my chemistry teacher swears by it

but how can it be a constant if some equasions come within like 90% accuracy?

As far as I remember, light travels at constant speed. However, it can not be measured exactly.

Welcome to sciforums!

... in a vacuum, yes.

Is the speed of light, even in a vacuum, slower for light coming off a very massive object than light coming off a less massive object? If so, then its not constant!

well there are some theories nowadays that the speed of light may have changed with the age of the universe. i think there may also be some data that shows the speed of light isnt as constant as we once thought

Is the speed of light, even in a vacuum, slower for light coming off a very massive object than light coming off a less massive object?

No, the mass of the object has nothing to do with the speed of the light propagating from it.

kanpeki:

It's not that the equations of physics are inaccurate. What is sometimes inaccurate are the measurements we make of physical quantities.

According to Albert Einstein's theory of Relativity, the speed of light in a vacuum should always have the same value. Experimental results tell us that this is true, once we allow for experimental errors (which in the best experiments are very very small).

There is theory that the speed of light varies.

http://www.discover.com/issues/apr-03/cover/

Edited to remove relatively worthless URL

"Still, not many physicists are ready to give up on Einstein"

I can't see that you have to give up on Einstein to consider a varying speed of light.

This was one area where I always saw a conflict. It seemed that if light couldn't excape (came to a stop) then it must also slow down before it stopped.

I developed this view of light escaping at v = c until it suddenly switched off, which made no sense. But as I undersatnd the jprocess now (correct me if I have that wrong too) is that the increaseing gravity doesn't slow light but causes a doppler (red shift) which lessens its energy.

It then reaches a point that its red shift becomes infinite to where there is no energy escaping but while it escapes it still does so at v = c.

those experiments may show a time dependence of the fine structure constant. the speed of light, as a dimensionful quantity, cannot vary.

also, the effect is probably not there. the experiment has not been verified.

Originally posted by kanpeki
my chemistry teacher swears by itI would assume that your chem teacher means the constant c, by definition. Nothing too earth-shattering there.

The wavelength of errandir is a constant. I swear by it. (I should also probably tell you that I am defining the wavelength of errandir to be 7.231985 nm)

Where can I find errandir?

Right here, at your friendly neighborhood sciforums. I'll be here all week.

But as I undersatnd the jprocess now (correct me if I have that wrong too) is that the increaseing gravity doesn't slow light but causes a doppler (red shift) which lessens its energy.

It then reaches a point that its red shift becomes infinite to where there is no energy escaping but while it escapes it still does so at v = c.

Holy hand-grenade Mac! You’re finally beginning to understand. Congrats!

How is the point that redshift becomes infinite defined? Is there a scale where we can detect light at one large redshift, then when it
exceeds that redshift we can no longer detect it?

The red shift becomes infinite when the light loses all it's energy.

speed of light = wavelength * frequency
energy = h * frequency

where h is Planck's constant

Q,

Holy hand-grenade Mac! You’re finally beginning to understand. Congrats!

That wasn't ever really in question now was it? :D

What about the point someone made one time that the speed of light is only a constant relative to space itself. In (or near) a black hole space itself is actually moving towards the black hole at speed and light always moves at c realtive to space so it looks a little slower to an outside observer.

The group velocity of light is always c, but the phase velocity of light can vary and even be supperior that c. I really don't understand this

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

Originally posted by Lucas
The group velocity of light is always c, but the phase velocity of light can vary and even be supperior that c. I really don't understand this

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.

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. :D

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

Doesn't the speed light vary depending upon the medium in which itis travelling? doesn't it slow down in water thes refractiion?

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?

Originally posted by Julixa
Doesn't the speed light vary depending upon the medium in which itis travelling? doesn't it slow down in water thes refractiion? 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.

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,

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?

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,

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.

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 :(

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. :confused: