# Question about the absolute limit to the speed of light

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#### captdude

##### Guest
Can somebody please correct my understanding of - or help me wrap my head around the mindbendingly strange fact that - the speed of light is absolute. :? How can it be that two beams of light travelling in opposite directions pass themselves with a reletive velocity equal to the speed of either light beam? :shock: (instead of the COMBINED velocity of both beams of light.)

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#### csmyth3025

##### Guest
captdude":1vx4t094 said:
Can somebody please correct my understanding of - or help me wrap my head around the mindbendingly strange fact that - the speed of light is absolute. :? How can it be that two beams of light travelling in opposite directions pass themselves with a reletive velocity equal to the speed of either light beam? :shock: (instead of the COMBINED velocity of both beams of light.)

To gain some insight into this phenomenon you may want to study the Wikipedia article on Special Relativity, which can be found here:

http://en.wikipedia.org/wiki/Special_re ... _beginners

Chris

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#### ramparts

##### Guest
That's always a good idea, but I'm not entirely sure if the question you're asking makes sense. Mathematically, you can't say what velocity one light beam measures for a light beam passing in the opposite direction. The equations blow up. This reflects the fact that if you're moving at the speed of light you can't measure velocities since a) time doesn't pass for you and b) you're massless. Only massless objects can move at the speed of light, and we have yet to figure out how to make a massless measurement device - mostly because it's impossible On the other hand, if you're moving at any velocity which isn't the speed of light (a requirement, if you have mass) then you'll perceive any light beam traveling in any direction as moving at the speed of light. Crazy stuff.

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#### captdude

##### Guest
Well the very fact that the equations blow up gives me a sense of satisfaction. Crazy stuff in physics often leads me to believe that there is a door of knowledge yet to be opened that will explain away the craziness.

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#### Kessy

##### Guest
Well, captdude, Relativity is fundamentally about how the same thing will look different, depending on where you're looking at it from. It's pretty easy to understand the idea of relative velocity, what's not so easy to understand is that when you're dealing with speeds near the speed of light, you actually get a distortion occurring. A good analogy would be the diffraction of light in water. Imagine two people, one of them standing by the side of a swimming pool, and the other swimming underneath the water. Obviously, when they look at each other, the images they see will be distorted by the water bending the light. In the same way two people looking at each other who are traveling at very high velocities relative to each other will also see a distortion. While the distortion caused by the water is governed by the water's index of refraction, the distortion caused by velocity is governed by the speed of light. Time and length are distorted so that light always appears to be moving at exactly c, no matter how fast you're going.

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#### darkmatter4brains

##### Guest
captdude":2t8b13m1 said:
Well the very fact that the equations blow up gives me a sense of satisfaction. Crazy stuff in physics often leads me to believe that there is a door of knowledge yet to be opened that will explain away the craziness.

Well, they don't really blow up depending on what direction you take the transformation - they can also just spit out zero.

You can also analyze the equations "in the limit" as you approach the speed of light - and they seem to say a photon sees ZERO time and ZERO space between any two events. It's everywhere before it ever left! Talk about mind blowing.

This relates back to what ramparts said too: that photons do not sense the passage of time. This is the same logic they used to deduce that neutrinos do in fact have a small mass. Since they experience neutrino oscillations, which is a time dependent phenomena, they must have mass, i.e. travel slower than c.

Oh, and nothing will ever explain away the craziness - it's what our Universe is made out of

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#### darkmatter4brains

##### Guest
Kessy":1olp5gfu said:
A good analogy would be the diffraction of light in water. .

I'd be careful with analogies like that, because it's based on an optical effect. True relativistic effects, like time dilation, length contraction, etc, are not due to optical effects and the finite speed of light. It's how the Universe really behaves.

That's why (despite the fact that inertial obsevers are depicted as human in text books) inertial observers are really an infinite lattice work of rods and clocks in Special Relativity. It takes humans (and their eyes) out of the picture.

That's not too say there aren't cool optical effects that do indeed happen at fast speeds, but it just confuses some people because then they think time dilation is just an optical effect (i.e an illusion).

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