How do Photons Reach the Speed of Light?

[sk3ptic76]

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?

 

[Rajini]

I guess it is the nature of photons..[but its velocity is very close to c]

 

[arivero]

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

 

[turin]

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]

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]

bapowell's analogy is better ;)

 

[LukeD]

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]

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.

 

[Dale]

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]

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.

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

Yep.

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.

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]

(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]

[...] 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]

thanks guys for the active responses and the great analogies.

 

[DaveC426913]

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]

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]

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]

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]

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?

 

[heldervelez]

Tanks Mr. Turin. I already knew the difficulties you mentioned.
I really hoped that no one take the analogy for granted fact, and used 'only a resoning, an analogy' to advertise. I forgot that the PF have a broad public with diverse degree of education on physics.
I use 'images' to think and use 'patterns' to make associations.
Having a starting point to a path to a solution, even if erroneous, is better than have none.

IMO a more correct answer is like this: The physical space has properties like ('c' ,G,alfa, ...). Motion of photons and matter follow the rules derived from those properties.
What more properties?

 

[LukeD]

This is incorrect.

Nope. I read it in one of Penrose's books.

It's on Wikipedia if you don't believe me: http://en.wikipedia.org/wiki/Zitterbewegung

Basically, a particle spirals around its "classical path" at light speed. Since it's moving in a circle around the path, it moves forward with a much slower speed than the speed of light.

 

[DaveC426913]

Nope. I read it in one of Penrose's books.

It's on Wikipedia if you don't believe me: http://en.wikipedia.org/wiki/Zitterbewegung

Basically, a particle spirals around its "classical path" at light speed. Since it's moving in a circle around the path, it moves forward with a much slower speed than the speed of light.

Well, it may be so, but this does not convince me. I'd say you're interpreting it imaginatively. I'll reserve judgement until an expert weighs in.

 

[lightarrow]

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.

If massless particles would move at speeds less than c, they would have zero energy so you couldn't detect them (and so they probably wouldn't even exist).

 

[turin]

It's on Wikipedia if you don't believe me: http://en.wikipedia.org/wiki/Zitterbewegung

Ah, Zitterbewegung mainly applies to bound states, I believe.

BTW, I love wikipedia, but it is always better not to assume that it is correct without verifying for yourself (either from other sources or from your own derivations). I think that you should only use wikipedia as either a starting point or a reminder, but never use it to determine or justify the correctness of a claim.

In this case I believe that the wikipedia article is correct, but please understand that a wikipedia article can change at any moment. Anyway, You did mention Penrose. I will just say that Zitterbewegung has been around for decades, and it can be found in many texts and papers. I would say that it is somewhat outdated, though.

Basically, a particle spirals around its "classical path" at light speed. Since it's moving in a circle around the path, it moves forward with a much slower speed than the speed of light.

I think that you are confusing "circular frequency" with "circular motion".

If massless particles would move at speeds less than c, they would have zero energy so you couldn't detect them ...

You are presuming that energy is used to detect the particles. The issue is stronger than this (alluded in your parenthetical). Even if detection does not depend on energy, there is another problem. A (classical) massless particle traveling at a speed less than c must be at rest (have zero momentum as well as zero energy). In other words, a (classical) massless particle cannot exist at any speed other than c (as you suggest in your parenthetical):

... (and so they probably wouldn't even exist).

 

[Galap]

As per relativity, all particles are traveling at c in the vector sum of all four dimensions (x,y,z,t). If something is at rest, it is moving at c through time. If it is acellerated through space, the velocity through time gets slower by the same amount. Photons do not travel through time, and therefore must travel at c through space.

The real question is why does mass affect the ability to experience time? Essentially this is what makes time different than the other dimensions.

 

[turin]

As per relativity, all particles are traveling at c in the vector sum of all four dimensions (x,y,z,t).

I believe that you're referring to what is called "proper velocity". I just want to clarify that this is not the kind of velocity that I've been talking about in this thread (and I don't believe anyone else has, either). Rather, I've been talking about "coordinate velocity", which is simply the derivative of the coordinate position of a particle w.r.t. the coordinate time, as measured by some observer.

If something is at rest, it is moving at c through time.

"at rest" means "zero (coordinate) velocity". The speed that something "moves through time" is ambiguous, because speed is a ratio of displacement to elapsed time, so that the speed of this motion becomes a ratio of elapsed times. I would argue that the speed at which an object/particle "moves through time" is, by definition, unity.

If it is acellerated through space, the velocity through time gets slower by the same amount.

This is again ambiguous, but probably incorrect if you mean what I think that you mean, in terms of proper velocity and acceleration.

<br /> du^{\mu}=a^{\mu}d\tau<br />

If the proper velocity is strictly in the time direction such that u^{\mu}=0 unless \mu=0, then the proper acceleration will have no immediate influence on the time component of the proper velocity, because a^{0}=0, and therefore du^{0}=0.

Photons do not travel through time, ...

I agree with this, but only because I claim that nothing travels through time, because traveling through time is an ambiguous concept. However, granting that traveling through time is meaningful, I argue that photons travel through time at the same speed as anything else, namely unity.

The real question is why does mass affect the ability to experience time?

Why do people keep changing the OP's question?

 

[Galap]

This is again ambiguous, but probably incorrect if you mean what I think that you mean, in terms of proper velocity and acceleration.

Is time dilation proportional to acceleration?

 

[DaveC426913]

Is time dilation proportional to acceleration?

No, it is proportional to velocity, more specifically, the square of the velocity.

 

[physixlover]

Again, the better question is: why do some particles not travel at c?

yes,that's a better question ...

 

[pallidin]

I thought this was already cleared-up.
Massless particles ONLY exists in a c state. Those with mass are propagated below c, the value depending on a wide variety of circumstance.

 

[lightarrow]

Nope. I read it in one of Penrose's books.

It's on Wikipedia if you don't believe me: http://en.wikipedia.org/wiki/Zitterbewegung

Basically, a particle spirals around its "classical path" at light speed. Since it's moving in a circle around the path, it moves forward with a much slower speed than the speed of light.

This is just a conjecture, nothing verified experimentally and not even theoretically accepted by the physics community.

 

[lightarrow]

You are presuming that energy is used to detect the particles.

If it's not energy, it's momentum and it would be zero the same (as you rightly say)

The issue is stronger than this (alluded in your parenthetical). Even if detection does not depend on energy, there is another problem. A (classical) massless particle traveling at a speed less than c must be at rest (have zero momentum as well as zero energy).

Don't understand this. Zero momentum doesn't mean "at rest".

In other words, a (classical) massless particle cannot exist at any speed other than c (as you suggest in your parenthetical):

Agreed.

 

[turin]

Is time dilation proportional to acceleration?

Not according to the assumptions of relativity theory. Perhaps you're thinking of gravitational time dilation. This is proportional to the difference in gravitational potential w.r.t. some reference, but not the local gradient. That is Δφ rather than gradφ.

If it's not energy, it's momentum ...

Not so. Charge can be used, for instance.

Zero momentum doesn't mean "at rest".

I guess we'll have to agree to disagree on this one. Would you care to explain?

My explanation is that momentum is the quantity of motion. When the quantity of motion is zero, that means "at rest". Of course, steady-state wave mechanics combined with deBroglie's relation (i.e. quantum mechanics) confounds the notion of rest because it describes (free) particles in terms of plane waves. Perhaps that is your complaint?

 

[lightarrow]

Not so. Charge can be used, for instance.

Ok.

Zero momentum doesn't mean "at rest".

I guess we'll have to agree to disagree on this one. Would you care to explain?

To me "at rest" means velocity = 0. Momentum can be zero even with v not zero.

 

[Naty1]

As some have noted there is no perfect answer to the posters question. right.

Post 24 is an interesting framework to think about speed and distance versus time...That post implies that everything (EVERYTHING) moves through spacetime at c...
massless particles move entirely in space and experience no passage of time; massive particles move in spacetime and so some of their speed is diverted from space to time..

If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, and massive particles move diagonally and are restricted from ever reaching the horizontal...

As a personal observation, I think the ambiguity results from our lack of understanding of exactly what space and time are...how can they transform, rotate, "become" one another as in Lorentz transforms...We accept Lorentz-Fitzgerald's and Einstein's mathematical formulations, because they work so well and have been observationally verified, but we do REALLY understand it completely??...

 

[Galap]

As some have noted there is no perfect answer to the posters question. right.

Post 24 is an interesting framework to think about speed and distance versus time...That post implies that everything (EVERYTHING) moves through spacetime at c...
massless particles move entirely in space and experience no passage of time; massive particles move in spacetime and so some of their speed is diverted from space to time..

If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, and massive particles move diagonally and are restricted from ever reaching the horizontal...

As a personal observation, I think the ambiguity results from our lack of understanding of exactly what space and time are...how can they transform, rotate, "become" one another as in Lorentz transforms...We accept Lorentz-Fitzgerald's and Einstein's mathematical formulations, because they work so well and have been observationally verified, but we do REALLY understand it completely??...

Yes, that is exactly what I was getting at. Personally I think it's a good way to look at it. IMO it makes more sense to use that interpretation. You did a much better job explaining it than me. Hopefully more people will understand it then.

 

[turin]

If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, ...

This is only correct from an interpretive QFT standpoint, but not if you consider photons as classical particles that follow a single, definite trajectory. I have personally done experiments that demonstrate a nonzero time of flight for light. Also, the design of the particle detectors used in the LHC (e.g. CMS) would not work (i.e. would not give meaningful results) if photons (and any other ultrarelativistic particles) had zero time of flight, because the readout of the detector excitationis based on a very accurate timing between the proton bunch collision and the different radii of the detector materials.

To put it in your terms, photons move diagonally with a slope of dy/dx=1/c; dy/dx=0 has been ruled out by countless experiments.

 

[heldervelez]

I read Post 24, and it offers no explanation, no cause. It only offers a math relation.
One can find at page 1506 de l' ACADEMIE DES SCIENCES (5 JUIN 1905) le doc. from M. H. Poincaré : ELECTRICITÉ . - Sur la dynamic de l'electron:
(en francais)
"en supposant que l'electron, déformable et compressible, est soumis à une sorte de pression constante extérieure don't le travail est proportionnel aux variation du volume"
in eng.:
"assuming that the electron, deformable and compressible, is subjected to a kind of constant external pressure whose work is proportional to the volume change"

It seems a 'cause', and one explanation is better than none.

 

[Matterwave]

I thought that usually the light cone is defined by the 45 degree line...(setting appropriate units where c=1). Space like separation is to the left and right of this cone, and time-like separation is top and bottom. In this way, events with space-like separations could not affect each other. I never read that the light cone is defined by the horizontal line...? Wouldn't that imply light moving at infinite speed?

Reference:
http://en.wikipedia.org/wiki/Light_cone

 

[jtbell]

Momentum can be zero even with v not zero.

Example?

 

[lightarrow]

Example?

mass = 0 (we were discussing hypotetical massless particles traveling at speeds less than c).