Understanding Relativity: Chat with a Physics Professor Online for Clarity

[Fredrik]

Simple. You can't add and subtract velocities as vectors.

He was asking why you can't do that. I posted a complete answer to that in #24. Unfortunately, he's not interested in an answer that uses mathematics or is based on special relativity.

 

[Dale]

if i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

Because velocities in one frame don't directly tell you anything about velocities in another frame. You have to transform into the other frame to get velocity in the other frame.

This has nothing to do with special relativity. This has to do with coordinate transformations. If a value is coordinate-dependent (like velocity) then the value in one coordinate system doesn't tell you anything about the value in another coordinate system, you must transform to the new coordinate system.

 

[Dale]

the speed of light is constant with respect to all frames of reference. okay i disagree with this part, because in my opinion it leads it impossible to detect a particle that could potentially be moving faster than the speed of light.

This is simply wrong. Here is a thread where we discussed this at length:
https://www.physicsforums.com/showthread.php?t=683408&page=2

See in particular posts 23 and 28.

It is not impossible to detect v>c, the fact that we have not detected any particle with v>c is due to the behavior of all known particles, not due to a limitation of our measuring devices.

 

[Samshorn]

If i can add and subtract v and u to get the velocity between v and u in a frame of reference, and time dilation and space dilation doesn't change velocity, why would v see a different velocity with respect to u?

It wouldn't, but there is more to Lorentz covariance than just time dilation and length contration. You're overlooking the relativity of simultaneity, and the physical basis for that.

Remember, speeds have meaning only in terms of a given system of space and time coordinates. To test your ingenuity, see if you can imagine two relatively moving systems of space and time coordinates such that a given pulse of light has the same speed in terms of both of them. Feel free to adjust the scaling AND tilt the space and time axes as necessary to achieve this. (Hint: It's possible.)

 

[phillip1882]

It is not impossible to detect v>c, the fact that we have not detected any particle with v>c is due to the behavior of all known particles, not due to a limitation of our measuring devices.

okay then use this measuring device relative two the two particle a and -a!
don't use light itself.
if i fire two light beams at each other, use this device to determine the speed between the two light beams.

in a vacuum, light has a medium. not matter, background radiation. you can never fully eliminate background radiation because it exists everywhere, and any sort of attempt to block it will generate background radiation itself. show me a way to completely eliminate background radiation, and fire a light beam through it. i would dearly love to know what happens.

 

[Dale]

okay then use this measuring device relative two the two particle a and -a!

That gives you the separation velocity in the device's frame, not the velocity of -a in a's frame nor the velocity of a in -a's frame.

in a vacuum, light has a medium. not matter, background radiation.

This is completely illogical. The background radiation IS light. It is the light given off by a black body at ~3 K, just like the light from a typical lightbulb is given off by a blackbody at ~3000 K. So what you are saying here is that light has a medium and that medium is light.

 

[phillip1882]

That gives you the separation velocity in the device's frame, not the velocity of -a in a's frame nor the velocity of a in -a's frame.

derive the equation for velocity addition (a1 +a2)/(1 +a1*a2/c^2)
with the assumption that two particles can detect motion faster than the speed of light; because apparently its possible.

This is completely illogical. The background radiation IS light. It is the light given off by a black body at ~3 K, just like the light from a typical lightbulb is given off by a blackbody at ~3000 K. So what you are saying here is that light has a medium and that medium is light.

in a round about sort of way yes. if you can believe the illogical assumption that light is not relative to an observer no matter the speed, than i can believe the illogical assumption that light requires a medium in a vacuum, even if that medium is light itself.

i think if you completely eliminated light from a medium, and then tried to put light through a medium, the light would not just go straight through. it would immediately scatter.
light is made up of particles. but those particles require other nearby particles in order to travel. you can't have motion without a frame of reference, therefore light creates it own frame of reference.

 

[Fredrik]

As everyone can see, we have closed this thread. The discussion was clearly not going anywhere.

 

[Dale]

Sorry Fredrik, I wanted to leave phillip1882 with a response to his last post, in case he checks in. I know it is probably futile, but who knows?

derive the equation for velocity addition (a1 +a2)/(1 +a1*a2/c^2)
with the assumption that two particles can detect motion faster than the speed of light; because apparently its possible.

Sure, here is a brief outline of the proof. Assume the following three things:
1) The principle of relativity
2) The invariance of c
3) Superluminal motion can be detected

From 1 and 2 derive the Lorentz transform. From the Lorentz transform derive the velocity composition formula.

If you notice, the third assumption is irrelevant to the proof. There is no need to assume it, but there is no harm in assuming it either. In fact, the third assumption need not be assumed at all, it can be proven as I did in the posts I linked to earlier on the other thread.

in a round about sort of way yes. if you can believe the illogical assumption that light is not relative to an observer no matter the speed, than i can believe the illogical assumption that light requires a medium in a vacuum, even if that medium is light itself.

The difference is that the first "illogical" really means "unintuitive to me" while the second "illogical" really means "self contradictory". If light needs a medium and the medium for light A is light B then light B still needs a medium, unless light doesn't need a medium, in which case light B is not the medium for light A. Your premise contradicts itself.

i think if you completely eliminated light from a medium, and then tried to put light through a medium, the light would not just go straight through. it would immediately scatter.
light is made up of particles. but those particles require other nearby particles in order to travel. you can't have motion without a frame of reference, therefore light creates it own frame of reference.

Pure speculation with no relationship to mainstream scientific knowledge whatsoever.

I am sorry that you couldn't wrap your head around relativity this time. Hopefully after some time some of this will sink in. Good luck!

Just FYI, the most important thing about relativity is how well it matches experiment. Any other theory is going to have to be equivalent to relativity in all regimes tested to date. So you need to get used to time dilation and velocity addition, they are simply part of how the universe actually works. When reality doesn't fit with your mental picture, then you need to change your mental picture: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html