# How does photon gain its speed in no time?

1. Jan 31, 2008

### mdeng

Photon can be emitted by an electron's changing of its orbit. As we know. photon always travels at speed c, never less. That means, a phone would acquire its speed c (relative to the electron/atom) instantaneously (i.e., in 0 elapse of time). What physical force could have made such a push? On the other hand, if we view light just as EM propagation instead of particles, then such question is moot, there would need to be no push, but some sort of media (e.g., vacuum) to allow the wave to propagate at that speed. Thus, if we view light as "beams of particles", what's the explanation of photon's "apparent" infinite acceleration?

2. Jan 31, 2008

### Mentz114

You seem to think that these particles of light have the same properties as a billiard ball. There is no evidence to suggest that light has a particulate nature between absorbption and emission. Your question is extremely tiresome because you haven't studied the subject and are using simple analogies to try and explain subtle phenomena.

Also, you make wild assumptions, like a 'photon' requiring acceleration, and that this 'acceleration' takes place instantaneously. There is no 'instantaneously' in physics, photons do not require acceleration, and the moon is not made of green cheese.

3. Jan 31, 2008

### HallsofIvy

No, it doesn't! In fact it can't mean that since any acceleration would imply that the photons start with 0 speed. A photon created by an electron is created at the speed of light- there cannot be any acceleration.

4. Jan 31, 2008

### lightarrow

Also, the photon is not created in zero time, as Mentz114 wrote. Typical average times in atomic transitions are 10^-7 - 10^-8 s, and even if there are faster, they however require different than zero intervals of time.

5. Jan 31, 2008

### Antenna Guy

In light of the above, i think it would be more appropriate to ask how quickly an electron must change its' orbit in order to emit a photon.

Regards,

Bill

6. Jan 31, 2008

### lightarrow

I intended exactly that (I wrote "atomic transitions").

7. Jan 31, 2008

### mrandersdk

also it seem like you are using

F = m*a so a = F/m

but for a photon m=0 so already there you can see that your question can't be answered.

8. Jan 31, 2008

### mdeng

I understand this as postulated by SR. I also understand that it will be ill-fated to try to explain a postulate using the theory that is based on it. So, perhaps I should rephrase my question as: is there any competing theory that offers some alternative explanation of this phenomenon?

Last edited: Jan 31, 2008
9. Jan 31, 2008

### mdeng

Thanks for pointing this out. Given that electron's orbit's radius is in the vicinity of 10^-11m, this gives the electron's orbit change a speed of around 10^3-10^4m.

So during this 10^7-10^-8 time interval, how does a "photon embryo" transform to a "full-grown" photon in terms of its speed? Since SR says photon can't have any speed < c, at which *instant* does this "embryo" attain speed c (I use "attain" to set aside the question of "acceleration")? At the very beginning, at the end of its creation, or somewhere in between? Is there any experiment or theory about this creation process? I am all for non-instanteneity (even though QT seems to suggest they have observed instanteneity). However, in this case, wouldn't a photon get its speed at some *moment* (0 time interval) during its birth process, and not any moment before?

IOW, if we could take high speed movie of this birth process of 10^-7 to 10^-8 time period, SR's postulate says that we would see one particular frame where the photon suddenly "matures" in terms of its speed. And in that frame, the photon is flying at speed of c, not before (I am not implying that the photon exists before that frame). But one would ask what it means to observe *speed* in a single movie frame? Perhaps we should look at two frames to talk about speed; and lay no claim about the photon's speed at the first frame where we we first see it. In that sense, the photon's initial speed at the instant of its being deemed a "full grown" photon, is undefined empirically (though we can postulate it). And before that movie frame we would see a "photon embryo" in the making that has 0 (or some other speed < c) speed (unless the embryo must have had the speed c at the instant when it was formed).

Last edited: Jan 31, 2008
10. Jan 31, 2008

### Meatbot

Dude, seriously. What's your problem? Relax. Some people use this forum to "study the subject". Why do you even ask questions yourself then? Shouldn't you have studied it first so that you don't have to ask?

Last edited: Jan 31, 2008
11. Jan 31, 2008

### mdeng

I sort of agree. I understand that from SR's perspective, I should take them for granted. However, I am also fascinated about how the nature pulls off such tricks. Just want to see whether anyone has come up with an answer/postulate/theory about such. Any pointer as to where such questions might be better received? Thanks.

Edit: Maybe Meatbot's question was not directed at me. Oh, well.

Last edited: Jan 31, 2008
12. Jan 31, 2008

### peter0302

At some level, SR is always going to require postulates. The invariance of the speed of light is an observed phenominon and therefore is a logical place to start. One can, of course, call other consequences of SR "postulates" and thereby derive the invariance of the speed of light. But, then we'd just be going in circles.

i.e. take the energy / momentum formula for light, also observed: E=pc

then:

E=ymc^2; p=ymv

ymc^2=ymv*c
v=c

So v=c for any massless particle _always_.

13. Jan 31, 2008

### mdeng

Yes, agree. My original question was not well worded. My question was, as you seem to have pointed out, about an explanation of the postulate of photon's property, not about how SR may explain it which as you have show would be going in circles. I hope to see a deeper understanding about photons that goes beyond what SR can tell us about.

14. Jan 31, 2008

### lightarrow

Sorry but I can't understand anything of what you have written.

The problem, as others have pointed out, is that we know very little of what is a photon in between emission and absorption. If you could say that the photon is the em wave, you could say that it's created during all this time, from the beginning to the end, but unfortunately the photon it's not the em wave!
See what I wrote up.

Last edited: Jan 31, 2008
15. Jan 31, 2008

### mdeng

It's a side track, just to illustrate that the electron's speed wasn't at c.

Thanks for a straight answer. Appreciated.

I think you have got a point here too.You are probably right that they are not being created continuously during all that time, and we know photons are not the same as EM waves. What is the theory about how photons are distributed within and moving along its EM wave field? I guess it's just a probabilistic wave.

16. Jan 31, 2008

### mdeng

Thanks. Could you point me to some readings on the part of photons (or EM, both?) "being liberated and pushed into this state"?

17. Feb 1, 2008

### pervect

Staff Emeritus

I'm not sure where the erroneous idea that "photons accelerate" came from - it seems to be based on something other than a textbook or a peer-reviwed paper, however.

The short easy answer is that photons don't accelerate. In fact, in most interpretations, photons don't even have a definite position.

The longer answer is that one has to understand what a photon is, before one can begin to attempt to address the question of whether or not it accelerates. This would involve a discussion of the interpretations of quantum mechanics, of which there are many - and such a discussion does not belong in the relativity forum.

Due to the fact that the thread is off-topic, and has other problems as well, I'm locking it.

There is one interesting but subtle point that I think is worth mentioning. The Schrodinger equation, as taught in most elementary courses on QM, is *not* compatible with SR, because it's not Lorentz invariant. One needs to use either the Dirac or the Klein-Gordon equation, instead, to have a fully relativistic treatment of QM.

Don't blame QM for not being relativistic, when the problem is not with QM, but rather with the Schrodinger equation.

The Schrodinger equation is quite useful, however, because the relativistic corrections are small.