Question about Light Speed in Relativity

[Liepar]

According to Einstein, all observers experience and measure the same light speed c, whether they're in stationary position or moving in motion.

 

Okay, this is one of the fundamental parts of relativity, and I'm new at it, so it's still having some difficulty in fully digesting the idea.

 

Let's start at the basic "real world" level first. If I'm driving on the road and going 50mph, and a car from behind me is catching up at 100mph, that car's speed, as it passes me, will not seem as fast relative to my perspective since I'm already in the same forward motion at half the speed. In other words, when it passes me, it won't be traveling 100mph faster than me since I'm not stationary.

 

But going to the area of light speed and advanced theories, all this idea goes out the window?! Let's say I'm in a spaceship, and I'm traveling impossibly fast at 0.5c. If light is coming from behind me and zips right by, I will feel see light traveling at 1 C faster than me. Whether i'm not moving at all, or walking, or driving in a normal car, or riding in a spaceship, I will observe light to come and go at the same constant c...

 

But my question is.... going back to the spaceship -- if my ship was already moving at 0.5 C, and this light from behind zooms by at that same rate of c, then wouldn't that light speed actually be 1.5c???

 

Or am I looking at this totally the wrong way because light is not an object? And that is the whole purpose of relativity?

 

Okay, going to something even more theoretically bizarre and/or unrealistic, what if I was traveling in a spaceship that was going at c? Since I'd be traveling at light speed myself, would light from behind even catch up to me? or would light still zip by my point of refence at c when I see it??

Edited November 3, 2008 by Liepar

[ajb]

It is correct to say that your separation speed is 1.5c. A third observer can say that as you move (lets say) 0.5c left and the other spaceship moves 1c* right relative to him, that the separation speed is 1.5c. This is not in violation of special relativity.

 

However, that is not the speed that you or the other spaceship will measure. This will always be less than c. This is where relativity comes in. You cannot simply add velocities when trying to calculate what one observer measures.

 

As for the purpose of relativity, I am not sure that is a well posed question. What is true is that it has been experimentally tested to some huge degree of accuracy

 

*lets forget if this is physical or not for a moment.

Edited November 3, 2008 by ajb

[swansont]

One key concept here is that there is no absolute frame of reference. You are not moving at 0.5c — that's an incomplete statement: you are moving at 0.5c relative to some reference. But it is just as legitimate to say that you are at rest, and that reference is moving at 0.5c relative to you.

 

Since you can't tell if you are moving, light moves at c with respect to you. There isn't a way to tell if you are moving or at rest. Measuring the speed of light to be something other than c would be a violation of that.

[Liepar]

One key concept here is that there is no absolute frame of reference. You are not moving at 0.5c — that's an incomplete statement: you are moving at 0.5c relative to some reference. But it is just as legitimate to say that you are at rest, and that reference is moving at 0.5c relative to you.

 

Since you can't tell if you are moving, light moves at c with respect to you. There isn't a way to tell if you are moving or at rest. Measuring the speed of light to be something other than c would be a violation of that.

 

Thanks for the response. I think I sorta understand it better now, but at the same time, I think I am also a little bit more confused as I try to digest the concept further. Question marks are orbiting around my head, man. lol

 

Okay, with regards to frame of reference, can light be used as a frame of reference? Would that change the outlook?

 

Let's say the light source is at point A, and I'm already somewhere away at point B and I'm continually traveling at 0.5c further away from point A, then wouldn't it mean I'm moving away relative to the light source (point A)?

 

I guess you already answered this question because you're suggesting I look at it another way and that perhaps I am the one who is actually in stationary position at point B, but light is thing that is actually moving away from me instead....

 

Okay, I'm really confused now. Because if light's supposedly moving 0.5c away from me from this example, but it later expands and catches to my location and hits me at speed of c, how was it really moving away from me in the first place? ?

 

 

 

However, that is not the speed that you or the other spaceship will measure. This will always be less than c. This is where relativity comes in. You cannot simply add velocities when trying to calculate what one observer measures.

 

As for the purpose of relativity, I am not sure that is a well posed question. What is true is that it has been experimentally tested to some huge degree of accuracy

 

Yeah, I'm learning that right now, but it's going against everything I learned from before.

 

Like back in driving school, they teach you that if you are going at 30mph, and an oncoming car is coming straight at you at 30mph as well, then the head-on collision would be a 60mph impact. But if the subject relates to light, then it's a new set of governing rules...

Edited November 3, 2008 by Liepar
multiple post merged

[D H]

Yeah, I'm learning that right now, but it's going against everything I learned from before.

That is just because everything you learned before is only approximately correct. Take the case of two cars colliding head on:

Like back in driving school, they teach you that if you are going at 30mph, and an oncoming car is coming straight at you at 30mph as well, then the head-on collision would be a 60mph impact. But if the subject relates to light, then it's a new set of governing rules...

The relativistic velocity addition rules are not a new set of governing rules. They are true even for a pair of cars traveling at speeds of 30 mph (relative to the road) but heading toward each other. The result is a head-on collision at 59.9999999999999 mph, not 60 mph.

 

That 1.2×10^-13 mph difference is of course unnoticeable. At the typical velocities we encounter in everyday life, the relativistic velocity addition rules simplify to the straighforward vector addition/subtraction rules you have been taught for some time.

[swansont]

Okay, with regards to frame of reference, can light be used as a frame of reference?

 

No. Relativity and our laws of physics are applied to inertial frames from things that have mass. These cannot move at c.