Has Time Dilation been tested?

[Saint]

Under the assumptions of relativity' date=' the speed of an object, relative to another, does not change the laws of physics. So the mechanism of the decay is unchanged - as far as the muon is concerned, it is at rest. And from this point of view, the decay rate is affected just as relativity predicts when viewed from another frame of reference. So there is absolutely no reason to exclude this observation from the list of things that confirm time dilation.

 

And, As CPL Luke has mentioned, there is GPS. And there is also the Hafele-Keating experiment of clocks on airplanes. And the Vessot experiment of a Hydrogen maser on a rocket (that's mostly GR, though). Atomic fine structure has relativistic terms in it, that's confirmed by spectroscopy. And so on and so forth.[/quote']

 

Has time itself has been dilated, or have the internal workings of the muon simply slowed do to movement through a gravitational field. Since we can't predict a single decay event (we only deal with averages), it's safe to say we don't understand the mechanism well enough to tell the difference.

 

Doesn't relativity suggest that the GPS system should correct for frequency differences between GPS satellites and the recievers based on the relative velocity between the two? The GPS system doesn't do that. It only corrects the satellite clocks for their relative velocity wrt the ECI frame. That's not exactly the same thing. And again, can you present any proof that the clock rates reflect an actual dilation of the local "time"? Can you tell the difference between the slowing of physical mechanisms, and a dilation of "time"?

[swansont]

Has time itself has been dilated' date=' or have the internal workings of the muon simply slowed do to movement through a gravitational field. Since we can't predict a single decay event (we only deal with averages), it's safe to say we don't understand the mechanism well enough to tell the difference.

 

Doesn't relativity suggest that the GPS system should correct for frequency differences between GPS satellites and the recievers based on the relative velocity between the two? The GPS system doesn't do that. It only corrects the satellite clocks for their relative velocity wrt the ECI frame. That's not exactly the same thing. And again, can you present any proof that the clock rates reflect an actual dilation of the local "time"? Can you tell the difference between the slowing of physical mechanisms, and a dilation of "time"?[/quote']

 

No, you correct for the difference with respect to the ECI frame. It turns out that the rotation doesn't have any net effect, because the change in the gravitational redshift (from the earth's deformation from a sphere) offsets it, so the frame you are using as a reference is a nonrotating one, and clocks on the geoid have no shift with respect to that frame.

 

The reason we don't suspect that the motion is causing a physical change is that the effect is the same, no matter what kind of oscillator is being used. This has been tested with different atomic transitions (Cs, Rb and H, at least) quartz oscillators and with various nuclear transitions. So if the change were due to changes in the "internal workings" you'd have to explain how the change is the same for different workings with different scales of interaction strength.

[Saint]

No, you correct for the difference with respect to the ECI frame. It turns out that the rotation doesn't have any net effect, because the change in the gravitational redshift (from the earth's deformation from a sphere) offsets it, so the frame you are using as a reference is a nonrotating one, and clocks on the geoid have no shift with respect to that frame.

 

I'm talking about receivers and satellites, not about the cancellation of the frequency shift on the geiod. What I'm saying is that the GPS system is composed of a number of clocks, all moving wrt each other (at least the satellites are). So you have a bunch of clocks moving around, and relativity would suggest that they should all be running at different rates relative to one another. If you took a GPS satellite and a GPS receiver and put them in the same orbit, but travelling in opposite directions, the GPS system would conclude that there is no frequency shift between the two (same relative velocity wrt the ECI frame). Relativity would conclude that they are running at different rates wrt one another (approaching each other at 2X orbital velocity). Both can't be true. The fact is that the receiver would not see a frequency shift even though it was approaching the satellite at twice orbital velocity. Because they are both moving at the same velocity wrt the ECU frame.

 

The reason we don't suspect that the motion is causing a physical change is that the effect is the same' date=' no matter what kind of oscillator is being used. This has been tested with different atomic transitions (Cs, Rb and H, at least) quartz oscillators and with various nuclear transitions. So if the change were due to changes in the "internal workings" you'd have to explain how the change is the same for different workings with different scales of interaction strength.[/quote']

 

So all physical processes and mechanisms are effected by velocity wrt a gravitational field. What's wrong with that? That would seem to be consistent.

[Tom Mattson]

So all physical processes and mechanisms are effected by velocity wrt a gravitational field. What's wrong with that? That would seem to be consistent.

 

Consistent with what? You talk about this gravitational influence on particle decay as if it is a well-established theory. But it's really just your ad hoc proposal to account qualitatively for that which relativity can already account for qualitatively and quantitatively. You have no idea if the experimental results are consistent with this proposal until you calculate the effect and compare it with measurements.

[swansont]

I'm talking about receivers and satellites, not about the cancellation of the frequency shift on the geiod. What I'm saying is that the GPS system is composed of a number of clocks, all moving wrt each other (at least the satellites are). So you have a bunch of clocks moving around, and relativity would suggest that they should all be running at different rates relative to one another. If you took a GPS satellite and a GPS receiver and put them in the same orbit, but travelling in opposite directions, the GPS system would conclude that there is no frequency shift between the two (same relative velocity wrt the ECI frame). Relativity would conclude that they are running at different rates wrt one another (approaching each other at 2X orbital velocity). Both can't be true. The fact is that the receiver would not see a frequency shift even though it was approaching the satellite at twice orbital velocity. Because they are both moving at the same velocity wrt the ECU frame.

 

No, your example of two different orbits isn't the same situation, because they are both accelerating wrt the inertial frame. You're changing too many variables at once to be able to do that comparison. If you want to do that, you'd have to go to a real inertial frame, as is done in the analysis of the Hafele-Keating experiment.

 

So all physical processes and mechanisms are effected by velocity wrt a gravitational field. What's wrong with that? That would seem to be consistent.

 

Nothing, if you could come up with a non-ad-hoc model that explains why this would happen. Consistent isn't enough. Why the same fractional change, independent of the strength of gravity? Why the same fractional change at all, for interactions of different strengths?

 

But under the postulate of a constant c, the effect is predicted by special relativity, and that is what is observed.

[Locrian]

This is from my blog, entitled "Physics Riddle." Of course all the big-wigs in the thread will know the answer immediately, but I remember finding it suprising way back when I was an undergrad, so maybe someone here will find it amusing as well. In any case, the information is on topic:

 

-------------------

 

One of the predictions of the special theory of relativity is that when two observers have a velocity with respect to each other, they percieve time as passing differently for each other. This is not just an optical trick; consider the infamous “twin paradox,” where one twin leaves planet earth traveling very, very quickly and then later returns. The one that left and returned would find themselves younger than the other.

 

Consider this experiment*: In 1972 two men took three atomic clocks to an airport. One left traveling east completely around the world on multiple flights. Another left traveling west and traveled around the world. A third atomic clock was left at the airport.

 

Based on the twin paradox, what do you think should happen? You might think those who are in motion have clocks that count slower. The results might (or might not) be suprising. The man traveling eastward was about 60 nanoseconds younger than the reference clock. The man traveling westward was 273 nanoseconds older than the reference clock.

 

And this is exactly what special relativity predicts.

 

So what gives?

 

*See Science 177, 166-170 (1972)

[swansont]

This is from my blog' date=' entitled "Physics Riddle." Of course all the big-wigs in the thread will know the answer immediately, but I remember finding it suprising way back when I was an undergrad, so maybe someone here will find it amusing as well. In any case, the information is on topic:

 

-------------------

 

One of the predictions of the special theory of relativity is that when two observers have a velocity with respect to each other, they percieve time as passing differently for each other. This is not just an optical trick; consider the infamous “twin paradox,” where one twin leaves planet earth traveling very, very quickly and then later returns. The one that left and returned would find themselves younger than the other.

 

Consider this experiment*: In 1972 two men took three atomic clocks to an airport. One left traveling east completely around the world on multiple flights. Another left traveling west and traveled around the world. A third atomic clock was left at the airport.

 

Based on the twin paradox, what do you think should happen? You might think those who are in motion have clocks that count slower. The results might (or might not) be suprising. The man traveling eastward was about 60 nanoseconds younger than the reference clock. The man traveling westward was 273 nanoseconds [i']older[/i] than the reference clock.

 

And this is exactly what special relativity predicts.

 

So what gives?

 

*See Science 177, 166-170 (1972)

 

To be precise about the story, it was more than one clock on each airplane, and the "third clock" was the ensemble at the US Naval Observatory.

 

The key to it all is [cue Obi-wan] "Luuuke. Use the ineeertial fraaame."

[Saint]

Nothing' date=' if you could come up with a non-ad-hoc model that explains why this would happen. Consistent isn't enough. Why the same fractional change, independent of the strength of gravity? Why the same fractional change at all, for interactions of different strengths?

 

But under the postulate of a constant c, the effect is predicted by special relativity, and that is what is observed.[/quote']

 

You're asking me "why"? How about, why would any velocity wrt anything else alter your local "time"? I'm suggesting that there might be a physical explanation. I'm saying that we should look into the physical mechanisms behind phenomena before accepting a curve fit with no mechanism that seeks to alter time itself.

 

What is the mechanism for relativity? What is it that drives the "time dilation". An equation is not reality. Does the universe really bend over backwards, and inside out, simply to accomodate the speed of light in all reference frames?

[swansont]

You're asking me "why"? How about' date=' why would any velocity wrt anything else alter your local "time"? I'm suggesting that there might be a physical explanation. I'm saying that we should look into the physical mechanisms behind phenomena before accepting a curve fit with no mechanism that seeks to alter time itself.

 

What is the mechanism for relativity? What is it that drives the "time dilation". An equation is not reality. Does the universe really bend over backwards, and inside out, simply to accomodate the speed of light in all reference frames?[/quote']

 

Light travelling at c in inertial frames is necessary for Maxwell's equations to work, so it's not like the postulate was pulled out of someone's rear. Time dilation is a consequence of that. It was not a case of fitting something to a curve.

[Saint]

Light travelling at c in inertial frames is necessary for Maxwell's equations to work, so it's not like the postulate was pulled out of someone's rear. Time dilation is a consequence of that. It was not a case of fitting something to a curve.

 

Really? Nothing else works with Maxwell's equation, AND fits experimental data for GPS, muons etc...?

 

 

And I never said it was pulled out of someone's rear. I just said it had no mechanism. And it doesn't. It is a curve fit, because, even though no one understands a mechanism, they will stand up and say that evidence has been found for relativity because relativity's equations fit this data curve from such and such an experiment. Unless you can provide a mechanism for a theory, it is a curve fit. You're building equations to model results, not looking for the mechanisms that drive the results.

[Locrian]

Suggesting that relativity is "a curve fit" doesn't make much sense. The only way I can interpret that is to say that the theory is built off large numbers of experimental results in such a way that it predicts them and all new ones.

 

Which is about the nicest thing you can say about any theory.

 

Yet, by your paragraph, it didn't sound like you were trying to give it a grand compliment. I'm afraid your meaning is eluding me.

[swansont]

Really? Nothing else works with Maxwell's equation, AND fits experimental data for GPS, muons etc...?

 

No. If c depends on the frame, you don't get a wave equation anymore, and light still seems to be a wave, even when I'm moving around.

 

 

And I never said it was pulled out of someone's rear. I just said it had no mechanism. And it doesn't. It is a curve fit, because, even though no one understands a mechanism, they will stand up and say that evidence has been found for relativity because relativity's equations fit this data curve from such and such an experiment. Unless you can provide a mechanism for a theory, it is a curve fit. You're building equations to model results, not looking for the mechanisms that drive the results.

 

F=ma is a "curve-fit" by that standard. Do you have a mechanism for it, or should we discard it?

 

You need a mechanism if it's a mechanical effect. As a description of how nature behaves, observation (i.e. curve-fitting) is all there is.

[insane_alien]

i thought muons decayed with a distinct cut-off rather than a halflife. or is that something elese i'm thinking about.

[Saint]

Suggesting that relativity is "a curve fit" doesn't make much sense. The only way I can interpret that is to say that the theory is built off large numbers of experimental results in such a way that it predicts them and all new ones.

 

Which is about the nicest thing you can say about any theory.

 

Yet' date=' by your paragraph, it didn't sound like you were trying to give it a grand compliment. I'm afraid your meaning is eluding me.[/quote']

 

OK, let's try this. Say you are going to predict the outcome of a soccer game. You develop an equation based on the strengths and weaknesses of each team. Your prediction is that, based on your equation, team A will beat team B by a score of 5 to 1. So the game starts, and team B's goalie breaks his arm, the starting striker's new pair of shoes are giving him blisters, and the coach is drunk. The score ends up being, just as you predicted, 5 to 1 in favor of team A. Now, based on the results, your equation works flawlessly. Your theory is sound, even though you never watched the game to figure out why the score ended up the way it did. That's how I view a curve fit theory. It's based solely on outcomes. It doesn't really care if the mechanisms are incorrect, or lacking entirely. A curve fit doesn't require one to understand what's really going on, only to be able to predict outcomes.

 

I would just think that physics as a whole should be more concerned with why things work the way they do, as opposed to simply predicting outcomes.

[Tom Mattson]

And I never said it was pulled out of someone's rear. I just said it had no mechanism. And it doesn't.

 

Why would you require a mechanism?

 

It is a curve fit' date=' because, even though no one understands a mechanism, they will stand up and say that evidence has been found for relativity because relativity's equations fit this data curve from such and such an experiment.

[/quote']

 

You've got it completely wrong.

 

The fact that relativity is based on two simple principles, and that it makes testable predictions that can be deduced from those principles, makes it much more than a curve fit. A curve fit is an ad-hoc model that is used to account for data after the fact. If you were to develop your idea about the alteration of particle lifetimes by gravitational effects, that would be a curve fit.

 

Unless you can provide a mechanism for a theory, it is a curve fit. You're building equations to model results, not looking for the mechanisms that drive the results.

 

No, relativity was built to predict results.

[Tom Mattson]

I would just think that physics as a whole should be more concerned with why things work the way they do, as opposed to simply predicting outcomes.

 

Physicists are concerned with that. The predictions of relativity work because we live in a universe in which no inertial observer's claim to reality is preferred over that of any other. Any prejudice against that idea can only come from judgments which are tainted by everyday experience. Admittedly, everyday experience is very persuasive, but it is also misleading.

[Locrian]

I would just think that physics as a whole should be more concerned with why things work the way they do, as opposed to simply predicting outcomes.

 

When you propose an idea that is why something works, how will you show it's correct? You'll show that it predicts things that are observed. If it predicts nothing that wasn't already predictable, then there is no reason for it to exist, and we'll just use occams razor to eliminate it.

 

You are creating a false dichotomy that isn't philosophically sound. Ontop of that, I'm not sure you are really fully aware of what physics as a whole does.

[Saint]

Physicists are[/b'] concerned with that. The predictions of relativity work because we live in a universe in which no inertial observer's claim to reality is preferred over that of any other. Any prejudice against that idea can only come from judgments which are tainted by everyday experience. Admittedly, everyday experience is very persuasive, but it is also misleading.

 

Tom and Locrian,

 

I should appologize for that one. I didn't mean to imply that physicists are incompetent. They're a whole lot smarter than I am, and I know they're doing their best to understand what's going on. I just don't buy relativity. I shouldn't make broad accusations like that.

[Saint]

On a related note, since we are talking about time dilation (Lorentz factor), does length contraction (Lorentz) work for the entire distance along the direction of travel? I mean, as you approach the speed of light, does the distance to an object you are leaving shrink as well? If you left Earth and accelerated to .9c, would the Earth appear to get closer? That's one that's always puzzled me.

[Tom Mattson]

I just don't buy relativity.

 

But it's plainly obvious that you've never studied it!

 

If you left Earth and accelerated to .9c' date=' would the Earth appear to get closer? That's one that's always puzzled me.

[/quote']

 

No, if you're moving away from the Earth then it gets farther away from you. There is a difference between the value of a function (in this case, distance) and the value of its rate of change.

[swansont]

No, if you're moving away from the Earth then it gets farther away from you. There is a difference between the value of a function (in this case, distance) and the value of its rate of change.

 

What I think Saint is asking, though, is what if you are e.g. 10,000 km from earth, and you are able to accelerate to .99c ([math]\gamma = 7[/math]) almost instantaneously, so a very short distance is covered. The earth should now appear to be 1430 km away by the equations of special relativity. But I don't think it will look that way, at least initially, because you've undergone an acceleration and can't blindly apply SR equations and hope they describe reality.

 

I think of it this way - the earth can't know you've undergone that acceleration until light from you reaches it, and so the observation of length contraction can't "reach" you, either. I suspect what you'd see is a distortion in what you observe, receding from you at c, with length contraction inside and the 'at rest' observations outside, and the thickness of the distortion related to how long you were accelerating. IOW, the sphere in which you can apply special relativity grows around you, and has a radius of ct. But I have to admit I haven't thought about this particular situation before, so I may be missing something.