How can someone moving be "on a equal footing" with someone at rest?

Question

Hypothetical scenario:

In outer space. Let's say a "void". No stars can be seen, it's pitch black and no gravity.

There's a two lost spacemen in a space suit. They both have a "light beacon", so they can see each other. One is moving let's say half the speed of light, but in a constant velocity. One is "sitting still" motionless.

As they pass one another, theory of relativity states that they are on a "equal footing". The guy that's moving at half of speed of light can claim that he is sitting still, the guy that's motionless can say he's moving at half light speed.

How can that be?? One is obviously moving and other one is not. But from what I read, they are both on a "equal footing". Either/or can claim that he is stationary or moving.

Can anyone explain?

Answer 1

The key word is RELATIVE.

If we are riding in a car together at 60MPH, wouldn't you say we are on "equal footing"? You appear to be sitting still (to me) because I am moving with you (even though you are travelling at 60 MPH).

Now, let's say a car drives past us at 61 miles per hour. From our viewpoint, they only "appear" to be going 1 mile per hour, but from someone who is standing on the side of the road, they zoom by at 61 MPH.

It's all relative!

Answer 2

You just said the background is pitch black and no gravity. The two parties can claim either one is moving or stationary and the statement is valid. Actually, there is way to show one party is stationary.

Einstein's point is that either party is in an inertial frame, ie, no acceleration. It means both both parties can write the same set of physics equations to describe any observed phenomena.

Traditional physics (including Newtonian) assumes that the universe is an absolute space and acceleration is relative to this space, regardless of reference objects as background. So this acceleration is "absolute". A system with this acceleration is NOT an inertial frame and the effect of the acceleration must be included in any equations describing motions of physical objects.

Answer 3

if you are floating in outer space there is no way to tell which one of the spacemen is moving. both of them will see the exact same thing - the other spaceman getting closer. you have to realize that in empty space there is really no such thing as "stationary". for example, if both spacemen where moving in the same direction at the same constant velocity, from their perspective they would both be stationary. so maybe when this experiment started they were both moving at half the speed of light and the one spaceman just stopped himself by going in the other direction at half the speed of light. i am sitting "stationary" at my computer right now, but really im circling around the sun which is circling around the center of the galaxy. i only appear still because everything around me is moving at that same velocity.

Answer 4

If no stars can be seen, it's pitch black and there's no gravity, how can you tell which is moving? Each of your spacemen will feel like he is standing still, and there is no experiment either of them can do that will demonstrate whether he is moving or not.

When you say one is moving and one is "sitting still", what do you mean by that? Who are you to say which is "sitting still"? There are no points of reference. "Sitting still" with respect to what? There's nothing out there.

Answer 5

I'll throw the question back at you:

How will YOU in the void of space determine which one is moving and which one is standing still?

Give up?

Right, you can't. That's what we mean when we say they're on equal footing.

Answer 6

They're in a void with no reference. How can you say one is moving and the other is still. That makes absolutely no sense.

Answer 7

Because, under your conditions, there is no way that they can decide who is moving and who is sitting still. Or if they are both moving.

It's not obvious that one is "not moving" -- what measurement could they make that would show which one is still, and which one is moving? If you can't measure it, then it doesn't exist.