# Speed of light

1. Mar 26, 2004

### picass

Has anyone an idea why speed of light is constant ? Has this something to do with lack of mass of a photon because I cannnot imagine it to be possible to add speed to something that has no mass. As light emerges from energy fall of an electron, does speed of light have anything to do with rotation speed of an electron ?

2. Mar 26, 2004

### gta-maloy]

I believe it is simply because of the lack of mass, it has a momentum and energie but I don't believe it has a mass, this would indeed seperate light from most laws attributed to speed/travel.

3. Mar 26, 2004

### Integral

Staff Emeritus
The speed of light was first recognized as a constant by Clerk Maxwell after he cast the fundamental equations of electomagnetism in the form of a wave equation. The term

$$\frac 1 { \sqrt {\epsilon_0 \mu_0}}$$

appeared as the propogation speed of electromagnetic waves. When he computed this constant the value was the same as the then experimental value for the speed of light.

Why is it that value?? That is not a question answered by physics.

4. Mar 26, 2004

### chroot

Staff Emeritus
Light must go either:
• infinitely fast
• finitely fast
If it went infinitely fast, it would cause all sorts of problems with causality -- we couldn't tell what happened before what, or what caused what. Since electromagnetism is the force at work in chemistry and biology, life would happen instantly -- you'd be born and die at the same instant. Stars would burn all their fuel instantaneously. The universe just wouldn't be very nice.

So, light has to travel at some finite velocity. Why does it travel at 300,000 km/sec? Because us humans just happened to define the kilometer and the second that way. Many people feel quite comfortable just declaring that the speed of light is 1, and work with light-seconds and seconds (or light-years and years) instead of kilometers and seconds.

- Warren

5. Mar 26, 2004

### jdavel

chroot said: "So, light has to travel at some finite velocity. Why does it travel at 300,000 km/sec? Because us humans just happened to define the kilometer and the second that way."

The speed of light depends on how humans define their units?

6. Mar 26, 2004

### chroot

Staff Emeritus
The speed itself does not, but its numerical representation certainly does.

For example, light travels at 299 792.458 kilometers per second, or 4.70279985 * 1014 furlongs per decade, or 9.71560349 * 10-18 parsecs per nanosecond. Or, if you prefer, 1 light-year per year.

- Warren

7. Mar 26, 2004

### jdavel

chroot said: "The speed itself does not, but its numerical representation certainly does."

Right. But if you measure the distance (any units) that light travels in a given time (any units) the ratio of distance to time (in those units) always has the same value. This raises two questions:

1) Why do you always get the same value?

2) Why is this particular value the one you always get?

I think the first quesiton is the one that started this thread. It can be answered much the same way you answered the question about why it's not infinite. Namely, if you didn't always get the same value, the world would be completely different, and we wouldn't be here asking about it.

Integral says the second quesiton isn't one answered by physics. Well, it certainly hasn't been yet. And there are probably some serious philosophical reasons to believe it never will be. But I believe physicists still hold out the hope of someday understanding why the values of c, h & G (light, planck & gravity) are RELATED the way they are. That is, if we can just accept one of them being what it is, we'll understand why the others have to be what they are. I think I saw that on Nova!

8. Mar 26, 2004

### chroot

Staff Emeritus
Well, the anthropic principle (the universe is the way it is, for if it were not, we wouldn't be here to talk about it) certainly is an answer, though not a particularly fulfilling one.

It is possible that one day the zoo of 26+ "free parameters" (numbers which must be estimated from experiment and cannot currently be predicted) will one day be whittled down to just a handful as theories progress. However, I feel it's very unlikely that the number of free parameters will ever be driven all the way to zero. We'll always have the question looming over us: why do those parameters have the values they do? No one may ever know, and perhaps the anthropic principle really will be the only answer we'll ever have, satisfying or not.

- Warren

9. Mar 28, 2004

### turin

This seems kind of trivial (misleading), though. If you want to say this is absolutely true, then you have to talk about proper time and displacement, which critically depends on the invariance of c. Otherwise, I don't think that you can say such a thing, since, the further you move away from the pole of your geodesic coordinates, the more distortion you will get, either in dl WRT dt, or dt WRT dl, or both.

10. Mar 28, 2004

### DrMatrix

The speed of light in meters per second is constant because the meter is defined as the distance that light travels in 1/299,792,458 second.

11. Mar 28, 2004

### Integral

Staff Emeritus
c was constant when the meter was defined as a fraction of the earths diameter, it was constant when lengths were measured in cubits, it was constant when man had no concept of measurement. The constancy of the speed of light is not an artifact of mans ability to measure it is a property of the universe.

12. Mar 28, 2004

### Jack Martinelli

Circular reasoning. Sorry, but this isn't useful.

Speed is the magnitude of some displacement of an object with respect to a reference displacement. Given that all possible displacements are contained in the universe we must choose one of them to measure the one of interest.

Your question could be restated: Why is a light-like displacement constant with respect to a reference displacement (clock) for all inertial frames. A complete explanation is surprisingly involved. But here's the simple explanation...

Because all of existence includes all places, there are no other places for the universe to travel to. I.e., the sum of all displacement vectors of all existents must sum to the null vector. ( I got to learn Latex sometime...) And so, if you write:

$$v_{net}= \sum_{i=0}^\infty v_i = {null}$$

implying...

$$|v_{j}| = | \sum_{i=0}^n {v_i} |$$

where $$n=\infty - 1$$ and $$v_j \ni v_i$$ (you get the idea)

Which means that:

$$\frac{|v_{j}|}{| \sum_{i=0}^n {v_i} |} = 1$$

(and I learned some Latex. Vector magnitude doesn't work?)

In english, single objects must travel at a constant speed with respect to all possible frames of reference.

Last edited: Mar 28, 2004
13. Mar 28, 2004

### DrMatrix

I agree that the speed was constant when the meter was defined as a fraction of the earth's diameter. But I'm not sure the speed of light is constant if distance is measured in cubits. Isn't a cubit the length of the forearm? My forearm is probably not the same length as yours and even if we use one person's forearm as the standard, people grow. So I'd say the speed of light is not constant if distance is measured in cubits. Please tell me how can a measurement be constant when it is not defined? In order to even ask the question "Is the speed of light constant?", you must have defined units of time and distance. And the answer depends upon the definitions.

Units of length are matters of definition. According to Special Relativity, when distance is measured by rigid rods, light has a constant velocity. This allowed us to define length in terms of the speed of light. With the meter defined as the distance light travels in 1/299,792,458 second, the speed of light is constant by definition.

14. Mar 28, 2004

### Integral

Staff Emeritus
Our ability to measure or even comprehend the speed of light is immaterial. The speed of light is a basic property of the universe, it appears to be constant. Our perception of it may change, but the property remains constant. (Unless, of course, there are long term slow variations which we have not been able to measure.)

Of course you must have a consistent unit of measurement to accomplish anything, If Noah had changed his definition of a cubit midway through construction the ark would not have floated. Do you think that the speed of light is different because the number of miles/year it travels is different then the number of m/s?

Your argument would lend itself to a blind man claiming color does not exist because he cannot see it. I personally believe that the universe exists independent of our observations.

15. Mar 28, 2004

### turin

I think DrMatrix and I are trying to make the same point. You must get the definitions straight. And, in order to have definitions, you must agree upon your standards. The speed of light c, last time I checked, was a fundamental constant that is used to define the meter, not vice versa. In this sense, asking whether the speed of light is a constant has a rather trivial answer and reason: "yes, by definition." Then, if one agrees upon a certain number of hyperfine oscillations of the Cs atom as a unit of time, the meter is defined using this unit and c. Thus, it is the meter that is the derived unit, no the speed. The way the mile and the year are defined requires agreement on further standards, at the very least a standard of conversion. I don't know how the mile was originally defined, but just think about the year. There are two equally valid definitions, sidereal and I can't remember the other one. But this already is quite ambiguous.

Last edited: Mar 28, 2004
16. Mar 28, 2004

### Michael D. Sewell

This is off subject, but you brought it up so...

At one time, an acre was the amount of land that a man could plow in one day.

Later an acre was then defined as 60 square chains. 1 chain is 66 feet. That's why an acre is 43,560 square feet. (isn't this system convenient?)

A mile is 80 chains, or 5,280 feet.(nice round number to work with)

So, that's why one square mile is 640 acres.(cool huh?)

Last edited by a moderator: Mar 28, 2004
17. Mar 28, 2004

### Integral

Staff Emeritus
I do not care what units are used, or what defines what. The speed of light is a constant. The only thing that the definition of the meter in terms of the wave length of light does is make the number used to represent the speed of light in meters a rational. This is merely a matter of convince and does not effect the speed of light in any way. While dinosaurs were walking the earth the speed of light was constant, it is the same constant now, the numbers used to describe it are of no real significance.

18. Mar 28, 2004

### DrMatrix

Suppose the speed of light were to double when its speed is measured by rigid rods. The meter is defined as the distance light travels in 1/299,792,458 second. What is the new speed of light in meters per second?

19. Mar 28, 2004

### Integral

Staff Emeritus
The speed of light does not, and cannot double. Only your units have changed. That has not effect on the speed of light.

20. Mar 28, 2004

### jdavel

DrMatrix,

Since E=mc^2, if the speed of light changed suddenly on the day we redefined the meter, then so did the temperature of the sun. But it didn't. Or are you saying that it did?