# Equivalency Principle

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Hello,

So let me start of by saying that I am a high school graduate, but not a college one. Personally, I blame the California school system. Anyway, I have been reading A Brief History of Time by Stephen Hawking, because I have a great interest in learning as an adult. I understand a great deal of the book, but some of it I'm more of the "well, if you say so." attitude. I would prefer more understanding on these subject, so I imagine I'll have more than a few questions for here.

So, my first bit of trouble is regarding Einstein's theory of general relativity, related to the equivalency principle. I get the basic gist of it, how light would behave the same way related to an accelerating object as it would to the same force of gravity. But my question is, is this something that could be observed, or simply a concept? I mean, is there anything that can be used on earth to demonstrate this? In my mind, I want to be able to demonstrate this with a car accelerating with a hole punched in an opaque cover on the side window, but I suspect that wouldn't work. How do we know this is true?

Also, would this be observable from within the accelerating object, or only something we know to be true when observing said object from a different vantage point?

I find this all very fascinating, and I am playing around with the idea of writing a book that simplifies much of this for a layman to understand. I looked at the "for dummies" book on the subjects, and it turns out that it isn't for dummies. I think these are more intimidating concepts then they might need to be, and I think more people could be interested if it were put into very simple terms.

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One of the key tests of general relativity was to observe stars during a solar eclipse, stars that nominally were close to being behind the sun. General relativity predicted that those stars should be visible because of the way gravitation affects light and because how the sun's mass curves space-time. The stars were visible, just as general relativity predicted. This experiment was first performed during the 1919 eclipse, and then again in 1922, and then again and again.

Another place astronomers see how gravitation affects light is when a galaxy is directly behind another. Here's one such image:

That bluish horseshoe-shaped object is a galaxy. The red object that it nearly surrounds is a another galaxy that is between us and that bluish one. The remote galaxy would be a little point of light were it not for gravitational lensing.

So far I haven't mentioned the equivalence principle. While the equivalence principle was central to Einstein's development of general relativity, it is not a part of general relativity. The equivalence principle does not fully explain how light is affected by gravitation. Regarding those stars observed during solar eclipses: The equivalence principle only tells half the story. In other words, it yields the wrong predictions for the curvature of the light by the sun. General relativity predicts twice the curvature, and it is the value predicted by general relativity that has been observed.

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Ah, I see. So the original description was theory, and then the demonstration (proof) was the stars around the sun. So, during the eclipse, the stars that astronomically should be directly behind the sun were seen as a circle around the sun? Is there any astrophotography of this phenomenon? ( mean directly, of the eclipse version)

Edited by Gobbleston

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Ah, I see. So the original description was theory, and then the demonstration (proof) was the stars around the sun. So, during the eclipse, the stars that astronomically should be directly behind the sun were seen as a circle around the sun? Is there any astrophotography of this phenomenon? ( mean directly, of the eclipse version)

What DH has shown you in the photo is an Einstein Ring and the effect seen in the 1919 experiment was not so spectactular; I think in that they were just saw normal looking stars in different apparent positions, as predicted by GR. The Einstein Ring is with the light source more or less bang in the middle behind the object and the observer and the other one is with the light source just to one side and behind the object.

Edited by StringJunky

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Ah, a shame there isn't a picture of the eclipse. Does anyone here know the name of the ring? (NGC###) I'd like to observe it myself, and perhaps get my own picture if it isn't too faint.

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I found this thread which says there are no Einstein Rings visible from Earth but you can see a Double Quasar with a 16" caused by the same phenomenon:

http://www.astronomyforum.net/deep-sky-objects-forum/135896-any-amateur-visible-einstein-rings.html

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So far I haven't mentioned the equivalence principle. While the equivalence principle was central to Einstein's development of general relativity, it is not a part of general relativity. The equivalence principle does not fully explain how light is affected by gravitation. Regarding those stars observed during solar eclipses: The equivalence principle only tells half the story. In other words, it yields the wrong predictions for the curvature of the light by the sun. General relativity predicts twice the curvature, and it is the value predicted by general relativity that has been observed.

I've gotten into this before and don't really want to again, nor do I want to derail the thread; however, I feel obligated to point out that what you're saying here is patently false.

The equivalence principle is certainly present in GR, and is in fact one of its core principles. It is equivalent to the statement that spacetime is locally flat and the "comma-goes-to-semicolon rule" which is mentioned in many GR texts. The EP itself does not yield bad predictions - it is the EP + Newtonian gravity which does so.

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I've gotten into this before and don't really want to again, nor do I want to derail the thread; however, I feel obligated to point out that what you're saying here is patently false.

The equivalence principle is certainly present in GR, and is in fact one of its core principles. It is equivalent to the statement that spacetime is locally flat and the "comma-goes-to-semicolon rule" which is mentioned in many GR texts. The EP itself does not yield bad predictions - it is the EP + Newtonian gravity which does so.

Question: I have read that an acceleration produces only half the bending of light of an equivalent gravitional field. For example, consider the famous accelerating elevator in zero gravity with a light beam traveling across the elevator. An observer outside the elevator sees the light bend due to the elevator's acceleration. But the amount of bending is only half what one would measure for a light beam in an elevator at rest in a gravitational field of the same magnitude.

Doesn't this say the Equivalence Principle fails? I'd appreciate your thoughts.

Ref: Ohanian, Einstein's Mistakes, p. 226.

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