# The Speed of Light, Gravity, and Why it all is what it is.

1. Jun 25, 2011

### Phyzwizz

I was recently watching part 1 of the Nova special The Elegant Universe based on Brian Greene's book of the same name. It was mentioned that through Einstein we learned that Gravity travels at the same speed of light. What relationship exists between Gravity and Light that makes them travel at the same speed?

Also besides the speed of light relationships between light and gravity, are there any other forces of nature that exist at this same speed? What aspect do all these things possess that make them travel at such a speed? Is it even known why light travels at the speed that it does?

Thanks I know there are a lot of questions in here.

2. Jun 25, 2011

### bcrowell

Staff Emeritus
The modern way of looking at it is that the c in relativity is fundamentally a maximum speed of cause and effect. It can also be proved to be the speed at which massless particles travel. Since light and gravity waves are both massless, they both travel at c. The gluon is another example.

The value of c has a defined value in the SI. The reason it has this value is that the meter is defined as the distance light travels in a certain fraction of a second.

3. Jun 25, 2011

### Pengwuino

The forces are mediated by massless quanta (although the graviton is still hypothetical). Massless anything travels at the speed of light exclusively. Photons are the only particles we have observed that are massless and are free (gluons have not been observed to be free, but they are massless) so everyone says "speed of light" when in actually, we really mean "speed of massless particles".

4. Jun 25, 2011

### Phyzwizz

Thanks.
And I assume that its unknown why the universe landed on the value it did for the speed of light(I'm not talking about the system of measurement but rather why light travels at 299 792 458 m / s or 670 616 629 mph or 1.07925285 × 109 kph or whatever other system of measurement is chosen, they're all the same speed, why that speed?

5. Jun 25, 2011

### Pengwuino

Yup that's one of those "why?" questions that can't be answered - that's just the way it is.

6. Jun 25, 2011

### bcrowell

Staff Emeritus
The question isn't meaningful. The reason it has the value it does is purely a matter of definition.

I disagree.

There's a good discussion of these issues in this paper: Duff, http://arxiv.org/abs/hep-th/0208093

It makes sense to ask why God made a unitless universal constant, like the fine structure constant, have a certain numerical value. It doesn't make sense to ask why God made a universal constant with units have a certain numerical value. The reason a unitful constant has a particular value is because the system of units was constructed by humans in a certain way.

If God doubled the speed of light at midnight tonight, but also changed e and h so as to keep the fine structure constant the same, there would be no empirically observable consequences.

7. Jun 25, 2011

### Pengwuino

The way I read the question is that is there a reason (if you independently define c = 3x10^8m/s) why it is the value it is and not say, 50m/s. Sure you could talk about a time-dependent structure constant, but you would still run into the problem of why the values are the way they are at some level of the analysis where you simply have to accept what is given to you by nature.

8. Jun 25, 2011

### bcrowell

Staff Emeritus
The reason it's not 50 m/s is that humans didn't define the meter to be the distance traveled by light in 1/50 of a second.

In fact, I could claim that it is 50 m/s today, as opposed to its value yesterday of 299792458 m/s. But it just happens that e and h also changed last night from their previous values to new values that preserve the value of the fine structure constant. As a side-effect, all our clocks have changed the rate at which they tick, and all our meter sticks have changed their lengths. Therefore, we think that c is still 299792458 m/s, even though it's really changed to 50 m/s.

This is the essential point being made in the Duff paper.

Last edited: Jun 25, 2011
9. Jun 25, 2011

### Pengwuino

No no, that's not what I meant. You could define the meter and second independently of the speed of light. Then you would say the meter is defined as this, the second is this, ergo by observation the speed of light is X. Then one could ask well, is there something special about X and the answer is no, X is simply what we measure the speed of light to be. I'm saying that whatever the speed of light is, it's an empirical observation and can't be derived from anything to say the value X is preferred over some other value Y.

10. Jun 25, 2011

### bcrowell

Staff Emeritus
No, X is what we forced the speed of light to be when we defined (in the original SI) the second to be 1/86400 of a mean solar day and the meter to be the distance bewteen two scratches on a metal bar. If the speed of light changed while keeping the fine structure constant (and all other unitless constants of nature) the same, then X would still have the same value, because the length of the mean solar day would change and the length of the bar would change. (Since the earth's orbit is controlled by gravity, I suppose you'd also have to readjust things so as to maintain the values of certain unitless constants related to gravity, e.g., the ratio of the mass of the electron to the Planck mass.) Again, all I'm doing is explaining the point of the Duff paper.

11. Jun 25, 2011

### atyy

How about phonons? Do they count as massless free particles? If they do, why don't they travel at the speed of light?

12. Jun 26, 2011

### Pengwuino

Nooooooooooooooooo I'm not talking about how you could make everything the same by changing everything else. Of course you could! I'm just saying if you just create the world independent of the speed of light, didn't let other constants change, and just observed the speed of light, that whatever you observed the speed to be has no deeper cause than what it is. Nothing tells the speed of light to be what speed it is. Take, for example, a blackbody spectrum. THAT has a deeper theory that tells you why the blackbody spectrum is the way it is. That is unlike the speed of light which is empirical; it has no deeper theory as far as we know.

13. Jun 26, 2011

### bcrowell

Staff Emeritus
They're a vibration of a medium that has mass.

The speed of light isn't empirical. It's defined. So are e and h and G. Their numerical values are all defined, directly or indirectly, by the system of units we use. The purpose of talking about changing the numerical values of the constants is to show that you can never determine them empirically, you can only define them. If you could measure them empirically, then you could tell when they changed over time. But you can't tell when they change over time.

14. Jun 26, 2011

### Pengwuino

We seem to be at a disagreement on what it means to define something vs. observe it and in what sense the OP is questioning what 'c' is. I know exactly what you mean, but I don't think that is the sense the OP is wondering why 'c' is what it is.

15. Jun 26, 2011

### bcrowell

Staff Emeritus
I think what we're disagreeing on is not what the OP was asking but whether the question the OP asked was meaningful.

16. Jun 26, 2011

### atyy

But is the vibration itself massless?

17. Jun 26, 2011

### GrayGhost

Bcrowell,

When you say c is a matter of definition alone, I take it you mean the 1-way speed of light, yes? I mean, we've measured the 2-way speed at only c. We don't know why it's 2-way speed is precisely its value, nor do we know why it is invariant. However, we do know that the value of c is determined. The design of SR reveals the symmetry of space and time required (ie Lorentz symmetry) if an invariant light speed exists, but that's like the chicken vs the egg deal. We first assume an invariant c, then we design the required math model for that to be the case. Then we (or many) claim that the reason light's speed must be invariant is because of the inherent symmetry in space and time. There is a reason for everything, whether we know why yet or not. There's something that causes both the invariance and symmetry to be as they are.

GrayGhost

Last edited: Jun 26, 2011
18. Jun 26, 2011

### Staff: Mentor

I agree fully with bcrowell. Any dimensionful universal constant has its value due to the choice of units. They essentially function as unit conversion factors.

The meaningful question is why does the fine structure constant have the value it does. And that one is indeed unknown.

19. Jun 26, 2011

### bcrowell

Staff Emeritus
What I'm saying applies to all measurements of c.

I would claim that we do know why it has the value it does -- because of our choice of units. Depending on what assumptions you start from, its invariance can be either an unprovable postulate (in Einstein's 1905 axiomatization) or a provable proposition ( http://arxiv.org/abs/physics/0302045v1 ).

20. Jun 26, 2011

### tkav1980

I think the question being debated back and forth is why is C a universal constant. What in the make up of space-time itself causes C to be C and remain constant for all zero mass particles/waves/ anything.

The way i always thought about C was little packets of vibrations of a specific frequency and amplitude "vibrating" through a medium. Sound travels through pure iron at a verry specific speed, Light does the same with the universe. Light would travel at this velocity with or without our observation or measurement, it simply wouldnt be labeld c or measured as roughly 300000km/s.

I think the op was asking if we understand the mechanism of space-time itself that created this constant. Similar to the idea that sound moves through steel at velocity x, through water at velocity n... and so forth. We know the reason soundwaves change depending on medium is density of that medium. But ive never seen an answer to the question of C being constant because space has a specific density, or some mechanism that is measurable and appropriate for space...etc.. I myself would love to be able to understand this.

I hope my rambling made some sence. It doesn't always come out on the screen the way its ordered in my head.