# What Propels Photons to c?

1. Feb 4, 2010

### 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?

2. Feb 4, 2010

### Rajini

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

3. Feb 4, 2010

### arivero

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

4. Feb 4, 2010

### 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.

5. Feb 4, 2010

### 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.

6. Feb 5, 2010

### turin

bapowell's analogy is better ;)

7. Feb 5, 2010

### 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.

8. Feb 5, 2010

### turin

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.

9. Feb 5, 2010

### Staff: Mentor

I would say conservation of energy and momentum.

10. Feb 5, 2010

### DaveC426913

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.

Yep.

Greater than c? Infinite speed? :uhh: I would like to see a reference for this.

This is incorrect.

11. Feb 6, 2010

### Per Oni

"On average travel at c" means sometimes below and sometimes above c. Where is the evidence for v>c?

12. Feb 6, 2010

### Rasalhague

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.

13. Feb 8, 2010

### sk3ptic76

thanks guys for the active responses and the great analogies.

14. Feb 8, 2010

### DaveC426913

15. Feb 8, 2010

### 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.

16. Feb 8, 2010

### DaveC426913

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

17. Feb 11, 2010

### 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.

Last edited: Feb 11, 2010
18. Feb 11, 2010

### turin

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?

Last edited: Feb 11, 2010
19. Feb 11, 2010

### 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?

20. Feb 11, 2010

### LukeD

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.

21. Feb 12, 2010

### DaveC426913

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.

22. Feb 12, 2010

### lightarrow

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).

23. Feb 12, 2010

### turin

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.

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

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 travelling 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):

Last edited: Feb 12, 2010
24. Feb 12, 2010

### 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.

25. Feb 12, 2010

### turin

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.

"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.

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

$$du^{\mu}=a^{\mu}d\tau$$

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 $du^{0}=0$.

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

Why do people keep changing the OP's question?