Verifying Time Dilation, Length Contraction & Space-Time Curvature

[shounakbhatta]

Equations or reality...

Hello all,

I am new to physics forum and very stupidly I don't have a background or formal education in science. I have one simple question:

Time dilation, length contraction, curvature of space-time, are they all mathematically deducable through equations or can they be physically verified by some experiments?

Thank you.

 

[Nick666]

Global positioning system (GPS) is the everyday experimental confirmation.

 

[ghwellsjr]

Why don't you back up in this forum and click on the link called "FAQ: Experimental Basis of Special Relativity"?

There're probably more experiments listed there than you will want to investigate. After all, it was the famous Michelson-Morley Experiment that got this whole thing started.

 

[Passionflower]

Time dilation, length contraction, curvature of space-time, are they all mathematically deducable through equations or can they be physically verified by some experiments?

Time dilation is experimentally verified, for instance GPS would not work if we would not account for time dilation. Length contraction is not and it is probably not even possible to experimentally verify length contraction.

The curvature of spacetime is a bit tricky. If you consider gravitational time dilation curvature of spacetime then the answer is yes, it has been experimentally verified by measuring the effect the Sun has on light rays from far away stars or the red- and blueshift between signals on Earth at different heights. However if you only consider the strong gravitational effects (e.g. the deviation from Euclidean spacetime) curvature of spacetime then it has not, at least not directly, as our instrumentation is not accurate enough to measure that.

 

[russ_watters]

Time dilation is experimentally verified, for instance GPS would not work if we would not account for time dilation. Length contraction is not and it is probably not even possible to experimentally verify length contraction.

That is clearly wrong as it is self-contradictory: you can't have one without the other.

 

[Passionflower]

That is clearly wrong as it is self-contradictory: you can't have one without the other.

If you think it is wrong, please provide an experiment, date and place, that shows length contraction.

 

[Doc Al]

That is clearly wrong as it is self-contradictory: you can't have one without the other.

I don't think Passionflower meant that length contraction isn't true, but that there is no direct experiment confirming it. (Of course, for consistency it must be true.)

 

[Passionflower]

I don't think Passionflower meant that length contraction isn't true, but that there is no direct experiment confirming it. (Of course, for consistency it must be true.)

Exactly.

 

[Nick666]

Knock yourself out http://en.wikipedia.org/wiki/Length_contraction#Experimental_verifications

 

[russ_watters]

I don't think Passionflower meant that length contraction isn't true, but that there is no direct experiment confirming it. (Of course, for consistency it must be true.)

What does "direct" mean? Is not a muon decaying after traveling a shorter than expected distance an example of it?

Or are you saying that since time is measured by counting events, but length is calculated from time, that makes time direct and length indirect?

 

[Doc Al]

What does "direct" mean?

Good question. Naively, measure the length of something moving to be contracted by the predicted amount.

Is not a muon decaying after traveling a shorter than expected distance an example of it?

I would say no.

 

[russ_watters]

Good question. Naively, measure the length of something moving to be contracted by the predicted amount.

I would say no.

So...because it is "distance", not "length" we're not seeing length contraction? How are they not the same thing?!

We measure the "length" of the atmosphere in a host of different ways and the muon sees that "length" to be vastly different, no?

This whole line of discussion seems like hairsplitting to me. In any case, there are references that disagree:

These observations provide direct evidence of relativistic time dilation and length contraction.

It's titled "High Energy Astrophysics" and looks like a textbook to me.

http://books.google.com/books?id=KG...ay direct evidence length contraction&f=false

 

[John232]

I have heard that you couldn't measure length contraction because your ruler would also become smaller, but you can't measure time dilation moveing with the frame of reference either. So length contraction could possibly be measured, but it would have to be done from another frame of reference that wasn't moveing with the object in question.

 

[shounakbhatta]

Hello All,

Than you very much for all your valuable replies. What I can understand is that some of the result of Relativity might be verified by experiments, while others are not. Say, Hawking radiation through black hole or the black hole rule of thermodynamics. Can they be seen through our physical eyes? Or they are just results of equations that lead to the entropic principle following with the mass of black hole multiplied by Boltzmann-constant? As black holes cannot be seen by eyes are the radiations just numerical figures or can they be seen during the emission process?

 

[ghwellsjr]

Time dilation is experimentally verified, for instance GPS would not work if we would not account for time dilation. Length contraction is not and it is probably not even possible to experimentally verify length contraction.

That is clearly wrong as it is self-contradictory: you can't have one without the other.

Russ_watters is exactly correct here. If you are going to accept the result of experiments comparing the accumulated times on a moving clock to a stationary clock as direct verification of time dilation, then you must also accept the result of those experiments as direct verification of length contraction.

The reason is that length contraction only applies along the direction of motion and if it were not happening, time dilation would change with the orientation of the clock and would not follow the theoretical prediction.

Think about this: a light clock in motion is usually depicted as having a beam of light that is bouncing at right angles to the direction of motion between a pair of mirrors. In this case there is no length contraction.

But now rotate the light clock so that the beam of light is bouncing along the direction of motion. Now if the mirrors stay the same distance apart, the light will take longer to bounce between them and our light clock will be sensitive to its orientation with respect to the direction of motion and will no longer follow the theoretical prediction and we will have proved that time dilation is in affect but length contraction is not.

So in order for us to get a consistent result from our experimental evidence of direct verification of time dilatation, we must also accept that it is a direct verification of length contraction.

 

[A.T.]

The reason is that length contraction only applies along the direction of motion and if it were not happening, time dilation would change with the orientation of the clock and would not follow the theoretical prediction.

The question is if you could show length contraction experimentally without invoking time dilation in the interpretation of the results.

It works the other way around: The twins will have different times on their clocks on reunion. That is a direct experimental proof that doesn't require the assumption of length contraction, and is not based on conventions of how to synchronize spatially separated clocks.

 

[Passionflower]

Russ_watters is exactly correct here. If you are going to accept the result of experiments comparing the accumulated times on a moving clock to a stationary clock as direct verification of time dilation, then you must also accept the result of those experiments as direct verification of length contraction.

It is not a matter of "must accept" things, I am not questioning that length contraction should happen conform relativity theory.

But that is not what we are talking about, we are talking about experiments. You can have all your faith in a theory but that does not replace experimental verification.

I can see experiments about the elapsed time of two clocks and notice it is different. Can you name the date and place of an experiment that compares the lengths and notice they are different?

 

[ghwellsjr]

It is not a matter of "must accept" things, I am not questioning that length contraction should happen conform relativity theory.

But that is not what we are talking about, we are talking about experiments. You can have all your faith in a theory but that does not replace experimental verification.

I can see experiments about the elapsed time of two clocks and notice it is different. Can you name the date and place of an experiment that compares the lengths and notice they are different?

The experiments you are talking about that you say confirm time dilation, by comparing accumulated time ticks of a moving clock to a stationary clock, also confirm length contraction, unless, of course, the clocks are designed to tolerate length contraction along the direction of motion without affecting their tick rate and the moving clock was always oriented along the direction of motion to be immune to the effects of length contraction.

Please note that I started my post by saying if you are going to accept these experiments as direct evidence of time dilation, then you also have to accept them as direct evidence of length contraction. The reason I say "if" is because these experiments are not directly observing time dilation which has to do with the tick rate of a clock, in other words, of a metronome. I could say to you that in order to be a direct confirmation of time dilation, you need to use metronomes, not a clocks, because you are demanding that I use rods, not odometers, to directly confirm length contraction.

 

[Passionflower]

The experiments you are talking about that confirm you say confirm time dilation, by comparing accumulated time ticks of a moving clock to a stationary clock, also confirm length contraction, unless, of course, the clocks are designed to tolerate length contraction along the direction of motion without affecting their tick rate and the moving clock was always oriented along the direction of motion to be immune to the effects of length contraction.

Looks like I am talking to a wall. You do know the difference between experiment and theory right? There are several experiments in the literature that compare the elapsed times on clocks that travel at a different average velocity between two shared events. At the end we simply look and record the elapsed times. Now name me one experiment where lengths are compared?

I know relativity predicts that lengths must also contract but there is no experiment that demonstrates that. Does not mean it is false, does not mean relativity is denied, something you apparently are so worried about on this forum, no, it just means that there is no experiment that demonstrates that lengths contract.

Please note that I started my post by saying if you are going to accept these experiments as direct evidence of time dilation, then you also have to accept them as direct evidence of length contraction.

Yes I noted it and it is completely wrong. You make inferences, and inferences I happen to agree with, about a theory, but that is not the same as measuring it by setting up an experiment.

The reason I say "if" is because these experiments are not directly observing time dilation which has to do with the tick rate of a clock, in other words, of a metronome. I could say to you that in order to be a direct confirmation of time dilation, you need to use metronomes, not a clocks, because you are demanding that I use rods, not odometers, to directly confirm length contraction.

So you claim clocks, for instance, atomic clocks, cannot be used to verify time dilation? If so you are completely wrong, there are many experiments in the literature that show time dilation.

 

[russ_watters]

Can you name the date and place of an experiment that compares the lengths and notice they are different?

What of my muon reference?

 

[Parlyne]

However if you only consider the strong gravitational effects (e.g. the deviation from Euclidean spacetime) curvature of spacetime then it has not, at least not directly, as our instrumentation is not accurate enough to measure that.

What about the Grav Probe B measurements of the geodetic effect and frame dragging?

 

[Passionflower]

What about the Grav Probe B measurements of the geodetic effect and frame dragging?

Good question, is there frame dragging in Newton-Cartan?

Let's take a simple example, have a test particle radially (e.g. zero angular momentum wrt to the mass and zero angular momentum wrt to itself) approach a rotating mass, what happens in Newton-Cartan does it go straight in and without self rotation? And if it does not is the difference wrt the Kerr metric a strong field effect only?

 

[ghwellsjr]

The experiments you are talking about that you say confirm time dilation, by comparing accumulated time ticks of a moving clock to a stationary clock, also confirm length contraction, unless, of course, the clocks are designed to tolerate length contraction along the direction of motion without affecting their tick rate and the moving clock was always oriented along the direction of motion to be immune to the effects of length contraction.

Looks like I am talking to a wall. You do know the difference between experiment and theory right? There are several experiments in the literature that compare the elapsed times on clocks that travel at a different average velocity between two shared events. At the end we simply look and record the elapsed times. Now name me one experiment where lengths are compared?

As I said in post #15, every experiment that demonstrates time dilation also demonstrates length contraction, unless you believe that the clock in motion is immune to the effects of length contraction along one of its axes and it remains oriented such that this axis is always along the direction of motion. Oh, I see I just said that. Maybe there is a wall in the way.

I don't know if the specifics in all the cases that demonstrate time dilation paid attention to this issue (I doubt it), but don't you agree that it would be extremely difficult, if not impossible to build a clock that would be immune to length contraction along one axis? So can't we assume that most if not all clocks would tick at a different rate if we changed their orientation with respect to their motion and length contraction were not happening? So don't you agree that the clocks that were used in the time dilation experiments would not conform to the predictions of SR and GR if length contraction were not also conforming to the predictions of SR and GR?

Furthermore, wouldn't a valid experiment be to place a bunch of clocks in many different orientations and track their tick rates through the day and through the seasons, (similar to the reasoning behind MMX) and see if we get a null result?

 

[Doc Al]

So...because it is "distance", not "length" we're not seeing length contraction? How are they not the same thing?!

We measure the "length" of the atmosphere in a host of different ways and the muon sees that "length" to be vastly different, no?

Sure, from the muon's frame the length is contracted. But the experiment was not done in the frame of the muon.

Does the muon experiment provide evidence for length contraction? Of course, since it all hangs together. One frame's time dilation is another frame's length contraction. You cannot have one without the other.

Nonetheless, is the muon experiment a direct demonstration of length contraction? I'd say no. That classic experiment is discussed in just about every textbook, yet I've never seen it described as particularly demonstrating length contraction. (Time dilation, yes.)

This whole line of discussion seems like hairsplitting to me.

I agree. That's why I have little interest in arguing the point. It just attracts crackpots--folks who think that the 'jury is not out' on the issue of length contraction. (But you did bring it up.) It depends on what one means by 'direct'.

Even our own 'FAQ" link https://www.physicsforums.com/showthread.php?t=229034" agrees with what I thought was the commonly accepted view:

7. Tests of Length Contraction

At this time there are no direct tests of length contraction, as measuring the length of a moving object to the precision required has not been feasible.​

That's what I would call a direct test of length contraction. Just not feasible. (That said, I think there are some cases--in accelerator physics, for example--where length contraction must be explicitly used to get the correct results. These would be closer to a 'direct test' than the muon experiment.)

In any case, there are references that disagree: It's titled "High Energy Astrophysics" and looks like a textbook to me.

That's interesting, but easily attributed to semantics. (As can much of this thread.)

 

[russ_watters]

Sure, from the muon's frame the length is contracted. But the experiment was not done in the frame of the muon.

Isn't that like saying you have to be with the slower clock to call a difference in elapsed time "direct evidence of time dilation"? ...which would make my favorite demonstration, the GPS clocks, "indirect"? Or worse, the same measurement is "direct" evidence of the GR dilation and "indirect" evidence of the SR time dilation? I think what makes the evidence "direct" is that we sit two clocks down next to each other and find them to have different times, therefore one was dilated. You don't have to be traveling with the dilated one (I realize that we have) to know it must be true: you can see it on the clocks!

For the muon experiment, we measure a difference in lengths in our reference frame.

Regardless, circling back to the original issue:

Does the muon experiment provide evidence for length contraction? Of course, since it all hangs together. One frame's time dilation is another frame's length contraction. You cannot have one without the other.

Ok, so the quote that started this line of hairsplitting was this:

Length contraction is not and it is probably not even possible to experimentally verify length contraction.

Ok, so yes, length contraction is experimentally verified - whether you want to call the evidence direct or indirect - right? So either way, the quote is still wrong...right?

 

[russ_watters]

Hello All,

Than you very much for all your valuable replies. What I can understand is that some of the result of Relativity might be verified by experiments, while others are not.

That's true, but not for length contraction. Whether one calls the evidence "direct" or "indirect", we have pretty strong evidence for it.

Say, Hawking radiation through black hole or the black hole rule of thermodynamics. Can they be seen through our physical eyes? Or they are just results of equations that lead to the entropic principle following with the mass of black hole multiplied by Boltzmann-constant? As black holes cannot be seen by eyes are the radiations just numerical figures or can they be seen during the emission process?

AFAIK, Hawking radiation has not been verified. And no, it's not black body radiation, it is something completely different.

 

[Doc Al]

Isn't that like saying you have to be with the slower clock to call a difference in elapsed time "direct evidence of time dilation"?

No, just the opposite! In the muon experiment we measure lengths in the Earth's atmosphere, which is at rest in the lab frame, and use that as a measurement of the muon's lifetime as seen by us. Nothing directly to do with length contraction. To measure length contraction directly, the something we are measuring must be moving with respect to us. Just like with time dilation.

For the muon experiment, we measure a difference in lengths in our reference frame.

True, but these are not contracted lengths of anything.

That's true, but not for length contraction. Whether one calls the evidence "direct" or "indirect", we have pretty strong evidence for it.

That's really the bottom line. The experimental evidence for SR, including time dilation and length contraction, is overwhelming. (This talk about 'direct measurement' of length contraction is a hair splitting side issue. I apologize if I added to any confusion.)

 

[zonde]

Ok, so yes, length contraction is experimentally verified - whether you want to call the evidence direct or indirect - right? So either way, the quote is still wrong...right?

It is SR that is experimentally verified. In SR time dilation and length contraction are not two independent effects and therefore experimental verification of time dilation gives us very good reason to believe that length contraction happens as well. But it's still not correct to say that length contraction is experimentally verified.

If we want to verify SR prediction about length contraction then we have to use setup where measurements can be interpreted independently from the theory we want to test.

So how we can possibly verify length contraction?
We have space telescope that is moving at some orbital speed and as telescope circles around Earth it should observe length contraction of universe in different directions. If we factor out aberration then we should observe different angle between two objects at different positions of telescope.
Say first position is where one object is in direction of motion of telescope but second object is at 45deg. angle from first object.
Second position is such that one of the objects is orthogonal to direction of orbital motion but other object is around 45deg. angle.
Then the angle as measured in second position should be smaller than it was in first position after we factor out aberration.