# Perceiving light/lighspeed (question)

1. Jun 4, 2013

### AnonymousTeen

Warning:The question might be asked in wrong terms, if it sounds wrong, it most likely is because I'm only in high school and I don't know the first thing about how special or general relativity, rather than the basic idea without the math (hysterical, I know).

I was watching a documentary on Einstein a few weeks ago and I just remembered.. they say that we are confined to see light traveling at an averaged 670 Million miles per hour no matter how fast we are going relative to it. Assuming we have the unlimited energy and the right technology, even if we are traveling near light speed, it still remains a constant towards anything that can perceive it.. Is there something missing that we don't know abut this phenomenon? ..or has this already been explained?

2. Jun 4, 2013

### Staff: Mentor

Special and General relativity explain it. There happens to be a maximum speed limit that anything can ever travel in our universe, and that speed is c, or 671 million miles per hour. This also happens to be the speed that light travels since it has no mass. The laws of physics prevent anything with mass from traveling at c, and since light always travels at c with respect to any inertial frame, weird things happen like time dilation.

There MAY be more we don't know at this time, but as it stands now it's pretty well understood.

3. Jun 4, 2013

The constancy of the speed of light to different observers is an *assumption* of the theory of special relativity. There is no way of explaining it at present. The consequences of this assumption have been well explored by physicists, and there have been some remarkable successes.

4. Jun 4, 2013

### Integral

Staff Emeritus
Just a little off the mark. The reason Einstein was able to state the constancy of the speed of light as a postulate was because of Maxwell. In the late 1860's Maxwell cast the equations of E&M in the form of a wave equation, the expression $\frac {1} {\sqrt {\epsilon _ 0 \mu _0}}$ appeared as the velocity of these electromagnetic waves. When he put in numbers the value was equal to the speed of light. This started the "Great Schism" in physics. The constancy of the speed of light and the issues it causes was topic of popular discussion for the next 50yrs, until Einstein published his Theory of Special Relativity.

5. Jun 4, 2013

### Staff: Mentor

You will find some threads on this particular historical point over in the relativity forum (I'll suggest that the mods move this thread to there in a moment). Einstein most certainly did state the constancy of the speed of light as a postulate (the second postulate of special relativity); on the other hand Maxwell's equations were a strong hint from nature that this would be a good postulate; but yet again that hint went unrecognized for a half century before Einstein picked up on it.

It's still possible to start an argument about just how "required" the second postulate is now, but as a matter of history there is no doubt that it was "required" in 1905 when Einstein published the classic "On the electrodynamics of moving bodes". (Google for that paper - it's worth the read).

6. Jun 4, 2013

### Integral

Staff Emeritus
Not sure what you mean by "unrecognized" here. As I said Maxwell's prediction of the constancy of the speed of light was a major issue in the last half of the 19th century. It was VERY recognized.

7. Jun 4, 2013

### Staff: Mentor

I should have said that the hint was "misunderstood" instead of "unrecognized". The incompatibility between Galilean relativity and Maxwell's electrodynamics was, as you say, well recognized; but it was still took fifty years for Einstein to realize that the answer was not to postulate a luminiferous ether but rather to rethink Galilean relativity.

8. Jun 5, 2013

### AnonymousTeen

Wow guys, my infinite thanks for these answers. They were more than what I had asked for, which is absolutely amazing. Thanks for the corrections as well. I will look further into this because I've been meaning to move that doubt bugging me during school..

"Are you interested in theoretical physics?" (filled.time)

as for that question...

I'm still just a teenager with very little experience with physics, I've only started Mechanics.. using Walter Lewin's 801 lectures on youtube because the class isn't available in my High school. The number of kids taking it there isn't sufficient for a whole class roster.
So far I'm really interested that I think I'll take E&M the second semester of senior year. That's if my 2 month head start at home for about 5+ hours a day is sufficient to cover classical mechanics right before second semester.
Part of the reason to why I'm doing such a thing is because since I was a kid, I've always found it awesome that things like earthquakes, orbits, star births, atoms, bombs, etc. each had a fundamental explanation of their own. So I really looked forward to that. Theoretical Physics? I don't know. I have to check up on my processing power, but I really want to be involved in the physicist community.

Once again, I want to give you my infinite thanks for helping me guys. Thanks∞

Last edited: Jun 5, 2013
9. Jun 5, 2013

### VantagePoint72

The constancy of the speed of light is not a postulate of SR. I'm not sure why there's such a widespread misconception on this point. It is the consequence of the two postulates: the relativity principle and the light postulate. As you say, you can see for yourself how Einstein defined his postulates in Electrodynamics. The relativity postulate is one everyone is well acquainted with and no more need be said. However, this is the light postulate:

Note the vast difference between this and positing that light is constant in all frames. All this postulate says is the speed of light in a particular frame is independent of the velocity of the body that emits it. This is the case for waves in a medium, e.g., sound propagates at the same speed regardless of whether the source is in motion. The reason Einstein had to postulate this is because he doesn't assume there is a medium for the light to propagate in. Thus, the result that follows automatically from waves propagating in media has to be put in by hand.

On the other hand, the statement that the constancy of the speed of light holds in all frames says that the speed of light is independent of both the thing emitting the light and the thing receiving the light. This is a theorem that follows from the light postulate and the relativity postulate, applied to Maxwell's equations. There is this odd notion I've heard that the constancy of the speed of light follows directly from the relativity principle applied to Maxwell's equations. Perhaps this is what you mean by saying that, "It's still possible to start an argument about just how "required" the second postulate is now." However, it is incorrect: it has to first be postulated that light propagates the same way as waves in media do.

So, it is true the constancy of the speed of light requires a postulate and does not follow directly from the Lorentz covariance of Maxwell's equations. The Lorentz covariance of Maxwell's equations don't say anything about what transformations are appropriate for observers, and it is only when you have established (prompted by experiments like Michelson-Morley) the constancy of the speed of light that you can deduce observer's coordinates should transform the same way Maxwell's equations do. However, it is not necessary to postulate the constancy speed of light in its entirety. The much weaker light postulate, combined with the relativity principle, is enough—and that is precisely what Einstein did.

10. Jun 6, 2013

### ghwellsjr

Since you state that the constancy of the speed of light is established by MMX, then you are talking about the two-way speed of light which I agree is not what Einstein's second postulate is stating. He also affirms this near the end of the first section of his 1905 paper where he states:

You call his second postulate the light postulate but it is more than what you claim.

You claim that it does not include the actual speed of light, only that it is independent of the motion of the source. But right there in your quote, Einstein also states the one-way speed is "the determined velocity c" and by that he means the universal constant that he just previously stated.

Why do you ignore his clear statement in your quote and this similar statement in the introduction?

11. Jun 6, 2013

### VantagePoint72

Why do you misleading quote only half the sentence and ignore the half that clearly shows it says what I said? The entire quote is:
I.e. once again talking about the independence of the propagation of light from the speed of the emitting body.

12. Jun 6, 2013

### ghwellsjr

I didn't disagree with that part of your claim, I said it is more than what you claim. So include his whole quote and my question remains, why do you ignore his clear statement in both quotes that light rays propagate at c?

13. Jun 6, 2013

### VantagePoint72

I'm not ignoring anything. The first half of the sentence is a dependent clause. You can't pick out half a sentence and then arbitrarily stop at a pronoun and say the part you're quoting means anything by itself. Light propagates at a definite speed which is independent of the speed of the emitter. That is not the same thing as, 'Light propagates at a definite speed. Also, that speed is independent of the speed of the emitter.' The second half of the sentence modifies the first.

14. Jun 6, 2013

### ghwellsjr

You didn't just ignore that statement in the quote that light propagates at c, you denied it:

You ignored and denied the phrase in bold.

15. Jun 6, 2013

### VantagePoint72

Again, the meaning the bolded phrase is modified by the second clause of the sentence. I am denying that you can take the dependent clause of a sentence and claim to know what it's saying without considering the part of the sentence it depends on. So if you're claiming I'm denying bad grammar, then yes, I am denying that.

16. Jun 6, 2013

### ghwellsjr

Well then, what is the meaning of the bolded phrase after it is modified by the succeeding phrase?