Understanding Frames of Reference in Relativity

[H0T_S0UP]

Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?

 

[JesseM]

Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?

Again, all motion is relative motion. As long as you are moving inertially, it is always possible to find an inertial frame where you are at rest, and in this frame it will always be other objects in motion relative to you that are shrunk relative to you. Nothing ever expands to a length greater than its rest length in any inertial frame.

 

[paw]

Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?

Try thinking of it this way. Nothing is really happening to the rod. No forces are acting on it. It's not being crushed or anything like that.

What is changing is the result you get when you measure it. If the rod goes past you at some speed v you will measure the rod to be shorter than if it was stationary from your point of view. If someone was riding on the rod they won't see the rod change in any way .

Now if you accelerate and pass the rod so it appears to have reversed direction and is now traveling some other speed, say -2v from your new frame of reference, the rod appears even shorter than it did before when it appeared to be doing speed v. Notice the direction doesn't matter, just the relative speed.

Say you now slow down and let the rod catch up with you then match speed with it. From your original frame both you and the rod are now traveling at v but it is easier to define a new frame where both you and the rod are not moving. In this case you measure the rod to be the same as its original length which is exactly what you'd expect.

The reason this works is there's no way to decide whether you, the rod or both are moving in any absolute sense. Because of this we are free to use whatever inertial frame (point of view) we want.

So it's all in how you look at it, from what frame of reference you are measuring the rod. This doesn't make length contraction any less real. It is a real effect. But hopefully this will help you understand that nothing is really happening to the rod.

 

[MeJennifer]

Try thinking of it this way. Nothing is really happening to the rod. No forces are acting on it. It's not being crushed or anything like that.

That is correct.

All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.

 

[Antenna Guy]

All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.

I never really thought of them as 3-planes, but I can see why you refer to them as such. However confusing they may be, I don't see them as inherently inaccurate if applied appropriately.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.

I suppose this begs the question of whether an inference made by one technique agrees with calculation based upon another approach to the same problem.

Regards,

Bill

 

[JesseM]

That is correct.

All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.

This is a pedagogical opinion, one on which all textbook authors seem to disagree with you since they all start out by introducing the notion of inertial frames (and as a matter of pedagogy, aren't the algebraic equations of SR in inertial coordinate systems a bit easier for beginning students than the Bondi k-calculus?) And can you express the idea that the laws of physics must be "Lorentz-invariant" without referring to the notion of inertial coordinate systems constructed in the standard way? If your answer is yes, please provide a reference. If not, that's a major weakness of your approach to thinking about relativity, since Lorentz-invariance is a very important symmetry in physics.

 

[MeJennifer]

And can you express the idea that the laws of physics must be "Lorentz-invariant" without referring to the notion of inertial coordinate systems constructed in the standard way?

You do realize that Lorentz transformations are coordinate transformations right?

 

[JesseM]

You do realize that Lorentz transformations are coordinate transformations right?

Sure, but the fact that the equations of the laws of physics are invariant under this transformation and not some other (such as the Galilei transformation) is a real physical symmetry of the laws of nature, just like spatial translation symmetry and time translation symmetry. See Lorentz covariance.