You could say it is the other way around: No matter what, the sped of light is the constant, because time and space is relative to it. Around the event horizon of a black hole, timespace may be sufficiently warped for c to seem different , but it is not. Hans
in the vacuum, there is no dispersion, and the group velocity and the phase velocity are both c. with matter present, all kinds of weird things can happen. you can get the group velocity to be slower (v=c/n, where n is the index of refraction), and with elaborate dispersion relations, you can also get the phase velocity to be less than or greater than c.
Group velocity In Wang at al's experiment, about 2-3 years ago, it was the group velocity that exceed c. This is strange.
Yep, I've just read about Wang's experiment and they reported a group velocity of light greater than c while light was passing through a chamber containing a cloud of gas. Seems that now what is considered that can't travel faster than c is another quantity called front velocity. I don't know what Einstein would say about this if he was alive
Let me tell you a little story the way I see it. I know I will be corrected. Those darn astronomers, look what they have done now. They keep finding distant galaxies receding from us faster than light, as measured by Doppler shift. Over a thousand now, everything outside the Hubble sphere. Can't have that, it would disprove Special Relativity. Can't say Doppler shift (red shift) does not indicate velocity, that would disprove SR. What to do? Ta Da! I know, we can say the universe is expanding at an ever increasing rate, that the EM waves are being "stretched" by the expansion while traveling in open space, not when they were emitted. Dang! Now we have to invent a mechanism to explain the expansion. Dark energy, that sounds good. It has to have repulsive gravity charateristics. Dang, that doesn't go with General Relativity, which says there can be no such thing. And how can you expand "nothing", empty space, and also those EM waves traveling in empty space? Can Mac's "substance" come next? Another problem. I'm sure everyone has heard of the object measured with red shift of z=6.4, the most distant yet measured. It calculates out to be traveling at 2.88c at the time the light was emitted. I think that was using the latest flat space model, using the latest Lambda and Omega inputs, which astronomers believe are quite refined. At least some astromers believe the the z=6.4 object is traveling SLOWER "now", about 2.03c, than it was "then." The only known "facts" are that many objects have a measured Doppler shift that indicate the objects are traveling faster than light, according to Special Relativity. I will now get down off my soapbox and hide behind it to protect against the onslaught. Please Register or Log in to view the hidden image!
2inquisitive: Mmm, what's you point? The universe is obviously d*mn complex and whenever we find an answer, it spawns two questions. So, we try to correlate what we know and see if it fits. This is called working assumptions. We know they are liable to be shattered by the next observation, thats what makes science so interesting. Of course, the halls of science are lined with disgruntled scientists who have watched half their life-work go south because some new guy happened to make a new discovery, and some of them take it badly. That's human nature. Hans
is the speed of light constant Doesn't the speed light vary depending upon the medium in which itis travelling? doesn't it slow down in water thes refractiion?
is the speed of light constant Doesn't the speed lf light vary in the medium in which it is travelling. I understood from high school physics that light slows down in water. true?
Re: is the speed of light constant The vacuum speed is a constant, c. The aggregate light is slowed down in a medium by absorption and reemission. As an analogy, the bus covers much less ground in the city in any given time because it makes a stop every other block or so. On the highway, the bus covers about 70 miles every hour. The highway is like free space. A city is like a medium.
is the speed of light constant Doesn't the speed of light slow down in water; thus the theory of refraction?
One other question I have wondered about for awhile. We all know that the vacuum of space is not empty. It seems to be accepted that the photon (wave?) experiences no time in its reference frame. Would not the photon see the molecules, etc. that it encounters during a 10 billion lyr. trip as a "sea" of molecules that would absorb and reemmit the photon? I know there would not be a great enough number of molecules to effect the velocity over shorter distances, but how might the number encountered over vast distances effect the photon, if it "sees" no time between them?
2inquisitive, ANS: This view is generally referred to as "Tired Light" and is an alternative view to Red Shift by velocity as a function of distance.
As I understand the tired light theory MacM, what I asked about is different. Tired light theory is based on the belief that light loses energy over vast distances which causes the observed redshift, with no known mechanism for that loss of energy. What I am asking is, if it is possible for there to be enough interstellar matter over vast distances, like billions of light years, to cause light to act as if it is moving through a medium. Light's energy is unchanged when passing through a medium, its velocity and wavelength are changed.
2Inquisitive, ANS: OK, I see your point and I think it is a good one. My inclination would be to say yes but then what do I know. I would only add that the mechanisism for tired light would be a transfer of energy to such particles, resulting in a Red Shift but no change in velocity.
I read an article which considered your idea about a year ago, but I can't find the link now Please Register or Log in to view the hidden image! this was as close as I could get: http://www.newtonphysics.on.ca/DOPPLER/Doppler.html the other article was better, as it compared it's expected red-shift number to expected redshift number found via expansion theories and actually readings. It was actually better at predicting the redshift measurments than the expansion theory was. Please Register or Log in to view the hidden image!
