Is the equivalence principle false (EP)?

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Main Question or Discussion Point

Is there a controversy over the EP? What I mean is: is it considered to be false beyond any doubts or on the contrary it is absolutely true and doubts about its validity are only misinterpretations?
I know that the question it's not so simple because there are many EPs, strong, weak, etc. but I would like to hear some opinions about it because I'm confused.
 

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  • #2
PeterDonis
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Is there a controversy over the EP?
Not to my knowledge.

is it considered to be false beyond any doubts
Certainly not.

it is absolutely true and doubts about its validity are only misinterpretations?
This would be my position, yes. The usual problem is that people don't state the EP correctly, and then criticize their incorrect statement of it instead of the actual EP. The actual correct statement of the EP requires getting a number of non-trivial technical details right, and doing that requires taking some time to understand the subject.

there are many EPs, strong, weak, etc.
Yes, there are. Experimentally, as far as we can tell, all of them are true (as GR predicts they should be).
 
  • #3
pervect
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The equivalence principle could be any of several related concepts in my experience. I usually hear "weak equivalence principle", "strong equivalence principle", and "Einstein equivalence principle". That may not be a complete list of variants of the equivalence principle, however.

The equivalence principle is subject to experimental test. For instance, if one found substances made of different materials falling at different rate in a "gravitational field", it would violate the weak equivalence principle and falsify general relativity.

I would hesitate to say that any theory is "absolutely true" or "beyond question". I believe there's some discussion of tests of the equivalence principle in some of our FAQ's/ sticky threads.

As far as controversy goes, I would say no, though I would say there is a certain amount of confusion about exactly what is meant by "the equivalence principle". There's less confusion if one identifies the specific variant of the principle one is talking about. When one starts looking closely at the experimental tests, it becomes important to distinguish exactly which of the several variants loosely referrred to is being talked about.
 
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Is there a controversy over the EP?
The presence or absence of controversy is a very poor indicator of the validity of a concept. After all, there is controversy about whether the earth is flat or not.

The equivalence principle has good experimental support and is theoretically sound. I am not aware of any controversy, but if it did exist it would have a lot to overcome.
 
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The presence or absence of controversy is a very poor indicator of the validity of a concept. After all, there is controversy about whether the earth is flat or not.
That's right, however I was talking about controversy in a more serious way, for instance, like the discussions about the interpretations of quantum mechanics, i.e., when even recognized experts don't agree. This is the kind of controversy I was refering to.
As for EP there seems to be tidal efects, that were not known in Einstein's times, and these effects are supposed to invalidate EP.
I don't know much about GR and EP that's why I wanted to hear your opinions.
 
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Vanadium 50
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As for EP there seems to be tidal efects, that were not known in Einstein's times, and these effects are supposed to invalidate EP.
That's not true. Einstein knew about tides. In fact, in GR, it's tides that determine the gravitational field.
 
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PeterDonis
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these effects are supposed to invalidate EP.
I don't know where you're getting that from, but it's not correct. Tidal effects are not detectable within a single local inertial frame, and the EP only holds within a single local inertial frame. So tidal effects do not invalidate the EP; they merely illustrate that the EP is only valid locally.
 
  • #8
stevendaryl
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The facts are pretty unambiguous, but the statement of what exactly is meant by the equivalence principle has a lot of slack.

If you look at the transformation from the equations of motion written in inertial Cartesian coordinates to the equations of motion written in noninertial, curvilinear coordinates, you will see that the form of the equations change by the introduction of new terms involving the connection coefficients (which are all zero for inertial Cartesian coordinates). Gravity according to GR is akin to the use of noninertial coordinates in the sense that gravity only affects the equations of motion through the connection coefficients (at least, the physics of lots of different phenomena in curved spacetime can be described this way).

But the specific form of the connection coefficients in curved spacetime---that is, the way they vary with location---is not achievable in flat spacetime through using noninertial, curvilinear coordinates. So in that sense, there is a difference between gravity and noninertial coordinates. That's what the curvature tensor captures---those aspects of the connection coefficients that do not depend on the choice of a coordinate system. These are the tidal effects. It's certainly not the case that Einstein was unaware of them---even before he realized the importance of the curvature tensor, he certainly knew that the force of gravity changed with location in a way that the pseudo-forces involved with accelerated motion did not.
 
  • #9
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That's right, however I was talking about controversy in a more serious way, for instance, like the discussions about the interpretations of quantum mechanics, i.e., when even recognized experts don't agree. This is the kind of controversy I was refering to.
Then there is no controversy about the EP.

As for EP there seems to be tidal efects, that were not known in Einstein's times, and these effects are supposed to invalidate EP.
I don't know much about GR and EP that's why I wanted to hear your opinions.
This isn’t a controversy and it is also wrong. Einstein knew about tides and dealt with tidal gravity in GR, and tidal effects don’t invalidate the EP they just limit its domain of applicability.
 
  • #10
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Ok, Thank you very much to all of you.
 
  • #11
stevendaryl
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There are some speculative areas of quantum gravity that may imply violation of the equivalence principle, as I understand them (which I don't). I think that some of the resolutions to Hawking's black hole information loss paradox might involve violations of the equivalence principle. The "firewall" idea doesn't seem like it respects the equivalence principle.
 
  • #12
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The "firewall" idea doesn't seem like it respects the equivalence principle.
Can you tell what's the "firewall" idea.
 

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