Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: lenadorap on 27/01/2010 10:34:39
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I've read in some "Relativity Simplified" article that example of accelarating lift which creates "gravity" which has a light source on one wall which will hit the other wall slightly lower position and the same should happen due to the natural gravity. The article says it's due to the accelaration of the lift. However my understanding is that is due to the velocity of the lift that the light beam will hit the other wall on a slightly lower position?
Am I misunderstanding something?
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You're absolutely right that if you're in an elevator moving with a constant speed relative to some light source shining through the wall, that the light entering one side will hit the other side at some point further down the wall than if you were in an elevator that wasn't moving at all. I think the point you're missing is that even though it hits the opposite wall further down, the light has taken a straight line path to get there. In special relativity, where the elevator isn't accelerating, light moves in a straight line.
If instead, your elevator is accelerating upwards, the light from outside will take a curved path on its way to the far wall. The argument usually goes from there to say that you can't tell acceleration of the elevator from a stationary elevator being pulled on by gravity, and so under gravity light must also take a curved path.
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Thank you very much Jpetruccelli, I undestood the phenomenon.
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This is the best question i have seen for a long time. We have horizontal starlight from a distant star entering a hole in the sidewall of the elevator (which is rising at constant speed), & the starlight hits the opposite wall a little below the level of the hole, a cloud of flourdust shows that the beam is straight (not curved). Einsteinians i think would say that the beam must be horizontal & must hit the opposite wall at the same elevation. Perhaps they might wriggle out of their problem by saying that the distant star is not rising with the elevator.
If a light source is close to the elevator & is at the same elevation as the hole & is moving up with the elevator then the beam seen in the elevator will be horizontal & straight (ie hits the opposite wall at the same elevation-height as the hole). Einsteinians might agree.
If that close light source is not moving up but is fixed then the beam seen in the rising elevator will have a downwards curve hitting the opposite wall above the level of the hole, but that curved beam will gradually swing upwards, & the spot of light on the opposite wall will gradually move upwards (that upwards swinging & movement will be accelerating).
If the light source is (briefly) above the hole then the spot of light on the opposite wall might be lower than the hole (briefly).
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I've read in some "Relativity Simplified" article that example of accelarating lift which creates "gravity" which has a light source on one wall which will hit the other wall slightly lower position and the same should happen due to the natural gravity. The article says it's due to the accelaration of the lift. However my understanding is that is due to the velocity of the lift that the light beam will hit the other wall on a slightly lower position?
Am I misunderstanding something?
If the light source is stationary to the elevator, then the light will hit the opposite side of the wall at the same height as the light as long as the motion of the elevator is a constant velocity. With such an arrangement, there is no way to tell if the elevator is moving or stationary. If the light is emitted by a source with a relative motion with respect to the elevator, and the light passes through a hole in the wall, the light will strike the opposite wall at some other height. This effect is called the aberration of light. However, it will not tell you whether it is the elevator or source that is moving, as the result is the same either way. It can only tell you the relative velocity difference between light source and elevator.
If the elevator is accelerating, the path of the light will curve relative to the elevator if the source is stationary relative to the elevator or moving relative to it and shining through the hole.
In the case the light shining through the hole, you can tell if its the the source or elevator that is accelerating. Like I said above, if the elevator is accelerating the light path will curve, however if it is the source that is accelerating, it will not.
The best way to envision this is to imagine a short flash of light leaving the source, passing through the hole, passing through a series of glass plates and then hitting the opposite wall. You can note where the pulse passes through each plate to trace its path from hole to opposite wall.
Accelerating elevator will show the pulse following a curved path relative to the elevator.
Accelerating source will show the pulse following a straight line relative to the elevator, the angle of this straight line will be determined by the relative velocity between elevator and source at the moment of emission from the source.
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I've read in some "Relativity Simplified" article that example of accelarating lift which creates "gravity" which has a light source on one wall which will hit the other wall slightly lower position and the same should happen due to the natural gravity. The article says it's due to the accelaration of the lift. However my understanding is that is due to the velocity of the lift that the light beam will hit the other wall on a slightly lower position?
Am I misunderstanding something?
If you were in a lift in the absence of gravity then shining a light from one wall to the other hits the wall on the exact opposite side. Viewing from a frame in which the lift is moving you would measure the speed of light to have a component in the direction the lift is moving so again, and having the same magnitude, the light still hits the opposite side.