[homeslice (OP)]

I understand that if one thing is moving at close to light speed relative to something else that is moving little, the one moving at close to the speed of light experiences a time slow-down - if it were to look at a clock held by the item moving little that clock would be ticking very, very fast from its point of view (and a clock held by the fast moving item would look to be ticking very slow to the slow moving item).

Question - let's say in the entire universe there are only those two items described above - how on Earth (lol) would "the universe" determine which is moving fastly relative to the other? From any point of view, they would be approaching each other, or moving away from each other, very quickly, but it would seem impossible to tell which is moving relative to the other. How does the "universe" "know" which one really has the relatively fast motion and thus which clock should be running fast and which slow?

Related question - I've always heard that relativity theory says basically that space is not an absolute - there is no "ether" for example. Space is not its own thing per se, but just describes relationships between different objects or other things. If that were the case, my question above would seemingly be valid.

But I distinctly remember listening to an audiobook some time ago that seemed to say that the motion of objects in the universe was actually RELATIVE to spacetime. This seemed weird to me, like almost inconsistent with everything I had ever heard (or thought I had heard). But, if that is right, then the "universe" should not have a problem determining which particle was doing the (fast) moving and which one was not.

Is what I heard right or wrong?

Thanks!

[Halc]

I understand that if one thing is moving at close to light speed relative to something else that is moving little, the one moving at close to the speed of light experiences a time slow-down

This is not the way to express it at all. If thing X is moving at nearly c relative to Y, neither of them is objectively moving fast or slow. Y is just as much moving at nearly c relative to X.
The other thing is the word 'experiences' in there. Per the principle of relativity, all inertial frames are equally valid, hence nobody 'experiences' anything unusual due to movement relative to some other arbitrary frame. For instance, Earth is moving at nearly c relative to some random muon in the upper atmosphere. Relative to that muon, our clocks run very slow, but it doesn't change our experience one bit. Physics is the same regardless of inertial motion.

if it were to look at a clock held by the item moving little that clock would be ticking very, very fast from its point of view

No. If from X one were to look at the other clock Y, that other clock would be moving at nearly c and would be measured to be running slow. Relative to the frame in which Y is stationary, the X clock runs slow.

Question - let's say in the entire universe there are only those two items described above - how on Earth (lol) would "the universe" determine which is moving fastly relative to the other?

Yes! That's the gist of the principle of relativity. There's no way to tell. There is no test for being absolutely stationary in Minkowskian spacetime of special relativity (a flat universe without mass or energy), so either item can validly consider itself to be stationary and the other one moving.

From any point of view, they would be approaching each other, or moving away from each other, very quickly, but it would seem impossible to tell which is moving relative to the other.

It is very apparent to both that they're moving relative to each other. It is quite easy to see something get smaller for instance as it moves away. But the other observer will see you get smaller. The situation is entirely symmetrical.

How does the "universe" "know" which one really has the relatively fast motion and thus which clock should be running fast and which slow?

Neither clock runs objectively faster or slower than the other. The universe doesn't need to 'know' anything. Each observer sees the other observer time dilated, again entirely symmetrical.

Related question - I've always heard that relativity theory says basically that space is not an absolute - there is no "ether" for example.

There is nothing measurable about aether. One cannot prove the nonexistence of something with no measurable properties.

But I distinctly remember listening to an audiobook some time ago that seemed to say that the motion of objects in the universe was actually RELATIVE to spacetime.

Well that's because spacetime is only locally Minkowskian. It is curved at larger scales, and thus looks different from one frame to the next. This suggests a preferred local frame, the one and only frame in which distant light appears isotropic. The inability to detect a preferred frame is relevant only to the Minkowskian spacetime. The actual universe has a lot of mass and energy.

Oh, and welcome to TNS!

[homeslice (OP)]

Thank you so much Halc!

So it sounds like my main flaw was not realizing that the "time dilation" effect works on both particles in my example.  So in an example where I tell you all that is in the universe is 2 particles, and they are approaching each other a close to light speed, if either was to look at one another's clock (putting aside my assumption that there were only 2 particles in the universe haha), the other's clock would both look to be running slow in each case.  I had not thought about that, but that makes absolute sense.

What if I kept the same example, only X and Y particles in the universe, but I changed the facts just a bit to tell you that it is 100% certain that a force acted upon particle X (not particle Y) to speed X up to close to the speed of light, and thus is was X, not Y that accelerated.  Would that change you answer at all?  I highly suspect you will say no!

Then if I went back to the first example, but told you X and Y are the only particles way out in space, but way away in space is the rest of the universe, and I were to tell you that X particle appears to be moving close to light speed both to Y particle and the rest of the universe (and vice versa), but neither Y particle nor the rest of the universe appear to be moving relative to one another, then I think you will just tell me an observer in the rest of the universe would have the same view as Y particle - both universe and Y particle would view the X clock as running very slow, and X would view both Y clock and rest of universe clock as running very slow (the symmetry you mentioned), but necessarily Y and universe clock would appear to be running at the same speed to each other.

Question about the "aether" or lack thereof.  I was under the impression that what we think of as empty space has various fields in it, along with virtual pairs of particles popping in and out of existence all the time.  Also the, I forgot what it is called, but whatever the energy in empty space is that seems to be causing the universe to expand.  Rest energy of space?  Anyways, none of that, to the extent true, means that relative motion needs to be considered against the background of space?

Thanks again Halc!!!

[Halc]

So in an example where I tell you all that is in the universe is 2 particles, and they are approaching each other a close to light speed, if either was to look at one another's clock (putting aside my assumption that there were only 2 particles in the universe haha), the other's clock would both look to be running slow in each case.

Each would be running slow relative to the frame in which it was moving. That's different than appearances. Doppler effect is more pronounced than time dilation, so a clock coming right at you will appear to run faster, just like the police siren sounds higher pitch when the vehicle is approaching.  But if it is moving away, it will be much slower since both dilation and doppler are combining effects and not cancelling.

What if I kept the same example, only X and Y particles in the universe, but I changed the facts just a bit to tell you that it is 100% certain that a force acted upon particle X (not particle Y) to speed X up to close to the speed of light, and thus is was X, not Y that accelerated.

Dilation is a function of relative speed. It isn't a function of acceleration at all. I can have two clocks moving at the same speed but very different acceleration levels and they'll stay in sync.

and I were to tell you that X particle appears to be moving close to light speed both to Y particle and the rest of the universe (and vice versa)

The universe hasn't a location, hence doesn't have a meaningful speed.

I was under the impression that what we think of as empty space has various fields in it, along with virtual pairs of particles popping in and out of existence all the time.

Yes. If space was empty, there'd be no light (among other things) and that means you'd not see anything, which contradicts the evidence.
You opened a new topic on this, so I'll let it take up the rest.

[Eternal Student]

Hi.

You ( @ homeslice ) seem to be asking sensible questions.   You're also getting good advice and replies from @Halc .

You do seem to be picking up some details ever so fast, well done, that is amazing.   However, I was also going to mention that you shouldn't worry too much if you would like to ask something a different way or clarify some details.   None of us were born already knowing about relativity.

What if I kept the same example, only X and Y particles in the universe, but I changed the facts just a bit to tell you that it is 100% certain that a force acted upon particle X (not particle Y) to speed X up to close to the speed of light, and thus is was X, not Y that accelerated.  Would that change you answer at all?  I highly suspect you will say no!

    That's the main bit where I wasn't certain you were given a sufficient answer and might get a little confused.
1.   First of all,  we'd rather say that X experienced an acceleration while Y did not.   This is only because, when using relativity, Force should really be replaced with a thing called a 4-Force.   We can just avoid any issue by talking about an ordinary acceleration.    Then, exactly as you would expect, it is easy enough for X to know that they are being accelerated.  They can use an accelerometer and tell that they are experiencing something different to Y.   In one important way this has broken the symmetry between X and Y and you might hear that mentioned in some science articles especially if they are talking about General Relativity (GR) instead of Special Relativity (SR).

2.    What Halc said is true.   At any instant,  all that matters is the relative velocity between X and Y.    However, if X is experiencing an acceleration but Y is not then the relative velocity of X compared to Y will not stay constant for long.   Just to say this another way,   for one instant,  the time dilation factor between  the rest frame of X and the rest frame of Y was precisely as you'd expect.  The fact that X was getting an acceleration doesn't matter.   At a later time, say 1 second later, the relative velocity has changed and then the time dilation between X and Y is different to what it was.    That's also perfectly fine provided you change to another set of frames where X and Y are both once again at rest (instantaneously only for X,  i.e.   X is at rest in that frame only at that instant,  you know it is accelerating so it doesn't stay at rest in that frame for long).
    Now General Relativity is a bit more advanced then Special Relativity,  it easily handles frames of reference that were being accelerated.   So with GR you don't have to keep throwing away the frame of reference for X and using a new one every instant.   Instead you can find a frame a reference where X is always at rest.    I think it's best not to worry too much about GR at the moment,  stick with the slightly simpler SR.   I've only mentioned it because you might find articles that suggest there is a gravitational time dilation effect (a GR effect) between X and Y which was only there because X was being accelerated differently to Y.   To say that another way,  the fact that the situation between X and Y is not perfectly symmetric  (one is being accelerated, the other is not) would be important  but  for just SR and where we keep throwing away the reference frame and using a new one then it doesn't matter at all.
   I hope that helps a bit.

Best Wishes.

[paul cotter]

Great question and highly informative replies. A big improvement on some of the recent nonsense afflicting the forum.