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Gravity/acceleration equivalent?

  1. Jan 31, 2006 #1

    Okay, so Einstein says that to an internal observer a room being pulled under a constant 1g acceleration is indistinguishable (by experiment) from a room hanging from a tree by a rope.

    Putting gravitational divergence aside, do you think this is true?

    If I (as an inside observer) can demonstrate by internal experiment that the room is either accelerating or is at rest in a gravitational field, would this be important?
  2. jcsd
  3. Jan 31, 2006 #2


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    Yes, that would contradict the equivalence principle, provided the room is arbitarily small so that differences in gravity from one internal region to another (tidal forces, for example) become negligible. Also, I think you'd have to assume the time period you're looking at becomes arbitrarily brief--the equivalence principle only applies to arbitrarily small regions of spacetime, although offhand I don't know how gravity could be distinguished from acceleration even given an extended period of time to make your observations.
  4. Jan 31, 2006 #3
    acceleration, gravity

    A look at
    Ling Tsai
    The relation between gravitational mass, inertial mass and velocity
    am.J.Phys. 54 340 (1986)
    could be illuminating
  5. Feb 1, 2006 #4
    So if I can really do this, it would really be important? Does anyone disagree or wish to add qualifications?

    I'll Google this since it sounds like something in the same vein as I'm thinking from the title, but do you have a link?

    Edit: Okay, I found it on my own. I'll do a little reading and let you know if it's applicable to my feat of derring-do...
    Last edited: Feb 1, 2006
  6. Feb 1, 2006 #5
    Even if your room was very small if you let enough time go by there would be a tidal effect which you could observe. Although I think this would only apply in the freefalling (or extremely low acceleration) case, since if you were accelerating at any decent rate your test particles would hit the floor of your box before you noticed any tidal effects.
  7. Feb 1, 2006 #6


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    Yes. As this page says:
    Or as http://scholar.uwinnipeg.ca/courses/38/4500.6-001/Cosmology/Principle%20of%20Equivalence%20in%20Mathematical%20Form.htm [Broken] puts it:
    So, a violation of this would certainly be important, although the fact that thousands of smart physicists have studied the issue and found no reason to doubt the equivalence principle should lead you to suspect there is very likely a flaw in your idea.
    Last edited by a moderator: May 2, 2017
  8. Feb 1, 2006 #7


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    How would these tidal effects manifest in the freefalling case, for a very small room falling for an extended period of time?
  9. Feb 1, 2006 #8
    JesseM et al,

    I need to make it clear that I'm referring to the hanging room case as per this paper written by Albert Einstein. I'm not saying I can make this determination in a free-falling room in space versus a free-falling room in a gravity field ("Except for the splat at the end... they're practically similar" -Tigger). Would this still be important?

    I feel I should forewarn you that this concept really works, but it's also a bit mischevious. That is; I have found a loophole to the conundrum, but I don't think it has a lot of practical considerations.

    Edit: I wish to add that I will start out with a simple and rather silly version of the test. You will then likely wish to qualify the experiment, so I strongly recommend you satisfy your qualifications now to the best of your abilities. I will accept later qualifications, but only if it is conceded that my first experiment works under the current treatise.

    Any qualifications you make will be examined for merit and depending on their limiting factors I will refine the experimental concept to compensate. If the restrictions get severe, I may need to ascertain that the measurements are purely hypothetical (as they may be very small) and have this still be accepted as valid. Of course, this will only be in response to restrictions that are not specified in the Einstein paper.

    Is everyone in accordance? Are you ready for the silliness to begin?
    Last edited: Feb 1, 2006
  10. Feb 1, 2006 #9
    The test particles would slowly seperate from one another due to the extremely minute difference in the gravitational force (or geodesics, if you prefer) due to their differing heights above the planet. Even with a very small room this effect would become evident given enough time, provided you don't hit the ground first. This seperation would not occur in the flat space room.

    Well don't keep us in suspense, let's hear it :wink:
    Last edited: Feb 1, 2006
  11. Feb 1, 2006 #10


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    I don't think it should matter--after all, a hanging room as seen by a freefalling observer passing it should look just like an accelerating room as seen by an inertial observer passing it. You could even imagine a small room in a box inside a freefalling lab, with the box using rockets to maintain a constant distance from the earth even as the lab falls so the box approaches the lab's ceiling--this should appear just like a small room-in-a-box inside an inertial lab in deep space using rockets to accelerate in the direction of the lab's ceiling.
    Again, the fact that this would violate such a basic principle should lead you to have some doubts about whether you have actually found such a loophole, I think. But anyway, let's hear it!
    OK, the best way I can put it is that if you compare the hanging room and the accelerating room, for any small but finite room over a finite time there will be some slight differences, but in the limit as the volume and time approach zero, the magnitude of these differences should also approach zero. I'd be very surprised if you could find something that violates this.
  12. Feb 1, 2006 #11


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    The tidal forces (aka Riemann curvature tensor) are different for the observer in the room and the observer standing on a spherical planet.

    This is essentially, however, a consequence of the gravitational divergence, so I'm not sure if you'd include it as a separate phenomenon or not.

    One should be able to suppress the tidal forces in theory by making the planet a huge disk rather than a huge sphere. The metric for an infinite flat plane should be equivalent to two Rindler metrics (the metric of an accelerating spaceship) "glued together" at the z=0 plane, as I've mentioned in another thread.

    Even with a finite disk rather than an infinite one, the supression of tidal forces should be good (but not perfect).
  13. Feb 1, 2006 #12


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    :cool: I had never heard that version of the EP thought experiment. (only the two basic ones, accelerating room in space vs. one sitting on the planet and free-falling room vs. one drifting freely in outer space, and in both comparisons what light or other free objects would be observed to do.) thanks for bringing it up.
  14. Feb 1, 2006 #13


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    I just made that thought-experiment up, to illustrate why it seems to me that the equivalence of free-falling frames and inertial frames would automatically imply the equivalence of an accelerating frame and a frame at rest relative to a gravitational field.
  15. Feb 1, 2006 #14
    OK ubavontuba - Lets hear the revelation
    Last edited: Feb 1, 2006
  16. Feb 1, 2006 #15


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    well, it's a good one. i'll have to remember to credit "JesseM" if i ever use it.
  17. Feb 1, 2006 #16
    How to test for gravity in the equivalence thought experiment


    Okay. Beginning with the treatise that the room is either hanging from an earthbound structure or an accelerating vehicle, I'd like to clarify one important aspect. That being that other than the fact that constant acceleration at the stated rate is currently impossible, all structures and systems obey all currently known laws of physics and that the structures are of reasonable parameters for either condition. Is this agreeable?

    The occupant begins by being completely unaware of the conditions of the room and he must perform all tests within the room. He cannot look outside (through a window or some such). Nor may he use devices that can perceive exterior conditions directly through the walls (like an X-ray observatory).

    Also, all effects of divergence are disallowed. That meaning that I will not fall on the effects of divergence as being a method to test for gravity (including tidal effects).

    Lastly, I will reitterate that this is not meant to be a serious examinination of equivalence. Rather this is simply an exercise in creative thinking. Enjoy!

    How to Test for Gravity in the Equivalence Thought Experiment

    Let's examine one of the most basic tenants of relativity... that being the equivalence principal.

    The equivalence principal basically states that it is impossible in a sealed room to conduct an experiment that could distinguish the difference between gravity and constant acceleration.

    Of course there is spherical divergence to consider (immeasureably small), but let's place that aside for now and look at this in a more fundamental way.

    Let's say I am in a sealed, hanging room (you'd like that, wouldn't you?) and that I want to find out if my room is part of an accelerating rocketship, or is situated comfortably on Earth. How might I easily determine this? Let's make this even harder by stipulating that initially, I don't even know that the room is hanging, or floating, or stable, or whatever.

    Remember, Einstein's equivalence principal states that I shouldn't be able to tell by using any experiment in the room. I'd have to look outside to tell. Here goes:

    To accomplish my feat, I need some very specialized equipment.

    I need a snifter of brandy (make it full to the brim) and myself.

    Let's proceed:

    Step 1. Drink half the brandy. "Ah... good stuff."

    Step 2. Place brandy snifter on the floor, but to the side a bit.

    Step 3. Begin leaping laterally (from side to side) in the room.

    Step 4. Observe the brandy.

    If the brandy sloshes, I am in a relatively low mass room and therefore must be accelerating, or at least be separated from the Earth (suspended). If sloshes, go to the next step. If not, then you are in a fixed room on a heavy mass. You are feeling gravity (end of experiment).

    Step 5. Start leaping from side to side. Build up as much pendulum acceleration as you can (swing the room like a child on a swing). Stop. Does the room continue swaying normally? Then you are hanging over a heavy mass and are experiencing gravity. If it stops rather suddenly, or it has an unusual and increasing resonance, then you are accelerating (Oh no! It's gone out of control!). In the latter case, disaster is soon to follow.

    You can qualify things by adding shock absorbers and whatnot to a suspended room or make the accelerating room's ship unusually massive, but this just defeats the spirit of the experiment and makes it so in that particular room it is hard to distinguish between gravity and acceleration. This wouldn't be applicable to a supposedly universal principal.

    Even so, you could still tell because a shock absorbed room must take time to settle (it's just quicker) Whereas an accelerating room and frame in free space is ALWAYS settled on it's center of mass (laterally) regardless of where you go in the room. The system will always stop moving laterally when you do. A massive room would just require more sensitive equipment.

    Therefore, you can determine the difference between acceleration and gravity in a sealed room, right?
  18. Feb 1, 2006 #17


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    You are trying to move the room. While technically not actually peeking out the window, you are still gleaning information about what is outside the room from within. You are not using the sensations of gravity or acceleration to do so. That's effectively cheating.

    There are umpteen ways in practice to determine the difference, especially if you allow the sort of real-world engineering effects (such as the fact that the room can swing).

    The effect holds in principle, if not in practice.
  19. Feb 1, 2006 #18


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    Actually, I disagree that your methods would even produce any different results depending on whether the box was being accelerated or held still in a gravitational field. It is crucial, though, that we assume the force on the box is coming from the same point in both cases--for example, if the box in earth gravity is being held up by a cable which is attached to a crane sitting on the surface of the earth, then we should assume the box in deep space is being pulled from a cable attached to its top which is attached to a crane which is being accelerated by a rocket whose inertia is just as great as the earth's (but assume in this case we are dealing only with the laws of SR so this rocket doesn't distort spacetime). Likewise, if the box is sitting on the top of the earth, then in the accelerating case it should be pushed from below by a rocket whose inertia is as great as the earth's. And if the box is being kept at a constant height from the earth by rockets attached to its bottom, then the box in space should be accelerating by means of identical rockets attached to its bottom. As long as we keep things analogous in this way, I don't think you would detect any difference in your brandy experiment.
    Last edited: Feb 1, 2006
  20. Feb 1, 2006 #19
    I can jump up and down in my apartment and get a half-full snifter of brandy to slosh, it isn't built so well :wink:

    Cute though.
  21. Feb 1, 2006 #20
    Too late! You didn't prequalify this.

    Besides, the dichotomy of the experiment clearly states that no test conducted within the room might distinguish the difference. It is not qualified as in; no experiment except___ (fill in the blank) can tell.

    It's a fundamental statement. It's not an; "He found a solution, so his solution can't count!" That's like kids saying, "Not it!" after they've been tagged already.
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