Undergrad What tests can falsify general relativity?

[francis20520]

TL;DR

falsification

I know that general relativity fails in singularities like the center of a black hole or the big bang.

GR also fails at fundamental particle levels like electrons, protons and neutrons etc. I.e. GR cannot explain interactions of various fundamental particles?? (Am I correct?)

But these does not make GR wrong because it works every where else. Above things don't falsify GR, right?

But my question is what test will falsify GR?

Like, what experimental setup can be done (theoretically) that if successful will prove GR wrong?

I.e. What are the falsification tests people have come up for GR?

 

[Ibix]

Loads of tests have been done. Gravitational deflection of light. Gravitational waves. Motions of planets. Redshift of distant galaxies. Tick rates of clocks at different altitudes. Frame dragging. Probably others that haven't occurred to me off the top of my head. Any one of these could have falsified general relativity, but has not done so.

On a point of language, I would say that what you seem to be talking about isn't falsification, precisely. It's finding the limits of applicability of GR as an approximation to a more fundamental theory. Falsification would be finding something right in the middle of GR's "known valid" regime that didn't behave as predicted.

GR keeps passing the tests we can do, in that its predictions match measurement to available precision. We are pretty sure that it's not quite right, that it's an approximation to a more general theory, but we can't reach a regime where it is measurably inaccurate, so we cannot gather data to help develop it.

 

[Ibix]

I see I haven't quite answered your question.

Better gravitational wave astronomy might show differences between the detected waves and the predicted ones in the final stages of a black hole merger, which is one way of probing near black hole event horizons. I believe that there are studies in the works to see how particles in superposed states fall under gravity, which might yield some information about gravity and quantum states.

 

[pervect]

Summary:: falsification

I know that general relativity fails in singularities like the center of a black hole or the big bang.

GR also fails at fundamental particle levels like electrons, protons and neutrons etc. I.e. GR cannot explain interactions of various fundamental particles?? (Am I correct?)

But these does not make GR wrong because it works every where else. Above things don't falsify GR, right?

But my question is what test will falsify GR?

Like, what experimental setup can be done (theoretically) that if successful will prove GR wrong?

I.e. What are the falsification tests people have come up for GR?

I'd suggest Will's review paper, "The confrontation between General Relativity and Experiment", https://arxiv.org/abs/1403.7377

There's too many tests to summarize them all, but I'll give a few.

The weak equivalence principle can be detected by testing that objects of different compositions all "fall at the same rate", and by Eotovos type experiments to measure the gravitational attraction of materials of different composition. If materials of different composition but identical masses attracted each other differently, GR would be falsified.

Tests that special relativity holds locally also test GR - for instance, the Michelson Morely experiment. Wills describes this as "tests of local Lorentz invariance". So if any of the standard SR tests of the speed of light failed, GR would also be falsified.

Gravitational redshift is another test of GR, though many other theories predict this phenomenon. Wills calsifies Pound-Rebka experiments in the same general category here.

To compare General Relativity with other metric theories of gravity, various tests using the PPN formalism have been done. The commonly known tests here are the deflection of light, and the "Shapiro time delay" tests.

The perihelion shift of mecury's orbit is another test that fits into the PPN framework.

The general idea is that metric theories of gravity have a set of parameters that can be measured (beta, gamma, and others), measuring the values of these parameters distinguishes GR from other metric theories such as Branse-Dicke gravity.

Gravity probe B tested for gravitomagnetic effects predicted by GR.

Moving onto strong field tests, the Ligo experiment detecting gravitational waves that fit with the predictions of GR is a good one, and we have some inspiral measurements that also indirectly indicate gravitational radiation of the expected amount.

 

[atyy]

I know that general relativity fails in singularities like the center of a black hole or the big bang.

Yes.

GR also fails at fundamental particle levels like electrons, protons and neutrons etc. I.e. GR cannot explain interactions of various fundamental particles?? (Am I correct?)

A quantum generalization of GR works for fundamental particles like electrons, protons, neutrons etc. These are all still at relatively low energies compared to where we think the quantum generalization of GR will fail. Quantum GR does not explain their interactions, but can be integrated with the standard model of particle physics that describes their interactions.

But my question is what test will falsify GR?

In addition to the experiments mentioned by @Ibix and @pervect, some ideas for tests that have not yet been done to test GR more stringently are discussed in https://arxiv.org/abs/0903.0100.

 

[martinbn]

I know that general relativity fails in singularities like the center of a black hole or the big bang.

No, GR predicts them. It doesn't fail at them. So far there are no obeservations or experiments about the singularities to see if GR fails or not.

GR also fails at fundamental particle levels like electrons, protons and neutrons etc. I.e. GR cannot explain interactions of various fundamental particles?? (Am I correct?)

GR is not a theory of everything, so it doesn't fail here either.

But these does not make GR wrong because it works every where else. Above things don't falsify GR, right?

But my question is what test will falsify GR?

Like, what experimental setup can be done (theoretically) that if successful will prove GR wrong?

I.e. What are the falsification tests people have come up for GR?

Any test whose outcome differes from the predictions of GR will show the limits og GR. So far GR passes all test.

 

[vanhees71]

I'd say, unavoidable singularities of a theory show clearly limitations of that theory.

 

[martinbn]

I'd say, unavoidable singularities of a theory show clearly limitations of that theory.

Not if the singularities are part of nature, then the theory makes good predictions.

 

[vanhees71]

Hm, well. How do you measure whether there's a true singularity? Maybe the singularities are all hidden behind a horizon (cosmic censorship). Then they wouldn't be observable at all and you could never falsify their existence, but we are drifting into philosophical questions which never have a definite answer ;-)).

 

[martinbn]

Hm, well. How do you measure whether there's a true singularity? Maybe the singularities are all hidden behind a horizon (cosmic censorship). Then they wouldn't be observable at all and you could never falsify their existence, but we are drifting into philosophical questions which never have a definite answer ;-)).

May be, but you simply reject them because of your philosophy, not scientific evidence.

 

[vanhees71]

True.

 

[Nugatory]

May be, but you simply reject them because of your philosophy, not scientific evidence.

There’s nothing wrong with rejecting an idea on the basis of philosophy as long as we know that’s what we’re doing. It’s how we keep our thought processes from being gummed up by an accretion of unfalsifiable ideas.

(This is commentary not argument)

 

[eaglechief]

If someone could show that velocity of light is a parameter in time and not a constant, than STR would be falsified and GTR, as well ... Is that assupmtion correct ?

 

[Nugatory]

If someone could show that velocity of light is a parameter in time and not a constant, than STR would be falsified and GTR, as well ... Is that assupmtion correct ?

Yes.
However it is also true that if someone could show that I had wings I would be able to fly; that true statement is of little relevance when I'm trying to get up onto the roof without a ladder. The equally true statement that relativity would be wrong if the speed of light were not constant is of equally little relevance when we live in a universe in which the speed of light is constant.

 

[Ibix]

Yes.

Do you think so? I was going to say no, because local Lorentz invariance requires that $c$ be invariant, not constant.

 

[eaglechief]

at least, wiki says so ...

"The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. "

 

[Nugatory]

Do you think so? I was going to say no, because local Lorentz invariance requires that $c$ be invariant, not constant.

I think that we're in a conversation that isn't far enough along to need that distinction yet, and that OP used "constant" when "invariant" might have been a better word.

 

[Ibix]

I think that we're in a conversation that isn't far enough along to need that distinction yet, and that OP used "constant" when "invariant" might have been a better word.

Not sure I agree. The question seemed to me to be asking what if $c$ varied over time. I think a measurement that concluded that $c$ (or $\alpha$, although happy to duck that distinction at this point) was different in the past wouldn't bother relativity. One that showed that $c$ was different for different local frames would be a problem, I agree.

 

[atyy]

Not if the singularities are part of nature, then the theory makes good predictions.

Classically, that is true. But I don't think it's true quantum mechanically, where we can treat general relativity as an effective field theory at low energies, but it's not even clear what we get if the energy gets high enough (string theory, asymptotic safety, fundamental failure of local Lorentz invariance?).

 

[atyy]

Hm, well. How do you measure whether there's a true singularity? Maybe the singularities are all hidden behind a horizon (cosmic censorship). Then they wouldn't be observable at all and you could never falsify their existence, but we are drifting into philosophical questions which never have a definite answer ;-)).

Maybe one of these theories with extra dimensions will be correct, then there could be a failure of cosmic censorship
https://www.livescience.com/53857-5d-black-holes-break-general-relativity.html
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.071102

 

[martinbn]

Classically, that is true. But I don't think it's true quantum mechanically, where we can treat general relativity as an effective field theory at low energies, but it's not even clear what we get if the energy gets high enough (string theory, asymptotic safety, fundamental failure of local Lorentz invariance?).

What makes you think that? Any theorems in that direction?

 

[martinbn]

Maybe one of these theories with extra dimensions will be correct, then there could be a failure of cosmic censorship
https://www.livescience.com/53857-5d-black-holes-break-general-relativity.html
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.071102

This is gramatically way too complicated for me. What is the proper term for conditional sentences of the form "If extra dimensions then possibly there might be numerical evindence that could perhaps be interpreted as suggesting that WCC may not be true."

 

[jbriggs444]

at least, wiki says so ...

"The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. "

Since it is not a unitless constant, it is as much conventional as physical. We can change its value over time with a flick of a pencil and eraser. And we have done so.

 

[pervect]

What definitions of the distance (say meters) and time (say seconds) are being used by the Original Poster (OP)? We can conclude that they are not using the most modern definitions, because they are interested in experimentally measuring the speed of light, and that's not compatible with the modern SI definition of the meter and second. So presumably the OP is using some other definition. I've found it unwise to make assumptions about what what defintitions people might be using in such threads.

Unfortunately there's no meaningful answer to experimentally measuring the speed of light without knowing the experimental basis of the distances and time standards being used. And the only conclusion we can be certain of is that they're not the modern one , which goes as follows;

The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency Cs.

So if the OP was using the modern standard, they wouldn't be talking about measuring the speed of light, because it is defined as a constant. So they are implicitly using some other definiton. We need to know what that is to attempt to answer the question.

The dimensionless approach would be to talk about the fine structure constant instead of the speed of light but we've had those discussions in the past and gotten "blank stares". I'm still willing to have such a discussion if the OP is interested and feels it's relevant, though.

Otherwise, we need to get into the nitty-gritty of the time and distance standards to attempt to provide an answer.

 

[Vanadium 50]

I am disappointed in the direction this thread has turned down.

Suppose I built a box with an LED, a mirror and a light sensor and set it up to look at the delay between emission and detection, and from that calculate the speed of light. Every second it blinks 300000. Now suppose one morning it, and every box in the universe like it, started blinking 600000. It doesn't matter if what "really changed" was the speed of light, our time standard, our length standard, or some combination. SR would be in trouble.

But the whole idea is immensely silly, and in my view should not have been brought up instead. SR is an amazingly well-tested theory - much better tested than, for example, the theory that Covid is caused by the Coronavirus - and bringing it up to the exclusion of real tests of GR is an ill-advised and ill-founded distraction. One might as well ask if the sudden appearance of Zeus with his energy-non-conserving thunderbolts would falsify GR.

There are at least ten parameters that can be measured and have values predicted by GR and with different values predicted by alternatives. As an example, the amount of space curvature (not spacetime curvature) is smaller in scalar-tensor theories than in GR.

 

[vanhees71]

Well, that example would lead us again to the unfruitful discussions of one-path light speed. As discussed at length recently, for that you need clock synchronization conventions, as was realized by Einstein already in 1905 when writing his famous paper on special relativity.

 

[Vanadium 50]

that example would lead us again to the unfruitful discussions of one-path light speed.

If I measure 300000 today and 600000 tomorrow with the same device, that's a problem. Synchronization conventions do not affect the number being read out.

 

[jbriggs444]

If I measure 300000 today and 600000 tomorrow with the same device, that's a problem. Synchronization conventions do not affect the number being read out.

You measure 300000 for what, exactly? A count per unit time? With time measured based on the period of the radiation associated with...? Perhaps the dimensions of the box changed. That does not affect special relativity.

If we want to avoid a debate on whether the speed of light changed or the size of the box changed, we get right back to worrying about unit definitions, the fine structure constant and rewording the question.

[Though I do understand a sentiment that these quibbles feel like cheats designed to avoid the thrust of the OP's question]

 

[Vanadium 50]

I am not going to answer. It's pointless quibbling, and a distraction from the question at hand. We could equally well argue about the color of Zeus' thunderbolts.

If you think less of me because I couldn't come up with an unquibbleable example in a paragraph, well, so be it.

 

[jbriggs444]

If you think less of me because I couldn't come up with an unquibbleable example in a paragraph, well, so be it.

No, no. I get where you are coming from and do not disagree. I'll be quiet now.