astronomical riddle

**nokhor** · 02-22-2005, 02:29

a physics buddy of mine give me this riddle.

imagine two starships in close proximity iin the void of space. there are no stars nearby or anything that can be used for reference or physical signs of it moving such as retro rockets etc. one of them decides to move. from a observer on starship A's standpoint, it is B that is moving and they are standing still. from an observer on B, it is the reverse. how do you prove which one is moving?

**Papewaio** · 02-22-2005, 02:35

If one of them decides to move it must go from a velocity of 0 to something else.

To do so would require acceleration. A simple accelerameter could measure that... ie a pendulum would have the weight drift backwards and helium balloons would float forwards in the accelerating ship.

**Big_John** · 02-22-2005, 02:36

only one of them would feel acceleration. the accelerating astronaut would feel the acceleration (or could just use a cup of water or something to observe it).

.. maybe?

edit: eh, beaten like a red-headed step-father :/

**Quietus** · 02-22-2005, 03:01

Reflect light of the off one ship then observe where the light is heading.

If you are closer to the light then your ship is moving.

Or you can set up reflecting mirrors inside the ships that reflect light perpendicular to the direction you are facing.

Travel really fast then the light deviates.

Don't move and the light stays the same.

**Ja'chyra** · 02-22-2005, 10:12

Ask them which one was moving :P

**lancelot** · 02-22-2005, 15:24

Is it some weird answer like- the one further out from the center of the galaxy 'orbits' (in a sense) the other and hence moves????

This isnt my guess BTW, I was just trying to guage how 'out there' the answer is...(I bet its something really simple after all that...)

**English assassin** · 02-22-2005, 16:42

I think the acceleration posts must be right. IIRC special relativity would rule out either observer being able to claim he was in a priviledged rest frame of motion compared to the other if they passed at constant velocity. Again IIRC that applies even to fancy effects of motion such as time dilation (ie each perceives the other's clock as running slow).

Acceleration can certainly be detected though.

**nokhor** · 02-22-2005, 17:31

yep, the acceleration posts were right. however it is done, helium ballons, cup of water or whatever, you can detect your motion if you have to overcome inertia.

**Quietus** · 02-22-2005, 22:07

You can do it inside the ship. In a moving ship, the light will have angled path as opposed to a straight line when not moving.

Once the light hits the mirror and bounces back to the other mirror. The other mirror will be at a different position and angle of reflection won't be 90 degrees to the surface.

It has to be done is super fast speeds though.

**A.Saturnus** · 02-23-2005, 00:11

Originally Posted by nokhor

yep, the acceleration posts were right. however it is done, helium ballons, cup of water or whatever, you can detect your motion if you have to overcome inertia.

Actually, you cannot detect the motion, only the acceleration. When the acceleration has stopped, the motion is relative. It would not be more correct to say the one ship moves than the other does.

Another riddle: how can you distinguish acceleration from gravitation (without reference to a source of gravitation or acceleration)?

**Papewaio** · 02-23-2005, 01:29

At a single point you cannot tell the difference between acceleration and gravity. Acceleration of one form is caused by gravity.

What you can do however is see if it is a field effect and has a exponential drop off akin to light intensity... ie acceleration proportional to the recipriocal of the distance squared.

**Quietus** · 02-23-2005, 07:03

Same as Pape's.

I forgot the exact equation with the big Mass and small mass.

**English assassin** · 02-23-2005, 11:26

If the acceleration in the orginal scenario was caused by gravity, could it then be detected? Assuming one ship was a lot further from the cause of the gravity than the other so there was relative acceleration.

(Sorry I did type out what I meant by all this but the reply crashed and I can't be bothered to do it again. if the qu isn't clear I'll have a go)

Seems to me no force would be perceptible, and you would have to do something fancy like measure the local curvature of space-time to see who was being accelerated more rapidly. But that may have violated the requirement for no reference points (in which case we couldn't have acceleration due to gravity anyway)

**A.Saturnus** · 02-23-2005, 21:19

Originally Posted by Papewaio

At a single point you cannot tell the difference between acceleration and gravity. Acceleration of one form is caused by gravity.

What you can do however is see if it is a field effect and has a exponential drop off akin to light intensity... ie acceleration proportional to the recipriocal of the distance squared.

That and the fact that the force vectors of gravity converge to one point, the center of gravity while those of acceleration are parallel. In the case of gravity. objects that are on an axis parallel to the force vector will move further apart from each other while those on an orthogonal axis will move closer. This is not the case when the acceleration is due to something else than gravity.