# Why does light travel so fast?

1. Nov 26, 2008

### questions4all

Why does light travel so fast???

I want to know about the speed at which light travels. Why can light travel so fast? is it the weight of the photons or what? And what accelerates the matter to that speed. Everything is propelled by some force so what is lights force? Thank you.

Questions4all

2. Nov 27, 2008

### pallidin

Re: Why does light travel so fast???

Well, a photon does not accelerate to c. It travels at c from the very instant it is a photon.
It goes from zero to c in zero time.

3. Nov 27, 2008

### pallidin

Re: Why does light travel so fast???

As for "why", I'm not sure if anyone knows. We know that a photon has no rest mass.

4. Nov 27, 2008

### stevebd1

Re: Why does light travel so fast???

Also, I think that one of the big question in respect of general relativity is not 'why does light travel so fast' but 'what exactly is mass', you'll find that there are about ten definitions of mass in GR while (to my knowledge) there is only one of light. You could say that there is nothing out of the ordinary about c (which is in a state of m=0, t=0) and the real strangeness is mass (which is in a state of m>0, t>0). There are people here who could probably explain this better but it's worth looking at it from a different angle.

Last edited: Nov 27, 2008
5. Nov 27, 2008

### pallidin

Re: Why does light travel so fast???

Indeed. It is rather curious that anything without rest mass somehow naturally and instantaneously propagates at c.
Too be sure, as you said, the real strangeness is mass, and appears to be interrelated.

6. Nov 28, 2008

### Gnosis

Re: Why does light travel so fast???

I view this matter from a different perspective. I realize that light-speed (c = 299,792,458 m/s) is roughly 900,000 times faster than that of sound however, compared to the colossal distances that light will traverse through the great expanse of the universe, the speed of light is comparable to the hour hand of an intermittent clock. For instance, point a powerful laser out into deep space and it still won't have arrived in some star systems even after traveling for the next 100 billion years. Light is painfully slow compared to the great expanse that it must traverse simply to reveal its ancient light information to us about the distant cosmos.

So rhetorically speaking, my question would be; "Why isn't light MUCH faster than it is in reality? Why is it so comparatively slow compared to the unimaginable vastness of the universe that it actually traverses?"

I have a theory as to why light only equals 'c' in a vacuum, but I think perhaps this is not yet the time for such a discussion. I will say this however, from this perspective, it would appear there is much potential for objects to travel far faster than 'c'. After all, there is more than enough starting and stopping distance available for such velocities.

In any case, this is a classic example where the terms "fast" and "slow" are relative to one's perspective. I tend to think in the expanded mode...

7. Dec 3, 2008

### Nick89

Re: Why does light travel so fast???

I would have expected people to correct your many false ideas but since nobody has explicitly done that I'm going to give it a go.

1) Why wouldn't it? See 4) also.

2) Photons have no mass, and thus no weight (at least not in the classical sense, they might have something that can be compared to weight in special relativity but I don't know much about that)

3) Photons are not matter. They are also not accelerated. A photon always travels at c, end of discussion. It cannot stop, it cannot slow down, it cannot accelerate.

4) As said, photons do not accelerate, hence no force is required to move them (they simply do).

8. Dec 10, 2008

### justwondering

Re: Why does light travel so fast???

Like why does light travel at the speed that it does? Does it have to have a 'memory' to do this every time or is there some 'physical' restraint that we don't/can't think about that causes it to travel at a certain speed? What 'fixes' its speed? It MUST be something!

Yes, light does travel at a very slow rate considering the size of the 'territory' that it has to move along in.

9. Dec 10, 2008

### harvellt

Re: Why does light travel so fast???

Do photons have a mass? We see that if they hit something it moves so they must have a mass. No matter how small they are wouldn't an instant acceleration to c take an infinite force?

10. Dec 11, 2008

### Nick89

Re: Why does light travel so fast???

Photons do not have mass. The force they exert on objects they hit is caused by the momentum. Photons do have momentum, but it is not equal to mv like it is for massive particles.

11. Dec 11, 2008

### Staff: Mentor

Re: Why does light travel so fast???

Just to avoid confusion, when we say photons are massless, we're referring to the "rest mass" (also called "invariant mass"), not what is often called "relativistic mass." The general relationship between (rest) mass, energy and momentum is

$$E^2 = (pc)^2 + (mc^2)^2$$

It's possible for something to have zero mass but nonzero energy and momentum, in which case E = pc.

12. Dec 11, 2008

### epenguin

Re: Why does light travel so fast???

A lot of people seem to think light travels fast.:surprised Nonsense! It travels painfully slow! Only to get to the other end of this Galaxy would take you 100,000 years at the speed of light. It is a good job the Universe is so old or we wouldn't know much about it.

There is another very recent thread called "reason for value of c". There you'll see you have to up-end questions like this for them to make sense.

13. Dec 12, 2008

### franznietzsche

Re: Why does light travel so fast???

Be careful with statements like these. You are trying to apply an old model (classical mechanics) to explain new observations. Observations always trump models.

Observation: Photons always travel at c regardless of the motion of the observer.

We don't know why. But we know that it is true, as far as we have ever been able to measure.

Newton's laws of motion, $$\vec F = m\frac{d\vec p}{dt}$$, is a model. Observations ALWAYS trump models.

Your questions make sense in classical mechanics. But classical mechanics is wrong, it makes assumptions that are not true. As others have said, it turns out that you don't need mass to have momentum, and momentum is what matters in terms of impact kinematics.

Of course, there are flaws in relativity as well (it doesn't work with very small things, just like classical mechanics doesn't work with very fast things).

14. Jan 6, 2009

### Dabonez

Re: Why does light travel so fast???

I thought light could be slowed down? I've read a couple articles about people that managed to even stop light and "restart" it.

http://www.wired.com/science/discoveries/magazine/15-11/st_alphageek

15. Jan 6, 2009

### Nick89

Re: Why does light travel so fast???

Light and individual photons are two different things. The speed of light can be slowed down, but the speed of the individual photons is always c.

16. Jan 6, 2009

### Shackleford

Re: Why does light travel so fast???

Well, 100,000 years in our reference frame. :tongue2:

17. Jan 6, 2009

### DaveC426913

Re: Why does light travel so fast???

The speed of light is the universe's way of not having everything happen all at once.

Seriously though, better questions might be:

Why does EMR* travel so slow?
Why does matter travel at a glacial pace compared to EMR?

(*electro-magnetic radiation, of which light is a tiny sliver)

18. Jan 6, 2009

### Shackleford

Re: Why does light travel so fast???

To the second question, noting pair production, actualized inertia for matter?

19. Sep 27, 2009

### tkav1980

Re: Why does light travel so fast???

could the constant speed of light in a vaccume be a property of space? in a remedial way of thinking about this could the "material" that space is made of be the cause of the constant c?

Off topic, but, i am simply interrested in physics. i have no educational background in the subject i am simply facinated by it so please excuse my ignorance when it presents itself.

20. Sep 29, 2009

### mikelepore

Re: Why does light travel so fast???

The value of c is a combination of the delay for empty space to respond to a changing electric field (due to the "permittivity of free space") and the delay for empty space to respond to a changing magnetic field (due to the "permeability of free space"). c can be calculated from those other two physical constants.

Maxwell used that fact to calculate, if an electromagnetic wave did exist, which they didn't know at the time, then that's the speed that it would have. When people looked at that calculated number, they noticed that it was the same as the measured speed of light. That's how people found out that light is an electromagnetic wave.