# Experiment to test the basic assumption of SR

• I
Has there ever been an experiment to support the principle assumption of relativity? For example an experiment where an atomic clock is sent on a probe to , let's say, Pluto. We on earth get transmissions of the time on that ship and, after taking account of the light travel time delay, we compare that clock to one on earth. We should see that clock ticking slower than ours as we do with GPS satellites (minus the gravitational effect).

The computer on board the ship would also be receiving transmissions of earth time. It would also post-process these and, according to relativity, it must see our clocks slowed in relation to its on board clock. This must be so as relativity forbids a preferred frame of reference, there's no way to tell if the ship is moving away from us or we are moving away from the ship.

But I think the experiment would show that the on board computer would see our time ticking faster, not slower than it's own.If the experiment proved this, then there would be a way to determine who is actually moving in a relative velocity scenario just by comparing the relative time rate between the clocks in the two frames. The one clock that sees the other ticking faster is the clock on the relatively moving timeframe and the other is the relatively stationary timeframe. I assume there's a huge gaping hole in my logic and it's been brought up countless times in the past? Wouldn't this experiment settle if the basic assumption of relativity is correct or not based on experimental evidence?

There are tons of tests that have been performed to test special relativity: https://en.wikipedia.org/wiki/Tests_of_special_relativity

GPS sats are a full range to tests for relativity in themselves, not just special relativity. They account for special relativity because they are moving at fairly high speed relative to us, but they also travel in ellipses and vary their speed, the clocks take that into consideration too. They are also a good test for general relativity, they're further out in the gravity well of earth which also affects their clocks. If any of these calculations were off, GPS would not work.

Ibix
2020 Award
Note that all experiments that test some aspect of relativity test the assumptions on which the theory rests. At a simple level the ability to explain how a charged particle moves near a current tests the assumptions of relativity. On its own it's not a particularly strong test, but all the different experiments add confidence each time we try something new and it goes as expected.

The basic assumption of relativity has been experimentally verified using planes and atomic clocks. See: https://en.wikipedia.org/wiki/Hafele–Keating_experiment

Relativity is also verified in the fact that GPS satellites must take time dilation into account every day.
The basic assumption of relativity is that there is no preferred frame of reference, that each inertial frame will see the others time dilate. In order for the Hafele-Keating experiment or GPS satellites to experimentally verify the basic assumption of relativity, there would have to be something monitoring the time rate of the Earth from the perspective of the satellite or plane. Relativity dictates that perspective should see the Earth's clock dilating. But as Wiki says about time dilation :

Common sense would dictate that if time passage has slowed for a moving object, the moving object would observe the external world to be correspondingly "sped up". Counterintuitively, special relativity predicts the opposite.

It is the experimental verification of this prediction I am specifically asking about. There has been experimental verification that the plane's or satellite's clocks tick slower in relation to earth clocks but has there been experimental verification that Earth clocks tick slower when seen from the satellite or plane's perspective. If the satellite or plane sees the earth's clock tick at a faster rate than its own clock then the basic assumption of relativity has been violated. Which would actually win, common sense or relativity?

No need to bring the twin paradox into this as there is no turn-around or reunification. As was mentioned in the FAQ, the clocks on the plane and satellites would only see the earth's clock running faster once they had reunited with the earth clock. My experiment dictates the plane and GPS clock would see the earth clock ticking faster from the beginning of the journey which is a violation of SR,

From FAQ :Lorentz time dilation is mutual for two inertial observers, in the sense that they will each regard the other's clock as running slow by the same factor.
On different circular orbits (of the same radius), there will be time dilation between them (though that dilation will cancel out once every orbit, as can be seen by comparison with any planetary clock). This latter statement leads to a whole other kettle of fish which can be discussed later once we get some agreement on the above point.

Last edited:
Ibix
2020 Award
pervect
Staff Emeritus
Have you looked at the PF FAQ on the experimental basis of special relativity, https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.229034/, which currently links to the external webpage http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html ?

There are a LOT of tests of special relativity. I can think of three reasons offhand these set of tests might not be satisfying. The first is that some things simply cannot be tested experimentally. The only possible resolution to this problem is to recognize that science, based on experiment, cannot answer questions that are not experimentally testable.

Another issue I see is that some of the experiments may seem indirect. Experimental design in most cases maximizes discriminatory power and has to deal with sources of error, making some of the experimental designs not as straightforwards as a student might like. There are, however, some simple direct experiments out there, however. The main one that comes to mind is Bertozzi experiment. It can currently be found online in a video from several sources, one of which is:

.

The third issue is the easiest to solve. This is a lack of familiarity with what has been done. The solution to this is relatively easy but still requires some work, it involves finding out what has been done.

Another issue. The scientific method works best if people are able to figure out the logical consequences of their assumptions for themselves. It also works on the basis of disproving ideas via what's called "falsification". This drifts into philosophy, an extended discussion of which is outside the scope of Physics Forums, but I'll refer the reader to authors such as Karl Popper for more on this issue if they're interested. The point I want to make is that it's not possible to falsify everything, because there are an infinite number of possible theories and only a finite amount of time. This is a matter of setting realistic expectations on what science , via experiment, can do - and what it cannot.

However, while one cannot claim or expect that all alternatives are falsified, special relativity has passed a LOT of tests, and it makes specific and testable predictions. An additional point I'd like to make here - a theory is considered to be falsified when the disagreement between the experimental evidence and the theoretical predictions can be replicated.. So if you read the FAQ, you'll see that there are some experiments where the results did not match experiment. However, these experiments could not be replicated. And replications were attempted.

One final issue, if I may. Science requires honesty to work, and the world is not always honest. However, technology as a whole and physics and engineering in particular can be seen to actually work. This wouldn't be possible if the whole system were essentially fraudulent.

You may also wish to check out http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html
which is linked from the jtbell's sticky thread at the top of this forum. Many of the experiments are repeated as the Earth rotates (i.e. in different inertial frames) and comes up with the same result. Thus there is no preferred frame.
I read all this and nowhere is the specific test I'm looking for to the question I'm asking. My original question was not, "Is there any experimental evidence for relativity?" I'm getting lots of answers to a question I never asked. Maybe I can clear up the confusion if you or someone else paraphrase the question you think I asked.

I read all this and nowhere is the specific test I'm looking for to the question I'm asking. My original question was not, "Is there any experimental evidence for relativity?" I'm getting lots of answers to a question I never asked. Maybe I can clear up the confusion if you or someone else paraphrase the question you think I asked.
The Hafele–Keating experiment first mentioned is exactly what you originally asked: one stationary clock and one clock in high-speed transport. They were perfectly synchronized when the experiment started and out of sync by the exact amount that SR predicts when the experiment concluded.

Mister T
Gold Member
Maybe I can clear up the confusion if you or someone else paraphrase the question you think I asked.

You asked, for example, if a probe were sent to Pluto to check and see if its clock ran slow compared to Earth clocks as GPS clocks do. And if it would see Earth clocks running slow. The simple answer is that that is also done with GPS clocks. The satellite clocks have to be compared not just with Earth clocks, but with each other. So after subtracting off the effects due to gravity, each clock will see the other as running slow. If this were not the case the engineers would not be able to maintain the location accuracy.

Nugatory
Mentor
never asked. Maybe I can clear up the confusion if you or someone else paraphrase the question you think I asked.
How is the GPS system not an answer? All the satellites and the earth stations (which are themselves in relative motion because of the earth's rotation) are continuously monitoring each other's clocks.

Ibix
2020 Award
I read all this and nowhere is the specific test I'm looking for to the question I'm asking. My original question was not, "Is there any experimental evidence for relativity?" I'm getting lots of answers to a question I never asked. Maybe I can clear up the confusion if you or someone else paraphrase the question you think I asked.
The point that you seem to be missing is that every test of relativity is a test of its assumptions to one degree or another. And many experiments testing many different predictions from the assumptions gives you strong confidence that the assumptions are correct and leaves very little wiggle room to construct a theory that isn't based on those assumptions but does give (very very nearly) the same predictions. Even if the GPS is not the answer you are looking for, all of the experimental evidence for special relativity is a test of the principle of relativity.

PeroK
Homework Helper
Gold Member
2020 Award
But I think the experiment would show that the on board computer would see our time ticking faster, not slower than it's own.If the experiment proved this, then there would be a way to determine who is actually moving in a relative velocity scenario just by comparing the relative time rate between the clocks in the two frames. The one clock that sees the other ticking faster is the clock on the relatively moving timeframe and the other is the relatively stationary timeframe. I assume there's a huge gaping hole in my logic and it's been brought up countless times in the past? Wouldn't this experiment settle if the basic assumption of relativity is correct or not based on experimental evidence?

You seem to be suggesting that the Earth represents a preferred reference frame and an observer on Earth may, in fact, be absolutely at rest; whereas, an observer travelling to Pluto is absolutely moving.

Is the Earth not spinning? Is the Earth not orbiting the Sun? Is the Sun not orbiting the galactic centre?

If there were a preferred reference frame, absolutely at rest, it wouldn't be on the surface of the Earth.

Ibix
Ibix
2020 Award
If there were a preferred reference frame, absolutely at rest, it wouldn't be on the surface of the Earth.
...which is the point I failed to get across in #6.

The Hafele–Keating experiment first mentioned is exactly what you originally asked: one stationary clock and one clock in high-speed transport. They were perfectly synchronized when the experiment started and out of sync by the exact amount that SR predicts when the experiment concluded.
In that experiment, when the plane returned to the earth bound clock, the earth clock had indeed run faster than the plane's clock.The twin paradox shows that as long as there's a turnaround and a reunification this will happen but the twin paradox also shows that so long as there is no turnaround both clocks see the other as dilating. Hence if the experiment was monitoring both clocks while the plane was flying away from the airport, the clock on the plane, when compared in the air to the earth clock should have seen the earth clock going relatively slower than its own. This assumption is to preserve the principle idea that there is no way to tell if the plane is flying away from the airport or the airport is moving away from under a stationary plane. If the plane sees the earth clock moving faster as the plane is moving away, that is a sure sign that the plane is moving relative to the earth. This is a determinarion that is forbidden by relativity.

Dale
Mentor
2020 Award
I read all this and nowhere is the specific test I'm looking for to the question I'm asking.
No, the specific test you are looking for (sending a probe to Pluto) has not been done and will not be done. There is no reason to do it. There is also no reason to paint the MM interferometer pink and rerun the MMX. Nor is there a reason to test the Haelfe Keating experiment with airplanes from different manufacturers.

The postulates of relativity have been well tested, and your proposed experiment would not be sensitive enough improve any of the experiential limits of relativity.

Instead of asking why some random experiment hasn't been done you should learn about the body of experimental evidence that already exists and ask why people believe it confirms relativity.

You seem to be suggesting that the Earth represents a preferred reference frame and an observer on Earth may, in fact, be absolutely at rest; whereas, an observer travelling to Pluto is absolutely moving.

Is the Earth not spinning? Is the Earth not orbiting the Sun? Is the Sun not orbiting the galactic centre?

If there were a preferred reference frame, absolutely at rest, it wouldn't be on the surface of the Earth.
I am not suggesting that at all. If you have two cars on a road in relative motion, one's going 60mph relative to the road and another is going towards it at 60 mph relative to the road, their relative velocity is 120mph to each other. I don't care what the road's velocity is relative to the rest of the universe.
Now change this to two spaceships going towards each other at .6c in space. There is a spacestation between them which they want to reach at the same time. Yes you can figure out their relative velocity using the relativistic velocity combination fortmula but spacetime path analysis would reveal that there is absolutely no time dilation between their clocks. Neither sees the other as dilating even though they have a high relative velocity.

Now assume one ship is already parked at the station. The other ship is now coming at them at the same relative velocity but it has 100% share of the relative velocity. Now time dilation is a factor. The fact that it is a factor means now there is a way to determine that in the first scenario both ships were moving at the same speed towards each other, 50% (approximately) of the relative velocity each. Now in the 2nd scenario, relativity prevents one from judging whether the station or the incoming ship has 100% of the relative velocity because both see the other dilating. But you can bet your bottom dollar that the incoming ship will not have aged as much once it reached the station. So just a minute ago it was impossible to tell who was aging slower? I say the incoming ship was able to see throughout his journey that the station's clock was going faster than his own onboard clock.No proof until the experiment with planes or gps satellites or the trip to pluto is done.

Ibix
2020 Award
Now change this to two spaceships going towards each other at .6c in space. [snip] Neither sees the other as dilating even though they have a high relative velocity.
This is wrong. Both will see the other travelling at about 0.88c, and time dilated with a ##\gamma## of 2.125.

Edit: Your general problem here is that you have forgotten about the relativity of simultaneity. Of the three observers, only one will say that everybody started their stopwatches at the same time. The difference of opinion over the start time accounts for the difference of opinion over whose watches are ticking slowly, and ends up with a consistent answer to what the stopwatches will read when everyone meets up.

No, the specific test you are looking for (sending a probe to Pluto) has not been done and will not be done. There is no reason to do it. There is also no reason to paint the MM interferometer pink and rerun the MMX. Nor is there a reason to test the Haelfe Keating experiment with airplanes from different manufacturers.

The postulates of relativity have been well tested, and your proposed experiment would not be sensitive enough improve any of the experiential limits of relativity.

Instead of asking why some random experiment hasn't been done you should learn about the body of experimental evidence that already exists and ask why people believe it confirms relativity.
So you're finding my logic so muddled that you think it's like I'm proposing to do a test to see if plane color would affect relativity. I'm really failing miserably at getting my point across then. Only another person who understands what I'm saying can save me now.

Dale
Mentor
2020 Award
the PF FAQ on the experimental basis of special relativity, https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.229034/, which currently links to the external webpage http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html ?
@ralfcis this is my favorite resource on the topic.

From your first post in this thread your suggested result on the Pluto-bound clock is excluded by tests of the relativistic/transverse Doppler effect as well the various Isotropy experiments. Sections 3, 4, and 8 of the review are particularly relevant as is the section on test theories of SR. In particular the paper by Robertson should be understood before proposing any new tests.

Dale
Mentor
2020 Award
So you're finding my logic so muddled that you think it's like I'm proposing to do a test to see if plane color would affect relativity.
Yes, your proposed experiment is only cosmetically different from existing evidence.

This is wrong. Both will see the other travelling at about 0.88c, and time dilated with a ##\gamma## of 2.125.
I'll give you an example of why it's not wrong. Consider 2 planes in a hafele-keating experiment doing orbits from pole to pole to avoid the spin of the earth making things more complicated. Now if the planes fly together, their relative velocity is zero and both will have dilated equally relative to the earth clock once they return to the pole. They will not have dilated with respect to each other. Now asssume both take off in opposite directions from the pole. Their relative velocity has doubled with respect to each other however their timedilation with respect to the pole clock must be the same as in the first scenario. Now their clocks can't differ in respect to each other so even though their relative velocity went from 0 to double their individual speed, their relative time dilation has not changed at all, it is still zero.

PeroK
Homework Helper
Gold Member
2020 Award
I am not suggesting that at all. If you have two cars on a road in relative motion, one's going 60mph relative to the road and another is going towards it at 60 mph relative to the road, their relative velocity is 120mph to each other. I don't care what the road's velocity is relative to the rest of the universe.
Now change this to two spaceships going towards each other at .6c in space. There is a spacestation between them which they want to reach at the same time. Yes you can figure out their relative velocity using the relativistic velocity combination fortmula but spacetime path analysis would reveal that there is absolutely no time dilation between their clocks. Neither sees the other as dilating even though they have a high relative velocity.

Now assume one ship is already parked at the station. The other ship is now coming at them at the same relative velocity but it has 100% share of the relative velocity. Now time dilation is a factor. The fact that it is a factor means now there is a way to determine that in the first scenario both ships were moving at the same speed towards each other, 50% (approximately) of the relative velocity each. Now in the 2nd scenario, relativity prevents one from judging whether the station or the incoming ship has 100% of the relative velocity because both see the other dilating. But you can bet your bottom dollar that the incoming ship will not have aged as much once it reached the station. So just a minute ago it was impossible to tell who was aging slower? I say the incoming ship was able to see throughout his journey that the station's clock was going faster than his own onboard clock.No proof until the experiment with planes or gps satellites or the trip to pluto is done.

Now, this is one of many examples where considering time dilation alone is not enough. You also have the relativity of simultaneity. There is actually a nice paradox here (better than the twin paradox in my view). If you can resolve it, you go a fair way to understanding SR.

However, sadly, I can see from this and others posts that you have chosen the path where "relativity must be wrong" and "I'm going to disprove it".

@ralfcis this is my favorite resource on the topic.

From your first post in this thread your suggested result on the Pluto-bound clock is excluded by tests of the relativistic/transverse Doppler effect as well the various Isotropy experiments. Sections 3, 4, and 8 of the review are particularly relevant as is the section on test theories of SR. In particular the paper by Robertson should be understood before proposing any new tests.
This is a thought experiment. Any extraneous physical results can be nullified and accounted for. The importance of the Pluto experiment is that the twin paradox shows that the away twin will age slower only if it turns around and returns to his twin. Before the turnaround, relativity forbids one from making a judgement on who is actually aging slower because they must both see each other as time dilating. The trip to Pluto is a one way trip. There is not turnaround or reunification. Hence, the twin paradox cannot be used to determine the away twin is aging slower because he will never return. But I'm saying during the journey, the clocks can be compared. Relativity states the Pluto bound twin will not see our clocks as going faster than his, he must see our clocks as going slower. But once he reaches pluto we can ask him how old he is and we can tell him how old we are and he will not have aged as much. Now how can relativity reconcile that? Proof of age.

Ibix
2020 Award
I'll give you an example of why it's not wrong. Consider 2 planes in a hafele-keating experiment doing orbits from pole to pole to avoid the spin of the earth making things more complicated. Now if the planes fly together, their relative velocity is zero and both will have dilated equally relative to the earth clock once they return to the pole. They will not have dilated with respect to each other. Now asssume both take off in opposite directions from the pole. Their relative velocity has doubled with respect to each other however their timedilation with respect to the pole clock must be the same as in the first scenario. Now their clocks can't differ in respect to each other so even though their relative velocity went from 0 to double their individual speed, their relative time dilation has not changed at all, it is still zero.
But this is a critically different scenario from your rocket one. In this case, the planes set off from one location and returned to it. They are able to unambiguously compare their clock readings at both start and end of the journey and agree that they started at the same time, and that they arrived at the same time. They will also agree that they were under acceleration at all times.

A simpler version of the above is a space station with two ships that set out at high speed in opposite directions and then return. In this case they will both agree that their clocks will end up showing the same time, although their reasoning for why this should be will be different for themself and for their opposite number. You should try it with maths.

However, in your spaceship scenario the ships start out separated so do not have an unambiguous way to determine whether or not they started at the same time. This is the mirror image of your one-way trip to Pluto, of course.

I am not wrong. I strongly suggest that you deploy some maths instead of hand waving.