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View Full Version : Speed of light was not always the same ?

2012-Jan-15, 09:58 PM
I've started to wonder the past year, if it's at all possible for the speed of light to change. As far I know we base the speed of light based on the time it takes for the light from the sun to reach earth. I'm aware the sun creates this massive shield like bubble called the heliosphere, that the solar wind can slow down when it reaches the edge of it. Would it be possible that this bow shock could also slow the light particles down ? I've also seen reports online that speed of light can be slightly slowed down when in enters an atmosphere. The amount it slows down is probably extremely small, but if you factor in how many planets between us and the stars, this can add up over time.

Edit: It was because of this sticky (http://www.bautforum.com/showthread.php/86593-Advice-for-Conspiracy-Theory-Supporters), that I decided it was probably best I post my question here.

pzkpfw
2012-Jan-15, 10:10 PM
Please clarify if this is just a question, in which case this thread will be moved to the Q&A forum, or if you are going to claim a changing of speed of light - in which case this thread will stay here and you will be required to defend the idea.

2012-Jan-15, 10:55 PM
It's a Q and A, I don't have any interest in stacking a claim, and I don't know have enough information on it to even start to defend a claim. I did read the rules, and I thought it would of been better to put it here because of the subject. Sorry, I'm not used to such strict rules on a forum. Next time time I can just pm a mod if I'm not sure ?

Cobra1597
2012-Jan-15, 11:17 PM
When we discuss the speed of light being constant, we are talking about the speed of light in a vacuum. Light traveling through other media has different speeds, this is something you observe when you look at a straw in a cup of water. Refraction is a visible expression of the different speed of light in water vs light in air. See Snell's law for the mathematics of this.

Is the speed of light traveling through the heliosphere different than the speed in a vacuum, or the speed the light was traveling up until it reached the heliosphere ? Yes. Is the universal constant "c," the speed of light in a vacuum, changing? No. Also note that this is not a cumulative effect. Once light exits the heliosphere and passes through something closer to a vacuum, its speed will return to something closer to "c." The same thing is happening with the light in that cup of water. When the light exits the cup and enters the air, its speed is that of light passing through air, not light slowed down by water and then air again. The effect does not add up over time.

pzkpfw
2012-Jan-15, 11:24 PM
Moved from ATM to Q&A.

Strange
2012-Jan-15, 11:28 PM
I've started to wonder the past year, if it's at all possible for the speed of light to change.

Not as far as I know. Not the speed of light in a vacuum anyway, which is what "speed of light" normally refers to. Light travels slower in a medium (air, glass, water, etc) with a refractive index greater than 1.

As far I know we base the speed of light based on the time it takes for the light from the sun to reach earth.

I'm not sure how that could be done. There are various ways of measuring and calculating the speed of light (which all give consistent results). Some examples here: http://en.wikipedia.org/wiki/Speed_of_light#Measurement

kevin1981
2012-Jan-16, 12:00 AM
Light travels slower in a medium (air, glass, water, etc)

If we measure the speed of light in water, we measure it traveling slower than it does in a vacuum. But i thought that was due
to the light being absorbed and then emitted by electrons rather than it actually slowing down per se.

I mean, from electron to electron, is it still slower than when it is in a vacuum ?

Why call it a constant if it is not ?

Cobra1597
2012-Jan-16, 12:24 AM
Why call it a constant if it is not ?
We call the speed of light in a vacuum a constant, and it is. We might often omit including the italicized in our speech because, much of the time, different media and their index of refraction is irrelevant to the conversation. If we are talking about light passing through space or air, such as an application like radar or communication, the index of refraction is essentially "1." If we are talking about calculating the wavelength of an emitted photon and using the equations E=hν and λν=c, we basically only care about the speed through a vacuum. If we are talking about E=mc^2, we always mean its speed in a vacuum.

C is constant. The speed of light might not be, but C is.

Strange
2012-Jan-16, 12:35 AM
If we measure the speed of light in water, we measure it traveling slower than it does in a vacuum. But i thought that was due
to the light being absorbed and then emitted by electrons rather than it actually slowing down per se.

That is roughly what happens if you consider it as photons. If you think of it as an electromagnetic wave, then the "slowing down" description works (although I suspect you can refine that by talking about the EM waves interacting with electrons...)

Tensor
2012-Jan-16, 01:47 AM
It might be well to remember that since 1983 the meter has been defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second. This effectively defines the speed of light, in a vacuum, as 299,792,458 m/s.

2012-Jan-16, 03:31 AM
Well I guess this throws my own person thought, that it might of been faster at one point and time. oh well :D

Tensor
2012-Jan-16, 05:09 AM
Well I guess this throws my own person thought, that it might of been faster at one point and time. oh well :D

Well, we know it hasn't changed for ~ 1.8 billion years. c is a component of alpha, the QED coupling constant. Evidence from the Oklo Natural Reactor (http://arxiv.org/pdf/hep-ph/0506186v3) shows alpha hasn't changed, so c hasn't changed.

Selfsim
2012-Jan-16, 05:45 AM
Well, we know it hasn't changed for ~ 1.8 billion years. c is a component of alpha, the QED coupling constant. Evidence from the Oklo Natural Reactor (http://arxiv.org/pdf/hep-ph/0506186v3) shows alpha hasn't changed, so c hasn't changed.
Hmm ... hi Tensor;
I was under the impression that there were some astronomical quasar measurements floating around at the moment, which appear to show some dipolar variation of alpha when viewed in two different directions of the sky (?) I'm not sure if the issue is fully resolved yet, as I think the initial measurement tolerances needed to be very fine, so they're re-verifying the measurements. (There are now, a few different set of figures around which seem to agree with the dipole observation).
Its all too early to conclude much from all these tests ... and I'd say that an unchanging 'c' is fairly safe (because o fthe weight of other evidence), ... but it is kind of exciting to see the evolution of alpha being put to the test. :)

Regards
PS: I hope I've got the right alpha here ...??

Tensor
2012-Jan-16, 06:26 AM
Hmm ... hi Tensor;
I was under the impression that there were some astronomical quasar measurements floating around at the moment, which appear to show some dipolar variation of alpha when viewed in two different directions of the sky (?) I'm not sure if the issue is fully resolved yet, as I think the initial measurement tolerances needed to be very fine, so they're re-verifying the measurements. (There are now, a few different set of figures around which seem to agree with the dipole observation).
Its all too early to conclude much from all these tests ... and I'd say that an unchanging 'c' is fairly safe (because o fthe weight of other evidence), ... but it is kind of exciting to see the evolution of alpha being put to the test. :)

Regards

First off, those are at a higher z than Oklo. Oklo is at z ~.16 all the astronomical numbers are at a minimum of z ~.5. There's are some controversy with the quasar measurements. For one, they use two different telescopes. The Very Large Telescope for the southern hemisphere, and the Keck for the northern. Now, there has to be a very careful analysis to correlate the differences in the two telescopes to make sure there are no systematic errors. Interestingly, when you compare objects that both telescopes have viewed, the effect goes away, or worse, there is one scope that sees the effect and the other doesn't see it. The effect also happens to align with the telescope in each hemisphere. Convenient, eh? Not to mention that the effect is not opposite to each other. Alpha would have to be changing by different amount in different directions. Finally, it is only one group that is seeing the effect. Other groups are not seeing the effect. See here (http://arxiv.org/pdf/1111.0092v4) for a an overview of current experiments, constraints and values.

2012-Jan-16, 06:54 AM
The speed of light can't be exceeded because it is defined to be 299 792 458 m/s. This is what I read over and over again, but it's not making sense to me..Why is is defined to this speed limit ? Nothing with mass can travel faster then light, ok fine I can work with that. What about Light itself then ?

Selfsim
2012-Jan-16, 07:08 AM
First off, those are at a higher z than Oklo. Oklo is at z ~.16 all the astronomical numbers are at a minimum of z ~.5. There's are some controversy with the quasar measurements. For one, they use two different telescopes. The Very Large Telescope for the southern hemisphere, and the Keck for the northern. Now, there has to be a very careful analysis to correlate the differences in the two telescopes to make sure there are no systematic errors. Interestingly, when you compare objects that both telescopes have viewed, the effect goes away, or worse, there is one scope that sees the effect and the other doesn't see it. The effect also happens to align with the telescope in each hemisphere. Convenient, eh? Not to mention that the effect is not opposite to each other. Alpha would have to be changing by different amount in different directions. Finally, it is only one group that is seeing the effect. Other groups are not seeing the effect. See here (http://arxiv.org/pdf/1111.0092v4) for a an overview of current experiments, constraints and values.
Thanks for the paper .. it might take me a while to digest it ..
I also agree that its most likely due to systematic measurement errors, but I think the latest I read on this (Oct 2011) is here (http://arxiv.org/abs/1008.3907) .. the intro says:

We previously reported Keck telescope observations suggesting a smaller value of the fine structure constant, alpha, at high redshift. New Very Large Telescope (VLT) data, probing a different direction in the universe, shows an inverse evolution; alpha increases at high redshift. Although the pattern could be due to as yet undetected systematic effects, with the systematics as presently understood the combined dataset fits a spatial dipole, significant at the 4.2-sigma level, in the direction right ascension 17.5 +/- 0.9 hours, declination -58 +/- 9 degrees. The independent VLT and Keck samples give consistent dipole directions and amplitudes, as do high and low redshift samples. A search for systematics, using observations duplicated at both telescopes, reveals none so far which emulate this result. (meaning the dipole observation cannot yet be put down to systematic errors .. but finer measurements are called for in order to eliminate the possibility).

If you're interested in exploring this further, I suggest we take this off to another thread, as we're kind of drifting away from what is most likely an unchanging 'c' (??)

Its a very interesting one ... worthwhile following though, (IMHO).

Best Regards

WayneFrancis
2012-Jan-16, 08:15 AM
I've started to wonder the past year, if it's at all possible for the speed of light to change. As far I know we base the speed of light based on the time it takes for the light from the sun to reach earth. I'm aware the sun creates this massive shield like bubble called the heliosphere, that the solar wind can slow down when it reaches the edge of it. Would it be possible that this bow shock could also slow the light particles down ? I've also seen reports online that speed of light can be slightly slowed down when in enters an atmosphere. The amount it slows down is probably extremely small, but if you factor in how many planets between us and the stars, this can add up over time.

Edit: It was because of this sticky (http://www.bautforum.com/showthread.php/86593-Advice-for-Conspiracy-Theory-Supporters), that I decided it was probably best I post my question here.

The speed of light can change as it passes through a medium but for all intensive purposes most of space is a vacuum or close enough not to slow down light. While individual photons may encounter a slow down that is probably immeasurable over all the speed of photons will be extremely close to c . If the speed of light did significantly change that would have some measurable effects on physics as we know it. From observations it doesn't seem that this is the case when we makes observations further out in space and therefore further back in time.

korjik
2012-Jan-16, 08:18 AM
The speed of light can't be exceeded because it is defined to be 299 792 458 m/s. This is what I read over and over again, but it's not making sense to me..Why is is defined to this speed limit ? Nothing with mass can travel faster then light, ok fine I can work with that. What about Light itself then ?

basically any particle without mass goes the speed of light, and any particle with mass goes slower than the speed of light. The actual quantity comes from quite a few different places, and has been confirmed by measurement to a large degree. Why it is that value specifically isnt really answerable. There isnt any real reason c cannot be almost any quantity, tho changing it would radically change the way the universe works.

WayneFrancis
2012-Jan-16, 08:45 AM
If we measure the speed of light in water, we measure it traveling slower than it does in a vacuum. But i thought that was due
to the light being absorbed and then emitted by electrons rather than it actually slowing down per se.

I mean, from electron to electron, is it still slower than when it is in a vacuum ?

Why call it a constant if it is not ?

Remember it is c that is referred to as constant. c is defined as the speed of light through a vacuum. But you can also say light is constant within a medium. Most often we talk about light in vacuum but given a medium of a uniform density light will remain constant within that medium. But that might be confusing the issue a bit.

I think one of the of the issues with just looking at photons just being simply absorbed and re-emitted is that absorption and re-emission is going to be in a random direction. Also absorption and re-emission only effects photons of specific frequencies which is dependant on the shells of the atoms the photon is travelling though.

Strange
2012-Jan-16, 09:54 AM
The speed of light can't be exceeded because it is defined to be 299 792 458 m/s.

It isn't defined to be that speed, it is measured to be that speed. Although, now the meter is defined in terms of the speed of light.

The "speed of light" is a bit of a misnomer, it is more the "universal speed limit". So one question is: why is there a maximum speed? It may be that is just the way things work. I did see an argument somewhere that there must be some upper limit to the speed with which information must be transferred (if there weren't then things could interact instantaneously across the width of the universe and this would mean you couldn't distinguish cause and effect - something like that anyway. I didn't really understand it!).

If there is an upper limit, then light will travel at that speed (because it is massless).

The second question is: why that particular speed? Well, given there is a maximum speed, then the question is meaningless. If it were one tenth of that, or 202.36% greater, then you could ask the same question. There may be some underlying reason for that particular speed that comes out of a theory of quantum gravity. But then it will probably depend on some other constants - so why do they have the values they do...

Jeff Root
2012-Jan-16, 10:48 AM
As far I know we base the speed of light based on the time it
takes for the light from the sun to reach earth.
Most measurements of the speed of light are done in laboratories.
Some are astronomical, but there is no way to accurately measure
the time it takes for light from the Sun to reach Earth. We know
it takes 8 minutes 20 seconds because we know the speed of light
and we know how far we are from the Sun, by various means.

One thing we can do, though, is measure the round-trip time for
a signal to go from Earth to a spacecraft in deep space and the
reply to reach Earth. The Sun's gravity causes a small delay,
called Shapiro effect, when such a signal goes near the Sun. It
is the same effect which causes gravitational lensing. Just as
light seems to slow down when it passes from air into water, due
to the interaction with electric charges, it also seems to slow
down when it goes through a gravitational field. It takes longer
for a signal from the Cassini spacecraft on the far side of the
Sun to reach us than if the Sun's gravity weren't there, because
the path the light takes is longer, due to the gravity. So the
light does not actually slow down because of the gravity, it just
has to travel a bit farther. Anyone measuring the speed of the
signal anywhere along the path would find that it had the same
value as always.

I'm aware the sun creates this massive shield like bubble called
the heliosphere, that the solar wind can slow down when it reaches
the edge of it. Would it be possible that this bow shock could
also slow the light particles down ? I've also seen reports online
that speed of light can be slightly slowed down when in enters an
atmosphere. The amount it slows down is probably extremely small,
but if you factor in how many planets between us and the stars,
this can add up over time.
Earth's atmosphere slows light slightly. It is easy to measure
with the right equipment, but it isn't obvious. The density of
particles in the heliosphere is way, way, way, way lower than
it is here at Earth's surface. The slowing would be very ...
miniscule.

If I recall correctly what I read just a few weeks ago, it has
been estimated that a typical photon of the cosmic background
radiation only encounters one or two hydrogen atoms in deep
space before it reaches Earth, while travelling for 13.7 billion
years. That has to be about the least effect anything can have
on anything!

-- Jeff, in Minneapolis

Strange
2012-Jan-16, 10:59 AM
Earth's atmosphere slows light slightly. It is easy to measure
with the right equipment, but it isn't obvious.

I had no idea what the refractive index of air is so I looked it up: about 1.003 (dependent on all sorts of factors including the wavelength). Which means that light will travel at 99.97% of c.

WayneFrancis
2012-Jan-16, 11:16 AM
The speed of light can't be exceeded because it is defined to be 299 792 458 m/s. This is what I read over and over again, but it's not making sense to me..Why is is defined to this speed limit ? Nothing with mass can travel faster then light, ok fine I can work with that. What about Light itself then ?

Light, and any other particle with zero rest mass has no choice but to travel at c. Forget for a moment the 299,792,458m/s. Particles with no rest mass, such as the photon, must travel at a specific speed. This speed is the c and is our "universal speed limit". Things with rest mass can travel with a speed = 0 up to a speed close to but not equal to c, this "universal speed limit". In equations c is often set to 1 since it makes the equations easier to manage. The final result is then in the form of a decimal number that can be multipled by c in any unit of measure, ie m/s mph etc, to give you a speed you are more used to.

2012-Jan-17, 12:20 AM
I think I'm starting to understand it alot better now. So where as something moving at the speed of light, may of taken longer to reach us. The speed in which it was trailing never actually changed, simply because the path it took was longer.

Middenrat
2012-Jan-17, 02:33 AM
Can one of the physicists here factor Time into the answer? I came to understand light was hauled along by Time, or that we're propelled into the future at the speed of light, if the two are birds of a feather, as it were.
disclaimer: this is not even the nostril hair of a Theory.

caveman1917
2012-Jan-17, 02:47 PM
I came to understand light was hauled along by Time, or that we're propelled into the future at the speed of light, if the two are birds of a feather, as it were.

I'm not entirely sure what you mean, but there is something that may be relevant. In relativity a spacetime background is used rather than seperate space and time. However the question arises, since space and time have different units (meters and seconds), how do you put both together as if it were a single concept? The answer is that you convert between seconds and meters using c. Suppose you stay stationary so there is no movement in space, but a second has passed on your clock so you moved "through time". To convert this to a spatial distance you multiply by c, so in a sense you could say that you travelled 1 lightsecond through the "time part" of spacetime in 1 second, or in other words that you move into the future at c.

As WayneFrancis said earlier, usually c is set to 1 so it disappears from formulae. The minkowski metric which you probably have seen around before is thus usually given as ds^2 = -dt^2 + dx^2 + dy^2 + dz^2, however the full form is actually (not setting c=1) ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2 where you can easily see the seconds-to-meters conversion going on with the time parameter.

ammer
2012-Jan-17, 10:37 PM
As WayneFrancis said earlier, usually c is set to 1 so it disappears from formulae. The minkowski metric which you probably have seen around before is thus usually given as ds^2 = -dt^2 + dx^2 + dy^2 + dz^2, however the full form is actually (not setting c=1) ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2 where you can easily see the seconds-to-meters conversion going on with the time parameter.

The key feature of c is that it is the same velocity for every observer. And along with that, as pointed out, comes the Minkowski metric to make sense of that, mathematically. The Minkowsky metric basically settles the speed of light. Let me explain.

With the Minkowski characteristic of spacetime, you also fix the lightike cone, that are the velocities that have a zero norm

ds^2 = 0

The norm is invariant for every observer and everyone agrees on the slope of the cone c=dx/dt. The actual choice of units is arbitrary. The pictures in textbooks favor the lightlike cone with a slope of c=1. In SI the metre is fixed at 1  ⁄   299,792,458th of a light second. Whatever suits the occasion.

This leaves the the conclusion that a different c is merely a different definition of the metre, which in itself is not so exciting. A varying c, as in photons 'speeding up' over time for example, does not make much sense either. Again, the metric sets the stage so to say and fixes c. There are no compelling reasons to meddle with the definition of the metre along the way.