Constant speed of light

I know there are some light speed threads here. I read them and don't see the answer there. I was hoping a physicist type person could help me out here.

I’m reading The Elegant Universe by Brian Greene. In it he talks about the speed of light being constant, no matter what. One of the analogies he uses is that if you were traveling in the USS Enterprise at 100 Million miles an hour away from something, you would intuitively expect to subtract your speed from the speed of light so that your apparent speed would be the speed of light (670 million miles an hour) minus your own 100 million miles and hour for a total of 570 million miles an hour. He says the converse is also true. If you travel at 100 million mph either toward or away from a fixed point, the speed of light from that object to you remains constant regardless of your speed. He says: (p.32)

Mounting evidence from a variety of experiments dating back as far as the 1800s, as well as careful analysis and interpretation of Maxwell’s electromagnetic theory of light, slowly convinced the scientific community that, in fact, this is not what you will see. Even though you are retreating you will still measure the speed of the approaching photons at 670 million miles an hour, not a bit less.

I don't get it. If I'm in an airplane going at 100 mph and another airplane is coming at me at 100 mph, our closing speed is 200 mph. Neither one of us is violating the 100 mph 'limit' or breaking any laws of physics. It's an apparent speed, but a 'real' one because one of us is going to have to take evasive action.

Um, sorry I forget the main details from my physics class from 2 years ago but, I will try and explain. First thing first, you need to picture space-time in your head, like an X and Y graph. Say X is time and Y is space. The faster you travel in space, the slower you travel in time.(please not I have no clue on how to tell this visualization with just words and no picture) Lights speed is set as a balance between the two, space and time. The faster you move in space the slower in time and vis versa. Light travels the same speed in both always so, regardless of how mush you put in speed, you loss the same amount in time thus, you never ketch up to light, to do so you would have to have 0 time but, then you won't be moving in time thus, not moving at all. Hope this helps.

Hope this helps.

Immensely! Thanks. I see what I was missing. They are related, so when one does something, the other must compensate accordingly. I got the gravity bending space part, but this was just killing me. Thanks again.

Great sigs, too!

I'd like to sum it up as the original postulate does, that may also help you, possibly:

Regardless of reference frame, the speed of light remains constant.

Perhaps the previous post clarified why this is true...

The concept of constant speed of Light is contradictory. It is dependent in the medium of measurement, and upon observation the properties of it changes.

The Heisenberg's Principal of Uncertainty explains this pretty clearly. Another example is the classic paradox of Schrodinger's Cat.

[edit on 8/29/2007 by Carnap]

[edit on 8/29/2007 by Carnap]

Originally posted by Carnap
The concept of constant speed of Light is contradictory. It is dependent in the medium of measurement, and upon observation the properties of it changes.

The Heisenberg's Principal of Uncertainty explains this pretty clearly. Another example is the classic paradox of Schrodinger's Cat.

[edit on 8/29/2007 by Carnap]

[edit on 8/29/2007 by Carnap]

Carnap, I don't think that Schrodinger's cat has anything to do with the speed of light in a vacuum -- which, is always the same.

It had to do with the idea of how an observation can effect an object. I know fully well that it has nothing to do with the speed of light. Although, that would be pretty cool if it did.

:-)

Wasn't it an argument of Einstein's that light moves at a constant rate, regardless of the medium in which it travels? I find that whole idea inherently flawed.

Originally posted by Carnap
Wasn't it an argument of Einstein's that light moves at a constant rate, regardless of the medium in which it travels? I find that whole idea inherently flawed.

And yet it has become a cornerstone of modern physics. I find it odd answering my own thread, but I finished Greene's book and am now reading his second, The Fabric of the Cosmos In that he has another analogy on the issue which, I think, is pretty close to the one offered by halfmask, whom I thank again for providing it. This is a paraphrase:

Say you are traveling North at 65 mph. Your speed directly North is 65mph. Your speed east (or west) is zero. Now say you change course to the northeast. You are still traveling at 65mph, but your speed to the North has dropped to 32.5mph because the North and East directions are now sharing, as it ere, your unchanged speed. Part of it "goes to" your travel North; part of it "goes to" your travel East.

If you attribute both space and time as dimensions the way North and East are, you see that 'spacetime' is related the same way. Thus if you go fast, your time slows down. This has been proven experimentally many times, first in 1970 with a jet airplane carrying a Cesium clock where its twin sat on the ground, and again in experiments conducted in orbiting vehicles. In both cases, the 'flying orbiting' clock 'lost' time. So where your intuition would lead you to believe this violated common sense and was 'inherently flawed,' the experimental data verifies the theory.

reply to post by schuyler


lol! That is the book I was reading. What was it called again? I new it was by Greene. I lost it on the shelves some where... I actually haven't read the last chapter yet... Its a great book, the first one I read before taking classes in school about physics.

About the speed of light constant... this is a bit of a miss conception though, it is not constant necessarily in any medium, it is just that it is the balance point or so, in a vacuum. Certain things do affect the speed of light but, they are special situations and whatnot, for the most part however, it is a constant for normal speed rules/conditions for objects with a mass.

Things that affect the speed of light;
-Gravity in large amounts, bending space making it appear light moves slower.
-Light on a universal scale slowing down due to a unknown reason, it keeps it special role but, is just at a different speed but, all rule equations and what not are the same but, with a new speed.
-Galaxy clock effect; look it up...
-Quantum tunneling; look it up...
these are some reasons I know of that affect the speed of light as a constant.

Hope it helps.

Also, there was another experiment just 2 or 3 years ago NASA did with isotopes that also proved this time dilation effect. Sorry, I don't have a link to an article with the story...

I will see if I can find it and I will post the url for it when I do.

We cannot be certain that the speed of light is constant in a vacuum though.

If gravity effect the speed of light, then surely 'C' is only relative to the observer. ALL instruments would read the speed of light the same in a localised gravity field.

This entire SOLAR SYSTEM is a huge gravity field, and we can only measure the speed of light from within.

We work out the distances of Nebulae/star clusters/ GALAXIES.. based on our 'supposed' knowledge of speed 'C' and the dopplar effect. But WHO KNOWS how many "Gravity wells" the light has had to pass through (maybe effecting it greatly) before reaching us.

We're looking from INSIDE the gravity of the solar system, how can we be sure things haven't "changed" by time they've reached us?

thats my 2 cents

PEACE
AoN

Originally posted by halfmask
reply to post by schuyler

 

Also, there was another experiment just 2 or 3 years ago NASA did with isotopes that also proved this time dilation effect. Sorry, I don't have a link to an article with the story...

Aww god damn, yeah...

That was something to do with particles comming from the sun (i think) to the earth at sea level.

I can't remember it exactly (as im sure you can tell ) But it was somethiong to do with their HALF LIFE, the rate at with they decay and the fact that there was (and DONT quote me on this) 15 or 30 times more particles reaching sea level than there should be.

This, they said was due to the particles traveling SO close to the speed of light that they were EXPERIANCING TIME SLOWER THEN WE WERE, so less decayed were expected.... this WAS actually predicted by (or at least confirmed) einsteins theory.

The particles i THINK.... were MUONS...but again, don't quote me on that (I know they began with the letter 'M', ..don't think they were Mesons though........sheesh i dunno, everythings on "the tip of my tounge..,,and my tounges getting pretty tired )

AoN

[edit on 26-9-2007 by Anomic of Nihilism]

Originally posted by Carnap
The concept of constant speed of Light is contradictory. It is dependent in the medium of measurement.

Actually, the speed of light is always constant. The reason why we manage to "slow light down" has nothing to do with actually making fotons move slower. Couple years ago, some scientists managed to make the light move only 18 m/s. How they did this was by using a medium with a huge refractive index (we are talking close to 8 000 000). The fotons always move with the speed of light, but when they are in a medium with a huge refractive index, they get "sucked" into electron clouds of the medium only to be released moments later. The conclusion is that the speed of the fotons is always constant. The reason why the light moves slowly is that they are held back by the medium. So it's wrong to say that light doesn't have a constant speed.

Should anybody actually read this and wonder what a refraction index is wikipedia offers good information.