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Old 09-18-2017, 09:45 PM
Ornery Bob Ornery Bob is offline
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Is spin a relative motion?

I understand relative motion by the old example of two boats on the ocean. Without visible land, or any other fixed point of reference, if you were on one of them and the distance to the other one was increasing, you couldn't say that he was moving away from you or that you were moving away from him, you are simply moving away relative to each other. I can visualize that, so I get it.

But if his boat sinks and mine starts spinning around, my spin isn't relative to him, it's relative to the ocean.

So my understanding is that space is like this - no fixed point of reference. A vast ocean with no land and no fixed reference points.

So where does spin come in? Jupiter is spinning relative to everything else in the universe, but if all that stuff went away, and Jupiter was the only thing in the universe, would it still be spinning and would that spin be generating magnetic fields?

Seems like I'm forced into saying Jupiter is spinning relative to space itself and doesn't that make space some sort of fixed point of reference?
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Old 09-18-2017, 09:55 PM
OldGuy OldGuy is online now
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You may get a better answer from someone whose physics is much more recent, but basically no spin is not a relative motion. When the Earth sins around its axis the parts on the equator (and elsewhere) are accelerating. While velocities are always relative to something else, acceleration is not. Acceleration requires a force.

Another way to think about it is If you wished to describe the Earth as not spinning, but all the stars revolving around it (a la Aristotle), then stars far enough away would have to be moving faster than the speed of light or you would have to describe the entire universe as revolving. I suppose you could do the latter without violating relativity just as expansion of the universe can be in some sense faster than the speed of light for points far enough away, but its' certainly not the easiest way to do it.

Last edited by OldGuy; 09-18-2017 at 09:55 PM.
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Old 09-19-2017, 02:22 AM
Derleth Derleth is offline
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You may get a better answer from someone whose physics is much more recent, but basically no spin is not a relative motion.
You dropped a comma, and all this is Newtonian physics, maybe late 19th Century if you want the more convenient mathematical formalisms.

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When the Earth sins around its axis the parts on the equator (and elsewhere) are accelerating. While velocities are always relative to something else, acceleration is not. Acceleration requires a force.
Right. One way to look at this is that in a rotating frame of reference, you observe frame-dependent forces; specifically, centrifugal force, which, from a non-rotating frame, isn't a force but is a body's linear momentum being thwarted by something. And, by definition, a change of momentum is a force. (p = mv, take the derivative of the velocity, F = ma.)

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Another way to think about it is If you wished to describe the Earth as not spinning, but all the stars revolving around it (a la Aristotle), then stars far enough away would have to be moving faster than the speed of light or you would have to describe the entire universe as revolving. I suppose you could do the latter without violating relativity just as expansion of the universe can be in some sense faster than the speed of light for points far enough away, but its' certainly not the easiest way to do it.
This is Mach's Principle in a nutshell: If you see the stars whirling around you, you will experience centrifugal force.

Here's an interesting page relating inertia to Mach's principle.
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Old 09-19-2017, 06:41 AM
Chronos Chronos is offline
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So far as we know, rotation is not relative. But there have been a few serious physicists who have suggested that in fact it is, and that the Universe is constrained to have exactly 0 angular momentum, and that therefore rotation is properly measured as relative to the entirety of all other matter in the Universe. Since we can't wave a magic wand and make all of the other matter vanish, this is very difficult to test.
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Old 09-19-2017, 06:45 AM
Snarky_Kong Snarky_Kong is online now
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So far as we know, rotation is not relative. But there have been a few serious physicists who have suggested that in fact it is, and that the Universe is constrained to have exactly 0 angular momentum, and that therefore rotation is properly measured as relative to the entirety of all other matter in the Universe. Since we can't wave a magic wand and make all of the other matter vanish, this is very difficult to test.
What does empirical evidence say? I know our solar system (and galaxy?) mostly spins in the same direction. What about other observable galaxies?
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Old 09-19-2017, 06:59 AM
Ludovic Ludovic is online now
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When the Earth sins around its axis
Sure, invoke Godwin's law
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Old 09-19-2017, 07:36 AM
Hari Seldon Hari Seldon is offline
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I thought that it was the fact that the galaxies were spinning too fast to hang together that led to the existence of dark matter. I think that suggests that rotation is absolute. Of course, that is just another form of centrifugal force.

Could anything be inferred from conservation of angular momentum in particles traveling at nearly c? Has this been measured? Can it be?
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Old 09-19-2017, 07:38 AM
Chronos Chronos is offline
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Any given astronomical object (planet, solar system, galaxy, etc) has a net angular momentum in some direction, but the directions are all randomized, and there's no correlation between scales (the Solar System doesn't have the same spin axis as the Galaxy, nor even particularly close).
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Old 09-19-2017, 08:32 AM
Mangetout Mangetout is online now
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Originally Posted by Chronos View Post
So far as we know, rotation is not relative. But there have been a few serious physicists who have suggested that in fact it is, and that the Universe is constrained to have exactly 0 angular momentum, and that therefore rotation is properly measured as relative to the entirety of all other matter in the Universe. Since we can't wave a magic wand and make all of the other matter vanish, this is very difficult to test.
We can perform closed-box experiments though. If you're inside a windowless box that is travelling in a straight line at any constant velocity, you cannot distinguish it from a stationary box.
If you're in a stationary box on the surface of the earth, you cannot distinguish it from a box accelerating through space at 1g.

If you're inside a windowless box that is rotating, you can easily determine that it is so, just by moving, spitting, whatever - and observing the coriolis effect, even if you start out spinning on the same axis as the box.

Now I don't expect that dismisses the question of absolute vs 'relative to the whole universe', but I think it does demonstrate that spin is not relative in any way that can matter, or be measured (which I think was the same point you made).
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Old 09-19-2017, 08:39 AM
LSLGuy LSLGuy is offline
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Originally Posted by Snarky_Kong View Post
What does empirical evidence say? I know our solar system (and galaxy?) mostly spins in the same direction. What about other observable galaxies?
In addition to Chronos' point, what does "direction" mean in the context of a star+planets system?

e.g. If we go a few light-months in the direction humans call North and look back at our solar system then whole shebang seems to be rotating counterclockwise.

OTOH, if we go a few light-months in the direction humans call South and look back at our solar system then whole shebang seems to be rotating clockwise.

Which direction is the "real" one and which is "backwards; just caused by looking at it upside down"?
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Old 09-19-2017, 08:44 AM
LSLGuy LSLGuy is offline
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Originally Posted by Mangetout View Post
...
If you're in a stationary box on the surface of the earth, you cannot distinguish it from a box accelerating through space at 1g.
...
Agree overall with your points, but the snip above isn't quite true.

For an ordinary human using ordinary human senses what you say is true enough.

But for someone equipped with some delicate tools it's not that hard to prove that your windowless box is not under 1G linear acceleration but rather is under 1G linear acceleration and slowly rotating.

One explanation for which is that your box is installed on the surface of a slowly-rotating planet. E.g. https://en.wikipedia.org/wiki/Foucault_pendulum

Last edited by LSLGuy; 09-19-2017 at 08:45 AM.
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Old 09-19-2017, 09:12 AM
watchwolf49 watchwolf49 is offline
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In addition to Chronos' point, what does "direction" mean in the context of a star+planets system?

e.g. If we go a few light-months in the direction humans call North and look back at our solar system then whole shebang seems to be rotating counterclockwise.

OTOH, if we go a few light-months in the direction humans call South and look back at our solar system then whole shebang seems to be rotating clockwise.

Which direction is the "real" one and which is "backwards; just caused by looking at it upside down"?
I believe "North" in this context is defined by the right hand rule, curl your right-hand fingers in the direction of the spin, your extended thumb points north ... perhaps there's a more sciency definition that isn't completely arbitrary?

I'm wondering if spin is relative to other spin? ... if I'm on the Moon looking at Earth, I would say it takes 24h 47m for the Earth to make compete rotation ... instead of the 23h 56m if I were stationary ...

Last edited by watchwolf49; 09-19-2017 at 09:13 AM.
  #13  
Old 09-19-2017, 09:39 AM
Ornery Bob Ornery Bob is offline
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So spin is absolute. It's simply built into the universe that any spec of matter can spin all by itself.
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Old 09-19-2017, 09:43 AM
Chronos Chronos is offline
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LSLGuy, what you just described is one direction, which is the real one. Though it's better to define what exactly you mean by "north", since the conventional definition is based on rotation in the first place (that is to say, if you fly north from any planet, you'll see it rotating counterclockwise below you, because that's what "north" means). A more careful description of the rotation of the Earth would be something like "If you fly in the direction of alpha Ursa Minoris, you will see the planet rotating counterclockwise below you".
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Old 09-19-2017, 10:13 AM
Andy L Andy L is offline
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Agree overall with your points, but the snip above isn't quite true.

For an ordinary human using ordinary human senses what you say is true enough.

But for someone equipped with some delicate tools it's not that hard to prove that your windowless box is not under 1G linear acceleration but rather is under 1G linear acceleration and slowly rotating.

One explanation for which is that your box is installed on the surface of a slowly-rotating planet. E.g. https://en.wikipedia.org/wiki/Foucault_pendulum
Yeah, the equivalence principle works for a small room over a small time (it's a "local" equivalence). Greg Egan using this idea in a novel ("Incandescence"), in which characters are in a small volume, but have a large amount of time, and shows how they figure out what's going on (including detecting differences between Newtonian gravity and GR) - http://www.gregegan.net/INCANDESCENC...ndescence.html
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Old 09-19-2017, 10:30 AM
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The world doesn't revolve around you, you know.

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Old 09-19-2017, 10:54 AM
Mangetout Mangetout is online now
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Originally Posted by LSLGuy View Post
Agree overall with your points, but the snip above isn't quite true.

For an ordinary human using ordinary human senses what you say is true enough.

But for someone equipped with some delicate tools it's not that hard to prove that your windowless box is not under 1G linear acceleration but rather is under 1G linear acceleration and slowly rotating.

One explanation for which is that your box is installed on the surface of a slowly-rotating planet. E.g. https://en.wikipedia.org/wiki/Foucault_pendulum
Correction accepted. Yes - what I meant to say is that acceleration from gravity is indistinguishable from acceleration due to motion. A box accelerating at 1g in a straight line in space, and a box standing on the surface of an Earth-sized, non-rotating planet, are indistinguishable from the inside of the box

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Old 09-21-2017, 01:32 PM
Snarky_Kong Snarky_Kong is online now
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In addition to Chronos' point, what does "direction" mean in the context of a star+planets system?

e.g. If we go a few light-months in the direction humans call North and look back at our solar system then whole shebang seems to be rotating counterclockwise.

OTOH, if we go a few light-months in the direction humans call South and look back at our solar system then whole shebang seems to be rotating clockwise.

Which direction is the "real" one and which is "backwards; just caused by looking at it upside down"?
Just define Earth North as positive X, the Earth - somewhere vector on a given date as positive Y, etc. Unless I don't understand, I don't see why this is a difficulty other than just defining it. What you call clockwise doesn't matter as long as it's consistent.

Chronos's answer that the rotational axes of different objects seems to be random is what I was looking for.
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Old 09-21-2017, 02:21 PM
CookingWithGas CookingWithGas is offline
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So spin is absolute. It's simply built into the universe that any spec of matter can spin all by itself.
Yes. Not just spin, but any motion other than moving at constant velocity (speed and direction).
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Old 09-21-2017, 05:24 PM
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I was taught, in my old cosmology class, that Einstein and Mach disagreed on what would happen if you had a planet....and nothing else in that entire cosmos. Could an isolated planet "spin?"

One of the two said that space-time itself provides a "metric" against which the spin can be measured...and the other didn't... But I don't recall which...and it's possible I'm remembering the story wrong anyway.

(I was taught that the GR equations had not been solved except for certain simplified-case cosmoses, such as a cosmos filled with a universally smooth density of matter. From what I've seen of tensors, I believe it.)
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Old 09-21-2017, 05:44 PM
Andy L Andy L is offline
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I was taught, in my old cosmology class, that Einstein and Mach disagreed on what would happen if you had a planet....and nothing else in that entire cosmos. Could an isolated planet "spin?"

One of the two said that space-time itself provides a "metric" against which the spin can be measured...and the other didn't... But I don't recall which...and it's possible I'm remembering the story wrong anyway.

(I was taught that the GR equations had not been solved except for certain simplified-case cosmoses, such as a cosmos filled with a universally smooth density of matter. From what I've seen of tensors, I believe it.)
As I recall, Mach would have said that a planet alone in the cosmos could not spin (and possibly not even accelerate) since there was nothing to spin (or accelerate) relative to. Einstein started out with a Machian view, but moved away from it later in life (again as I recall).
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Old 09-21-2017, 07:32 PM
Chronos Chronos is offline
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Of course, Gödel then went on to point out that if you have a planet alone in the cosmos, and it is spinning, you'd get all sorts of weird effects (to put it in context, time travel would be one of the less weird effects).
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Old 09-21-2017, 08:46 PM
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So far as we know, rotation is not relative. But there have been a few serious physicists who have suggested that in fact it is, and that the Universe is constrained to have exactly 0 angular momentum, and that therefore rotation is properly measured as relative to the entirety of all other matter in the Universe. Since we can't wave a magic wand and make all of the other matter vanish, this is very difficult to test.
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Of course, Gödel then went on to point out that if you have a planet alone in the cosmos, and it is spinning, you'd get all sorts of weird effects (to put it in context, time travel would be one of the less weird effects).
The juxtaposition of these two ideas opens up some fascinating rabbit trails. Do you have any approachable readings for us undergrads?

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Old 09-22-2017, 12:02 AM
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Would the spin be a quantity of energy, even if there was nothing else in the universe? Would there be effects of relativity from the center to the surface? The surface is moving faster inner parts.

If suddenly something else came into existence in the universe and collided with the spinning object. There would be a different exchange of energy depending if the collision was with or against the spin. So the energy of the spin was there, even if nothing else was. The existence or nonexistence of another object would not change that.

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Old 09-22-2017, 10:34 AM
Trinopus Trinopus is offline
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Would the spin be a quantity of energy, even if there was nothing else in the universe? Would there be effects of relativity from the center to the surface? The surface is moving faster inner parts.
Yes...for the reason you note.

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If suddenly something else came into existence in the universe and collided with the spinning object. There would be a different exchange of energy depending if the collision was with or against the spin. So the energy of the spin was there, even if nothing else was. The existence or nonexistence of another object would not change that.
You can't have it both ways. If something "came into existence" then that changes the universe, so you cannot use that new situation to argue about what the situation is in the case of the "nonexistence" of another object.

(The magical disappearance of an object is a commonplace thought-experiment in relativity, especially relating to the "speed of gravity." If the sun suddenly disappeared, how long would it take earth's orbit to straighten out? Nearly everyone says gravity would still "travel" through space at the speed of light, but there are a tiny handful of dissidents who say it propagates instantaneously. Unfortunately, physics makes no provision for masses "suddenly" disappearing, so the thought-experiment is null and moot.)
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Old 09-22-2017, 11:59 AM
Asympotically fat Asympotically fat is offline
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Of course, Gödel then went on to point out that if you have a planet alone in the cosmos, and it is spinning, you'd get all sorts of weird effects (to put it in context, time travel would be one of the less weird effects).
The Godel solution is probably most accurately described as a swirling homogeneous dust with a cosmological constant, but it is general regarded as unphysical, not least because it is it allows closed timelike curves.

A lone spinning planet would be expected to be described by a Kerr solution in the exterior matched to a suitable interior solution (though nobody has yet found such an interior solution).
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Old 09-22-2017, 12:16 PM
OldGuy OldGuy is online now
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Of course, Gödel then went on to point out that if you have a planet alone in the cosmos, and it is spinning, you'd get all sorts of weird effects (to put it in context, time travel would be one of the less weird effects).
If you have a planet alone in the universe, then what is time traveling? It seems unfair to say a part of the planet is time traveling as you're then treating the single planet as at least two things. And if it's not a part of the planet doing the time traveling, there's nothing else there at all to do it.
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Old 09-22-2017, 03:48 PM
Chronos Chronos is offline
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One can imagine a piece of the planet (say, a spaceship) separating from the planet, flying along an appropriate path, and then returning.
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Old 09-23-2017, 09:53 AM
Asympotically fat Asympotically fat is offline
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If you have a planet alone in the universe, then what is time traveling? It seems unfair to say a part of the planet is time traveling as you're then treating the single planet as at least two things. And if it's not a part of the planet doing the time traveling, there's nothing else there at all to do it.
As I said the Godel solution isn't really anything like a planet alone in the Universe - it is full of matter, whereas an isolated planet is surrounded by a vacuum. Also it is homogeneous (the same everywhere), whereas you'd expect the field of an isolated planet to change with proximity to the planet. It is more like a cosmological solution where the particles swirl around each other.

It would be worrying if such an eminently physically reasonable situation like an isolated planet resulted in time travel as we'd probably have to accept that time travel was a generic physical prediction of GR or that the properties of a region of spacetime depended so heavily on what was going on outside of it that the whole theory had little predictive value.

However there are solutions which describe isolated objects which lead to time travel. A Tipler cylinder is an infinitely long rotating rod and hence unphysical, but in theory there is a region surrounding the cylinder that a hypothetical observer could go into and emerge from and meet a past version of themselves.
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Old 09-23-2017, 10:17 AM
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Don't the time-travel equations work for a truncated Tipler cylinder? It doesn't have to be infinitely long, just "sufficiently" long. And massive. And spinning insanely fast.

Still never gonna happen in the real world, but at least physically possible, which an infinitely long rod isn't.
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Old 09-23-2017, 10:52 AM
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Direction is relative. In space which way is North?
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Old 09-23-2017, 03:00 PM
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You can define a direction as North in many different ways, such as the direction of the vector from Sol to alpha Ursa Minoris.
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Old 09-23-2017, 03:31 PM
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If you know the axis of rotation of some object, there are not that many ways to define north, just two.

If you are navigating in deep space, it is wise to pay attention to details like whether your coordinates are with respect to galactic north or something more local, which galaxy you are actually in, etc.
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Old 09-23-2017, 03:51 PM
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You'd have to create North to get a direction. When we say Polaris is North then that way is North.

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Old 09-23-2017, 07:07 PM
Asympotically fat Asympotically fat is offline
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Don't the time-travel equations work for a truncated Tipler cylinder? It doesn't have to be infinitely long, just "sufficiently" long. And massive. And spinning insanely fast.

Still never gonna happen in the real world, but at least physically possible, which an infinitely long rod isn't.
It's not known if a long, but finite length, cylinder could reproduce the CTCs seen in a Tipler cylinder and it is at least plausible that the CTCs are a result of the unphysical aspects of the Tipler cylinder.
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Old 09-23-2017, 07:17 PM
Andy L Andy L is offline
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It's not known if a long, but finite length, cylinder could reproduce the CTCs seen in a Tipler cylinder and it is at least plausible that the CTCs are a result of the unphysical aspects of the Tipler cylinder.
On the other hand, the gravitational field near a large but finite cylinder is very close to the field for an infinite cylinder (and become even closer to the infinite cylinder result as you get closer to the finite cylinder) as long as you avoid the ends of the cylinder.
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Old 09-23-2017, 09:56 PM
Asympotically fat Asympotically fat is offline
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On the other hand, the gravitational field near a large but finite cylinder is very close to the field for an infinite cylinder (and become even closer to the infinite cylinder result as you get closer to the finite cylinder) as long as you avoid the ends of the cylinder.
A Tipler cylinder has very weird asymptotic properties so it is dangerous to assume too much about it's similarity to a finite length cylinder. In addition the theorem Hawking used to argue for his chronology protection hypothesis suggests a link between the infinite length/age of a Tipler cylinder and the closed timelike curves in the vacuum surrounding it.

That doesn't completely kill the idea that general relativity might predict that it is possible to construct a time machine from a rotating cylinder, but there are good reasons to suspect that time travel does not occur in the physically reasonable solutions of general relativity.
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Old 09-24-2017, 08:24 AM
Andy L Andy L is offline
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A Tipler cylinder has very weird asymptotic properties so it is dangerous to assume too much about it's similarity to a finite length cylinder. In addition the theorem Hawking used to argue for his chronology protection hypothesis suggests a link between the infinite length/age of a Tipler cylinder and the closed timelike curves in the vacuum surrounding it.

That doesn't completely kill the idea that general relativity might predict that it is possible to construct a time machine from a rotating cylinder, but there are good reasons to suspect that time travel does not occur in the physically reasonable solutions of general relativity.
Thanks.
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Old 09-25-2017, 12:53 PM
Doubticus Doubticus is online now
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One of the two said that space-time itself provides a "metric" against which the spin can be measured...
I remember a PBS program where a theoretical theorist claimed as much. Paraphrasing: "Space isn't the absence of everything, but the "medium" in which matter can exist." Thus objects spun relative to the space "medium".
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Old 09-27-2017, 09:15 AM
Jackknifed Juggernaut Jackknifed Juggernaut is offline
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I was watching an episode last night of "How the Universe Works" about exploding stars, supernovas, gamma ray bursts, etc. and also wondered the same. Some the more violent explosions seem to be the results of stars spinning very rapidly. Presumably, these would happen regardless of the existence of any other bodies in the universe.
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Old 09-28-2017, 10:00 PM
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I think the core of this thread, is based around the challenge that awareness of relativity effects, have caused a lot of less than completely informed people (which includes most of us) to get lost here and there. Even to the point where it can seem that we are being told that NO motion is real, and that it's ALL relative.

Perhaps the better question would be to ask what EFFECTS of spin, are dependent on the spinning object being a part of a larger system, and what effects are entirely due to the object and it's spin by itself.
  #42  
Old 09-28-2017, 10:18 PM
Chronos Chronos is offline
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The answer is the same: So far as we can tell, the effects of spin don't depend on any other objects. But it's possible that they depend on the entirety of all other objects in the Universe.
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