**Ask a monkey a physics question thread**

[OngBonga]

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 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.

Dude. I got this. It's not in the Standard Model of Particle Physics.

Well that's the point, isn't it? If the SM is missing gravity, it's incomplete.

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.

Found me something to half arsed research.

I can't help but point out that string hypotheses

haha I've found your kryptonite.

[MadMojoMonkey]

haha I've found your kryptonite.

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.

[OngBonga]

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?

[MadMojoMonkey]

:/

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?

[OngBonga]

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.

[MadMojoMonkey]

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.

[OngBonga]

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.

[MadMojoMonkey]

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.

[OngBonga]

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'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.

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.

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.

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?

This is an arbitrary choice and leaves us a factor of 2pi discrepancy between 2 possible choices for the value of 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.

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.

[OngBonga]

Fun fact - protons taste sour.

[MadMojoMonkey]

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.

Fun fact - protons taste sour.

I saw that. I freaking love Steve Mould's YouTube channel.

[OngBonga]

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.

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.

But there must be a way for GR and QM to unite. The universe is proof of that.

I'm wondering if there are other ways to probe such a small distance, like perhaps gravity waves.

Maybe, though it seems the necessary measurement accuracy is ridiculous and well beyond what we can hope for any time soon, if ever.

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.

I wonder how close the 2 black holes ina merger get before they merge into a single entity.

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.

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.

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.

I saw that. I freaking love Steve Mould's YouTube channel.

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.

Veritasium, on the other hand, is clean as a whistle.

[MadMojoMonkey]

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.

Remember that QM is as accurate (more in some aspects) as GR.

But there must be a way for GR and QM to unite. The universe is proof of that.

This is now my favorite quote.

Maybe, though it seems the necessary measurement accuracy is ridiculous and well beyond what we can hope for any time soon, if ever.

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.

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.

We've talked about this before. I'm unconvinced any such merger actually happens.

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.

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.

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.

I really like Steve Mould. He has the glazed look of a man that has enjoyed his life.

Veritasium, on the other hand, is clean as a whistle.

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.

[OngBonga]

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 QM, if gravity has a quantum particle, a graviton, it wont be a photon.

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.

I'll read your post properly tomorrow after work.

[MadMojoMonkey]

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.

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?

[OngBonga]

Remember that QM is as accurate (more in some aspects) as GR.

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.

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.

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?

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.

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.

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.

We consider the fundamental constants to be meter, second, kilogram...

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.

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.

So the exact value of the Planck Length may not be meaningful

Ok, but the exact value of the Planck constant certainly is meaningful.

BTW, the wind car on Veritassium stirred up a whole lot of controversy in the physics world

I saw this vid, it's interesting that it caused a stir in the physics community. I'll look into this.

No, my gut says there wont be a graviton, but there is room in the Standard Model to account for such a particle.

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.

[MadMojoMonkey]

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.

[OngBonga]

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.

[MadMojoMonkey]

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.

[CoccoBill]

It's just a lazy hack in our poorly designed simulation.

[MadMojoMonkey]

This is one of the best Veritasium videos ever.



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.

[oskar]

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.

[oskar]

I think he might be talking about induction, but if he is he should say so.

[OngBonga]

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.

[OngBonga]

I knew I'd seen something else on this subject...

[OngBonga]

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.

[MadMojoMonkey]

I think he might be talking about induction, but if he is he should say so.

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.

[oskar]

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?

[OngBonga]

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.

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.

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.

[OngBonga]

^ 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.

[OngBonga]

So the entire circuit is just misdirection?

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.

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.

[oskar]

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.

[OngBonga]

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.

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.

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.

[MadMojoMonkey]

(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.

[oskar]

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:

[MadMojoMonkey]

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?

[oskar]

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°.

[Keith]

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.

[MadMojoMonkey]

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°.

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.

[oskar]

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?

[MadMojoMonkey]

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.

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.

[MadMojoMonkey]

Interesting new experiment to find dark matter, brought to us by Veritasium.

[OngBonga]

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.

[Poopadoop]

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

[MadMojoMonkey]

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.

[Poopadoop]

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.

[Poopadoop]

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.

[MadMojoMonkey]

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.

[MadMojoMonkey]

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.

[Poopadoop]

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.

[OngBonga]

Interesting fact - according to observation, there are 7% more galaxies rotating in one direction than the other.

Implication - the universe already had angular momentum when it was born. But angular momentum comes from an external torque force.

It's possible that our region of the universe is special, and that if we observed all galaxies in the universe then we'd have a 50-50 split (thus conserving angular momentum), or it's possible that there are external forces outside of what we consider to be our universe.

[MadMojoMonkey]

I mean... unless they've measured the angular momentum of dark matter, I'd say there's plenty of margin of error in that to be unable to rule out a total angular momentum of 0.

[OngBonga]

I mean, idk about dark matter, it has always felt like a mathematical hack to explain gravity as we observe it. So what, now it's also a hack to explain angular momentum as we observe it?

Dark matter is of course one possible explanation. But it's no different to anything else unobservable, such as a parallel universe, or Planck-scale events, or something happening in the first fraction of a second after the big bang. Personally the most likely solution to me is that our observable universe is, essentially, too small for us to draw conclusions. If we "zoom out" then balance is restored. That's my intuition. But maybe you're right, maybe dark matter is real and has angular momentum.

One potential explanation I saw for dark matter was a "Planck relic", basically the remnants of evaporated black holes. The idea is that a black hole cannot fully evaporate since energy is quantised, so when a black hole reaches the lowest possible mass (1 fundamental unit of mass/energy), it cannot radiate more energy. These "Planck relics" would be far too small for us to observe directly, but if there's enough of them we could certainly observe their gravitational influence on other objects.

I like that idea. Maybe these Planck relics exist and have angular momentum, if so then your idea that dark matter is where the missing angular momentum is would potentially be correct.

[MadMojoMonkey]

Well... almost all of physics is mathematical hacks to predict the future, insofar as it can be predicted.
The rest is cartoons and so-called "laws" which are only distinguishable from the math hacks in pedantic ways.

I didn't really mean to hypothesize that the disparity in angular momentum would be resolved with dark matter.
I only meant that they cannot possibly have enough data to make the claim they're making.

The %-age of even just the Milky Way that we have actually observed is relatively tiny. Much less the actual inner workings of distant galaxies.

My point was more to say that there is simply not enough data gathered by all humans throughout history to make claims about something on this scale.

The best they can claim is that given the laughably less than 1% of 1% of the universe we've observed, we find this trend... maybe that's indicative of large scale imbalance, or maybe its indicative that normal fluctuations should be expected and various regions of spacetime are expected to have different local angular momenta. Like whirlpools in a river. The total rotation in the river is expected to be 0, but at any location, it could be 1 way or the other. How big those volumes of spacetime should be we don't really know. Or at least, I don't know how to define that.

As you said, our entire observable universe may be only a tiny fraction of all the contiguous spacetime - not even counting "parallel" universes, just the same universe, but so far away as to be unobservable... we don't know how big those pockets "should" be.

[Poopadoop]

Well... almost all of physics is mathematical hacks to predict the future, insofar as it can be predicted.
The rest is cartoons and so-called "laws" which are only distinguishable from the math hacks in pedantic ways.

Funny because this is kind of how statistics work too. People expect it to give them definitive yes/no answers when really it's a lot of educated guessing.

[OngBonga]

Ok so recently I've been thinking about how the universe is expanding. We know this because distant galaxies are redshitfted. But I have a problem with this. Distant galaxies don't exist anymore. When we look very far away, we're looking very far back in time. We're not observing the universe expanding now, we're observing it expanding in the past. And it seems to be expanding more in the distant past than in the recent past. Which implies the expansion isn't accelerating, on the contrary it's slowing down (technically still acceleration but you get the gist).

Of course, shortly after the big bang, the universe experienced a rapid period of insane expansion. Is that not what we're observing when we see red shift of distant galaxies?

[OngBonga]

I've just cancelled heat death.

[Vinland]

If they solve quantum gravity, what do you think would be some quick advances?

[OngBonga]

idk what it even means to "solve" quantum gravity. Presumably it means to unify General Relativity with Quantum Mechanics. If it is solvable, what does that imply? That there's a particle for gravity? If the graviton exists, then what happens if we can stop it from interacting with its intended target? Does that mean something can be "shielded" from gravity? If so, welcome to insane propulsion. Probably not useful for moving people around a planet, but great for leaving the planet. Gravity is pulling me down. If I could "turn it off" for me, I'd shoot up at terminal velocity. The acceleration might kill me though, probably 0-60 in a nanosecond.

I'd say the gamechanger for humans will be the accelerating development of AI. Chances are AI will solve quantum gravity, not humans. And when that happens we'll be kind of overwhelmed by all the other crazy things happening, like an energy revolution, medical revolution, nanobots and macrobots doing work instead of humans, cities growing further into the sky, moonbases, Mars tourism, I'm probably going a bit far here but it's more fun to speculate about what AI can do for us than quantum gravity.

[MadMojoMonkey]

Distant galaxies don't exist anymore.

This kinda depends on what you mean by a galaxy's existence.
Galaxies don't "burn out" like stars do. Galaxies are self-sustaining in a sense. Exploding stars send out incomprehensibly massive shock waves through the galaxy. They spread out fused and more importantly, not fused atoms into a nebula.

Exploding stars create a zone with a "high concentration" (on galactic scales - its still more sparse than most man-made vacuum chambers) of materials that could form into a star, as well as other shizwaz that could form planets and other stuffs. They also send out a shock wave that presses other nebulae from other supernovae into each other, and cause new star formation. In this sense, galaxies are efficient at using up some fuel, separating out unfusable materials and re-mixing the fusable materials into new stars.

The process of fusing Hydrogen into heavier elements is actually very slow compared to the amount of Hydrogen fuel available in the Galaxy. For example, the Milky Way is ~13.4 billion years old (in "proper time" i.e. an observer in the Milky Way the whole time would say that's how long they've been here - time is wonky). The age of the universe in proper time is only 13.7 billion years. So you can see that the age of the universe hasn't been very long, yet, on the time scale of galactic evolution.

Andromeda and the Milky Way will collide in ~ 4 billion years or so (IIRC). Would you say that both galaxies cease to exist and a new galaxy or collection of galaxies is born? Serious question. If you would say that, then it is fair to say that some distant galaxies no longer exist, due to mergers with other galaxies. If you would not say that, then even the most distant galaxies probably still exist.

But this is playing fast and loose with time. What we see of those distant galaxies, here and now, is what they are, here and now. There's no way to relate "here and now" to "there and now" without describing how you plan to traverse that spacetime. To the photons we observe of those distant galaxies, no time has passed between leaving that galaxy and being observed on Earth, due to photons experiencing infinite time dilation in their (non-inertial) rest frames.

So the manner in which you cross spacetime makes a difference in how you relate the time between 2 locations in spacetime. The time between events is relative. The order of events is relative - unless they are causally linked.


If you and I coordinated our clocks, and flashed a bright light at each other at the same time from some distance apart, we'd see our own light happen before we saw the other's light happen. So the order of events is relative. But in this case, the turning on of each of our lights was not caused by the other person turning on their light, so there is no causal link between these events.

If we change our experiment such that the moment I see your light, I turn on my light, then the time between these events will vary based on who's observing (I see your light an my light simultaneously, you see a distinct gap in time between them), but no observer would ever see my light turning on before your light, as now there is a causal link in the order of events. The time between your light turning on and my light turning on will vary, but the order will not.

When we look very far away, we're looking very far back in time. We're not observing the universe expanding now, we're observing it expanding in the past. And it seems to be expanding more in the distant past than in the recent past. Which implies the expansion isn't accelerating, on the contrary it's slowing down (technically still acceleration but you get the gist).

There's more than 1 expansion in this story (or more than 1 phase in a single expansion).
There's the big bang. First expansion.
A short time later (to us / on a human scale - it was the entire age of the universe back then) the universe went through a phase of exponential expansion called cosmic inflation.
After that, the expansion rate slowed down dramatically over a relatively short period.
The rate of expansion dropped dramatically after inflation, but it didn't stop. It has continued ever since.


When we look out in space, we look back in time. This is correlated; closer objects in space are closer in time. Further in space means further in time. So we are probing the history of the universe when we look at galaxies of varying distances. As we look at different distances, we look at different times. As we explore the red shift of different distances, we explore the expansion rate of the time between us/here and there/then. So we can build a map in time that shows the rate of expansion as it changes over time.

The part where you said, "it seems to be expanding more in the distant past than in the recent past," is correct and incorrect. It was expanding more during the period of cosmic inflation, but then the rate dropped (inflation ended) to an apparent minimum. As it has been increasing since then, as well, albeit at a far lower rate. That rate has still been increasing since then, just at a much slower rate than what lead to cosmic inflation.

Of course, shortly after the big bang, the universe experienced a rapid period of insane expansion. Is that not what we're observing when we see red shift of distant galaxies?

No, it is not. The period of cosmic inflation happened prior to the formation of atoms, let alone stars and galaxies.

The formation of stars and galaxies doesn't happen for another 200 - 500 million years.

[MadMojoMonkey]

If they solve quantum gravity, what do you think would be some quick advances?

Ong eventually made some decent predictions in his response.

It'd be a new way to understand energy and how that energy is transferred, and that has historically lead to explosions in technological advances that utilize this new knowledge.

The understanding of thermodynamics was a huge shift in tech. The unraveling of Maxwell's Equations and the understanding of electromagnetism spurred the industrial revolution. Advances in Quantum Mechanics and General Relativity have allowed us to make microwave ovens and nuclear power plants.

When we learn a new way to manipulate energy, it tends to have dramatic and unforeseeable consequences.


I'd imagine new state-level power plants and military weapons that make nuclear look like baby toys to eventually come of it. I'd expect advances in space travel would go wild. I imagine the scientific instrumentation we could make and data we could collect would be astounding.

I'm not really a futurist, though, in the sense of imagining what could be if [dot dot dot].
It's a fun game, but not one I devote much time to.

[OngBonga]

I was a bit lose with my language when I said "distant galaxies don't exist anymore", but that was a really interesting correction so I'm glad I said that! A lot of the stuff about simultaneousness I was already aware of, but it's still nice to read. And I knew nothing about the efficiency of galaxies. It's kinda crazy to think that galaxies survive for so long.

I have been thinking about this more since my post, and the concept and a "here and now" is really messing with my head. If I could instantly teleport to this distant galaxy, what would I see? Probably nothing. The galaxy might exist, but it will have moved, a lot, in the time it has taken for its light to reach us. So in this sense I am kinda applying my own sense of "now" in an effort to see the universe as it is in my "now" rather than looking back in time. Which I appreciate is absurd. I might as well visualise it from the timeless photon's pov, which flattens everything and renders distance irrelevant.

I'm still mulling this over.

Although, one more thing. Heat death isn't inevitable. You just told me that the universe has changed its rate of expansion over time. Why would we assume the rate of expansion to play nice with us in the future if it hasn't in the past? We still don't know why it expands. We just make up concepts like dark energy and hope to one day understand what we actually mean by that. So just because it might look like heat death is inevitable, well observers in the very early universe would probably have expected heat death much sooner and are probably surprised to see that things calmed down a bit.

[MadMojoMonkey]

I don't think there's any evidence-based reason to say Heat Death is inevitable.

It's the most boring of the long-term analyses of the universe's evolution. It assumes that we know enough to basically know "everything" now. And what we know now is entropy wins all contests (or at worst ties).

[Poopadoop]

What the highest-order derivative that has a use in the real world?

For example, in movement we sometimes measure jerk (second derivative of velocity, or third of position) as a measure of "smoothness" of a movement, but nothing beyond that. Is there anything in any domain that goes higher than that and what's it used for?

[MadMojoMonkey]

What the highest-order derivative that has a use in the real world?

For example, in movement we sometimes measure jerk (second derivative of velocity, or third of position) as a measure of "smoothness" of a movement, but nothing beyond that. Is there anything in any domain that goes higher than that and what's it used for?

x -> position
(d/dt) x -> velocity
(d/dt)^2 x -> acceleration
(d/dt)^3 x -> jerk
(d/dt)^4 x -> snap
(d/dt)^5 x -> crackle
(d/dt)^6 x -> pop

and no, I'm not even joking about the last 3.

Off the top of my head, elasticity theory frequently uses the 4th derivative, but it's a spacial derivative, not a time derivative. I.e. (d/dx) instead of (d/dt). Elasticity theory covers how materials change shape when forces (loads) are applied. This describes how an object in bending will bend such that it minimizes the curvature for a given loading.

It also applies in civil engineering when determining how a road should bend. Minimizing the 4th derivative (w.r.t. space) makes for smooth turns.


I do not know of any branch of physics or engineering that regularly uses a higher order derivative.

[MadMojoMonkey]

There is some discussion in the scientific community about what time it is on the moon.
In the past, the time used on the moon was the same as whatever country launched the mission. That's not going to be so easy when we start establishing permanent moon bases funded by international efforts.

Furthermore, Einstein taught us that the passage of time depends on local acceleration. The moon's gravity being less than Earth's amounts to the length of a second on Earth not being equal to the length of a second on the moon.

Earth-based clocks on the moon will not stay synchronized with Earth-based clocks on Earth.


There's no answer to how this will be handled, and no real urgent need to solve it. Just an interesting fact that humans will have to start dealing with in the coming years.

Apparently, there's talk of putting GPS satellites in orbit around the moon, so simply ignoring the issues and using GMT (UTC) simply wont work at that point.

[CoccoBill]

Yeah knew about gps satellite time needing to be adjusted, I seem to recall some tens of microseconds per day. Wonder what the difference is on the moon.

[OngBonga]

What's also crazy is that a moon day (sunrise to sunrise) is over four weeks of earth time, so that's two weeks of daylight and two weeks of night, every single "day". I'm not sure I'd enjoy that.

Presumably the solution to the time dilation problem is to simply synch earth-moon clocks whenever they need it. It won't be often. A dirty google search suggest the "lost" time is 0.66 seconds per billion seconds. A billion seconds is just over 30 years. idk how often they'll need to be synched in order for things to work properly, but GPS works so they have the mathematical and technical solutions already.

[MadMojoMonkey]

Yeah knew about gps satellite time needing to be adjusted, I seem to recall some tens of microseconds per day. Wonder what the difference is on the moon.

Ground level on the moon is about ~56 microseconds per Earth day shorter than ground level on Earth, IIRC.

Ong said 66 microseconds, which is close enough for my poor memory to accept.

[MadMojoMonkey]

What's also crazy is that a moon day (sunrise to sunrise) is over four weeks of earth time, so that's two weeks of daylight and two weeks of night, every single "day". I'm not sure I'd enjoy that.

Sure, but you'd be living on the Moon, and that's gotta count for some cool points.

Like people working in Antarctica... it's not the most pleasant environment, but the cool factor is off the charts.

Presumably the solution to the time dilation problem is to simply synch earth-moon clocks whenever they need it. It won't be often. A dirty google search suggest the "lost" time is 0.66 seconds per billion seconds. A billion seconds is just over 30 years. idk how often they'll need to be synched in order for things to work properly, but GPS works so they have the mathematical and technical solutions already.

They'd need it often. Synchronizing Earth-based computers and moon-based computers for communication will be wonky right from the start. Nothing we haven't solved already, though.

There's talk of putting GPS satellites in orbit around the Moon. Once that happens, the notion of any hodge-podge synchronizing between Earth and Moon will be gone. Same reasons as the initial GPS wasn't working on Earth. The Moon will need its own time system, separate from the Earth's to allow GPS to work there.

[OngBonga]

We're talking about two different types of "time" here though. There's actual time, as in spacetime, which is relative, and then there's the human concept of time, which is universal. It might be 9pm for me and 3pm for you (that's a guess) but in five minutes it'll be 9.05 for me and 3.05 for you, with no lost time. Of course, we have our own time, presumably you're at a higher elevation than me so I expect time is very slightly faster for you, but we don't change our clocks based on elevation to account for this and our technology works just fine despite this discrepancy.

Now if that figure of 0.66 seconds per billion seconds is accurate, I can't see why they would need to do anything other than add a microsecond to the slower clock every year or whatever. Maybe the technology is too sensitive for a microsecond difference and so maybe we need to go down an order of magnitude or two, but we can do that. Maybe adjusting the slower clock every hour is sufficient to keep it within a nanosecond, which is acceptable for technology to work.

Whatever time we decide it is on the moon is completely arbitrary and has no roots whatsoever in science. The time we decide is the human concept of time. If we say it's 0:00 right now on moon, in one hour of earth time it'll be 1:00 plus a nanosecond (or whatever) on the moon. As long as our computers expect this, and are coded to account for it, I don't see why it's an issue. It's entirely predictable. The only issue I see is if we need greater time accuracy than our technology allows. That doesn't seem to be a problem for us in 2023 so I don't see why it's a future problem. I guess the need for accuracy could outpace the evolution of accuracy, but that seems unlikely.

[CoccoBill]

Ground level on the moon is about ~56 microseconds per Earth day shorter than ground level on Earth, IIRC.

Ong said 66 microseconds, which is close enough for my poor memory to accept.

Just checked that for gps satellites the adjustment is 45 microseconds per day. The difference between earth local - earth orbit - moon local doesn't sound intuitive considering gravity on the moon is 1/6 of the earth's. I'm sure there's mathemagics to explain it but not intuitive.

[OngBonga]

Of course it's not just gravity that's causing time dilation, things orbiting a body are moving faster than the thing they are orbiting. So there's another source of time dilation and that's probably more significant than gravity.

[CoccoBill]

Moon's mean orbital velocity: 1.022 km/s
GPS satellites move at 3.9 km/s.

Now someone calculate how much of the time dilation disparity is due to gravity and how much due to velocity.

[MadMojoMonkey]

@ong if I'm understanding your long post correctly, you may have misunderstood something.

It's a minor point, and on these scales, not noticeable, but it's there.

The human concept of time is dilated as well. The flow of time is different, and that means your every biological process is happening in a different time flow. Your perception of how long a second lasts is unchanged, no matter your time dilation. You feel 1 second as 1 second, no matter what.

The fact that what you call and feel and measure as 1 second is not the same as all humans is part of GR. However, on the difference-in-time scales we're talking, it would not be anything you could notice.

It matters to computers synchronizing. If a computer operates and sends signals at X Hz, but those signals are being received at X+delta Hz, then the computers will not be able to keep in sync with each other. There will be data loss of the kind we first had to fix when our GPS satellites were fading out of sync with the ground below them.

It's perfectly solvable, and as you said, kinda irrelevant to physics. What humans call a time is perfectly subjective. We're not solving a universal phenomenon. The conversation isn't so much about how to account for the time dilation, it's about ... how do we incorporate what is intuitive about time to humans into the way we track time on the moon when humans will be living there?