Special Relativity - Page 3

On April 25 2011 03:35 Dagobert wrote:

Nope, they can be slowed down (Akopian et al., 2011).

On April 25 2011 03:35 Dagobert wrote:

Nope, they can be slowed down (Akopian et al., 2011).

There is no universal reference frame that we know of. Everything about relativity is relative (surprise surprise). It's common to use the sun or the Earth as a 'fixed' reference point.

On April 25 2011 03:58 Luddite wrote:

only when traveling through a medium, not in a vacuum.

On April 25 2011 03:53 Mr. Wiggles wrote:


I'm not incredibly knowledgeable on the subject, but isn't whether photons have mass or not still an experimental question? Also, this would only mean that c is a constant for maximum possible speed, rather than the speed of a photon per se, right?

On April 25 2011 04:07 Dagobert wrote:

Irrespective of whether that's the case - what part of "only when travelling in a vacuum" amounts to, and I quote, "ALWAYS"? :/

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On April 25 2011 04:14 Pleiades wrote:
Relativity is interesting, but there are many factors and variables in theory that are not true or doesn't reflect the way things behave in the normal universe and is also incomplete.

As for the first one, light doesn't behave like that and doesn't apply in real world, only in theory. We shouldn't even use light as an example of relativity to sight, because we are dependent on light as sight. Or light = sight

You can't see the photon of light you are chasing. It's like taking a picture in completely dark space with a flash of light and running after it. You will never see the light again if you don't have enough speed to get in sync with it in the same direction. Theoretically, if you travel faster than that flash of light, you can't even see as well since it's too slow to hit your eyes. In order for you to see the same light source twice, you would have to see it first, move faster than it in the same direction, then stop and wait for it to hit you again, or move in the speed to stay in sync with it for it to hit your eyes constantly.

For the stationary person, all he will see is a flash at most of both the photon and the person chasing it, depending on whether his brain can comprehend the little amount of time to see it.

So the question is, what are we using to see sight, if light or something to the speed of light is used as a reference in relativity?

I'll get back to the second one later.

On April 25 2011 04:14 QuAnTuM314 wrote:


Its one of those things which agrees so overwhelmingly with all theories and tests, that it is nigh on impossible for it not to be true.

But if through some bizarre circumstance it turns out photons have a minuscule amount of mass, then yes. c would cease to be the speed of light, and return to being a cosmological speed limit (of sorts).



Its still interesting that it's slowed down in a medium. I seem to remember that you could slow down light to 24 miles an hour in sodium crystals.
But, yes. All things assume travelling in a vacuum, unless stated otherwise. Which I guess should have been stated earlier

On April 25 2011 04:26 QuAnTuM314 wrote:


Ahh, you're ruining all the fun of though experiments. One of the major advantages of thought experiments is that the don't have to feel the restrictions that real experiments do.
It is true that you cannot effectively chase a photon, because you cannot observe it. But you can certainly use the situation in a thought experiment. In fact I defy you to try to imagine the real life circumstance. I doubt that anyone could realistically imagine some of the situations that come up when you talk about these topics.

On April 25 2011 04:06 QuAnTuM314 wrote:
Here is a time-space graph depicting movement through space-time. Dont ask me why they do a time-space graph instead of a space-time graph, I never understood.

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The red arrow depicts every day objects, that move at non-relativistic speeds. Because everything moves through space-time at the same rate, and they aren't moving through space very quickly, the majority of their movement is through time. And this is the time you measure on your watch whilst going about your life.

The green arrow shows what would happen as we accelerate. Because the object is moving (much) faster through space, and has to move the identical amount through space-time, it moves less along the time axis. I.e. It experiences less time (relative to space-time) than the red arrow.

The yellow arrow is for photons, and anything travelling at the speed of light. Here, all of its movement through space-time is going through space (you can prove this by putting in v=c in the equation for gamma/time dilation). Therefore, it experiences no time at all.

On April 25 2011 04:37 Misder wrote:

I'm pretty sure that the speed of the photon doesn't slow down, but it only appears to. This is because of the mass about of molecules in a medium, meaning that the photons will hit those molecules. They, in turn, produce another photon after the collision, and it continues on. So the speed of the photon doesn't really change but it looks like it because of the multiple collisions.

On April 25 2011 04:46 rubio91 wrote:
I'm pretty sure that the light produces in the nucleus of the stars takes years to get on the surface, this is pretty much impressing isn't it?

So the speed of the photon doesn't really change but it looks like it because of the multiple collisions.

On April 25 2011 05:06 garbanzo wrote:

I'm not entirely sure what exactly you're trying to say in your post. I think you're saying that there is an absolute frame and I'm pretty sure that's not true. It sounds like you're describing the concept of an aether. But like I said, I don't really get what you're saying so I will just comment on what I quoted above.

I'm not really sure if there's a "real" reason why spacetime diagrams always have time on the vertical axis. It really does just make sense that way, especially in general relativity. Horizontal cones are just awkward to look at.

Your diagram is also not correct. Something traveling at the speed of light is not the same as something traveling at infinite speed (your yellow line). Light still takes a certain amount of time (relative to a stationary observer) to travel a certain distance. For instance, light takes about 8 mins to get from the sun to earth. The green arrow is what is actually traveling at the speed of light. The usual point of that diagram is that if you are an observer at (0,0) then you can't affect anything outside of the light cone (the lines x=t and x=-t (c=1)). Likewise anything outside your light cone won't affect you until their light cone cross your light cone.