Thread: Relativity for dummies (as in, me)

Quick question about relativity, not sure if it's exclusive to either special or general, but whatever. Here it goes...

So my friend and I are racing in the same direction. He takes the rusty, mechanically unstable rat rod, while I take a rocket-powered super-car (cause I'm a selfish greedy bastard). We start the race, and my friend finds himself creeping at about 10mph. Meanwhile, I'm racing with my rocket propulsion about 100 times faster than him, blazing down the track while an ecstatic audience cheers to my awesome racing powers, but I digress. Here's the confusing part. From what I've learned, light travels at the same speed relative to any person regardless of his/her own speed. So relative to my friend, light is traveling at c (just pretend it's a vacuum). And though I'm going stupidly faster than him, light still travels at the same speed relative to me.

Why is this? I don't doubt this in the least bit, I'm sure it's true. But it doesn't make sense in the least bit intuitively. Answers anyone?

One very simplistic way to think of it:

When you are at rest in relation to someone else, you are only moving through time in relation to them. Once you are moving through space in relation to someone else, you have converted some of your motion through time into motion through space, in relation to them. From their point of view, you are now moving through less time (your clock is slowed in relation to them) whilst moving through more space (your ruler is shorter than theirs) so you will still measure the speed of light to be the same.

The reason that observers will always measure the speed of light to have the same value, regardless of any motions they have relative to anything else, is because space and time are relative rather than absolute.

Originally Posted by epidecus

Why is this?

There is no reason, other than the fact that this is the kind of universe we live in. The alternative would be a universe with a preferred reference frame.

Originally Posted by epidecus

Why is this? I don't doubt this in the least bit, I'm sure it's true. But it doesn't make sense in the least bit intuitively. Answers anyone?

There are two ways to answer that :

1. Through special relativity. Speed of light is simply a measurement of distance/time for a photon. If you move very fast you will experience two phenomena in relation to your ( nearly ) stationary friend : time dilation and length contraction. What this means is that for the very fast moving observer his clock runs slower than the clock of your stationary friend, and also distances appear to be shorter than measured by your stationary friend. The relation between the two, i.e distance/time = speed of light, remains the same because the two measurements change by exactly the same factor ( called the "relativistic gamma factor" ).

2. Through electromagnetic properties of the vacuum. Both of you ( the moving observer and the stationary observer ) are seeing the same vacuum around them. Speed of light is a function of two properties of this vacuum, called permittivity and permeability. These remain the same, independent of your state of motion. Therefore both observers see the same speed of light.

Originally Posted by SpeedFreek

One very simplistic way to think of it:

When you are at rest in relation to someone else, you are only moving through time in relation to them. Once you are moving through space in relation to someone else, you have converted some of your motion through time into motion through space, in relation to them. From their point of view, you are now moving through less time (your clock is slowed in relation to them) whilst moving through more space (your ruler is shorter than theirs) so you will still measure the speed of light to be the same..

Thanks, SpeedFreek. So is this basically about time dilation and length contraction? Or are those for a different matter?

The reason that observers will always measure the speed of light to have the same value, regardless of any motions they have relative to anything else, is because space and time are relative rather than absolute.

I'm not that familiar with reference frames and the whole approach of relative view points, so I guess I'm just really used to the idea that everything is absolute. Einstein's relativity has to be the most counter-intuitive theory ever made. At least, that's how it seems to me.

Originally Posted by Markus Hanke

1. Through special relativity. Speed of light is simply a measurement of distance/time for a photon. If you move very fast you will experience two phenomena in relation to your ( nearly ) stationary friend : time dilation and length contraction. What this means is that for the very fast moving observer his clock runs slower than the clock of your stationary friend, and also distances appear to be shorter than measured by your stationary friend. The relation between the two, i.e distance/time = speed of light, remains the same because the two measurements change by exactly the same factor ( called the "relativistic gamma factor" ).

Okay. I get the idea now. Thanks MH.

2. Through electromagnetic properties of the vacuum. Both of you ( the moving observer and the stationary observer ) are seeing the same vacuum around them. Speed of light is a function of two properties of this vacuum, called permittivity and permeability. These remain the same, independent of your state of motion. Therefore both observers see the same speed of light.

You say there are two approaches to answering the problem. Are these two answers related?

Originally Posted by epidecus

Einstein's relativity has to be the most counter-intuitive theory ever made.

No, that would be quantum mechanics.

Originally Posted by epidecus

You say there are two approaches to answering the problem. Are these two answers related?

They are indirectly related. The second one is a consequence of Maxwell's Equations, which originally sparked off the theory of relativity. The premise that Maxwell's equations are valid in all frames of reference leads directly to SR, and then through generalisation to non-inertial frames to GR.