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Thread: The strong equivalence principle

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    The strong equivalence principle

    According to Wikipedia:
    https://en.wikipedia.org/wiki/Equiva...ence_principle

    "The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime."

    This means if I am in a freefalling elevator and I hold a glass of wine then I should notice no difference.

    This is not true because the reality is the wine will form a bubble.

    Did I misinterpret anything or is the definition ill-formed?

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    Quote Originally Posted by philippeb8 View Post
    According to Wikipedia:
    https://en.wikipedia.org/wiki/Equiva...ence_principle

    "The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime."

    This means if I am in a freefalling elevator and I hold a glass of wine then I should notice no difference.

    This is not true because the reality is the wine will form a bubble.

    Did I misinterpret anything or is the definition ill-formed?
    Whether you are near the Earth and falling rapidly toward it or adrift far from any massive object at any old velocity, the wine will behave the same way.

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    Quote Originally Posted by Hornblower View Post
    Whether you are near the Earth and falling rapidly toward it or adrift far from any massive object at any old velocity, the wine will behave the same way.
    Thanks but falling towards the Earth implies an acceleration and if there is an acceleration (change in velocity) then the wine won't stay still.

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    To stay on the same topic, I have a second question. In the following article:
    https://phys.org/news/2018-07-einste...vity-fall.html

    Why are we referring to Einstein's equivalence principle when Galileo really was the first to coin that term?
    Last edited by philippeb8; 2018-Jul-07 at 01:56 AM. Reason: wrong article

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    Quote Originally Posted by philippeb8 View Post
    To stay on the same topic, I have a second question. In the following article:
    https://phys.org/news/2018-07-einste...vity-fall.html

    Why are we referring to Einstein's equivalence principle when Galileo really was the first to coin that term?
    Galileo observed that it holds within the uncertainties of his experiments. Einstein's GR predicts that it is exact, while some alternative modern theories predict a slight discrepancy which Galileo would have had no means of detecting. The latest tests uphold Einstein.

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    Quote Originally Posted by philippeb8 View Post
    Thanks but falling towards the Earth implies an acceleration and if there is an acceleration (change in velocity) then the wine won't stay still.
    No, the wine and the glass accelerate in unison.

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    Quote Originally Posted by Hornblower View Post
    Galileo observed that it holds within the uncertainties of his experiments. Einstein's GR predicts that it is exact, while some alternative modern theories predict a slight discrepancy which Galileo would have had no means of detecting. The latest tests uphold Einstein.
    Ok so it's a generalization to the whole universe. Ok thanks, it's clear now!

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    Quote Originally Posted by Hornblower View Post
    No, the wine and the glass accelerate in unison.
    I understand what you mean but I still think the definition is ill-formed or perhaps I am simply tired.

    Thanks again!

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    Galileo was writing in a diary, as if the equivalence principle was already known. It was, at least to him. He didn't bother to name it. Kepler expanded the usage and then Newton piled on. Its about invariance and inertia.

    Einstein's theory is about universal gravitation and covarience. It does something different.

    In certain contexts, it would be important to name which one you mean but not in all. As a history type person, I would hammer out "Einstein" or "Galileo" every single time because I am not inclined to go into math or theory. But in a science field that would be overkill because in the context, it's obvious.
    Solfe

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    Quote Originally Posted by philippeb8 View Post
    Thanks but falling towards the Earth implies an acceleration and if there is an acceleration (change in velocity) then the wine won't stay still.
    Thatís not true at all. The wine will not form a bubble. I promise you that if you get on a escalator going down, put a scale on the step, and weigh yourself, you will weigh the same...


    Sent from my iPhone using Tapatalk
    As above, so below

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    Quote Originally Posted by Jens View Post
    That’s not true at all. The wine will not form a bubble. I promise you that if you get on a escalator going down, put a scale on the step, and weigh yourself, you will weigh the same...


    Sent from my iPhone using Tapatalk
    On the escalator in normal operation you are not in free fall. If you are on an elevator and the cable breaks, you will come closer.

    My guess is that by "bubble" he meant a globule. I have seen it demonstrated with liquids aboard spacecraft in orbit.

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    Quote Originally Posted by Jens View Post
    That’s not true at all. The wine will not form a bubble. I promise you that if you get on a escalator going down, put a scale on the step, and weigh yourself, you will weigh the same...
    You're right... yesterday I was tired indeed.

    If you are in an elevator with a scale and the cable breaks, you will weigh the same but the weigh will be null.

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    Earth-normal gravity is sufficient to overwhelm the surface tension of the wine, so on Earth it sits in its glass.

    Without gravity, surface tension is then the dominant force acting on the wine.

    I imagine the material the container is made from is very important in the outcome: glass is hydrophilic, so the wine may well stay put. On the other hand, a plastic vessel is probably hydrophobic so the wine will detach and form globules.

    Next (different) point: what if you had an e x t r e m l y l o n g elevator in free fall? It's not precisely equivalent at the top and bottom of the elevator ?

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    If we could detect gravitons then their abundance would tell us that we were in free fall in a gravitional field and not out in space far from any massive object.

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    Quote Originally Posted by Chuck View Post
    If we could detect gravitons then their abundance would tell us that we were in free fall in a gravitional field and not out in space far from any massive object.
    That might be true. But of course, if gravity turns out to be mediated by gravitons, general relativity would need to be replaced by some kind of quantum theory of gravity anyway, so in that case it might be reasonable to imagine that the equivalence principle would have exceptions.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by Chuck View Post
    If we could detect gravitons then their abundance would tell us that we were in free fall in a gravitional field and not out in space far from any massive object.
    If there are real gravitons passing through the laboratory, then it is not what they mean by "free fall." The same would hold if there was real light passing through the laboratory. It's not well stated, but what they mean is that the laboratory must be shielded from all external influences, other than spacetime itself.

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    Quote Originally Posted by philippeb8 View Post
    According to Wikipedia:
    https://en.wikipedia.org/wiki/Equiva...ence_principle

    "The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime."

    This means if I am in a freefalling elevator and I hold a glass of wine then I should notice no difference.

    This is not true because the reality is the wine will form a bubble.
    The problem is you are not making the comparison they have in mind. You are comparing a free-falling situation with a non-free-falling situation, the principle is trying to say you have to compare two free-falling situations, located in different places and/or at different velocities. They forgot to mention you cannot have any outside influences at all, other than gravity, and they also mean there is not a fluctuating spacetime due to the merger of nearby black holes, for example. (Fluctuations like that would be essentially tidal, violating what they mean by a "local" experiment). So it's not stated very clearly, but not because of what happens to wine in an elevator.

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