1. May 5, 2014

### Kkangliu

AS we all know, the speed of light is constant wherever and whenever you observe it. Nevertheless, how could that happen? In the first place, why could the light travel. in other words, how could light travel from one place to another? secondly, why the speed of light keep constant, to put in another way, why the speed of light never slow down? Furthermore, if I could travel at the speed of light will I never die? Or could I become younger if I can travel with the speed great than the light? Finally, if I could travel with the speed great than the light, Could I change the history or see the light from the past?

2. May 5, 2014

### iDimension

I may not be the best person to explain as I tend to simplify things too much but nevertheless I'll take a stab at it. The speed of light is constant for one reason and one reason only, because it doesn't have any mass. The instant a photon is created it travels at 300,000,000m/s and light can slow down. There have been tests in the lab where they slowed light down using glass and they even stopped light dead in it's tracks for an entire minute.[1] [2]

If you were to travel at the exact speed of light then you would not age, the blood in your veins would stop flowing, your heart would stop beating, the electrical impulses in your brain would stand still, even the electrons that orbit the nucleus in your atoms would stop "orbiting".

Nothing can travel faster than light as you probably know but if you did travel faster than light, time would not go backwards but according to the equations time would become imaginary. I don't quite understand this part so someone else please weigh in on that bit.

You could travel at the speed of light for 1trillion years and you would not age a second, and when you stopped, it would be like you just blinked... not even that. From your perspective you'd experience 0 time passing by.

Last edited: May 5, 2014
3. May 5, 2014

### bapowell

Light is the propagated disturbance of the electromagnetic field. Why do you think it strange for light to travel?

Light does slow down in different media, as already pointed out. The key is that the speed of light in vacuum is constant for all inertial observers. Why is this? It's just the way the universe works.

It is not possible for massive objects to attain light speed, and so it is meaningless to even speculate. This is not for fear of speculation -- it is simply not correct to apply Einstein's special relativity to massive objects traveling at the speed of light because the equations don't make sense anymore. It is true that there is a time dilation effect for objects in relative motion: moving clocks run slow. But to the person moving (we're all moving relative to something, aren't we?), things appear as normal.

4. May 14, 2014

### DjEqualizee

I think what he's getting at is that from the photon's perspective time stands still, and if time stands still for the photon then it would make movement impossible.

And to answer that simply, infinite time dilation (time at a stand still for photons), which would seemingly make movement impossible, fits in with infinite length contradiction (would appear as 0 distance for a photon).So the photon is traveling no distance in no amount of time.

I'm not really an expert on time dilation or length contradiction, but i'm fairly sure that they're both made possible by relativity. It has to do with the photon's point of reference. Although you already explained the general idea of it.

Last edited: May 14, 2014
5. May 14, 2014

Does this mean that my understanding is wrong?
I thought light always travel at c. Can you explain why I am wrong?

6. May 14, 2014

### DjEqualizee

When we talk about "c" we are talking about the speed of light in a vacuum. The speed of light waves can be changed by the medium they travel through. This is why when you shine light through a prism it separates into colors, or why when you look at the straw in your drink it seems to be in a different place than it should be.

Different mediums have different indexes of refraction, expressed by n = c/v where v is the velocity of light in the medium. For example, The reactive index of a vacuum is 1, while the refractive index of air is 1.000277

Last edited: May 14, 2014
7. May 14, 2014

### bapowell

Light travels at c in vacuum.