# Why doesn't light move at an infinite speed?

1. Oct 24, 2012

### Astro_Will

I understand it is physically impossible for anything to move at an infinite speed simply because infinity can never be reached but...

My understanding of physics is that as something interacts with the Higgs Field it is given mass and therefore requires more energy to move. However I'm also under the impression that photons are massless and therefore do not interact with the Higgs Field. But if this is true then what limits them to moving at c? Why not faster?

(I'm pretty new to physics so I apologise if this question is based off misinformation)

2. Oct 24, 2012

### phinds

How fast would you like them to move? If they moved at twice what we now know as the speed of light, would you be happy? How about 100 times as fast ? Where do you stop?

It's either infinite or it isn't, and it isn't.

3. Oct 24, 2012

### Daniel1992

I think the OP is asking why there is a speed limit on the speed of light. Saying 'beacuse it is' is not a very good answer.

4. Oct 24, 2012

### ShayanJ

The point is that the OP thinks there is something acting on photons causing them to stop increasing their speed when they reach the speed of light.But that's wrong.Photons move as fast as possible.There is just no speed more than the speed of light in the observable universe and matter and energy always travel with a speed equal to or smaller than the speed of light.
Another point is that,photons just don't accelerate to reach the speed of light.At the moment of their creation e.g. in a pair annihilation,they're just created in motion with the speed of light.So maybe,just maybe,this question is irrelevant.

5. Oct 24, 2012

### bossman27

C is just a physical constant of the universe, much like the gravitational constant, Planck's constant, etc. They are fundamental in the sense that you can't prove them from something else underlying them, they simply are the way they are and were discovered by careful observation.

It is worth noting that while massless, relativity implies that photons do have an energy/momentum corresponding to wavelength or frequency. This comes from the regular relativistic energy equations for a particle, where v = c and thus m = 0 by necessity.

6. Oct 24, 2012

### phinds

It is THE ONLY answer. The posts subsequent to mine spelled it out with slightly more formality, but the bottom line is "it is what it is"

7. Oct 24, 2012

### kamenjar

Think of the universe as a surface of an ocean with fixed depth.
Think of "EM waves" and "event propagation" as waves in that ocean.
Now think of anything else as a surfer.

Maximum wave speed in an ocean with fixed depth is constant.
Surfer can not travel faster than waves.

This, somewhat awkwardly, may give you the answer.

8. Oct 25, 2012

### ShayanJ

I should say that I agree with phinds
In physics,sometimes we should say,its like this and there is no reason(at least yet)
The best example is QM.No one really knows what's happening but only it works.
And just maybe we're gonna understand someday,what is QM.
That's because physicists are looking for a theory of everything.I guess they don't like it too.

9. Oct 25, 2012

### Fredrik

Staff Emeritus
Light has speed c by definition in both classical and quantum electrodynamics. So to ask why it has speed c in the real world is to ask why those theories are so good, i.e. why their predictions are so accurate. The only thing that can answer that is another theory. At present, there's no theory that answers that specific question. If we ever find one, people will just change the question and start asking why that theory is so accurate. Then we'd need another theory to answer that, and so on.

10. Oct 25, 2012

### bossman27

"There's still a school of thought, that cannot believe, that atomic behavior is so different than large scale behavior. I think that's a deep prejudice; it's a prejudice from being so used to large scale behaviors... and they're always seeking to find, or waiting for the day we discover that underneath the quantum mechanics, there's some mundane, ordinary balls hitting or particles moving and so on. I think they're gonna be defeated. I think nature's imagination, is so much greater than man's, she's never gonna let us -- relax!"
-- Richard Feynman

This just seemed like an opportune time to drop an ol' Feynman quote. The moral of the story is that, as far as we can tell, nature just is the way it is, and we have to accept the fact that there may not be a satisfying "reason" that we have a particular value of c and not some other; but it's all part of the mystery that makes science (and particularly physics) so exciting and interesting.

11. Oct 25, 2012

### JustinRyan

If you consider the time dialation experienced by a photon, they ARE moving at infinite speed. It is only the constraints of time as we experience it that c is finite. In the inertial frame of the photon, the begining and end of its journey are simultanious.

12. Oct 25, 2012

### Fredrik

Staff Emeritus
There is no "inertial frame of the photon" (see e.g. this post), and therefore no meaningful way to make sense of "the time experienced by a photon".

13. Oct 25, 2012

### JustinRyan

Apologies for the conjecture contained in my post. And the spelling...eww

14. Oct 25, 2012

### tiny-tim

Welcome to PF!

Hi Astro_Will! Welcome to PF!
That's not logical, and not true.

In special relativity, there is no reason why something going faster than light cannot reach infinite speed.

The only prohibition is on anything crossing the speed of the light.
I think everyone is missing the point.

A muon, say, interacts with the Higgs field, and therefore requires more energy to move than an electron does.

But that interaction does not specify any speed for the muon, nor does it limit the speed of the muon.

That limit (the speed of light) has nothing to do with the Higgs field, and the same limit applies to every particle.

15. Oct 25, 2012

### harrylin

Hi Astro, welcome to physicsforums!

As you mention "Higgs field" I guess that you like to get a more meaningful answer than "because it is so". An answer that relates to models would be first of all that an infinite speed is not perceived as physical by most; a model that has infinite speed might be called magical. If we model everything as fields and waves (as is the case in SR and I think QFT), then this implies a finite velocity constant c for vacuum. And of course, no gravitational lensing could occur with c=∞.

16. Oct 25, 2012

### harrylin

Re: Welcome to PF!

I'm aware that there have been papers making claims as you do. According to "orthodox" SR, c is the limit speed of nature for matter and the only speed of EM radiation. Thus it was said that "the velocity of light in our theory plays the part, physically, of an infinitely great velocity."
-http://www.fourmilab.ch/etexts/einstein/specrel/www/

17. Oct 25, 2012

### Naty1

So far we have no formal reason, no first principles, why the speed of light is what it is, nor why the electromagnetic force is so much stronger than the gravitational force, nor why the mass of the electron happens to be what we observe.

What we do understand is that if any of those were even slightly different values, we'd probably not be here...because either our universe would have come to an end before we had time to develop, or the universe might never have developed.

http://en.wikipedia.org/wiki/Fine-tuned_Universe

18. Oct 25, 2012

### tiny-tim

yes we do …

the speed of light is the invariant speed of space-time

(ie, the speed which is the same for all inertial observers)

and everything else is measured relative to it

19. Oct 25, 2012

### phinds

But what gives it the exact value that it has? THAT, I think, is what everyone means when they say we don't have a reason for it being what it is.

20. Oct 25, 2012

### PAllen

What gives it that value? Our choice of units (how big we are, how big things on earth are, etc.). The only constants for which it is at all meaningful to ask "why that value" are dimensionless constants.

For light, the only meaningful question is actually the OP one (re-phrased a bit): why is there any finite speed which is observer invariant? I don't see any better answer than:

Under some broad symmetry assumptions, there are only two ways for space and time to behave: Galilean relativity (no finite speed is observer invariant) and Special Relativity (there is a finite observer invariant speed). Our world happens to be the latter.

21. Oct 25, 2012

### pervect

Staff Emeritus
We mostly try and point people who ask that into looking at why the speed of light has the value it does to look instead at why the value of the fine structure constant is what it is, because that's where they'll find the most serious discussion of the question in the literature.

The answer won't be too satisfying - we don't have any theory that predicts the value of the fine structure constant AFAIK. On the bright side, we do have some experiments testing the constancy of the fine structure constant, though the results aren't terribly conclusive (again AFAIK).

I'm not sure that we have succeed in this goal, I rather suspect from these remarks that we haven't. Maybe you can provide some insight as to why people ignore the FAQ? Or other ideas which might lead to an improvement of the FAQ? Umm - assuming you've read it. That might be the first question, have you actually read the thing?

22. Oct 25, 2012

### Naty1

I, for one, rarely remember about FAQ's.....I don't have the lists memorized regarding topics covered and for this one you said it:

This existing FAQ explanation is too obtuse. A novice asking about the speed of light will never get it....In fact don't get it. I have yet to see anyone asking the question who is interested in the units used....

I read an explanation in one of the popular physics books for the public [by Kaku, Greene, Susskind, one of them] which still makes good sense too me...I can try to find it if it would be helpful, but it seems some knowledgeable people here don't like it....

Every time I post something along the lines of what I read, because it made sense and helped me understand there are missing pieces in our understanding, something like

"no one knows from first principles why we have the four forces, the mass of the particles we observe, nor why space and time even exist" etc somebody in the forums objects. It's as if the Standard Model of particle physics, for example, emerged from some overarching fundamental theory rather than largely from empirical observations.

Another idea might be to work the big bang singularity into a reply, along these lines:

"At the moment of the big bang which it is generally believed initiated this universe, conditions emerged which led to the universe we observe today. Why we see the exact characteristics from those initial conditions is not yet understood. It may be that when general relativity and quantum mechanics are reconciled at that initial condition, we will have a better insight. Right now we don't have good mathematics to describe exactly what happened; all we have is what occurs after that moment."

23. Oct 25, 2012

### tiny-tim

Hi Naty1!
There's two faqs (in two different threads) …

I think pervect meant the other one.

24. Oct 25, 2012

### PAllen

Think about what it means to ask this without referring to units that relate to happenstance features related to humans and earth. You are forced to think in terms of dimensionless ratios that factor out units. Further, they must be fundamental dimensionless ratios - asking how many times faster light moves than I can run may be dimensionless but is totally silly. You are then led to something like the fine structure constant which is a fixed dimensionless quantity in all systems of units that involves c.

25. Oct 25, 2012

### pervect

Staff Emeritus
The FAQ I had in mind was "Why does c have a particular value, and can it change?"