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What Propels Photons to c?

by sk3ptic76
Tags: photons, propels
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sk3ptic76
#1
Feb4-10, 01:21 PM
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I am a student currently doing astrophyics as a independent study at a high school. My question is related to photon "birthing". When an atom drops from a high energy to a low energy state it releases photons. When these photons are "created" what propels them to c? I am quite aware that photons do not accelerate as they do not have mass. But what allows photons to achieve that instantaneous speed of light with no acceleration and break out of the electron cloud of the atom?
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Rajini
#2
Feb4-10, 01:37 PM
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I guess it is the nature of photons..[but its velocity is very close to c]
arivero
#3
Feb4-10, 03:01 PM
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Wrong question. Read Lucretius, he already notices it. The problem is what makes all the other matter to go slower.

turin
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Feb4-10, 07:10 PM
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What Propels Photons to c?

A photon is an excited mode of electromagnetism. As a very simplified analogy, think of the electromagnetic field as a violin string, the excited atom as the string being pulled on, and the emission of the photon as the release of the string. Don't read too much into that analogy, though. The idea is that the photon is a quantity of disturbance (as are all particles, supposedly); it is not a literal tiny dot that flies out of the atom.
bapowell
#5
Feb4-10, 08:01 PM
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Turin's analogy is good. Just like throwing a rock into a pond, the disturbance generates ripples that instantaneously move away from the point of impact at a certain speed. If you think of light simply as a disturbance, you should have no problem with such a disturbance instantaneously achieving its speed of propagation -- the ripples in the water don't need to 'speed up'. This is one of those cases where it's more helpful to think of light as a classical wave than as a particle, even though it *is* really a particle.
turin
#6
Feb5-10, 01:44 PM
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bapowell's analogy is better ;)
LukeD
#7
Feb5-10, 03:34 PM
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From what I understand, you can understand all particles as always traveling at c. However, massive particles are constantly backtracking and zigzaging along their path. Their path on average goes slower than c. (By the way, when I say "travel at c", I really mean "on average travel at c" because we actually have a wave function that describes the velocity). A massless particle cannot zigzag along its path because the frequency of oscillation between going in one direction and the other is proportional to the mass.
turin
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Feb5-10, 06:24 PM
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Quote Quote by LukeD View Post
From what I understand, you can understand all particles as always traveling at c. However, massive particles are constantly backtracking and zigzaging along their path. Their path on average goes slower than c.
Maybe. A picture that I infer from QFT is slightly different. A particle travels along every possible path at every possible speed, including speeds greater than c all the way up to infinite speed, simultaneously. Most of these paths interfere with each other destructively. The paths and speeds that are consistent with the endpoints of the path and the mass of the particle interfere constructively.
DaleSpam
#9
Feb5-10, 07:07 PM
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Quote Quote by sk3ptic76 View Post
But what allows photons to achieve that instantaneous speed of light with no acceleration and break out of the electron cloud of the atom?
I would say conservation of energy and momentum.
DaveC426913
#10
Feb5-10, 07:16 PM
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Quote Quote by sk3ptic76 View Post
When these photons are "created" what propels them to c? I am quite aware that photons do not accelerate as they do not have mass. But what allows photons to achieve that instantaneous speed of light with no acceleration and break out of the electron cloud of the atom?
The fact that they are massless means that their inertia is zero.

Take any massive particle and reduce its mass to zero and it will fly off at the speed of light.

Quote Quote by arivero View Post
Wrong question. Read Lucretius, he already notices it. The problem is what makes all the other matter to go slower.
Yep.

Quote Quote by turin View Post
Maybe. A picture that I infer from QFT is slightly different. A particle travels along every possible path at every possible speed, including speeds greater than c all the way up to infinite speed, simultaneously. Most of these paths interfere with each other destructively. The paths and speeds that are consistent with the endpoints of the path and the mass of the particle interfere constructively.
Greater than c? Infinite speed? I would like to see a reference for this.


Quote Quote by LukeD View Post
From what I understand, you can understand all particles as always traveling at c. However, massive particles are constantly backtracking and zigzaging along their path.
This is incorrect.
Per Oni
#11
Feb6-10, 03:39 PM
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Quote Quote by LukeD View Post
(By the way, when I say "travel at c", I really mean "on average travel at c" because we actually have a wave function that describes the velocity).
"On average travel at c" means sometimes below and sometimes above c. Where is the evidence for v>c?
Rasalhague
#12
Feb6-10, 04:43 PM
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Quote Quote by LukeD View Post
[...] you can understand all particles as always traveling at c. However, massive particles are constantly backtracking and zigzaging along their path. Their path on average goes slower than c. (By the way, when I say "travel at c", I really mean "on average travel at c" [...]
This seems like a contradiction: the average speed of each particle is always c, but the average speed of a massive particle, as well as being c, is also less than c.
sk3ptic76
#13
Feb8-10, 09:03 AM
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thanks guys for the active responses and the great analogies.
DaveC426913
#14
Feb8-10, 09:12 AM
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Quote Quote by Rasalhague View Post
This seems like a contradiction: the average speed of each particle is always c, but the average speed of a massive particle, as well as being c, is also less than c.
Don't worry about the contradiction; the statement's just plain wrong.
pallidin
#15
Feb8-10, 04:25 PM
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Why does a massless "particle" exist only in a state of c propagation?
Well, I'm not sure if anyone really knows. It just does.
DaveC426913
#16
Feb8-10, 07:41 PM
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Quote Quote by pallidin View Post
Why does a massless "particle" exist only in a state of c propagation?
Well, I'm not sure if anyone really knows. It just does.
Again, the better question is: why do some particles not travel at c?
heldervelez
#17
Feb11-10, 06:33 PM
P: 253
Originally Posted by arivero (my bold):

" Wrong question. Read Lucretius, he already notices it. The problem is what makes all the other matter to go slower."

this is not an answer, and I can not say if there is or not some reference (formal or informal) elsewhere.
It is only a resoning, an analogy.
yesterday I was searching for 'terminal velocity' (TV), used for instance to calculate the theoretical max velocity an object can get when moving thru a fluid. Example 'para-chute'.

the (TV) is dependent on several factors (see WP 'terminal velocity', and follow links) including (in a gravitational field like at Earth): Mass, effective area, ...
the area seems irrelevant here but the variation with mass is compelling.
start with some large parachute and a small object-> the TV is low.
accelerate the object-> more kinetic energy->more mass-> more (TV)
accelerate again the object-> more kinetic energy->more mass-> yet more (TV).

elaborating this toy model may be nice.
turin
#18
Feb11-10, 06:54 PM
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Quote Quote by heldervelez View Post
Originally Posted by arivero (my bold):
what makes all the other matter to go slower."

this is ... only a resoning, an analogy.
...
'terminal velocity' (TV), used for instance to calculate the theoretical max velocity an object can get when moving thru a fluid. Example 'para-chute'.
...
elaborating this toy model may be nice.
I don't believe that this "analogy" is applicable. 2000 years ago, Aristotle believed that a state of motion requires force. However, in our modern conception of physics (i.e. what these forums promote), motion does not require force.

The terminal velocity results from the balance of two forces acting on an object. If not for some external force (e.g. gravity acting on the skydiver), an object would come to rest with respect to the fluid that it is in, due to the drag force of that fluid. This is perhaps what lead Aristotle to his belief. However, in "empty" space, there is (presumably) no drag force (although there is a supposedly a GZK limit).

What is the terminal velocity of a parachute in vacuum, on the Moon, say?


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