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Suppose You were put into a box with all the instruments you might require.
The box moves with constant velocity on the surface of the earth.
We are asked to detect inside the box whether we are moving or not.

This question was asked by our physics teacher and thrown to discussion to the class.
I know Einstein's first postulate that prevents detection of absolute motion. But we came up with arguments that seemed to tell us that we were in motion.

  1. When a policeman fires his 'speed tracking instrument' at a car the light produced reflects from the car and according to the speed of the car frequency changes to an apparent observed frequency. (Doppler Effect)
    So we could just fire up a light beam to a wall and detect the reflected light. If it has another frequency we detect motion of the box.
  2. The box moves around earth with us on it. So the centripetal acceleration must be provided by the gravitation pull of the earth as no other force acts downward. This Force must be greater than the normal force for it provide centripetal force which is otherwise equal to the normal force if we weren't moving.
  3. Suppose a gigantic movable wall moving in air then intuitively the pressure at the side of the wall in front (that is facing the air molecules) to have a little bit more pressure than the other side (behind the wall).
    Similarly we could measure the pressure at points close to the 4 walls. I expect the pressure a little less on one side then the other.

I would like to know why the above won't work.

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  • $\begingroup$ You cannot make any measurements outside of the box. So 1 and 3 would not be allowed. (The teacher did not say what the box is made of, so you could assume that it is make of perspex. Then you just look out. You don't need any instruments.) $\endgroup$ – sammy gerbil Feb 4 '18 at 22:23
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Lets tackle each argument one at a time...

When a policeman fires his 'speed tracking instrument' at a car the light produced reflects from the car and according to the speed of the car frequency changes to an apparent observed frequency. (Doppler Effect) So we could just fire up a light beam to a wall and detect the reflected light. If it has another frequency we detect motion of the box.

What you say about the policeman firing a radar gun at a car is entirely correct: the Doppler Effect will certainly have an effect in this situation and thus show that the car is moving. However where the problem in your translation of this argument to the situation of a moving box lies is within the idea of different Frames of References. In the car situation, the car and the policeman are in two different frames of reference, given that the policeman is stationary and the car is in motion. Therefore in the policeman's frame of reference the car is moving, and this relative motion will give rise to the Doppler Effect. In the moving box situation, however, the person and the box are in the same frame of reference. Therefore when the ray is fired off from the radar gun, it will be moving at the speed of light relative to both the person and the box. As a consequence, there will be no Doppler Effect and hence it is not possible to observe that this frame of reference is moving.

Now imagine that there are windows on this box which is flying through space at a constant velocity. If the person inside were to look out of them, he would see the Doppler Effect acting on all objects outside his box. Why? Because these objects are in a different frame of reference. However it is important to realise that this would still not tell him if he was moving - it could very well be the objects outside moving rather than him, for which the effects would be the same. It is therefore impossible for him to know if he is in motion at a constant velocity.

The box moves around earth with us on it. So the centripetal acceleration must be provided by the gravitation pull of the earth as no other force acts downward. This Force must be greater than the normal force for it provide centripetal force which is otherwise equal to the normal force if we weren't moving.

First of all, there is no normal force - the only force acting here is the force of gravity towards the center of gravity of the planet. In regards to relativity, you are in this instance in a non-inertial frame of reference: you are moving in a circle, which means you must be accelerating, a fact which you yourself pointed out, saying that "the centripetal acceleration must be provided by the gravitational pull of the earth." In this case the postulates of Special Relativity no longer hold, so you cannot use this argument to disprove it.

Suppose a gigantic movable wall moving in air then intuitively the pressure at the side of the wall in front (that is facing the air molecules) to have a little bit more pressure than the other side (behind the wall). Similarly we could measure the pressure at points close to the 4 walls. I expect the pressure a little less on one side then the other.

This argument has more or less the same flaw as the previous: moving through the air will exert a force on the wall (hence the presence of the pressure you mentioned) and as a result you are no longer in an inertial frame of reference - you will experience an acceleration due to this force. Hence Special Relativity does not apply here, and thus this argument also cannot break the postulates of SR.

Hope this was helpful. (If there are any mistakes, please point them out in the comments.)

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Assuming that you are moving in a straight line at constant velocity, you would be unable to detect your own motion. In the Doppler Effect argument, you forget that the police officer is at rest relative to the moving object in his frame, but because there is no motion in your frame, and thus and Doppler Effect measured by the instrument would be counteracted by the fact that you are moving at the same speed.

In your other two arguments, you are correct. This isn't a flaw in Einstein's Relativity Principle, but because you are moving on a curved geometry, and moving through a fluid outside of your frame. In the case that you move through the air, the frame of the air is accelerating, and inertial frames can't accelerate relative to each other, your our frame (the box) can no longer be intertial, thus it is not restricted by relativity in this context.

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You can technically find the velocity of the box if it is moving AROUND THE EARTH because there is a slight curvature so centripetal acceleration will cause the acceleration of gravity to be slightly smaller :) It all depends on how sensitive your instruments are and how fast you are moving.

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  • $\begingroup$ This answer is not meant to give a complete answer to the question, but it does give a possibility :) $\endgroup$ – NL628 Feb 4 '18 at 20:27
  • $\begingroup$ My mistake :P I'll fix it right now $\endgroup$ – NL628 Feb 5 '18 at 6:58

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