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Htorne
2009-Apr-30, 06:45 AM
Hi people.

Something has been bothering me for a long time.

When you look up the speed of light its constant speed is always given in a vacuum? Why is this - if the speed of light is a constant, is the mention of a vacuum not then a bit redundant?

I do however suspect that there is a very logic reason for this mention of vacuum, just canít seem to figure it out.

Whatís the explanation?
Thanks :)

WayneFrancis
2009-Apr-30, 07:02 AM
It is constant because no how much energy the photon has it will travel at the same speed.

A red photon has less energy then a blue photon but it travels at the same speed as the blue photon.

It is not that it is constant only in a vacuum. Its just its speed in a vacuum is a maximum speed that something can travel that makes it important. There are other massless particles that travel at C. If we where blind and discovered the gluon first then we wouldn't call it the speed of light but the speed of the strong force or something like that.

Jens
2009-Apr-30, 07:05 AM
When you look up the speed of light its constant speed is always given in a vacuum? Why is this - if the speed of light is a constant, is the mention of a vacuum not then a bit redundant?


Simply, because the speed of light is constant in a vacuum, but is not as fast in other materials. So it slows down when going through water, for example (or air, for that matter).

Htorne
2009-Apr-30, 07:11 AM
Simply, because the speed of light is constant in a vacuum, but is not as fast in other materials. So it slows down when going through water, for example (or air, for that matter).

Thatís what I thought, but then again? Does it really slow down or does it just appears to have slowed down due to refraction / reflections?

To WayneFrancis

The energy explanation was brilliant thanks - it helped a lot :)

slang
2009-Apr-30, 07:50 AM
Thatís what I thought, but then again? Does it really slow down or does it just appears to have slowed down due to refraction / reflections?

Individual photons still travel at c inside a material. However photons interact with (IIRC) the electrons of the atoms, which takes a miniscule amount of time. But since there are a lot of atoms, it adds up to some significant delay. That's how it was explained to me, anyway. :)

astromark
2009-Apr-30, 08:24 AM
Individual photons still travel at c inside a material. However photons interact with (IIRC) the electrons of the atoms, which takes a minuscule amount of time. But since there are a lot of atoms, it adds up to some significant delay. That's how it was explained to me, anyway. :)

If the space your light is transversing is a vacuum then its traveling at c.
Air water or any other combination of molecular mater slows your light photons... reducing the flow... interestingly it, on clearing the medium and back into that vacuum returns to c.
If we could unlock that little trick, Ummm.
and by trick I mean from where does the energy come to accelerate back to c. Not forgetting that light only exists at c. yes, thats right, and now I have confused myself... Cough, Cough, Sneez... oh boy, here we go....its May.
My understanding of this subject is a little unclear so my opinion is just that.

Nick Theodorakis
2009-Apr-30, 02:15 PM
Hi people.

Something has been bothering me for a long time.

When you look up the speed of light its constant speed is always given in a vacuum? Why is this - if the speed of light is a constant, is the mention of a vacuum not then a bit redundant?

I do however suspect that there is a very logic reason for this mention of vacuum, just canít seem to figure it out.

Whatís the explanation?
Thanks :)

Others have talked about the vacuum part, but the significance of c being constant (and why it's different than say, the speed of a baseball thrown in a vacuum) is that it is constant (and always the same value) for any observer in an inertial frame of reference. For example, if you are travelling on a spaceship moving past me at half the speed of light (relative to me) and are shining a flashlight out the front, both you and I would measure the same speed of light for the photons coming from the flashlight.

Nick

phunk
2009-Apr-30, 03:25 PM
If the space your light is transversing is a vacuum then its traveling at c.
Air water or any other combination of molecular mater slows your light photons... reducing the flow... interestingly it, on clearing the medium and back into that vacuum returns to c.
If we could unlock that little trick, Ummm.
and by trick I mean from where does the energy come to accelerate back to c. Not forgetting that light only exists at c. yes, thats right, and now I have confused myself... Cough, Cough, Sneez... oh boy, here we go....its May.
My understanding of this subject is a little unclear so my opinion is just that.

It doesn't ever decelerate really, whenever photons exist they are moving at C. What happens when photons pass through a material is that they run into electrons and get absorbed, then are reemmitted still travelling at C. It's the time that the energy is in a different form than photons (in this case, as kinetic energy of the electron) that makes it take longer to pass through the material.

nokton
2009-Apr-30, 03:41 PM
Simply, because the speed of light is constant in a vacuum, but is not as fast in other materials. So it slows down when going through water, for example (or air, for that matter).
Correct, also glass, curiosity is, why does it then revert to its original speed? I think Albert
avoided this question, it takes energy to accelerate, where is the power source?
Nokton

Hornblower
2009-Apr-30, 04:09 PM
Correct, also glass, curiosity is, why does it then revert to its original speed? I think Albert
avoided this question, it takes energy to accelerate, where is the power source?
Nokton
The incoming light is the energy source. The energy is momentarily tied up in the glass, and is restored to the light in the process of transmission. Some of it is absorbed, so fewer photons come out the other side.

rommel543
2009-Apr-30, 04:51 PM
I thought that the light itself doesn't slow down, rather it just is bumping off of the atoms/molecules and therefor takes longer to get through. Kind of like driving a car. If you set the cruise in your car at 60mph and drove in a straight line you should be 60 miles from your starting point in 1 hour. Now if you started at the same point and drove in curves and circles once in awhile it would take you longer to get to the same point 60miles away. You didn't slow down, it's just your path took longer to get there.

Cougar
2009-Apr-30, 06:31 PM
I thought that the light itself doesn't slow down, rather it just is bumping off of the atoms/molecules and therefor takes longer to get through.

Well, at the quantum level, any "bump" is an interaction...

Jeff Root
2009-Apr-30, 06:52 PM
It isn't really known whether light is "actually" absorbed and re-emitted
in the same direction at each interaction, but that is exactly how it acts.
Light behaves precisely as if it always travels at a constant speed, and
when it runs into an electric charge -- such as while going through the
atoms in the air -- it is very briefly absorbed and then re-emitted. The
length of time between absorption and re-emission determines the index
of refraction for the material the light is passing through. The index of
refraction varies with wavelength, so the numbers you will see given
for various materials are the indices for some standard wavelength.

Light cannot speed up or slow down. It cannot accelerate. It always
moves at the constant speed c. It takes energy to create a photon,
but once created, it moves forever at c without any additional energy.

-- Jeff, in Minneapolis

kleindoofy
2009-Apr-30, 08:10 PM
... interestingly it, on clearing the medium and back into that vacuum returns to c. ...


... why does it then revert to its original speed? ...
This one of the many reasons why I personally think it would be much more advisable to speak of the "state of light" and not of the "speed of light."

The "speed" of light is part of its state of being. Upon leaving a denser medium, it doesn't "speed back up" to c or need any additional energy to do so.

One should define the speed by light's state of being, and not the state as a speed. I.e., light is not fast or have a speed, light just *is* and that state can have a speed as part of its description.

korjik
2009-Apr-30, 11:21 PM
This one of the many reasons why I personally think it would be much more advisable to speak of the "state of light" and not of the "speed of light."

The "speed" of light is part of its state of being. Upon leaving a denser medium, it doesn't "speed back up" to c or need any additional energy to do so.

One should define the speed by light's state of being, and not the state as a speed. I.e., light is not fast or have a speed, light just *is* and that state can have a speed as part of its description.

The speed of light is a speed, as in the magnitude of a velocity. All of those word have specific meaning in physics. State does also, and the speed of light isnt a state.

This also has to do with the differences between classical and quantum based physics. In classical physics, the speed of light in a vaccum is the velocity of an electromagnetic wave. wave interactions treated classicly end up giving you a velocity in a medium that is the speed in a vaccum divided by the index of refraction.

In quantum physics, a photon is a bundle of quantum numbers that travels at the max speed of information, namely the speed of light. Photons are really too simple to have a state of their own. All they are is a bit of energy, momentum, and angular momentum. They can change the state of other things, usually changing the state of an atom.

So, calling the speed of light a state is likely to cause far more problems than calling it a speed.

kleindoofy
2009-May-01, 01:08 AM
@korjik

Thank you for making that clearer for me. Not having advanced education in the physical sciences, I tend to see things from a different perspective.

In calling the physical attributes of light a state, my intention was to move more away from the common notion of "speed," i.e. something that is attained through applied energy and resulting acceleration and which can be increased or reduced by putting more energy into the system or removing it.

When speaking of the "speed" of light, the common sense notion is often "well, why not just go faster?" (e.g. "faster than the speed of light," "warp speed," etc.). Many of the questions asked in this very forum are based on that notion, the notion derived from our daily experience with movement and time.

Thus, in proposing the use of "state" instead of "speed," I was pointing towards a method of describing a basic principle of light which does without using a term that has so many other connotations in normal human understanding.

Understanding that the speed of light isn't *a* speed but *the* speed is one of the hinges in beginning to grasp the concept of relativity, i.e. exchanging the constants of our human experience (space and time) with that which is relative in our daily lives, speed.

Do you see now what I meant?

Jens
2009-May-01, 01:37 AM
Correct, also glass, curiosity is, why does it then revert to its original speed? I think Albert
avoided this question, it takes energy to accelerate, where is the power source?


Don't ask me why, but apparently, photons only exist at the speed of light. They don't need to accelerate.

kleindoofy
2009-May-01, 01:47 AM
@korjik

Quod erat demonstrandum. ;)

korjik
2009-May-01, 03:42 AM
@korjik

Thank you for making that clearer for me. Not having advanced education in the physical sciences, I tend to see things from a different perspective.

In calling the physical attributes of light a state, my intention was to move more away from the common notion of "speed," i.e. something that is attained through applied energy and resulting acceleration and which can be increased or reduced by putting more energy into the system or removing it.

When speaking of the "speed" of light, the common sense notion is often "well, why not just go faster?" (e.g. "faster than the speed of light," "warp speed," etc.). Many of the questions asked in this very forum are based on that notion, the notion derived from our daily experience with movement and time.

Thus, in proposing the use of "state" instead of "speed," I was pointing towards a method of describing a basic principle of light which does without using a term that has so many other connotations in normal human understanding.

Understanding that the speed of light isn't *a* speed but *the* speed is one of the hinges in beginning to grasp the concept of relativity, i.e. exchanging the constants of our human experience (space and time) with that which is relative in our daily lives, speed.

Do you see now what I meant?

Yeah. I was more pointing out that your terminology has already been taken.

Plus, calling it 'the' speed is a better way of putting it than making up some other term for it. c is a speed. Length over time. It comes out in several different places where the units can be just as important as the magnitude. If you solve maxwells equations to get a wave equation, you have a speed term pop out, the gamma factor of special relativity has a c^2 term that indicates that frame speeds are the governing factor. c pops up in QM alot, that is why masses for subatomic particles are expresses in energies instead of masses.

There is nothing inherently wrong with your idea, it just would add a bit of complexity where you dont really need it.