A method to determine movement?

Simplex0
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Topic 193334

I am trying find a method that allows me to determine if I am moving in space and how fast.

Assume that I am in a non rotating rocket in space with the engine turned off and I emit a laser beam from the wall across the rocket so that a light dot is projected on wall at the other side of the rocket and that I mark that position.

Then I leave a buoy to mark the position and start the engine and accelerate with 1 g in a straight line. until I reach a relative speed of 0.5 c relative the buoy and then turn off the engine.

If I now turn on the laser beam will the light dot be projected at the same position as in the first case?

Devilogic
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A method to determine movement?

Quote:

I am trying find a method that allows me to determine if I am moving in space and how fast.

Assume that I am in a non rotating rocket in space with the engine turned off and I emit a laser beam from the wall across the rocket so that a light dot is projected on wall at the other side of the rocket and that I mark that position.

Then I leave a buoy to mark the position and start the engine and accelerate with 1 g in a straight line. until I reach a relative speed of 0.5 c relative the buoy and then turn off the engine.

If I now turn on the laser beam will the light dot be projected at the same position as in the first case?

If by the same position you mean the same position relative to the rocket, then the answer is Yes. Special relativity sates that the "laws of physics" are the same in any non-accelerating (inertial) frame, which means that you can perform no experiment inside the rocket that will tell you if the rocket is "moving" or is at "rest". The question of the (absolute) speed of the rocket then becomes meaningless, since there is no inertial frame that is "special" in any way, that could serve as a basis for measuring "absolute speed". This is the basis for the famous statement that "all motion is relative" :)

P.S. It could be argued that by using a variant of Mach's principle, the special inertial frame could be chosen to be the one where the center-of-mass of the entire universe is at rest - but for all practical purpuses this is irrelevant, since all inertial frames have the same laws of physics in special relativity anyway and the CoM frame would only serve as a curriosity and would not affect measurements in any way.

Simplex0
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Thank you Devilogic. I

Thank you Devilogic.

I thought that as the rocket moved faster in the later scenario the projected spot from the laser beam should hit the wall at a lower position than in the first scenario.

peanut
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If Tomas turns his laser on

If Tomas turns his laser on while he is accelerating, does the beam hit lower?

Simplex0
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In that case the laser beam

In that case the laser beam will hit the wall at a different place, se this page http://ffden-2.phys.uaf.edu/213.web.stuff/Scott%20Kircher/relativity.html

The fact that it does not if the movement is non accelerating indicate that the laser beam moves side ways with a speed that is the same as the rocket, truly strange. It's seams like the space inside the rocket moves with the rocket if the movement is non accelerating.

peanut
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So, it appears you can tell

So, it appears you can tell if you are changing speed. Though you would not need the laser to do that. You would likely feel the change in speed.

Odysseus
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RE: So, it appears you can

Message 75519 in response to message 75518

Quote:
So, it appears you can tell if you are changing speed. Though you would not need the laser to do that. You would likely feel the change in speed.

Certainly. I believe the â€œequivalence principleâ€? says that you canâ€™t tell the difference between acceleration and gravityâ€”without looking out the window. ;) So the results of any experiment you could conduct inside the rocket while itâ€™s accelerating at one gee would be the same as if done on Earth (excepting rotational effects, e.g. Foucaultâ€™s pendulum), but while â€˜coastingâ€™â€”at whatever speedâ€”youâ€™d be in free-fall, as if in orbit.

ML1
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RE: I am trying find a

Quote:
I am trying find a method that allows me to determine if I am moving in space and how fast...

Sounds like:

Michelson-Morley Experiment

You may be able to do as you wish by the careful use of gyroscopes as in inertial navigation?

Keep searchin',
Martin

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Simplex0
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I just try to understand the

I just try to understand the scientific explanation of light, space and time of today ML1. As I do not have higher education I do it by 'experiments' and try to find the answers by asking higher educated people of what the outcome of that experiment would be.

Regarding the Michelson-Morley Experiment I know that they could not find any evidence of a medium or luminiferous aether.

But the space itself sometimes behave like a medium. I think of the expansion of space that can be seen trough the red shifted Quasars and the fact that the space can be curved, it's hard to think that 'nothing' can expand and be curved. maybe space could have mass?

Now back to the measurements of movement. Could the micro wave background be used to determine in which direction and speed you are moving?

Say that you could verify that the background radiation was more blue shifted in one direction would not that be an indication that you moved in that direction and how fast?

Odysseus
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RE: Say that you could

Message 75522 in response to message 75521

Quote:
Say that you could verify that the background radiation was more blue shifted in one direction would not that be an indication that you moved in that direction and how fast?

Such measurements have indeed been made: in the WIkipedia Milky Way article thereâ€™s a reference to a paper that reported a velocity of 552 km/s for our Galaxy WRT the CMB, based on a pattern found in red-shift measurements from all directions.

Simplex0
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Sounds Interesting

Sounds Interesting Odysseus.
Maybe this is a method to define absolute movement in the local space.