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I have a question on pseudogravity as invoked in thought experiments

  1. May 18, 2012 #1
    I have a question on pseudogravity as invoked in thought experiments related to the principle of equivalence.
    One invents ‘gravity’ opposite to an accleration to explain weightlessness, the bending of light beam, the dilatation of time period, and twin paradox. I can explain all of the above without invoking pseudogravity. Has actual experiments been done to confirm that actual new gravity was present opposite to an acceleration? Can you please cite me references? No theory, please.
  2. jcsd
  3. May 18, 2012 #2
    Re: Pseudogravity

    I don't think so, but the consequences of this (General Relativity) are absurdly well tested. Perhaps an example of a completely equivalent experiment is astronauts experiencing true weightlessness when they're in freefall.
  4. May 19, 2012 #3
    Re: Pseudogravity

    I don’t understand as to which statement your “I don’t think so” applies!?
    And, regarding “… but the consequences of general relativity are absurdly well tested.” I have not seen related experiments. Can you please cite me a book or a journal article?

    I will approach this problem another way. Please find a hole or two in the following statements:

    Acceleration. We take an accelerating rocket far from any mass. The rocket chamber has an observer in it. We then examine what happens to a beam of light propagating across the rocket chamber from one wall to the other – from the point of view of the observer.

    As the light beam traverses the rocket chamber, the observer is accelerating toward it. Relatively, the observer finds the light beam tracing a parabolic path and may conclude that gravity is present. In reality, however, the light beam never changed its direction as no [new] gravity was mediated!

    It is useful but not necessary to invent pseudogravity opposite to the acceleration to explain the bending of the light beam.

    Weightlessness. There are two aspects to weight. In one aspect, the weight of a body is the force with which the earth attracts it. In the other aspect, weight is the ‘feeling’ a body gets in gravitational field from the reaction force from the ‘floor’ on which it rests; weightlessness is the feeling when that reaction force is unavailable.

    Weight may be increased or decreased by adding to or subtracting from the reaction force from the floor, such as in an ascending or descending elevator. When the elevator is in free fall, the gravitational force of the earth is being ‘used up’ in accelerating it downward; an observer in it gets no reaction force and feels weightlessness. Oblivious of any external environment, the observer in the elevator may conclude that gravity is present. In reality, however, no new gravity is being mediated!

    It is useful but not necessary to invent pseudogravity opposite to the acceleration to explain weightlessness. Weight is real; weightlessness is apparent.

    Centrifugal force. A force normal to a body’s uniform velocity keeps the body in a circular orbit; that force is called centripetal force.

    A body in a satellite around the earth is under the centripetal force of the earth’s gravity. The centripetal force is being ‘used up’ in keeping the satellite and the body in orbit; the body gets no reaction force and feels weightlessness.

    A body on a merry-go-round must have three reaction ‘agents’ to keep it in balance: a seat to push it up against the downward gravity; a backrest to accelerate it to the needed uniform tangential velocity; and a side rest to push it toward the center providing centripetal force to keep it along the circle. When the centripetal force is turned off, the body moves along the tangent with the current velocity.

    It is useful but not necessary to invent pseudogravity (centrifugal force) to cancel the centripetal force in order to avoid radial motion. Centripetal force is real; centrifugal force is fictitious.

    Twin paradox.
    Gravity actually dilates the time period of an atomic clock and the length of a material rod. According to special relativity, to an observer, a clock and a rod in relative motion appear respectively to be running slower and getting shorter in the direction of motion. The observer in motion with the clock and the rod detects gravity’s effects on their time period and length but no such special-relativistic effects.

    There is no special-relativistic paradox. Each observer sees other observer’s clock slowing down.
  5. May 19, 2012 #4


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    Staff: Mentor

    Re: Pseudogravity

    You are correct it's not necessary - if you're willing to do all your physics in the reference frame of an inertial observer who sees the rocket accelerating. But that's damned inconvenient for the guy inside the rocket.

    I could as easily run your argument the other way: My rear is solidly planted in a chair sitting on the surface of the Earth... This chair is accelerating towards the sky at 1G... Now I want to calculate what happens when I let go of the mug of delicious cold frosty beer in my hand... Of course the beer mug will remain at rest in its inertial frame while the floor accelerates towards it at 1G, hits the mug, breaks it, and spills the beer. Of course it's way more natural for me to introduce a pseudoforce that I'm going to call "gravity" or "the Earth's gravity" or even "real gravity" that pulls the mug towards the floor - but it's not necessary, any more than it was necessary to introduce pseudogravity in the rocket case.

    The point of the equivalence principle is that the two models are equivalent. There's no a priori reason to label some gravitational/accelerative forces "real" and others "pseudo" - although if you live and drink beer on the surface of the Earth you'll probably find yourself thinking in terms of a "real' gravitational force more often than a fictitious one.
  6. May 20, 2012 #5
    Re: Pseudogravity

    First, an accelerating rocket is not an inertial frame.
    As I understand "equivalent" -- it means "as if" but not real. There are so far four fundamental forces: the strong, the weak, electromagnetic, and gravitational. In that sense I understand that the pseudogravity directed opposite to an applied acceleration is not a fundamental force.
    On the earth real gravity pulls down your beer can. In an accelarating rocket, the floor of the rocket accelerates toward your beer can.
    Sometimes, physics invents pseudo terms to make calculations easier and straightforward. Centrifugal force is another example. Sadly, some of those terms become "reality."
  7. May 20, 2012 #6


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    Staff Emeritus
    Science Advisor

    Re: Pseudogravity

    Classical GR is more about concepts about "geodesic deviation" and less about things like "pseudoforces".

    Basically, one has a curved space-time, which can probably be most readily visualized as drawing all of your space-time diagrams on spatially curved surfaces rather than on a flat piece of paper. This takes the approach that the easiest way to visualize time is to draw it on a piece of paper and represent it by a spatial dimension.

    Straight lines on the flat piece of paper are replaced with geodesics on the curved sheet of paper. A formal defintion of geodesics gets mildly involved, but "walking in a direction in which you follow your nose" contains the essence of the concept of generating a geodesic curve.

    On a flat piece of paper, parallel lines never meet. However, on a curved surface, parallel geodesics (which replace lines) can and do meet. It's worthwhile studying the concept of "geodesic deviation" to understand how this mimics a force in a flat space-time.
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