Special Relativity question?

Question

Special Relativity tells us that we can't go light speed. But speed can only be measured in relation another reference frame. Since there's no absolute speed what reference frame do we measure speed against to know if wer're approaching light speed?

I hope that makes sense.

Gary H -- Correct me if I'm wrong but I think you're always at rest in your own reference frame.

Answer 1

speed is measured in time and distance, light speed is the time it takes light to travel from one place to another, its a constant. light speed is that measure compared to the relative speed of another object.

I hope that makes sense, if not, comment and I will add more.

Answer 2

Great question!

The answer is that you can't go at lightspeed relative to ANY frame.

Since, as you rightly pointed out, you can measure your own speed only in relation to another reference frame, there is never any absolute answer to the question "how fast am I going?" So, the only way you could say that you're "approaching light speed" would be to look out the window and choose a reference frame that's moving at near light speed relative to YOU.

Since speed is always relative, all of the effects that are associated with speed (such as time dilation, increased mass, increased energy, etc.) are also relative. Those things happen to the "other guy" as measured from your point of view; but they happen to YOU as measured from the other guy's point of view.

Answer 3

Relativity is the idea that the laws of physics do not change from one inertial reference frame and another. Any experiment that you can do in your laboratory at home should work equally well in a train coach or the stateroom of a ship (provided the train/ship is just cruising along and not accelerating, bumping, rolling, or turning). This was known since before the time of Galileo. Newtonian mechanics incorporates relativity. Specifically, Newtonian mechanics work under the following transformation (called the Galilean transformation) from one reference frame (x,t) to another (x',t') moving at a speed v. x -> x' - vt' t -> t' That's pretty straightforward. Everybody measures the same time and their positions are off by a reasonable, intuitive amount based on the relative motion. Everyone agrees on the time and distance between two events. Now the trouble is that the laws of electricity and magnetism (discovered by various folks and compiled by Maxwell) are NOT invariant under this transformation. Some folks thought that the old concept of relativity does not apply to electricity and magnetism--that the laws of E&M only work with respect to an ether. Einstein, however, assumed that relativity SHOULD apply to E&M and that Newtonian mechanics must be flawed. He thereby developed special relativity and derived new transformation laws (which had already been figured out by Lorentz, so they have his name). x -> gamma (x' - vt') t -> gamma (t' - vx' / c^2) Under these new laws, not everybody measures the same times and distances between events. That factor of gamma goes from one at slow speeds up towards infinity as you approach the speed of light. Moving clocks run slow and moving rulers are short!

Answer 4

Gary is right --it is measured against the frame of reference in which one begins the acceleration.

This is the same as the big question in the famous example of the twins: Two kids born at the same moment. Put one in a rocket ship and accelerate him to near the velocity of light, then bring him home. The twins are now two different ages. Here's the question: Observing from an independent frame of reference, the Earth moves relative to the rocket ship just like the rocket ship moves relative to the Earth. Who can tell which one is being accelerated to near the velocity of light?

The answer is that within the original frame of reference, energy was applied to one to change the velocity (accelerate it) and not the other. The rocket ship has the engine, therefore the twin in the rocket ship experiences time dilation, therefore he is the younger when he gets back to Earth.

The twin in the rocket ship's velocity is measured relative to Earth, his frame of reference at the outset.

James --On that last "frame of reference" question: It depends on how you choose it. Say you are in your car, in the garage. If you choose the car as your frame of reference, you might be at rest through your whole Sunday morning drive. If you choose the garage, you are in motion relative to your frame of reference throughout the same Sunday drive.

Answer 5

Take your pick. Just make sure you are consistent in taking all other related measurements in the SAME frame....whichever one you choose.

A common error among amateurs in analysing relativistic problems is unconsciously switching to a different reference (inertial) frame for different measurements and 'mixing' the results.

Answer 6

Light speed according to Einstein first relativity postulate, basically indicates that the velocity measurement of light has only any meaning relative to its moving source ,if its measured from the same frame of reference that the source started its motion.

Its very vely simple.

Answer 7

The only thing I can come up with is the speed of light is in reference to the source of the light.

Answer 8

The reference frame you measure against is the one you are in right now. Seriously.