**Ask a monkey a physics question thread**

[wufwugy]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

[OngBonga]

I would have thought it's analogous to two volcanos going off, or hundreds. Nuclear winter happens as a result of full-scale nuclear war. A couple of nukes, I assume the cooling effect is negligible, at least on a global scale.

[OngBonga]

I'd also theorise that the best way to initiate a nuclear winter would be to drop a nuke on Yellowstone.

Well, it'd be a nuclear/volacanic hybrid winter, but the nuke started it.

[Savy]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

It's a hypothetical thing that probably doesn't ever happen.

[wufwugy]

You think the hypothesis is likely wrong?

[Savy]

You think the hypothesis is likely wrong?

Given a certain set of criteria it probably does happen. are those criteria likely to ever manifest in reality? Probably not. Very short term cooling effects are likely & happen to some degree but long term nah.

[MadMojoMonkey]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

We (humans) have not yet made a bomb that big. Even the Tsar Bomba isn't big enough for that.

We're talking energy release on the order of a 15 km diameter asteroid impact, such as is likely to have caused (or at least heavily contributed to) the Cretaceous Paleogene extinction event which killed off all the non-avian dinosaurs.

It would take a full scale international nuclear war to release that kind of energy, and I'm not sure it would be a long enough winter to compare with the one mentioned above.

Hiroshima and Nagisaki were altogether tiny explosions on the scale of nuclear weapons. We had much better technology in the following decades (see Tsar Bomba), and are many more decades past that, now.

IDK about the density of the cities as pertains to this discussion. The bombs themselves didn't create much burning. The widespread destruction of buildings and power lines is what created the ensuing firestorm. The Japanese didn't even believe that the bombs were a problem. They were convinced that we sent in massive waves of fire bombers behind the nukes, which of course we did not have the logistical capacity to have done.

I don't think any single nuclear explosion can trigger nuclear winter.

I don't think you can fit an actual nuclear weapon in a suitcase, either. You can fit a dirty bomb in a suitcase, but initiating a multi-stage nuclear event is not child's play.

[BananaStand]

I've picked out a few potential mountain tops from which to view and photograph the Supermoon this Sunday.

I see LOTS of articles posted saying "How to view the Supermoon this weekend", and they all say pretty much "look up"

The mountains I've picked do not all of 360 views of the horizon. Some of them are wooded and only have outlooks in one or some directions. And nothing I can find online is telling me from which direction, the moon will rise.

[Poopadoop]

And nothing I can find online is telling me from which direction, the moon will rise.

Well Timmy, the Earth spins around so that the Sun rises in the East and sets in the West.

So where do think the Moon rises and sets?

[BananaStand]

Your sister's ass hole?

[Poopadoop]

Your sister's ass hole?

I don't have any sisters. So fuck you.

And next time instead of posting your question in the physics thread, maybe start a new one called 'dumb ass questions from someone who failed grade 2'

[OngBonga]

Well Timmy, the Earth spins around so that the Sun rises in the East and sets in the West.

So where do think the Moon rises and sets?

Well, considering it's a "supermoon", it's a full moon, which means it's the opposite side to the sun. As the sun sets, the moon rises. So you want an easterly view. We're approaching winter (or summer) solistice, so bear in mind the further north (or south) from the equator you are, the further south (or north) from true east the sun will set. Which means the moon will rise further north (or south) from true east.

It's not rocket surgery.

[MadMojoMonkey]

I can't leave you guys alone for a few hours without this?
Who pooped in your cheerios, poopadoop?

Anyway.

The Earth spins from West to East, so celestial objects appear to rise from the East and set in the West.

I totally encourage anyone to get out and do some sky watching, but don't get too, too worked up about a perigee syzygy (super moon). It's basically just a full moon unless you're an avid moon watcher. You kinda want to compare it to a full moon from a month or two ago or a month or two from now to get a sense of how it was "super."

So plan on going back to said mountain in a month or two to really appreciate how it's not too much less super under any conditions.

[OngBonga]

The Earth spins from West to East, so celestial objects appear to rise from the East and set in the West.

Mojo's right, I got my brain science wrong.

[Poopadoop]

So you want an easterly view.

No, you got it right. The moon rises in the east.

[MadMojoMonkey]

@ ong:
Aside from the obvious typo, you got it right.
(You said, "[...] from true east the sun will set," but everything else you said indicates you fully well meant to say the sun sets in the west.)

[OngBonga]

I confuse myself sometimes, like a dog shocked by its own fart.

[MadMojoMonkey]

Dat poetry.

[pantherhound]

Hello Monkey

I've found a number of these types of videos that zoom in on things with an electron microscope, and I love them.

https://www.youtube.com/watch?v=dNvdrpEmS48

What sort of preparation would one have to do to recreate a process like this, and is it because it's complicated that we don't see very many of them?

[MadMojoMonkey]

Hello Monkey

I've found a number of these types of videos that zoom in on things with an electron microscope, and I love them.

https://www.youtube.com/watch?v=dNvdrpEmS48

What sort of preparation would one have to do to recreate a process like this, and is it because it's complicated that we don't see very many of them?

Hi, Panther.

The short answer is money. A cheap, used, low resolution microscope is going to cost in the tens of thousands. Those jobs can't go anywhere near the zoom shown in the video.

Something capable of producing that video definitely cost in the millions, perhaps up to 10 million... for the microscope. That's just one piece of the puzzle, and while that cost is a big chunk of the budget, it's not more than half the lifetime cost of building a lab around that machine, maintaining it and staffing it with high dollar technicians.

[hlester25]

I'm not 100% convinced that nuclear weapons even exist... haha troll me now

[OngBonga]

I'm not 100% convinced you exist. You could be a Russian bot.

[BananaStand]

http://www.foxnews.com/tech/2018/02/...ace-apart.html

It can tear a hole in space?? Huh??

The idea is to achieve a phenomenon known as "breaking the vacuum", whereby electrons are torn away from positrons (their antimatter counterparts) in the empty vacuum of space.

So is space an empty vacuum or not?? If it is, then how are there electrons in it? Wouldn't that make it not-empty??

[OngBonga]

China is building a mega-laser that's so powerful it could literally tear space apart.

You think a Fox News article with this as the leading headline is "physics"?

Dude.

[OngBonga]

I will give points to Fox though for the use of the word "literally" to describe something that noone can even define.

[Savy]

So is space an empty vacuum or not?? If it is, then how are there electrons in it? Wouldn't that make it not-empty??

There is no such thing as a perfect vacuum. You are correct in what you say.

Even if there are no particles there energy is enough to make matter so yeah vacuums don't exist.

[OngBonga]

"That would be very exciting. It would mean you could generate something from nothing," he explained.

What absolute fucking twaddle.

You're not creating something from nothing... you're creating a stable atom from energy. So it's not like it was pulled out of a magic vacuum hat. If there's energy there, it's not a true vacuum. So he's talking out of his arse.

Rip space apart indeed. You might as well be worried about blinding god with that fucking giant laser pen.

[MadMojoMonkey]

It can tear a hole in space?? Huh??

I'm not sure what is lost in translation from the Chinese to English, but that's odd phrasing for pair production, which is all they seem to be talking about. However, pair production happens just over 1.022 MeV, which is way less than their current laser, so it's possible the article isn't talking about anything associated with this, even though it specifically mentions pair production.

My gut says this is just an intersection of QM definitions being used colloquially by the reporter.

There's literally not enough of a hint at actual physics to even guess at what the scientists are actually trying to accomplish.

So is space an empty vacuum or not?? If it is, then how are there electrons in it? Wouldn't that make it not-empty??

Is the vacuum empty? No, not really.
We cannot measure absolute energy, only relative energy. We can measure if and how much one thing's energy is higher than another thing's energy, but we cannot really know if any of the values is exactly 0 J of energy. This is obvious to anyone who has studied electronics. You can't measure the potential (short for: electrical potential energy) at a point, you must measure the potential across 2 points. It is common to set ground to 0 V as a convenience, but it is not possible to prove that ground is 0 V. This is why we only really talk about the delta-V, or change in potential, but in short-hand, this specific use of language is thrown by the wayside.

The best way to describe the vacuum is not that it's "empty," but that it bears the minimum possible allowable energy in this universe (at this time).

One of the odd universe-ending physics things is spontaneous tunneling of the vacuum energy to a lower state. Meaning that the vacuum is tiny, but could be tinier if not for some unknown barrier keeping it from falling. However, QM says that all non-infinite barriers have a non-0 probability of not working (in a process called quantum tunneling). This means that IF the universe's current vacuum energy is not its absolute minimum, AND IF the barrier preventing the universe's vacuum energy from reaching its minimum energy is non-infinite, THEN at any given moment, the universe as we know it will cease to exist at some infinitessimal point. That ceasing of existence as we know it will expand out radially from that point at the speed of light, and will annihilate the universe with an event horizon that expands at the speed of light without anything to oppose it. Our universe would simply end in one instant without any warning that it was going to end.

[MadMojoMonkey]

You think a Fox News article with this as the leading headline is "physics"?

Dude.

More telling was that the article was originally reported in The Sun. Not a bastion of scientific literature.

There is no such thing as a perfect vacuum. You are correct in what you say.

Even if there are no particles there energy is enough to make matter so yeah vacuums don't exist.

In general the energy is not enough to precipitate particles out of the "empty" space. However, in certain environments, like the extreme spacetime curvature near a black hole's event horizon, particle creation from the ambient, sloshing energy of that curvature can create particles, carrying away some of that energy.

What absolute fucking twaddle.

You're not creating something from nothing... you're creating a stable atom from energy. So it's not like it was pulled out of a magic vacuum hat. If there's energy there, it's not a true vacuum. So he's talking out of his arse.

Rip space apart indeed. You might as well be worried about blinding god with that fucking giant laser pen.

Despite the tone, this seems all good.

We can dicker over the phrase "true vacuum" and whether that should be a theoretical ideal or if that should be an observed quantity, but we're not really disagreeing on what anything is or means, just which collection of syllables means what.

Nothing in the article said it was going to create stable atoms. It'd be far, far more likely that it created random ions and isotopes, the vast majority will decay in nuclear reactions over time. Most of those decays will happen basically immediately, but plenty will have longer half-life times and will linger about slowly irradiating their surroundings.

[OngBonga]

Despite my tone, this has tweaked my curiosity a little.

I mean, taking energy and mashing it all together to make, well either an ion or even a stable atom, that's kind of like collecting all the ash up and making a log out of it. Is this actually possible, in theory? I was always told it wasn't possible, but I was never really convinced. I mean it's impossible because we're going to lose ash, which means we never reproduce exactly what was burned, but let's assume we can collect 100% of the ash and heat... I'm not asking if we can do it, because obviously we can't, but could a super-intelligent alien?

[MadMojoMonkey]

Despite my tone, this has tweaked my curiosity a little.

I mean, taking energy and mashing it all together to make, well either an ion or even a stable atom, that's kind of like collecting all the ash up and making a log out of it. Is this actually possible, in theory? I was always told it wasn't possible, but I was never really convinced. I mean it's impossible because we're going to lose ash, which means we never reproduce exactly what was burned, but let's assume we can collect 100% of the ash and heat... I'm not asking if we can do it, because obviously we can't, but could a super-intelligent alien?

In theory, the log can be unburned at a cost of even more entropy than it originally produced in burning, plus all the released energy in the form of heat and light and noise, etc. and you have to have all the original matter to reassemble into the pre-combusted molecules.

Maybe a talented chemist could coax that chemical process to run in reverse on the microscopic level, but IDK.
I suspect that un-rusting iron would be easier to do. Rust and fire are chemically similar in that they're both exothermic oxidation processes.

Quantum Mechanically, all processes are reversible, so entropy doesn't exist at the particle level. Entropy is an emergent property of many-particle systems. "Many" is still poorly defined and understood. Active research in this field is called mesoscale (in-between-sized) physics.

In theory, everything we see happening "forward in time" can be seen "backward in time," but dissipating energy in the form of heat, light, sound, etc. has an incredibly low probability of happening in reverse. A thing exploding from a central source is easy to create. Unexploding a thing by trying to get all the particles moving exactly just so to come together in a dynamite stick is, like, super hard [citation needed].


***
Upon further reflection, all those unstable isotopes will decay into stable atoms or ions, so the assertion that it creates stable atoms isn't really that terrible.

[OngBonga]

So this "incredibly low probability"... am I to assume that while I sit by my fire, the vast majority of the heat is moving from a warm place to a cool place, but not quite all of it? I assume the heat moving from the cooler area to the warmer area is utterly overwhelmed by the heat moving in the opposite direction, rather like trying to piss up a waterfall.

What causes that "incredibly low probabiltiy"? Is it environmental, or completely random? Or do we not know?

[MadMojoMonkey]

So this "incredibly low probability"... am I to assume that while I sit by my fire, the vast majority of the heat is moving from a warm place to a cool place, but not quite all of it? I assume the heat moving from the cooler area to the warmer area is utterly overwhelmed by the heat moving in the opposite direction, rather like trying to piss up a waterfall.

What causes that "incredibly low probabiltiy"? Is it environmental, or completely random? Or do we not know?

Heat transport is a function of relative temperature between 2 sources, and some coefficient of heat diffusion, which characterizes the specific material through which the heat is flowing.

Newton's Law of Cooling:
dT/dt = -k(T(t) - T_inf)

The time rate of change of the temperature of an object is proportional to the difference in temperature between that body and its "ambient" environment.

Solving this 1st order differential equation yields an equation of the form
T(t) = T_inf + (T_init - T_inf)*e^(-kt)

That e^(-kt) part is what we're looking at now.

I'm running out of time.
I'm trying to get to a point where I state that the transport of heat from low temp to high temp is exponentially less than the transport from high temp to low temp the further apart the high and low temps are. When the high and low temps approach the same value due to this exchange process, the amount of transport approaches equilibrium. I.e. when things are the same temperature, the amount of heat flowing between them is the same.

(I need to add a lot of caveats to pin that down to a specific case, but I hope you can do some of that yourself. Feel free to ask for clarity where I trailed off. I'll come back to it later.)

[OngBonga]

I'm trying to get to a point where I state that the transport of heat from low temp to high temp is exponentially less than the transport from high temp to low temp the further apart the high and low temps are. When the high and low temps approach the same value due to this exchange process, the amount of transport approaches equilibrium. I.e. when things are the same temperature, the amount of heat flowing between them is the same.

You might be surprised how logical this is. Two objects of the same temperature are still exchanging heat, it's just the heat transfer cancels out to net neutral. Something that is hot is losing more heat than it gains, and the hotter it is the faster it loses its heat relative to the cool object.

Is this a function of radiation? Is it simply the case that the hot object radiates more than the cool object?

[OngBonga]

I'm also very much glad this isn't a "random" function, because to be quite honest, whenever science talks of "random", what I actually read is "unknown dynamic".

[OngBonga]

The lovely Holly Krieger.

Dr Holly Krieger.

10/10

[MadMojoMonkey]

Yes, Dr. Holly Krieger is a delightfully attractive person with an equally delightfully bubbly demeanor.

Anyway, she makes a good point. The Earth is not exceptionally big as far as planets go, but it is much bigger than a drop of water [citation needed].
The fact that the altogether weak intermolecular interactions which cause surface tension are strong enough to overcome the gravitation pull of an entire planet is noteworthy.

Gravity plays a noticeable role on the largest scales due to the fact that mass-charges of the same sign are attractive, and not repulsive like electric-charges of the same sign. This means that the tendency of an ensemble of electrically charged particles tends to 0, as the ensemble acquires a charge disparity, the forces act to bring it back to equilibrium. With gravity, this isn't the case, so mass-charge tends to accumulate into planets, stars, black holes, etc.

Since more mass is always more attractive to other masses, and there is no counter-gravity caused by negative mass, the altogether tiny force of gravity has dominance on large scales.

[OngBonga]

Gravity plays a noticeable role on the largest scales due to the fact that mass-charges of the same sign are attractive, and not repulsive like electric-charges of the same sign.

This is all a little simplistic for me.

I just had an idea while rolling this spliff.

Space is expanding, right? What does that actually mean? Can it be that if space is expanding, then not-space is not expanding? Let's call not-space something nicer... how about energy?

Where energy exists, expansion does not happen. Where energy does not exist, expansion happens. In this way, spacetime curvature is created. Gravity is lack of spacetime expansion caused by inertia.

Holly can have my Nobel prize for inspiring me (strings attached).

[OngBonga]

Inertia = resistance. What is mass resisting? Expansion.

/solved

[MadMojoMonkey]

This is all a little simplistic for me.

It was an overly simplistic explanation, but I think I've gone into deeper detail at some point in this thread.

I just had an idea while rolling this spliff.

Strap in, boys, this gon' go good.

Space is expanding, right?

so the data shows

What does that actually mean?

It's based on Hubble's Law (empirical), which states that (simplisticly) the further away from us something is, the faster it's moving away from us*.

* on average. Objects in a rotating galaxy will have a bell-curve of recession velocities, centered around the average velocity of the whole galaxy. So, while they are for the most part all the same distance from us, they will not all be moving at the same speed relative to us. Those deviations are tiny compared to the average recession velocity, but my original statement can't be true if we don't take things like this into account.

Also, the Andromeda Galaxy is racing toward us, despite it's distance, but, again, this is not contradictory with Hubble's Law.

Can it be that if space is expanding, then not-space is not expanding?

Ummm... what, now?

Sure. I guess. IF something that we can't measure is doing something, then I cannot measure anything which refutes your assertion. It's just outside the purview of physics until it can be measured.

Let's call not-space something nicer... how about energy?

Not a great choice, since energy is already well-defined and complicating it with a new, altogether different definition will muddy up conversations. Especially when the already defined version of energy is a measurable scalar, and "not-space" is a dearth of anything known to mankind.

Where energy exists, expansion does not happen. Where energy does not exist, expansion happens. In this way, spacetime curvature is created. Gravity is lack of spacetime expansion caused by inertia.

There is plenty of energy in so-called "empty" space. Photons zipping hither and thither, plus the energy of the quantum ground-state of vacuum. Not to mention the non-0 gravitational curvature at all locations in space, as well as all the other fields which permeate and define space-time.

Maybe there are transient locations where the total energy in a finite volume can be 0 J, but I can't think of any way to produce that using any known methods. I could be well under-informed on the finer points of GR, though.

Holly can have my Nobel prize for inspiring me (strings attached).

As soon as I'm empowered to hand out Nobels, I'll let her know.

[MadMojoMonkey]

Ugh. I shouldn't have said there's non-0 gravitational curvature at all locations in space.
That is wrong.

The curvature is the derivative of the gravitational field, and the derivative can certainly be 0 between mass charges. E.g. the slope on the top of a hill is always 0.


I was thinking that the absolute value of the potential is always negative, but that's not really meaningful unless we assert that the gravitational energy is 0 J an infinite distance away from all mass, which sounds reasonable, but can't be proven via experiment. That said, what I really meant was that whatever the value of the gravitational energy an infinite distance away from all masses, all other values must be less than that, since all masses have the same charge. So the absolute magnitude of the field a non-infinite distance from all masses can never equal the "infinity magnitude."
... which again, was NOT what I said, and not really relevant to the discussion.

[OngBonga]

...but that's not really meaningful unless we assert that the gravitational energy is 0 J an infinite distance away from all mass, which sounds reasonable, but can't be proven via experiment

Well it's neither reasonable nor provable because the concept of infinite distance is itself flawed.

Surely every region of spacetime is curved? Surely your original comment was techincally correct? I mean the top of the hill, it's not "flat" except from the frame of reference of those on earth looking at it. And even then, down to the molecular level, it's not flat at all.

[Poopadoop]

The surface of the Earth is smoother than the surface of a billiard ball, scaled to size.

Fact.

[MadMojoMonkey]

Well it's neither reasonable nor provable because the concept of infinite distance is itself flawed.

Yes. Whenever we handle infinities, we have to be careful not to use them foolishly.

In this case, we're asking, "What is the gravitational potential energy of a mass when you measure it from 'very far' away?"
And to do so, we write the equation for the effect as a function of distance. In this case, it's U = GMm/r, where U is the potential energy in J, G is Newton's gravitational constant in N*m^2/kg^2, M is the mass of the object in question in kg, m is the "test mass" in kg, and r is the distance between the 2 masses, in m.

The "test mass" is a mathematical construct, but unimportant to explain, as it is a constant and the only variable in the equation is the r in the denominator.

We can see that as r increases, U decreases. There is no limit to the bigness of r, so no limit to the smallness of U. What we're really saying is that if r is arbitrarily large, then U is arbitrarily slight, which is perfectly reasonable.

Surely every region of spacetime is curved? Surely your original comment was techincally correct?

:/
I'm now torn, because I'm thinking I got it wrong 2 times, and I'm hesitant to put anything out there at this point.
My GR is weak.
I'm currently leaning toward you being correct, here.


OK, I flagged down a grad. student to make sure I'm not putting my foot in my mouth for the 3rd time on this.
Curvature is related to the 2nd derivative of the field in question. Since all mass charge has the same sign, there are no inflection points within the field, meaning there is nowhere where the sign of the curvature flips. Therefore there is nowhere that the 2nd derivative is 0, and therefore there is nowhere that the curvature is 0.

I mean the top of the hill, it's not "flat" except from the frame of reference of those on earth looking at it. And even then, down to the molecular level, it's not flat at all.

OK, not for nothing, but you inspired me to take a question you didn't pose a bit too seriously, and after talking to a couple of doctorates in physics who work in GR, I can only say that it's an excellent question which exposes a contradiction in my understanding and which I haven't rooted out as of now. I.e. something that I think I'm saying is not what I'm saying and no one I've spoken to has figured out exactly which of the many things I'm saying is the wrong one(s).

So. Your statement that "what is the top of the hill is a matter of perspective" is boneheaded UNLESS you're talking about observers moving at relativistic speeds, and in that case, it's a doozy.

OK, so how shall we define the "top" of the hill. I propose that we generalize away from a hill (made of molecules) to a field which is curved. Instead of talking about the "top" of a hill, let's talk about an equilibrium position in the field. This does describe a hill, but generalizes to a more broad collection of cases.

Now, a reasonable definition of an equilibrium position is one where the slope of the field is perpendicular to the force caused by that slope. This is nice, as it means that we can definitively say that an object at rest w.r.t. the field is invariant to all observers, as predicted by GR. I.e. no observer should see the object moving w.r.t. the field, no matter how that observer is moving or accelerating.

However, that annihilates the previous definition of the force being perpendicular to the field, because angles are not preserved in Lorentz transformations. So something is wrong. That definition of an equilibrium position seems to not fit, but it has to fit because no observer should observe the object moving w.r.t. the field, which means no portion of the force is parallel to the slope of the field, which gets us right back where we started.

Summary: This is a fun question to ponder, and I wouldn't suggest holding your breath for me to deliver a concise explanation any time soon. There's a clear contradiction in my understanding and even the people I spoke to didn't see the flaw right away.

Kudos for this. It was an otherwise boring day until you got me thinking about this.

[OngBonga]

I can barely follow you unfortunately. Maybe I'll start to get it as I quote and reply...

We can see that as r increases, U decreases. There is no limit to the bigness of r, so no limit to the smallness of U. What we're really saying is that if r is arbitrarily large, then U is arbitrarily slight, which is perfectly reasonable.

Yes this is very much reasonable. Basically 1/x... just don't divide by zero.

Therefore there is nowhere that the 2nd derivative is 0, and therefore there is nowhere that the curvature is 0.

I have no idea what you mean by this, but I like the conclusion. I'm ok with expansion into infinity, but I see the universe as finite. Like a Koch snowflake...



What's the perimeter of a Koch snowflake of a given area? Knowing it's finite, despite expanding into fractal infinity, is actually quite profound. That's how I see universal expansion... infinity bounded.

So. Your statement that "what is the top of the hill is a matter of perspective" is boneheaded UNLESS you're talking about observers moving at relativistic speeds, and in that case, it's a doozy.

Well why is moving at "relativistic speeds" important? I'm being technical and pedantic here. Motion at 1/1000000000th of c is motion, and subject to fractional time dilation. You can say it's negligible if you wish, but even the slightest bit of curvature is curvature.

Now, a reasonable definition of an equilibrium position is one where the slope of the field is perpendicular to the force caused by that slope. This is nice, as it means that we can definitively say that an object at rest w.r.t. the field is invariant to all observers, as predicted by GR. I.e. no observer should see the object moving w.r.t. the field, no matter how that observer is moving or accelerating.

I really don't like the term "at rest". Everything, literally everything, that has mass, is in motion. The idea that something is "at rest" might be useful when it comes to doing sums, but it's misleading imo. And it could be partly to blame for the contradiction you seem to face.

I have to be honest though, I'm still nonethewiser, I don't understand your contradiction.

[MadMojoMonkey]

I can barely follow you unfortunately. Maybe I'll start to get it as I quote and reply...

I think you get most of it.

I'm getting a bit mentally exhausted trying to unravel this whole thing, so I'm going to pause thinking on it for the evening.

Yes this is very much reasonable. Basically 1/x... just don't divide by zero.

Yep.

I have no idea what you mean by this, but I like the conclusion. I'm ok with expansion into infinity, but I see the universe as finite. Like a Koch snowflake...



What's the perimeter of a Koch snowflake of a given area? Knowing it's finite, despite expanding into fractal infinity, is actually quite profound. That's how I see universal expansion... infinity bounded.

The area of a Koch snowflake is finite, but the perimeter is infinite. I think you know that, but it's worded funny.

How do you account for time dependence? I.e. the Koch snowflake doesn't change in time, but the universe does.

If the universe exists on the perimeter of the Koch snowflake, then it's either infinite, and the notion of expansion is absurd, or the perimeter is finite and ever-increasing in crinkliness over time, expanding without bound.

Well why is moving at "relativistic speeds" important? I'm being technical and pedantic here. Motion at 1/1000000000th of c is motion, and subject to fractional time dilation. You can say it's negligible if you wish, but even the slightest bit of curvature is curvature.

You got me. I have no critiques of this.

I really don't like the term "at rest". Everything, literally everything, that has mass, is in motion. The idea that something is "at rest" might be useful when it comes to doing sums, but it's misleading imo. And it could be partly to blame for the contradiction you seem to face.

This makes me terribly uncomfortable as well, but we just have to deal with it for the foreseeable future.
QM says nothing with mass is at rest in any inertial reference frame.
GR says only photons cannot be at rest in an inertial reference frame (because a reference frame moving at c is explicitly non-inertial).
They don't exactly agree on everything. It's an ongoing investigation.

Nonetheless, we're not talking about QM at all in this discussion. So the GR assertion that an object can be at rest in an inertial reference frame is fine.

I have to be honest though, I'm still nonethewiser, I don't understand your contradiction.

I'll get you there. In a bit.
I'm mentally tired from trying to think through this, and I need a break.
I'll come back on this later, but I'm gonna play some Skyrim for a bit.

[OngBonga]

Do I get a gold star for making you think so hard?

The area of a Koch snowflake is finite, but the perimeter is infinite.

Incorrect. This is the same as saying one plus a half plus a quarter plus an eighth plus a sixteenth etc equals infinity. It doesn't, it equals two. The "infinite" perimeter is only infinite in the fractal sense, like a circle having an "infinite" number of sides while having a finite circumference.

How do you account for time dependence? I.e. the Koch snowflake doesn't change in time, but the universe does.

The Koch snowflake is self similar, the universe is not. Take the Mandlebrot set for an example.. there's a fractal that is *nearly* similar at different scales.

If the universe exists on the perimeter of the Koch snowflake, then it's either infinite, and the notion of expansion is absurd, or the perimeter is finite and ever-increasing in crinkliness over time, expanding without bound.

The latter. Meanwhile, the area of the snowflake remains constant. If the volume of the universe is increasing due to expansion, what remains constant? Net energy? It must, but this implies an ever decreasing density. It's 9.30am and I'm already beginning to get exhausted myself thinking about this!

GR says only photons cannot be at rest in an inertial reference frame (because a reference frame moving at c is explicitly non-inertial).

Weird. I would expect the opposite to be true... photons are "at rest" for all intents and purposes, since they have the same velocity to all observers.

Nice start to the day.

[OngBonga]

The area of a Koch snowflake is finite, but the perimeter is infinite.

The perimeter is a simple curve. Ok you construct the snowflake with traingles, ie straight lines, but one can never construct a perfect Koch snowflake because we can never replicate the fractal detail. Same with circles... we're essentially constructing a circle with a very large number of very small straight lines. One can never construct a perfect circle... find me a circle, and a good enough magnifying glass, and I'll prove it's not a circle. Eventually, at a small enough scale, the curvature will be broken.

We know a circle's circumference is directly proportional to its radius... so if the radius is finite, then so is the circumference. What makes the snowflake different? I'm going to confidently propose that the perimeter of a Koch snowflake is directly proportional to its radius (probably at a ratio related to pi), which therefore makes it very much finite.

[OngBonga]

I'm wrong.

Incorrect. This is the same as saying one plus a half plus a quarter plus an eighth plus a sixteenth etc equals infinity. It doesn't, it equals two.

This is true, but what we're doing here with each iteration is adding a factor of 0.5... it's less than one, so our sum is always decreasing in magnitude... we're tending towards a finite number... in this case 1/0.5, which of course is 2.

The Koch snowflake's curve increases with each iteration at a factor of 4/3... it's higher than one, so the amount we add each time increases, it's more than what we added last time. It therefore tends towards infinity.

Mind blown.

[OngBonga]

There's one thing that strikes me... the number 4/3.

That makes me think of the volume of a sphere. This can't be a coincidence?

[OngBonga]

Looking at the triangle to star iteration... we can see where the 4/3 comes from.

The star has four times as many sides, and each side is three times smaller than the traingle's sides.

Ok, so why 4/3 when it comes to spheres?

The volume of a sphere is pi * radius cubed * 4/3, while the area of a circle is simply pi * radius squared.

Where does that 4/3 come from? This must be directly related to the snowflake.

[OngBonga]

Ok so if we didn't mutliply by 4/3, are we left with a cube? Is that extra 1/3 of volume the "rounding" of a cube to make a sphere?

[MadMojoMonkey]

Do I get a gold star for making you think so hard?

Incorrect. This is the same as saying one plus a half plus a quarter plus an eighth plus a sixteenth etc equals infinity. It doesn't, it equals two. The "infinite" perimeter is only infinite in the fractal sense, like a circle having an "infinite" number of sides while having a finite circumference.

That's a convergent series. The perimeter of a Koch under finite iterations is a divergent series, as you've noted below.

The Koch snowflake is self similar, the universe is not. Take the Mandlebrot set for an example.. there's a fractal that is *nearly* similar at different scales.

I'm not sure how this point relates to the greater discussion.

The latter.

This is a different statement than your original position, "That's how I see universal expansion... infinity bounded."
"The latter," is unbounded expansion.

Are you changing your position?

Meanwhile, the area of the snowflake remains constant. If the volume of the universe is increasing due to expansion, what remains constant? Net energy? It must, but this implies an ever decreasing density. It's 9.30am and I'm already beginning to get exhausted myself thinking about this!

Everything for which we have a conservation law is a viable candidate for the "constant area."
Net energy, net momentum, net charge, etc. There are others, but those are pretty solid.
The momentum and charge conservation laws are perhaps the most robust statements in physics, with no known violations existing on any scales. The same cannot be said for energy, depending on your interpretation of the following uncertainty law:
{delta_E}*{delta_t} >= h/2pi
The uncertainty in the energy associated with a quantum interaction times the uncertainty in the time it takes for that interaction to happen is always greater than or equal to some finite value (Plank's reduced constant). Some physicists interpret this to mean that particle interactions can "borrow" energy from the quantum vacuum, provided they pay it back really quickly.
This is a hand-wavey explanation to me. It tends to come up when discussing Feynman diagrams and virtual particles. There are energy imbalances within the diagrams (unobservable), but never outside the diagrams (observable).

Weird. I would expect the opposite to be true... photons are "at rest" for all intents and purposes, since they have the same velocity to all observers.

I'd like to think that the one thing you know about photons is that they are never at rest, but always (ALWAYS!) moving at c, for all observers.
How do you go from a statement that all observers agree it's moving, to the conclusion that it is at rest?

Nice start to the day.

Yeah.

[MadMojoMonkey]

The perimeter is a simple curve. Ok you construct the snowflake with traingles, ie straight lines, but one can never construct a perfect Koch snowflake because we can never replicate the fractal detail. Same with circles... we're essentially constructing a circle with a very large number of very small straight lines. One can never construct a perfect circle... find me a circle, and a good enough magnifying glass, and I'll prove it's not a circle. Eventually, at a small enough scale, the curvature will be broken.

We know a circle's circumference is directly proportional to its radius... so if the radius is finite, then so is the circumference. What makes the snowflake different? I'm going to confidently propose that the perimeter of a Koch snowflake is directly proportional to its radius (probably at a ratio related to pi), which therefore makes it very much finite.

You may be technically right about circles, but not about the line-segments.
Fields are continuously valued and exist at all points in spacetime.
At any rate, approximating things as circles is quite powerful and while it may not be a perfect description, the predictions are good, so we roll with it.

The Koch is different because it has no curvature at any location on its "curve." It lacks smoothness on any scale.
This explains it well:
https://www.youtube.com/watch?v=D2xYjiL8yyE

[OngBonga]

I'd like to think that the one thing you know about photons is that they are never at rest, but always (ALWAYS!) moving at c, for all observers.
How do you go from a statement that all observers agree it's moving, to the conclusion that it is at rest?

Just because you *observe* something to be in motion, doesn't mean it is in motion. Have you ever been sat on a train at a station, and there's a train on the next platform that starts moving, and for a second you're not sure if it's you or them moving? Forget the fact that both are moving through spacetime, it's time for me to conveniently disregard spacetime curvature.

The photon can be "at rest", while we move away from it. This is expansion... imagine a singularity of light, which then expands inwards at c. Well, anything caught in the slipstream of that newly created space (anything with mass) will see the light shoot away from them, but the opposite happened... the new space we presently occupy is getting further and further away from the light source.

And for good measure, this can explain quantum entanglement... two entangled photons that *appear* a great distance apart actually occupy the same region of spacetime. This makes sense from the pov of the photon, since it experiences no time. So certainly from its pov, it's not moving. How can it be? Motion happens over time.

[OngBonga]

This is a different statement than your original position, "That's how I see universal expansion... infinity bounded."
"The latter," is unbounded expansion.

Are you changing your position?

idk, I need another smoke before I can be certain wtf I'm talking about.

[MadMojoMonkey]

Just because you *observe* something to be in motion, doesn't mean it is in motion.

Careful.
Motion is relative, not absolute, yes. However, I've stipulated what the motion is relative to, an observer, any observer, under any observing conditions.

NO observer (whether that observer is an atom or a person or a microscope) ever observes a photon moving at any speed besides c.
That is the empirical datum.

The photon can be "at rest", while we move away from it.

A photon at rest in an inertial reference frame has never been observed.
Nothing is at rest with respect to (w.r.t.) us "while we move away from it." That's contradictory. If you're asserting it's at rest, you need to state w.r.t. what.

A photon can only be at rest in a reference frame which is comoving with it, which means that reference frame is moving at c, which is a non-inertial reference frame.

This is expansion... imagine a singularity of light, which then expands inwards at c.

I don't know what you mean by a singularity which expands inwards. A singularity is already an infinitely localized object, i.e. it exists at exactly one point.
That definition seems to make any "inward expansion" absurd, but ... black holes are weird, so maybe you're trying to make use of that?

Inside a black hole, the singularity is a location toward which all spacial directions point, and time is all weird-like. All we can say for certain is that an object inside a black hole's event horizon will never move away from the singularity. We cannot say that anything will reach the singularity in a finite amount of time.

So it's weird.

Well, anything caught in the slipstream of that newly created space (anything with mass) will see the light shoot away from them, but the opposite happened... the new space we presently occupy is getting further and further away from the light source.

I'm finding it hard to reconcile how light can traverse any space in that model.
If photons just sit still, and space expands around them, then how does light from the sun ever reach the Earth?

And for good measure, this can explain quantum entanglement... two entangled photons that *appear* a great distance apart actually occupy the same region of spacetime.

If 2 things appear (have been measured) to have distance between them in spacetime, then that is data.
A theory with is contradictory with the data is false.

Which is not to say that these particles may not occupy the same state in another field, but that's already beyond the conversation because we're talking about relative positions in spacetime.

This makes sense from the pov of the photon, since it experiences no time. So certainly from its pov, it's not moving. How can it be? Motion happens over time.

No matter how fast your reference frame is moving (or any other GR effects), you always observe your own time to be "normal." A photon isn't frozen in its own frame, everything BUT the photon is at frozen in time, due to infinite time dilation.

[OngBonga]

If photons just sit still, and space expands around them, then how does light from the sun ever reach the Earth?

Let me try and make sense of my stoned thoughts. Imagine we have a light source that shines in all directions, so that one second later we have a sphere with a radius of 1 light second. What happened? Did the light move through existing space to create this sphere? Or did new space get created at light speed? How can we tell the difference? And if it's the latter, did the light move through space?

I guess that's what I mean whan I say "at rest"... wrt to space.

So how does light get to Earth? I guess it doesn't... the Earth gets to the light. That light basically is a stain in spacetime, and we're heading in its direction at c because that's how fast space is expanding, and we're moving in its direction because the sun is so massive and curves space in such a manner that we move more toward it that anything else. Of course, the sun itself is collapsing into this newly expanded space, so when we "collide" with the stationary light, the sun is over 8 light minutes away from us.

Do I believe this? I don't think so. But... I don't really feel that we're all that close to understanding nature. This idesa of light being constant to all observers... does that inlucde light itself? Are two photons moving very close and parallel to one another observing each other to be moving at c? What happens if you're on a spaceship moving at v=c and you turn the headlights on? I know what happens if you're at v<c, the light moves away from you at c. But if you're already at c?

Something doesn't feel intuitively right. There's something I like about a theory that incorporates spacial expansion, gravity and entanglement, it's just a shame I have absolutely no idea how to go about proving or disproving such a claim.

[MadMojoMonkey]

Do I believe this? I don't think so. But... I don't really feel that we're all that close to understanding nature.

Yeah. It sounds pretty bunk to me, but ya know... I'm so clearly biased that my opinion can't really matter. Who ever let a physicist be in charge of a physics thread? nonsense. You'll never get an unbiased opinion on photons, here.


As for understanding nature... well... we've got a lot of reason to think we've pretty much got atoms sussed out. That's a good deal of what we currently think of as "nature." Having sussed out atoms means we've got photons and gluons and the W and Z bosons pretty much nailed.

Dark matter, on the other hand... and the accelerating expansion of the universe (dark energy)?
You're really right. We don't understand them at all, and they simply dominate the structure of the universe.
(I'm walking back my statement that gravity is king on the largest scales. Up to the scale of accelerating expansion, but then that takes over.)

This idesa of light being constant to all observers... does that inlucde light itself?

What we know about photons is expressed in equations, and interpreting those equations beyond their exact statements is risky business.

Are two photons moving very close and parallel to one another observing each other to be moving at c?

How are the photons observing each other? (What are they exchanging?)
Don't forget infinite space contraction in the direction of motion, along with infinite time dilation.
The concept of simultaneity is tricky, too.

Also, a photon does not have a well-defined position function, since its energy is well-defined and therefore its momentum can be known exactly. The HUP says if the momentum is defined exactly, then the position is undefined.

What happens if you're on a spaceship moving at v=c and you turn the headlights on? I know what happens if you're at v<c, the light moves away from you at c. But if you're already at c?

The Higgs mechanism is a beast.
If you're already at c, then you are massless.
There is no way to accelerate to c from less than c.
Either a particle is massless and moves at c, or it is not massless and moves at less than c.
So "you" can't be on a [thing made of atoms] at v=c.

but that's no fun.
Let's simply apply conservation of momentum and say that if you're moving at c and you emit a photon, then you have less momentum than before you emitted the photon ('cause headlights presumably face your direction of motion), but the same mass.

A photon can change it's wavelength to change momentum without changing speed. You'd need to do something similar, but IDK what that means for a non-photon.

Something doesn't feel intuitively right. There's something I like about a theory that incorporates spacial expansion, gravity and entanglement, it's just a shame I have absolutely no idea how to go about proving or disproving such a claim.

You are trying to unify QM and GR in an ex-poker forum.

We've got the deck stacked against us, but it's still a fun game.

[OngBonga]

I thought of another way to explain my thinking. Consider the following... in every direction we look, we observe the past. Perhaps a nanosecond ago, perhaps 8 minutes ago, perhaps billions of years ago. Every single direction in 3d space is the past. Consider walking from A to B. When you get to B, are you in the same place that B was when you started your journey? Of course, B has moved, in time (and therefore space).

So in what direction is our motion? We're moving into the 4th dimension... the future. Everything (with mass) moves into this unseen dimension. Time is expansion.

We don't move into pre-existing spacetime. Two objects with mass (let's say the Earth and Moon) are both moving into new spactime, and both resist this motion, creating curvature. This curvature keeps both bodies entangled in space, which is why we don't expand away from each other at light speed.

A photon can change it's wavelength to change momentum without changing speed. You'd need to do something similar, but IDK what that means for a non-photon.

Yeah I mean this is excellent. I can't think of another way for something with mass to change momentum other than to accelerate.

You are trying to unify QM and GR in an ex-poker forum.

It's more fun than massaging my ego on flat earth youtube videos.

[MadMojoMonkey]

I thought of another way to explain my thinking. Consider the following... in every direction we look, we observe the past. Perhaps a nanosecond ago, perhaps 8 minutes ago, perhaps billions of years ago. Every single direction in 3d space is the past. Consider walking from A to B. When you get to B, are you in the same place that B was when you started your journey? Of course, B has moved, in time (and therefore space).

Yes. I agree.

Except that being at a different location in time does not imply being at a different location in space, necessarily. Here on the surface of a planet, it certainly does, but not necessarily in the abstract discussion of the nature of motion.

So in what direction is our motion? We're moving into the 4th dimension... the future. Everything (with mass) moves into this unseen dimension. Time is expansion.

Sure. We can say that the 4-vector which describes our motion has a time component which is of constant direction and non-0 length. We can still have our 3 spacial components as 0-vectors, though, showing that motion in time, but not space is possible.

We don't move into pre-existing spacetime. Two objects with mass (let's say the Earth and Moon) are both moving into new spactime, and both resist this motion, creating curvature. This curvature keeps both bodies entangled in space, which is why we don't expand away from each other at light speed.

I'm not entirely sure I follow this part.
You're equating inertial mass and gravitational mass, which isn't a huge deal, since they agree to like 30+ decimal places, and I'm not certain if recent developments in the Higgs mechanism hasn't shown them to be the same, but as I currently understand it, they don't have to be the same. At any rate, if they are the same, then I think your description is pretty good. I think it breaks down at the end because you don't need the 2nd object to rule out motion at light speed. The curvature is enough (again, if inertial mass and gravitational mass are the same).

Whether or not the future exists before we arrive in it seems unmeasurable. I'm not sure how one could measure something "in the future" in that regard. Of course, I can measure something tomorrow, but by the time I measure it, it will be the present, not the future. I cannot acquire the result of tomorrow's measurement before tomorrow.
If we stipulate faster then light travel, then we have tachyon particles (FTL particles), which would for all observational purposes be moving backwards in time. The existence of tachyons would break causality.

The many worlds interpretation of QM would imply that all possible futures exist, but that's not science, so much as intellectualizing how hard it is for our human brains to accept the counter-intuitive statements of QM.

You're kind of on to something big, though. In a very real and practical sense, the reason for gravitational attraction toward massive bodies is due to time dilation. In short, the closer you are to a massive object, the slower time flows. So the rate of time for your feet is slower than the rate of time for your head, and this culminates in an acceleration toward your feet (assuming you're standing up).

It's more fun than massaging my ego on flat earth youtube videos.

Thanks?
Being slightly above the intellectual rigor of a YouTube comments section is better than being slightly below, I guess.

[OngBonga]

Except that being at a different location in time does not imply being at a different location in space, necessarily. Here on the surface of a planet, it certainly does, but not necessarily in the abstract discussion of the nature of motion.

Well, surely it does imply different locations if time and space are one and the same, or to put that in better language, coordinates of the same field.

We can still have our 3 spacial components as 0-vectors, though, showing that motion in time, but not space is possible.

But that 4th dimension of time isn't a 0-vector. So there is always motion through spacetime.

...inertial mass and gravitational mass... they don't have to be the same.

Sure they do. Gravity is acceleration, it's basically resistance to a change in state of motion caused by this acceleration. How is that not exactly the same as inertial mass? Gravity just does the acceleration for us.

Surely you at least expect it to be the same? I mean isn't this what GR is? An attempt to unify graviational and inertial mass? That's my interpretation of GR.

Whether or not the future exists before we arrive in it seems unmeasurable.

Surely we only need to prove space and time are essentially one and the same? Because once we do that, then how can we arrive in a pre existing region of spacetime? Something else existed in that part of space at that moment in time.

If space and time are seperate, then ok we can be in a region of pre existing space. But if we can't say space without actually meaning spacetime, then how can the region of spacetime that I am approaching already exist? If it does, why can't I observe it? One nanosecond in the future isn't that far away, but every direction I look is in the past. Where is one nanosecond in the future? Why can't I see it?

And of course, we can measure a great deal that we once would have called "immeasureable". What does atom mean? Indivisible. When we named the atom, it was unthinkable that there was anything smaller, let alone that we could measure such things.

I'm not sure how one could measure something "in the future" in that regard.

What does it tell us if we prove that we cannot measure the future?

The many worlds interpretation of QM would imply that all possible futures exist, but that's not science

Yeah I don't get on with this. I'm happy enough to say all possible futures have an equal probability, and this one is happening.

You're kind of on to something big, though. In a very real and practical sense, the reason for gravitational attraction toward massive bodies is due to time dilation

This only strengthens the idea that space and time are the same thing, in the same sense that electricity and magnetism are the same. If space is curved, then so too is time, to a proportional degree. If this is an unavoidable truth, then space and time are one, and motion through spacetime is, for everything with mass, into the unseen.

[MadMojoMonkey]

Well, surely it does imply different locations if time and space are one and the same, or to put that in better language, coordinates of the same field.

They are coordinates in the same field, but moving in any one direction doesn't necessitate movement in any other direction.
That's like saying, "You moved forward, so therefore you've moved up and to the side." but in a way that says there is no rotation of the coordinate system in which a straight line lies parallel to one of the coordinate axes.
That's bonkers, even to a quantum physicist.

Give me at least one line (the direction of motion) and I can call it the z-axis in cartesian, cylindrical, or polar coordinates. Then my first guess is to see if I can solve it in 1 dimension (or 2 if we're counting time) by that choice. By choosing an orientation of the coordinate system, I can eliminate motion in 2 axes.
Therefore, motion in one axis does not necessitate motion in another axis.

Space and time are coordinates of the same field, but they are not one and the same. Stuff can move freely in any direction in space, but not time.

But that 4th dimension of time isn't a 0-vector. So there is always motion through spacetime.

Yes, but motion through spacetime does not necessarily imply motion through space.

Sure they do. Gravity is acceleration, it's basically resistance to a change in state of motion caused by this acceleration. How is that not exactly the same as inertial mass? Gravity just does the acceleration for us.

Surely you at least expect it to be the same?

There is nothing in the theory which says the [something] which causes an effect will be, itself, affected by the effect.

Particles are not affected by their own electric charge. If you tell me the direction of the electric field in some region, I can tell you how a charged particle will accelerate in that region due to that field. I do not need to work out how that particle's charge alters or adds to the incident field, because all of those effects cancel out for that particle.

I mean isn't this what GR is? An attempt to unify graviational and inertial mass? That's my interpretation of GR.

No. GR is an attempt to answer the question, "What do all observers agree on?"
Specifically, and differentiating it from Special Relativity, GR considers accelerations.

Importantly, SR describes gravitation, and does not consider acceleration. Object follow orbits because those are logically straight lines in curved spacetime.

Surely we only need to prove space and time are essentially one and the same? Because once we do that, then how can we arrive in a pre existing region of spacetime? Something else existed in that part of space at that moment in time.

We cannot move freely in time the way we can in space, though. Proving that they are the same means we need to be able to do that.

If space and time are seperate, then ok we can be in a region of pre existing space. But if we can't say space without actually meaning spacetime, then how can the region of spacetime that I am approaching already exist? If it does, why can't I observe it? One nanosecond in the future isn't that far away, but every direction I look is in the past. Where is one nanosecond in the future? Why can't I see it?

These are juicy questions.
Why can I send information to the future, but not the past?

Arguments for the arrow of time never really satisfy me. I don't have any good answers to these questions.

And of course, we can measure a great deal that we once would have called "immeasureable". What does atom mean? Indivisible. When we named the atom, it was unthinkable that there was anything smaller, let alone that we could measure such things.

Of course this is an excellent point, and maybe someday we will figure out how to move freely through time.

I kinda like Stephen Hawking's cheeky one-liner on the subject.
"If time travel is possible, where are all the tourists from the future?"

What does it tell us if we prove that we cannot measure the future?

That all the science fiction about time travel was fiction after all?
That the grandfather paradox is truly untestable?

Yeah I don't get on with this. I'm happy enough to say all possible futures have an equal probability, and this one is happening.

:/
Surely futures in which at least one major world government will be probing Uranus are more probable.

This only strengthens the idea that space and time are the same thing, in the same sense that electricity and magnetism are the same. If space is curved, then so too is time, to a proportional degree. If this is an unavoidable truth, then space and time are one, and motion through spacetime is, for everything with mass, into the unseen.

ummm... what?

[OngBonga]

They are coordinates in the same field, but moving in any one direction doesn't necessitate movement in any other direction.

No, but it does mean an overall different location. The problem is you're thinking in 3d space. Well yeah, we can be motionless through 3d space. But why does motion only apply to 3d space? It applies to spacetime. If you move through time, you're moving through spacetime. Maybe not space, but that's only like saying you're not moving on a 2d graph if your x axis moves but not your y axis. Well, all we can say is we're not moving along the y axis. But we are moving through the graph.

That's like saying, "You moved forward, so therefore you've moved up and to the side."

Not at all. It's like saying you moved forward, therefore you moved.

Therefore, motion in one axis does not necessitate motion in another axis.

Ok, but my argument is that motion through one axis, whether that be x, y, z or t, is motion.

"What do all observers agree on?"

Sure, but wasn't the underlying theory to this question based on the geometry of spacetime, and how we move through it?

Specifically, and differentiating it from Special Relativity, GR considers accelerations.

Indeed, it argues gravity is acceleration caused by geometry... so when gravity accelerates an object, why is this different to another source of energy accelerating this object? What's the difference between graviational and intertial mass from a GR pov? Best I can tell, there isn't one... it's the same mechanism.

Object follow orbits because those are logically straight lines in curved spacetime.

Yes, and this is provable because the same side of the moon constantly faces us (tidal locking), yet the moon doesn't rotate as it orbits in order to maintain this relationship... if it did, it would lose energy and crash into us. The moon is moving in a straight line through curved spacetime. I certainly do not dispute this.

We cannot move freely in time the way we can in space, though. Proving that they are the same means we need to be able to do that.

Perhaps we can move freely in time if we have no mass. Can you swim against a current in a river that flows at light speed?

Arguments for the arrow of time never really satisfy me. I don't have any good answers to these questions.

These questions are why I tend towards the idea that we're moving into expanded space. The future simply doesn't exist yet.

Of course this is an excellent point, and maybe someday we will figure out how to move freely through time.

I'm not buying time travel, not backwards anyway. Of course we can travel to the future by moving fast. To move freely through time, perhaps it can be done in theory if you move at c, which we know is impossible in practise. But how do you ever prove such a thing? I guess in the same way we know it's impossible to move at c... we don't need to try to move that fast in order to figure out we can't.

"If time travel is possible, where are all the tourists from the future?"

In the future future!

[OngBonga]

ummm... what?

idk

I mean if we change an electric current, the magnetic field responds instantly. We can say it's the same thing... we do, we call it electromagnetism. Spacetime is no different. If you suddenly plonk an object of mass in empty space, well anything that suddenly experiences this gravity will also immediately experience time dilation. It's unavoidable, like the magnetic field changing when we change the current. Space and time, they're the same just like just left and right are, sort of, the same thing... left and right are different directions that rely entirely on each other... you can't define one without immediately defining the other.

So if space and time are essentially the same, then it can be seen as a 4d graph with x, y, z and t axes. And when we consider our motion relative to this 4d graph, well our motion is in the direction where nothing exists, because all points we can travel to are in the future.

[MadMojoMonkey]

I think the first part is simply a definition thing. When I talk about position in space-time, that is different than me talking about position in space.
Movement in spacetime is not necessarily the same as movement in space, is all I'm really saying.
Frankly, I may be in the wrong (I'm getting over a head cold, and have been a bit grumpy lately).
For a non-physicist, your knowledge of 4-vector spacetime is remarkable. Kudos.

Sure, but wasn't the underlying theory to this question based on the geometry of spacetime, and how we move through it?

Well, kinda. I mean, re-defining what we understand about the geometry of spacetime was the only logically consistent way to answer the question in a way that was consistent with known data. This is beside the point, though.

You're right that I would have guessed that they are the same thing, but intuition is an untrustworthy tool in science. It guides us, but we must remain vigilant to not trust intuition when an experiment can be performed instead.

Bottom line is that I don't know of anything in the Lambda CDM Model* or in the Standard Model** which can prove or derive that m_grav = m_inertial.

*Lambda CDM is Lambda for the term in Einstein's equations which describes universal acceleration (I.e. Lambda = dark energy), and CDM for Cold, Dark Matter. It's basically Einstein's GR + the more recent data, which at least the Lambda part was in Einstein's equations, but he thought it was an erroneous term because data had not yet suggested universal acceleration, which is still not explained, even if there is a term for it in Einstein's GR equations.
** The Standard Model is all of QM.

Indeed, it argues gravity is acceleration caused by geometry... so when gravity accelerates an object, why is this different to another source of energy accelerating this object? What's the difference between graviational and intertial mass from a GR pov? Best I can tell, there isn't one... it's the same mechanism.

Gravitation is explained by SR, not GR. Einstein explained gravitation without any accelerations.

Yes, and this is provable because the same side of the moon constantly faces us (tidal locking), yet the moon doesn't rotate as it orbits in order to maintain this relationship... if it did, it would lose energy and crash into us. The moon is moving in a straight line through curved spacetime. I certainly do not dispute this.

Tidal locking is due to gravity, and therefore an effect of time dilation.
I'm pretty sure the moon still rotates in GR, with it's axial rotational period equal to it's orbital rotational period.

Motion does not require energy. Acceleration requires energy.
Otherwise the fact that the Earth is not tidally locked with the sun would mean we are crashing into the sun.

Perhaps we can move freely in time if we have no mass. Can you swim against a current in a river that flows at light speed?

Photons do not move freely through time.

Hard to say with simple math, because a river moving at c is non-inertial, and whatever is flowing in the river is massless. IDK against what you'd be pushing to swim, but ignoring that, I'm pretty confident in saying that you can swim against the current all you like, but you'll always be moving downstream w.r.t. the shore.

These questions are why I tend towards the idea that we're moving into expanded space. The future simply doesn't exist yet.

I thought your position was that the future must exist, else how can we move into it?
(Sorry if that's a fever dream of my sick mind)

At any rate, my position is that whether or not the future exists before it is the present is unmeasurable, so beyond the purview of physics.

I'm not buying time travel, not backwards anyway. Of course we can travel to the future by moving fast. To move freely through time, perhaps it can be done in theory if you move at c, which we know is impossible in practise.

But how do you ever prove such a thing? I guess in the same way we know it's impossible to move at c... we don't need to try to move that fast in order to figure out we can't.

Observe something which is at odds with causality. This implies FTL motion, which would tell us the future exists, as FTL motion is a particle moving backwards in time, sending information to the past and therefore able to influence the past.

IDK if we can prove FTL motion is impossible aside from asserting that what we currently know is all there is to be known, which no credible scientist would ever claim.

In the future future!

Are we there, yet?

[OngBonga]

For a non-physicist, your knowledge of 4-vector spacetime is remarkable. Kudos.

Thanks, I assure you it's an accident.

I think the first part is simply a definition thing. When I talk about position in space-time, that is different than me talking about position in space.

Ok, but if we're talking about motion, well you're disregarding an important coordinate when you only talk of space, and this changes the subject from "always moving" to "potentially at rest". That's a profound difference. I don't feel we can accurately describe motion in 3 dimensions alone, it's just useful to for real world considerations. But we cannot say something is "at rest" and be accurate, any more than we can say I'm "at rest" while I relax and smoke a spliff as a fly around the sun. I can see that I am sort of "at rest", but I know I am not, and never will be.

You're right that I would have guessed that they are the same thing, but intuition is an untrustworthy tool in science. It guides us, but we must remain vigilant to not trust intuition when an experiment can be performed instead.

Fair enough. This is where I'm liberated by simply being a layman... I don't care for vigilance, I prefer intuition. I'm approaching this more from a philosophical pov than sceintific. I'm quite comfortable saying they're the same thing.

Gravitation is explained by SR, not GR. Einstein explained gravitation without any accelerations.

You're probably right, however I understood SR to be essentially an approximation of GR, in that SR deals with negligible curvature... ie it assumes no curvature, which is still only an approximation of the most remote region (in terms of mass) in the universe. GR fixes these approximations. Please correct me if I'm wrong, it's not like I've studied either beyond their respective wikipedia pages and the occasional youtube video. I kinda bunch the two together and call it GR (I know, it's sloppy), I mean SR wasn't even called SR until he published GR.

I'm pretty sure the moon still rotates in GR, with it's axial rotational period equal to it's orbital rotational period.

Yeah when I read that back I thought it was probably a poor understanding of the mechanism, but I decided to not edit it out and see what you replied.

Otherwise the fact that the Earth is not tidally locked with the sun would mean we are crashing into the sun.

I guess my point was that the moon should crash because it is tidally locked. It has to constantly "turn" to face the Earth as it moves through space. How is this not acceleration? It would be if this "turn" was a deviation from a straight line.

Photons do not move freely through time.

No, they are frozen in time. Assuming the photon has eyes and a watch, does it experience time like we do, but instead observes a flat universe?

I thought your position was that the future must exist, else how can we move into it?

No, it WILL exist. My position is that the future is soon-to-be-expanded space. It doesn't exist yet, but it will do.

At any rate, my position is that whether or not the future exists before it is the present is unmeasurable, so beyond the purview of physics.

But not philosophy. Suck it, physics.

Are we there, yet?

Never!

[OngBonga]

At any rate, my position is that whether or not the future exists before it is the present is unmeasurable, so beyond the purview of physics.

I actually do not think this is true. It's like saying we can never know what happened before the big bang, or what happens past the event horizon. We may never be able to directly observe these things, but that won't stop us trying to indirectly observe them.

I mean, have you ever actually seen the moon? No, you've just seen light reflecting off it. That light came from the sun, not the moon. So how do you know it's there? Well, we could just study the tides on this planet.

[MadMojoMonkey]

Hey, ong.... here's a thought to roll a spliff over:
(My dad's having triple bypass surgery and I'm doing anything to try to keep my mind off of worrying for him.)

If the "many worlds" interpretation of quantum mechanics is true, then there are universes in which the laws of physics are identical to ours, but in which you would never be able to deduce any laws of physics. This is because, given the interpretation that all results of any interaction between quantum states exist in "parallel" universes, then there are some universes in which the least probable events are the ones which happen most frequently, purely by random chance. I.e. if all futures exist, then then there is a subset of all futures in which physical laws are the same, but are not manifest.

E.g. there would be a universe in which whenever I flush my toilet, rather than flushing, all the atoms in my poo spontaneously become a poo sized diamond. Sure, in plenty of those universes, this only happens once, and cannot be reproduced, but in others, this happens every time, despite that universe following the same physics are our universe. It's just an extremely improbable event that happened, and then is followed by another extremely improbable event. No test or measurement would ever reveal that this is improbable, after all, it keeps happening in a very repeatable way.

So... if that's the case... what physical laws appear to be "normal" in our universe, but are really just extremely improbable events which keep happening, simply because there is a non-0 chance that they're happening somewhere?

[Poopadoop]

Maybe we're the ones in that universe and all the laws we think we know are false.

[oskar]

It would be possible if the infinity of universes is the same size infinity as the number of possible universes. Which I hope isn't true for the sake of the inhabitants of the possible universe where every time you cough, Bea Arthur slaps you on the ass.

[MadMojoMonkey]

Maybe we're the ones in that universe and all the laws we think we know are false.

Yeah. That was what I was getting at in my final paragraph.

I guess the only thing we can rely on in that case is that the universe truly isn't deterministic after all, and that part of QM is right, if nothing else. Since this whole theory relies on the same circumstances leading to different outcomes, that is.

I'm not really a fan of the many worlds interpretation, but I'm not really a fan of any of the interpretations of QM. I'm pretty happy to just assert the math makes sense and any intellectual dissonance is a psychological problem, not a physics problem. That's kind of an agnostic position on interpretations of QM.

[OngBonga]

Pretty good explanation for why the "many interpretations" theory is (very very probably) a load of bollocks.

Also, who's to say if that theory is viable that there are an infinite number of alternative universes? Why not simply approaching infinity (but finite at any given point in time)?

On an unrelated sbuject, here's something that blows my mind...

Imagine first a "table" with edges that bounce balls perfectly elastically, ie no energy is lost... all energy and momentum is conserved.
Take two balls, ball A, which has a mass of x, and ball B, which has a mass of 16x. Now roll them towards each other at the same velocity. When they hit each other, ball A will bounce off ball B, because B is much larger. Ball A will then hit the edge of the "table" (losing no momentum), and then bounce right back at ball B and hit it again.
The question is, how many times does ball A bounce off ball B before ball B loses its energy and starts to move backwards?
The answer is 3.
Now scale the big ball up x100.
Now the answer is 31.
Scale up x100 again
314
and again
3141
next
31415
then
314159
and then
3141592

And on and on... so long as we're scaling up x100 from an intial ratio of 16:1 then we get the digits of pi. This works all the way.

Where the fuck is pi coming from here? This has nothing to do with circles, it would happen with cubes, under the same conditons, ie conservation of energy and momentum, which means no friction.

THAT shit blows my mind, not thought experiments about the improbability of many worlds.

[CoccoBill]

Where the fuck is pi coming from here? This has nothing to do with circles, it would happen with cubes, under the same conditons, ie conservation of energy and momentum, which means no friction.

https://en.wikipedia.org/wiki/Pi#Rol...in_mathematics

Pi is freaky. I'd wager it's the random seed for our universe simulation.

[MadMojoMonkey]

@ong on pi
That's discussed in an episode of Sixty Symbols, right? From a few years ago?

I created a spreadsheet after watching that video and brute-force crunched numbers to show it was the case. However, since I let Excel do all the calculations, and I never worked out the algebra, I never revealed to myself where the pi comes from.

So I still don't know why pi shows up in that example.