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peter eldergill
2007-Oct-28, 02:53 PM
I'm just wondering two things. Suppose a photon is emitted from a source

1. Was the photon "already there", as in ejected by the source, or is it somehow "created"

2. Does it accelerate to the speed of light from rest or is it somehow already at the speed of light as soon as it is emitted

Pete

trinitree88
2007-Oct-28, 03:58 PM
I'm just wondering two things. Suppose a photon is emitted from a source

1. Was the photon "already there", as in ejected by the source, or is it somehow "created"

2. Does it accelerate to the speed of light from rest or is it somehow already at the speed of light as soon as it is emitted

Pete

Peter. Photons, carrier of the electromagnetic force, are born at c, travel at c and die at c, never varying from it. The value of c, however, may depend upon the medium in which the photon travels....Snell's Law stuff. Pete.

Kaptain K
2007-Oct-28, 04:25 PM
The value of c, however, may depend upon the medium in which the photon travels....Snell's Law stuff. Pete.
"c" is defined as the speed of light in vacuo. The speed of light in a medium is not "c".

Len Moran
2007-Oct-28, 04:53 PM
Peter. Photons, carrier of the electromagnetic force, are born at c, travel at c and die at c, never varying from it. The value of c, however, may depend upon the medium in which the photon travels....Snell's Law stuff. Pete.

I'm not sure we can say a photon is "born" at c in the way we can say light seems to travel at c. We can certainly model it as such, but we can never test the model. Quantum uncertainty would never allow any finite acceleration of a generated photon to be measured, so I would say the question is not something experimental physics will ever be able to answer.

peter eldergill
2007-Oct-28, 05:05 PM
After I asked the question I thought of thinking of a photon as a wave rather than a particle, and I can see a wave being "born" at c (I am aware that the speed of light varies depending on the medium)

If light was thought of as a particle (even massless), woudn't it start from v=0 and accelerate to c? I think this is where I'm a bit confused. I've not studied any quantum mechanics, and I only have a superficial knowledge of the subject (Heisenberg, plank's constant and so forth)

Pete

peter eldergill
2007-Oct-28, 05:11 PM
"c" is defined as the speed of light in vacuo. The speed of light in a medium is not "c".

What about Einstein's equations then, which involve 1/sqrt(1-(v/c)^2)?

Is the "c" supposed to be the speed of light in a particular medium? If not, then i suppose you could come up with some contradictions. I'm on the assumption that when people use the symbol "c", they are (perhaps unknowingly) referring to the speed of light in that particular medium. and not to the absolute vacuum

Pete

Fortunate
2007-Oct-28, 05:13 PM
Suppose light travels through a vacuum and then enters a "transparent" medium with an index of refraction different from 1. The light certainly "slows down." Do we say it decelerates? I suspect quantum effects are involved in the transition, but I need some help here.

peter eldergill
2007-Oct-28, 05:16 PM
I think that is basically the same thought that I had, but certainly worded more clearly and easier to understand as an example! Thanks

Pete

Jeff Root
2007-Oct-28, 07:44 PM
The letter 'c' is the symbol for the fundamental constant in Maxwell's
theory of electromagnetism. As I understand it, 'c' is the speed of
light in any medium. My understanding is not necessarily the same
as that of others.

However, light can be considered as massless, particle-like photons
which travel always at a constant speed between interactions with
electric charges. In this way of considering light, the photons always
move at the speed of light in vacuum, while a light beam traveling
through a medium is slowed by interactions with electrons in the
medium. When a photon interacts with an electron (or other charged
particle), it is absorbed and then re-emitted. If nearly all the emitted
photons travel in the same direction as the absorbed photons did,
the material is transparent. If they go in random directions, the
material is translucent. The time delay between absorption and
emission determines the speed of light in the material, and the index
of refraction.

In this way of describing the behavior of light, the photons do not
accelerate or decelerate. The photons travel at their characteristic
speed from the moment they are emitted until the moment they are
absorbed. My own personal view is that the emission and absorption
events take some time to occur, because the forward end of the
photon is emitted before the back end is emitted. However, it is not
possible to detect a portion of a photon. A photon is either detected
or not detected, so the duration of emissions and absorptions
cannot be measured. It appears just as if the events occurred
instantaneously.

-- Jeff, in Minneapolis

Fortunate
2007-Oct-28, 07:53 PM
Thanks, Jeff.

Noclevername
2007-Oct-28, 07:57 PM
I'm just wondering two things. Suppose a photon is emitted from a source

1. Was the photon "already there", as in ejected by the source, or is it somehow "created"

Photons are energy, and when they are emitted (say, by a hot object) it's because the object is losing or "leaking" energy. They aren't created, just converted from one form of energy to another. In this case, from molecular motion to radiant heat.

EvilEye
2007-Oct-28, 08:37 PM
One more thought. Shoot me if I am wrong.

c is c from the persepective of the observer.

But the universe is also expanding at c (or near it)

So I would guess that the speed of light is c+x (x being expansion), but c+x is useless, because we are expanding with it.

peter eldergill
2007-Oct-28, 08:49 PM
When a photon interacts with an electron (or other charged
particle), it is absorbed and then re-emitted. If nearly all the emitted
photons travel in the same direction as the absorbed photons did,
the material is transparent. If they go in random directions, the
material is translucent. The time delay between absorption and
emission determines the speed of light in the material, and the index
of refraction.

My bold.

I have taught basic properties of light to grade 11 physics (and will again second semester) and have never known an explanation for the different speeds (only that there are different speeds). I've told the students that I don't know much about quantum mechanics and just think about it as "optical density"

It was a running joke in my physics class that any answer to a a question that I couldn't explain was always "quantum mechanics"

However, it is not
possible to detect a portion of a photon

I'm guessing that's Heisenberg again?

Pete

sirius0
2007-Oct-29, 03:31 AM
Excellent question!
Sticking though with the idea of light passing from one medium to another; does it accelerate (decelerate) as it crosses the interface?
Maybe an analogy with sound will help.

The speed of sound is found by v=sqrt(K/rho).
Where v is the speed, K is the appropiate modulus. Say bulk modulus for a liquid and rho is the density of the medium. When sound passes from one medium to another there is a change of speed that matches the change of modulus and density. this change is as 'sharp' as the edge of the respective materials. The equation for light speed is not all that different when you look at it http://upload.wikimedia.org/math/5/7/4/574e79fcd783f402f69907b6acbf7cd1.png.Light. when it goes from say vacuum to glass will imediately respond to the permistivity and permeabillity of the glass and sloww down imediately. Yes it is a good idea I think, to be thinking in terms of waves for this one.

speedfreek
2007-Oct-31, 07:15 PM
One more thought. Shoot me if I am wrong.

c is c from the persepective of the observer.

But the universe is also expanding at c (or near it)

So I would guess that the speed of light is c+x (x being expansion), but c+x is useless, because we are expanding with it.

The universe is only apparently expanding at c at a certain distance - something under 10 billion light years (the Hubble limit). Anything closer than that is apparently receding at less than c, and anything further away than that is apparently receding at more than c.

I do wish people would stop saying the universe is expanding at c without qualifying the statement as it can be very misleading. At close distances it is hardly expanding at all, whereas at the furthest distances it is apparently expanding at many multiples of c.

The universe is theorised to be expanding at something between 71(km/s)/Mpc and 77(km/s)/Mpc