# Is it really not possible to tell which object is moving?

• B
The Special Theory of Relativity is based on two principles. The first one is, that if you have two objects, then it is not possible to tell which object is moving, and which object is standing still. So someone moving in a spaceship relative to observer, can't say who is actually moving and who is stationary.
But how can this be possible?
If person in spaceship is approaching the speed of light, he will notice how observer ages very fast...and observer will see person in a spaceship as almost frozen in time. Therefore person in a spaceship could actually tell that he's moving, because he'll notice that he's aging slower than things around him...where is my misconception?

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A.T.
If person in spaceship is approaching the speed of light, he will notice how observer ages very fast...and observer will see person in a spaceship as almost frozen in time.
No, they both observe the other age slower. What they actually see is a whole different issue.

PeroK
Homework Helper
Gold Member
The effects of relative inertial motion are symmetric. The proposition that a spaceship is "moving at the speed of light" assumes absolute velocity. You can only have a velocity relative to something.

The spaceship and The observer in your example have equal and opposite relative velocities. Each is moving at near the speed of light relative to the other.

Orodruin
Staff Emeritus
Homework Helper
Gold Member
In general, you should not expect that one of the assumptions going into a theory is violated by one of its predictions. Time dilation is a prediction and a result of making the given assumptions and so cannot violate them. What you are missing is the relativity of simultaneity, which is another result of the assumptions.

Dear Jan Nebec

Rather than saying -
if you have two objects, then it is not possible to tell which object is moving, and which object is standing still. So someone moving in a spaceship relative to observer, can't say who is actually moving and who is stationary.
it's more helpful to reformulate the principle of relativity as follows-

The laws of classical (and quantum) mechanics are invariant under a uniform boost transformation.

A boost transformation is just transformation to a frame of reference in uniform motion relative to the first. So the principle of relativity should really be interpreted as a symmetry principle of the laws of physics with respect to choice of a particular class of coordinate systems. If you have 2 observers whose frames are related to one another by an boost transformation, then for instance, the form of Newton's Laws is exactly the same in both frames (both observers agree on the accelerations of objects and consequently the forces acting on these objects). This is what one really means by the statement "It's not possible to know whether you're moving or not ". It's that the laws of physics don't betray the motion.

Dale
The first one is, that if you have two objects, then it is not possible to tell which object is moving, and which object is standing still.
Oh, you can easily tell that. It's just that the answer depends on the frame of reference you're using, and all inertial frames of reference are equal (with respect to the laws of physics).

ZapperZ
Staff Emeritus
The Special Theory of Relativity is based on two principles. The first one is, that if you have two objects, then it is not possible to tell which object is moving, and which object is standing still. So someone moving in a spaceship relative to observer, can't say who is actually moving and who is stationary.
But how can this be possible?
If person in spaceship is approaching the speed of light, he will notice how observer ages very fast...and observer will see person in a spaceship as almost frozen in time. Therefore person in a spaceship could actually tell that he's moving, because he'll notice that he's aging slower than things around him...where is my misconception?
May I suggest you look at the videos in this post?

Zz.

Jan Nebec
robphy
Homework Helper
Gold Member
It might be good to start at the beginning since it was Galileo who first formulated the principle of relativity...

...in the case where you aren’t looking outside

Start at #t=4m10s

Then you can try to look outside... and compare your measurements with someone else doing the same thing.

m4r35n357, Sorcerer and Jan Nebec
It might be good to start at the beginning since it was Galileo who first formulated the principle of relativity...

...in the case where you aren’t looking outside

Start at #t=4m10s

Then you can try to look outside... and compare your measurements with someone else doing the same thing.
Love that video.

But did you notice who the executive producer was (@27:20)? Kevin Smith. How is this possible?!!

lol just kidding. But this is the kind of thing that got me interested in relativity. I love the ingenious contraptions they used in this.

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PeroK
Great thanks for this video! It's a really good one :)