distance" loses meaning.
I thought for a moment about implications to QM if the distance between particles cannot be known exactly in any frame, and I came up with a kind of irrelevance to that kind of thing in QM. What matters is the many fields incident on a particle and the available paths (under all applicable conservation laws) that the particle can take to change its quantum state. The fields incident on the particle - or more specifically the fields interacting with the particle's wave function - may originate from other particles or their movements, but that's kind of a 2nd order association.
So did I just kick the ball down the road to what happens in the fields?
I don't think so, but it's a whole - one problem with GR and QM is that GR assumes you can know the exact position of ... well, stuff... in spacetime and QM emphatically says you cannot.
And this is right there on the boundary where QM weirdness (the Planck Length) and GR weirdness (warping of spacetime) come together in a way that causes us to scratch our heads.
I'm wondering if there are other ways to probe such a small distance, like perhaps gravity waves. I wonder how close the 2 black holes ina merger get before they merge into a single entity. I wonder if there's a way to tease anything about that out of the gravitational waves we're picking up, now.
***
The factor of 2 pi comes from making an arbitrary choice of whether you choose h as a "fundamental" constant or of you choose h_bar as one. What you choose as "fundamental" constant has been under debate in the physics community for a long time. It is an arbitrary choice. We could have chosen the distance from the Earth to the Sun as a fundamental unit of length, and then the "Planck" Length would just be that distance - the juggling of numbers in the powers of fundamental constants to tease a length unit out of it would be bone simple. There would be a length unit that is considered fundamental already.
In choosing fundamental constants, you get the same units from either h or h_bar, as those only differ by a factor of 2pi. So if you choose to include either as a fundamental unit of action (this is widely regarded as an epicaly good move), then as far as the completeness of your dimension space (a math term that means you can get ANY needed unit from juggling the fundamental units) - as far as that goes, it's identical.
So you have an arbitrary choice within your game of arbitrary choices that lets you pick whether or not to include that factor of 2pi with no mathematical ramifications to the completeness of your units in your "fundamental" constants.
I've explained this to you before, and provided links to pages explaining the Buckingham Pi method of juggling numbers. You can see in that method that there is not necessarily physical meaning in that number juggling. Maybe there is, but the mere presence of a number that comes out of it being "large" or "small" is as likely coincidental.
I saw that. I freaking love Steve Mould's YouTube channel.
GR seems to be an incredibly accurate approximation. Only when we get to the Planck scale does this accuracy break down. It of course doesn't mean GR is wrong, just that it is yet to incorporate QM.Quote:
I don't think so, but it's a whole - one problem with GR and QM is that GR assumes you can know the exact position of ... well, stuff... in spacetime and QM emphatically says you cannot.
But there must be a way for GR and QM to unite. The universe is proof of that.
Maybe, though it seems the necessary measurement accuracy is ridiculous and well beyond what we can hope for any time soon, if ever.Quote:
I'm wondering if there are other ways to probe such a small distance, like perhaps gravity waves.
Gravity waves propagate at the speed of light. That kinda implies that these waves behave similar to light. Gravity waves are a form of energy, just like light, and so maybe they're the same thing at the fundamental level. Maybe gravity waves are photons. If so, then gravity waves aren't helping us any more than light.
We've talked about this before. I'm unconvinced any such merger actually happens. I'm inclined to think they basically orbit each other forever at the speed of light (or close to). How do two singularities collide? Of course this is based on nothing more than a sense of intuition, which is kinda ridiculous in these extreme conditions, but if a singularity takes up precisely zero volume, then no such collision seems possible. Rather, they continuously miss each other.Quote:
I wonder how close the 2 black holes ina merger get before they merge into a single entity.
Ok, I don't get any of this to be honest. I don't understand why there's debate about which of these values is "right". The Planck constant is derived from the relationship between photons and energy. The Planck length is derived from this constant. idk why there would be two possible values. But the Planck constant is... constant.Quote:
The factor of 2 pi comes from making an arbitrary choice of whether you choose h as a "fundamental" constant or of you choose h_bar as one.
I really like Steve Mould. He has the glazed look of a man that has enjoyed his life. I suspect he used to have a cocaine problem. There's something about him that makes him... interesting? idk, I don't intend to glorify cocaine use, but he looks like he's partied at university, let's just say that.Quote:
I saw that. I freaking love Steve Mould's YouTube channel.
Veritasium, on the other hand, is clean as a whistle.
Remember that QM is as accurate (more in some aspects) as GR.
This is now my favorite quote.
:D
According to QM, if gravity has a quantum particle, a graviton, it wont be a photon.
Photons are spin-1 and gravitons (though not yet observed) are spin-2.
I believe if anyone can perform a repeatable experiment in which it is shown that a massless particle of spin-2 exists, that will earn them a Nobel for the discovery of the graviton.
My bad. I was talking about the distance between the event horizons the moment before they combine into a single event horizon. Though, now that I say it that way, I suspect we have the same problem with the extreme curvature of spacetime making the notion of distance meaningless.
That relationship is E = hf, where E is the energy of a photon, h is Planck's Constant, and f is the frequency of the photon.
So if we're using that equation as the origin of h, then we should not use the reduced constant, h_bar, in calculating the Planck Length, and yet... we find that when anyone talks about the Planck Length, they've used h_bar in the calculation, and not h.
Why?
Because h_bar seems to capture something about angular momentum in it. The units are the same, but it comes up with the reduced constant in many QM equations, like Schroedinger's Equation. And that seems relevant.
And when picking which constants are "fundamental" ... that's a human game. The universe has these constants. They are inter-related. There are way more of them than are strictly needed, in a mathematical sense, to define everything. Mathematically, these constants are not all independent. So we can play a game in which we determine the minimum number of constants such that they are all independent, yet form a complete set by which ANY constant could be calculated.
Like in SI. We consider the fundamental constants to be meter, second, kilogram, Ampere, Kelvin, mole, and (totally unnecessarily, as it is redundant, dependent on other constants already) candela. Candela is even dumber than the Planck Length, so don't get me started. Candela is so dumb, even spellcheck rejects it.
So in SI, the "Planck" length is 1 meter. The "Planck" energy is 1 kg m^2/s^2 = 1 J. OK, so what am I getting at? Maybe it's more clear to look at electric charge. The "Planck" charge is 1 As (amp-second), which is 1 Coulomb, or 1.6(10)^19 times the charge of a proton. What is the significance of this charge? History, really. Our ability to measure currents, kinda... when the definition of an Ampere and a Coulomb were invented.
So the mathematical method of deriving "Planck" units is just a game to be played.
Playing the game with base units of c, the fine structure constant, h or h_bar, the charge of a proton, the Boltzmann constant, etc.. when you do that, we expect some cool surprises. Like to find meaning in the Planck Length, because the units we've chosen as fundamental aren't tied to humans aside from our ability to accurately measure them.
BUT, the meaning in those things is not guaranteed. And just because we can juggle some measurable numbers to come up with a length unit doesn't mean the magnitude of that length unit is relevant to the universe as a limit.
As Matt said in the video, it's not *at* the Planck Length that the QM weirdness suddenly happens. The quantum wierdness happens when the certainty in a finite volume containing a particle over-constrains a volume to the point that the number of particles in the volume becomes uncertain happens "near" the Planck Length, but its exact volume varies for each particle.
So the exact value of the Planck Length may not be meaningful, even if there is a meaningful something going on near that length scale. The juggling of numbers to derive the Planck Length is not based on physics, but math. The numbers being juggled are based on physics, but the way they're being juggled is not.
LOLOL.
I know exactly what you mean.
BTW, the wind car on Veritassium stirred up a whole lot of controversy in the physics world, with qualified doctorates in physics betting thousands of dollars against each other that the other was wrong. Apparently, Derek from Veritassium is about to receive a $10k payout from Alex Kusenko.
https://docs.google.com/presentation...0eb9892c_0_180
The green pages are Kusenko's critique of the claim in the Veritassium video. The white pages are the rebuttal to the critique.
I'm about to go to bed so I'm not replying in depth yet, but I just want to ask you a question.
According to the Standard Model, I believe. Do you believe the graviton exists? GR tells us gravity is merely (lol) warped spacetime. That is, it's not a "thing" like a photon. It's an illusion, like the g-force you get when you accelerate (which I always thought should be called the i-force for inertia, but I digress). Does the g-force have a particle? That sounds ridiculous. So why would gravity? I've never been comfortable with the idea of a graviton.Quote:
According to QM, if gravity has a quantum particle, a graviton, it wont be a photon.
I'll read your post properly tomorrow after work.
No, my gut says there wont be a graviton, but there is room in the Standard Model to account for such a particle.
It's hard as a scientist to say something doesn't exist. Just because it hasn't been observed, yet, it could just be that we're bad at observing some things. We've only been able to observe gravitational waves for a few years, now. Maybe if we knew exactly how to look for particle nature in those waves, we may elucidate something.
Like, could we find a way to perform Young's Single- or Double-Slit experiment on ripples in spacetime? What could we conceivably use as a boundary?
QM and GR are both incomplete on their own. But together they seem to describe the universe as accurately as we're able to measure. They must be compatible.Quote:
Remember that QM is as accurate (more in some aspects) as GR.
Distance is "meaningless" anyway. We don't agree on distance unless we're moving with exactly the same velocity. Distance is entirely relative. You're in USA at a different latitude to me. You are therefore spinning at a different rate to me. Our velocities are different. So we observe different distances to the moon, for example. Of course, the discrepancy is completely unmeasurable to us without serious equipment, but there is a discrepancy. So what does distance mean even in the classical universe?Quote:
My bad. I was talking about the distance between the event horizons the moment before they combine into a single event horizon. Though, now that I say it that way, I suspect we have the same problem with the extreme curvature of spacetime making the notion of distance meaningless.
It just becomes more meaningless as velocity increases, or space decreases. But it was already meaningless.
btw, "meaningless" is probably the wrong word. Not sure what the right word is though. But distance has meaning in your own frame of reference.
Yeah this is well beyond me. But so long as the Planck constant remains constant, then all is well. idk which of the lengths is "right". It's interesting this is related to angular momentum though. That's usually a 3D thing, but the 2pi ratio is very much a 2D thing. You're turning a line into a circle (radius to circumference), but not into a sphere. Curious.Quote:
Why?Because h_bar seems to capture something about angular momentum in it. The units are the same, but it comes up with the reduced constant in many QM equations, like Schroedinger's Equation. And that seems relevant.
I understand why these "fundamental" constants are arbitrary. I also understand that once one arbitrary value is chosen, that affects others. Like the millilitre and kilo, a gram is the weight of a millilitre of water. Change a millilitre, and you need to change the gram, too, which in turn changes the kilo.Quote:
We consider the fundamental constants to be meter, second, kilogram...
But the Planck isn't like this. The Planck is derived though equations, it's not arbitrary (at least when we talk of the constant rather than length). So there's something more fundamental about the Planck constant than the meter.
Ok, but the exact value of the Planck constant certainly is meaningful.Quote:
So the exact value of the Planck Length may not be meaningful
I saw this vid, it's interesting that it caused a stir in the physics community. I'll look into this.Quote:
BTW, the wind car on Veritassium stirred up a whole lot of controversy in the physics world
The graviton doesn't make sense to me. If such a particle exists, then surely these particles are produced whenever anything accelerates. To argue that the graviton only exists because of gravity is to argue that Einstein's equivalence principle is wrong.Quote:
No, my gut says there wont be a graviton, but there is room in the Standard Model to account for such a particle.
The notion that space is expanding tends to make me ask, into what is it expanding?
But the way we talk about the universe expanding is totally different than the way we talk about everything else expanding.
When we talk about anything else expanding, we are talking about looking at an expanding thing from the outside, and describing the way it expands into an embedding volume. A sphere doesn't simply exist alone. It exists within a volume from which we can describe the sphere. If we say the sphere is expanding, we're saying that the sphere is expanding from the outside, and displacing that embedding volume.
It is a subtle point, but the way we measure the universe is from the inside. We do not observe any embedding medium in which the universe exists. As such, what we are observing and how is actually very different.
The notion that the universe is expanding on the inside does not imply anything about what the universe is doing from the outside, neither expansion nor contraction nor even stasis. Nothing at all.
Mind blown.
That's definitely interesting to think about. My first thought is that we can't really talk about space expanding without thinking about time. Is spacetime expanding? Or is space expanding while time contracts? Does space expand into time, and vice versa? The relationship between space and time seems reciprocal, so I'd be inclined to think space is expanding into time. In a contracting universe, the opposite would be true. There's a way to think about this, and it implies the universe is finite in time. If the universe is finite in time, then as space expands, there is less time. So we can imagine space expanding into time in that sense. But in an infinite universe, well I guess time could be an infinite empty void, and space is a bubble within that is destined to expand forever. But that seems even more bizarre to think about.
I've seen an episode of PBS that makes claims along the lines of space and time switching places inside a black hole... that is, space is one dimensional and time is three dimensional. I can wrap my head around the concept of one dimensional space, all geodesics point in the same direction... towards the singularity... and it is only in this direction you can go. But I do not understand how time can be three dimensional. I mean sure, there's the past, present and future, but if you can go back in time inside a black hole then why not go back to a time when you weren't inside the black hole? It kinda implies that it's not possible to go inside a black hole because there is no part of your past when you were inside the black hole. So that's a mind bender too.
I remember those PBS spacetime vids.
Keep in mind that they also said that while you are free to move about in time, you are irrevocably bound to move forward in space toward the singularity. I can't wrap my head around that, either.
It's just a lazy hack in our poorly designed simulation.
This is one of the best Veritasium videos ever.
https://www.youtube.com/watch?v=bHIhgxav9LY
I'm just picturing the energy flowing through my walls and into my computer and lights and other shizwaz.
So mesmerizing.
Like walking in the snow after I finally understood vector fields.
All these tiny arrows flitting about in their little dance. So cool.
This is frustrating. It's 12 minutes of thanks for the refresher and then he spends no time on how he arrives at 1/c, or that electrical signals travel faster than the speed of light according to him.
I think he might be talking about induction, but if he is he should say so.
Well I correctly guessed it would be 1/c, but I don't really know why, other than to say that I know what's not happening is electrons are speeding around the circuit. What I don't get is where this 1/c takes into account the distance between the switch and the bulb. The time I think it should take for the bulb to come on is the time it would take a photon to travel in a straight line from switch to bulb. That depends on the distance between the two. but 1/c seconds is just a fixed 3x10^-9 seconds.
But c isn't just 300000000, it's 300000000 m/s. c is a measure of distance over time, not just a plain number.
I think when he says 1/c that he's saying the bulb comes on at the speed of light in a straight line between switch and bulb, not the time it takes light to complete the circuit. I think 1/c is the speed of light, written as time rather than distance.
I knew I'd seen something else on this subject...
https://www.youtube.com/watch?v=C7tQJ42nGno
Basically, charge and energy are not the same thing. Both are flowing when there's a complete electric circuit, but while the charge moves with the current. along the wire, the energy flows perpendicular to the charge, into the electromagnetic field.
The complete circuit means electrons begin to flow. Moving charge creates a magnetic field. Energy flows in this magnetic field. Energy is not flowing along the wire. It's flowing radially out of the battery, perpendicular to the wire, and into the device. The energy flows through space. The current merely creates the electromagnetic conditions needed for the flow of energy to be able to happen through space. The device takes energy out of the EM field, and the battery replenishes it. When the battery has no power left, electrons stop moving, and the EM field collapses. Energy flow ceases.
It's all a bit crazy though, not remotely analogous to water flow.
He's vague about induction, and as ong pointed out 1/c is not a time, so his conclusion there is, without further explanation from him, nonsense.
The induction thing is like... If the wires go out 1/2 a light second in each way, then sure, some of the energy flow goes straight across almost no distance and the light bulb can start to glow. But other energy paths are longer, and the energy spends more time to travel those paths. So the light bulb will slowly turn on over time, as all those paths of energy eventually reach the light bulb.
Similarly, if the battery were to be switched off, the light bulb would immediately start dimming, but it wouldn't be fully off for a longer time.
So he's talking purely about the EF from the battery inducing a theoretical current in a theoretical 0 ohm lightbulb? Induction wouldn't be instant, neither would be capacitive coupling, and both would only make the bulb blink once.
So the entire circuit is just misdirection?
I suspect it's an accurate use of the term. He said 1/c, not 1/c seconds. The time is already factored into c, it's just not obvious.Quote:
He's vague about induction, and as ong pointed out 1/c is not a time, so his conclusion there is, without further explanation from him, nonsense.
If it takes 1/c, that's the same as saying it takes 1/300000000m/s - the "per second" is there. But it has distance, too. So it takes 1 second for every 300000000m in distance... the speed of light. If the distance between switch and bulb is 1 meter, then it takes 1/300000000 of a second in time.
1/c simply seems like the correct way to write down the speed of light as a function of time over distance, rather than distance over time.
^ that is...
1/c tells us how much time it takes something to travel a given distance at the speed of light.
c tells us how much distance something travels in a given time at the speed of light.
Sort of. The circuit allows electrons to flow, and the flowing electrons create the electromagnetic field necessary for the flow of energy. Without a complete circuit, electrons don't flow and energy cannot be transferred electromagnetically. So the circuit is an essential part of this system. But it's misdirection in the sense the energy does not flow round the circuit. Well, some does, very slowly, and it's a tiny amount related to the charge of the electron, not the power of the source. The energy flow from battery to bulb is through space, not through the circuit.Quote:
Originally Posted by oskar
What I struggled to get my head around is how an electron can instantly "know" a circuit is complete. But the conclusion I came to is that the circuit was completed, except for the switch, in advance, and so the entire circuit is already electrically primed, for lack of a better phrase. The electrons have already "communicated" the electrical status of the circuit. The switch is merely the final piece of the jigsaw, and that information moves through space at c. If you were to cut the circuit in space, 0.1 light seconds away, then the EM field would collapse, starting where the break is, and propagating at c, reaching the bulb in 0.1 seconds.
Not sure if you're following what I'm trying to say, it's not easy to explain, especially given I'm basically guessing.
It isn't clear to me what he's talking about. If you assemble his rig with non bullshit components and replace the lightbulb with a detector of some kind, i'm almost entirely positive that you would see a charge only after [wire length]/c.
But I could imagine that there is something happening, because electric charge weirdly does seem to know where to go. It seems obvious in a wire, but it has always weirded me out that a fault current will return to the transformer it came from. It's should be no different inside a wire, but my monkey brain thinks it makes a difference.
Well, aside from pointing out that a light bulb is a detector of some kind, I'm thinking this is categorically wrong. But I'm basing this off what he's saying, and what Nick Lucid says too (the link I posted). I'm taking their words for it. But you do have to abandon any idea of how you think electricity flows, you have to stop thinking it's like water flowing through pipes. That's a useful analogy for simplistic purposes, but misleading when you scratch under the surface.Quote:
Originally Posted by oskar
When you understand that charge and energy flow perpendicular to one another, you realise that if charge flows along a wire, energy cannot. So it's clear to me that the simple model isn't a realistic comparison.
(1 m)/c is a time. 1/c is not. He may have meant to say that the battery and the light bulb are 1 m apart, but either didn't or edited that part out for other reasons.
He does list the answer as 1/c s, which is even more absurd, but I'll ignore it, as I have a sensible interpretation on this already.
He says the wire is ideal, and I think we can assume the entire setup is ideal. I'm not fresh on my circuits knowledge, so I'll ask: Does that allow us to ignore the self-induction of the circuit and (I don't know what this one is) capacitive coupling?
Ong hit on something with the "primed" circuit. The battery has delivered charge out both of its ends to the wires connected there. before the switch is closed, the system is in equilibrium, so the voltage at the bottom of the battery is the same as the voltage along the entire wire connected to the bottom, and vice versa for the top and its voltage.
Where I get confused is when the switch closes, the different potential meet, and current begins to flow. That current flow will expand out in a wave in both directions along the wire (which is the path of least resistance). The current in a wire creates a magnetic field that circulates around the wire. (The "in a wire bit" is important, as there is no reference frame w/o moving charges, so the creation of the magnetic field is there in all reference frames.)
Ohhhh. I see, I see!
As the induced magnetic field at the battery is created, that changing magnetic field propagates to the light bulb at c. That changing magnetic field creates changing electric fields, according to Faraday's Law. So it will induce a changing electric field when it arrives at the light bulb, which will induce a current flow in the bulb that opposes the changing fields. That magnetic field's direction is the same as its direction under the steady-state operation of the circuit.
So the transient will arrive at the light bulb as a sphere expanding at c, rather than a signal contained within the wire at c.
But until the signal can propagate through the wire at c, the circuit cannot be at steady state.
I think that's it.
EEVblog made a video response. The first 25 min. is him reviewing the original video. The interesting part starts at 25:40 when he starts breaking it down as an EE problem:
https://youtu.be/VQsoG45Y_00?t=1540
Way to educate the monkey, oskar!
This ties together so many ideas in my head that I haven't thought about in years.
I only took an intro to circuits course in my undergrad, so the self-inductance of the wire and the capacitive coupling weren't in that 1 semester.
I get it, now.
I wonder if that transient spike he plotted in the simulation was so brief because he only used 4 elements.
Would the actual plot spike up "immediately" and hold?
Or gradually rise to steady state?
Or spike, dip and rise again?
Do you have an intuition on this?
The wire acting as a capacitor would be effectively shorted at first and then increase in resistance as it's charging until it's fully charged and then it stops conducting. If it's DC you'd only get a short spike. If it's AC it would act as a conductor with the phase shifted by 90°.
Is there enough mass within a typical solar system to supply the raw materials required to build a dyson sphere ? Would the dyson sphere rotate around the sun to maintain its orbit during construction.
Yes, but the "short spike" from the DC happens over the entire wave passing down the wires from the switch/battery, right?
So like... there's the "immediate" response due to the distance between the switch and the light bulb. But then that wave of voltage/current flow expands out in the wires at c_wire. Causing a wave of those capacitave coupled responses that arrive at the light bulb. Each little nanosecond of spreading current creates another pulse that is overlapped with all the others.
But then, eventually, all the current gets out to that furthest distance (1/2 light second away), and starts coursing toward the light bulb.
Does the "short spike" last 0.5 s? Is that an 0.5 s plateau that then decays, or an impulse spike that has decayed to 0 after 0.5 s?
And how does our picture of the circuit change as we round those furthest distant points in the circuit and the current wave is now heading back toward the bulb?
Like, in terms of current at the bulb.
Is it a spike, dip and rise to V-0?
Is it a quick spike that holds at V_0?
Is it a gradual climb to V_0?
Something else? The transient ripples of the switch's discontinuity in time manifesting over presumably many seconds?
I'm fascinated by this.
Good point, I think you're right, it would probably spike up in exactly m/c and then actually stay up for the entire second it takes for the circuit to close. One thing is giving me a headache: If you think of it as a series of capacitors, then the last one will conduct at 0,5s, but the current will not see our load until 1s after the switch is turned on, or just as the actual real world usable circuit would be completed. So that means our series of capacitors have been conducting for only half a second, but they have been creating a current at the load for a full second. What does that mean for what we would measure at the load for that first second?
Not probably?
Given all the non-solar mass in the solar system, and assuming average density, and a shell with inner size equal to Earth's orbital size, that spherical shell would only be ~3 - 8 in (~8 - 20 cm) thick. Given the stresses involved, I can't imagine a material we could manufacture through any possible means to hold itself together with only that thickness to work with.
But then... all of this is theoretical fantasy land compared to the current human tech.
Other planetary systems (technically, only our star, Sol, has a Solar system) that we know of seem to be pretty on par with our own, though with fewer planets in general. But the ones they have are big. Though, we can only see planets that are "big enough" from this distance, so we're not too, too confident that we can see every planet around another planetary system. Right now, it looks like having 8-ish planets is a pretty big number of planets for a star to have, but also having 0 planets is unlikely. It's hard to say with any certainty how "like ours" those planetary systems are, or how "normal" ours is compared to all the rest.
Do note that Dyson Sphere's are unstable. There would be no net gravitational force between the sphere and the star at its center. Nothing would hold the star from drifting into the wall of the sphere, or vise versa. There would need to be active controls to prevent that. But hey, we're daydreaming, so why not just have those, too?
A Dyson swarm is more practical. Where you have a bazillion objects in orbits around the star, such that (nearly) all its light is captured by the swarm. These can be built independently, on "small" scales, creating a colony at a time.
Interesting new experiment to find dark matter, brought to us by Veritasium.
https://www.youtube.com/watch?v=6etTERFUlUI
We might actually have been watching that at the same time, assuming you posted shortly after watching. Can't say precisely what time I saw that but it was a few hours ago!
I mean, there's a whole universe out there we can't see. That much we already know. Dark matter is basically "stuff with gravity that we otherwise can't see". Find a more concise definition than that!
What kinda fascinates me is the thought that to dark matter aliens, we're the dark matter. They'd look at their galaxies and think "there's something wrong with this picture". They know we're here because they can see our gravitational effect, but they can't see us. They don't even know that we have a whole variety of elements, molecules, planets, stars, life. We're just stuff, a theory, a mathematical mystery.
How do you send Morse code by using gravitational waves? We can say hello that way.
I'm guessing the vibrations impact the dark fluid differently than the medium it's in because of something to do with density.
amirite?
https://twitter.com/wonderofscience/...22502353547272
Nah, bro. It's totally magnets.
Ferrofluid is basically iron shavings in oil. Except there a much harder to pronounce word for that.
Ferrofluid is that, but when the iron particles are down to nano-particle size.
They're suspended in the oil kinda like coffee is tiny particles rinsed off a coffee bean suspended in water.
And speakers work by magnets pushing and pulling a cone back and forth to wiggle the air just so.
There's a video on the YouTubes where the artist shows how he made the thing, but doesn't show anything of what the actual magnet interacting with the ferrofluid is.
It looks like that is a single unit behind the dancing blob, and not multiple bits n bobs arranged around it, but the shots could be manicured to look like that, too.
That single unit looked like a speaker magnet with the speaker removed.
So it could be the AC magnetic field created by the normal speaker magnet behind the thing
combined with the oil and water fluid surface and surface tension interactions at the boundaries between the blobs
is enough to get those crazy dancing shapes.
IDK what all the circuitry is for if that's the case, though.
Oh yeah, I guess the ferrofluid wouldn't hover in the medium like that if it had a different density.
Magnetism makes much more sense.
Ok, wtf is going on here?
https://twitter.com/wonderofscience/...68093719904258
My guess is the pressure is compressing the H20 molecules so more space is taken up by the starch, effectively the same outcome as if you just removed some of the water.
That stuff is called Oobleck. It's literally just corn starch and water.
If you've never played with it, then you definitely 100% should fucking make some when you get home. If you don't have any corn starch, then buy some. It's cheap AF. Almost free.
Just mix that shit in water. Guess the amount and adjust as you go. Literally can't get it wrong.
In the video you posted, there's frankly just a bit too much corn starch, and adding a bit of water would allow it to really go back into the runny state when he stops working it.
Mythbusters made a tank of the stuff and one of them stands on it... as long as he's pumping his legs like he's running up a mountain. As soon as he stops, he sinks.
Fun fact: a physicist at my work invented speed bumps out of the stuff. It's only a bump if you're driving too fast. Go over it slowly, and it squishes out of the way. Go over it too fast and it hardens into a solid bump. That's cool AF, but it never really caught on. Only place I see them is on campus.
I'll look into the deeper physics of it, though, to try to explain what is actually happening on the molecular level.
I'm not actually sure.
This is really long, and I only had time to scan it and read the intro and conclusions and pause to look at some pretty pictures.
I'm putting it here for reference to dig into it later.
https://www.pnas.org/doi/10.1073/pnas.1908065116
Seems that it all comes down to shear strain moving the suspended particles into close contact with each other, where friction between those particles can dominate.
Basically, when the fluid tries to move out of the way, it causes the suspended particles to touch each other and lock up. That locking up stops the shear strain, and the particles relax back into the fluid, losing their contact forces with each other.
Good, succinct explanation, thanks.
I saw that mythbusters. There was another one where they tried to swim through syrup. Seems they're quite keen on fluid dynamics lol.