# Initial Acceleration of Photons

1. Aug 9, 2012

### RiddlerA

When an electron and positron collides, they annihilate and produce photons, right?
No massive particle can travel faster than light,
So here is my confusion...
Electrons and Positrons have mass and hence can never reach the Speed of light But
When they collide they create photons. Which means When photons are born, they must have some initial acceleration to reach the speed of light... This means photons can actually travel at a speed lesser than c even in vaccuum? (Assume the collision happens in vaccuum)

Analogy for the above question:
Imagine two race cars(e- & e+) accelerating towards each other and finally collides, and when they does, a tire(photon) is thrown off of the collision. Lets say the tire travels at speed c, but the collision accelerated the tire to reach speed c.. Which means the tire had an initial acceleration..

So My real question is
- Does a photon has initial acceleration?

2. Aug 9, 2012

### Staff: Mentor

How do you get that? That is wrong.

3. Aug 9, 2012

### bobc2

No. You can draw a straight vertical line on a piece of paper, then continue the line at an angle (without curving). You can start a straight line at a 45-degree at some arbitrary point on a piece of paper, then arbitrarily end the line at some other arbitrary point.

We don't really know the details of how the 4-dimensional world line of a photon is created (for us, it is just there), but it seems to initiate at a point in the 4-dimensional continuum, be oriented at a 45-degree angle with respect to an observer's rest frame, and terminate (be absorbed) at some other point.

You have posed an intriguing quesion, RiddlerA.

4. Aug 9, 2012

### ghwellsjr

I think the idea of a photon having an initial acceleration is basically expressing the idea that it must take time for a photon to get going. But I think this is based on the false idea that a photon occupies a point in space, that is, it has no length. However, it does have a length and it does take time for it to be emitted but each part of it is traveling exactly at c.

5. Aug 9, 2012

### RiddlerA

So that means Photons are 4D entities?
And the rest of the matters that has mass are 3D entities?
Does this mean that all energies(especially EMR) are 4D entities?
And without Time(4th D) matter dont possess energies and hence no work can be done by the matter?
So If the 4th D vanishes all of a sudden, everything on the universe will freeze and remain just the way it was during the last moment of the 4th D's disappearnce?

Thank you very much for the compliment.. i really needed that.. :)

6. Aug 9, 2012

### RiddlerA

Im having difficulty in visualizing a photon travelling at c soon after its creation without having any initial acceleration at all...

7. Aug 9, 2012

### HallsofIvy

No, that's not what it means. All objects, with or without mass, exist in both space and time and so are "4D entities".

8. Aug 9, 2012

### Staff: Mentor

Would it help to think of the photon as an EM wave instead of a particle?

9. Aug 9, 2012

### bcrowell

Staff Emeritus
Amplifying on this point, consider the following question:

When a sound wave is emitted from a drum, how does the sound wave accelerate up to the speed of sound?

The answer is basically the same as for light waves. The laws of physics require the sound wave to travel at the speed of sound. As soon as it's created, it's traveling at the speed of sound. It's not a preexisting object like a bullet that needs to be accelerated up to c.

(It's irrelevant that the air is a preexisting object. The air is the medium for the sound wave, but it doesn't move at the speed of sound. It's the wave *pattern* that travels at the speed of sound.)

10. Aug 9, 2012

### bobc2

Yes.

They are also 4-dimensional entities.

No. That's getting off track.

Now you're really crossing the speculation line for this forum (check the forum rules). However, given your IF (which would not normally be considered a fruitful IF on this forum) I suppose your notion might make sense in a way. However, the main problemn has to do with exactly where and how the 4-D universe is to be cut. You seem to be kind of new to special relativity, so you would need to read up on the problem of simultaneity. The "future" beyond some point in time is not so easy to identify--it could be different for different observers. In any case, with a 4-dimensional universe you don't need to eliminate time for the "past" universe to be "frozen." It already is (if you embrace a 4-dimensional universe concept).

By the way, the 4-dimensional universe concept alluded to here is not embraced by many physicists, which probably includes the majority of those on this forum. So, you probably want to sample quite a few responses before forming your ideas about what's going on with photons and what's going on with time.

Last edited: Aug 9, 2012
11. Aug 9, 2012

### Staff: Mentor

If it were not so, then momentum would not be conserved. A photon has to have momentum so that the "parents" momentum is conserved, and it has to go at c to have momentum.

12. Aug 9, 2012

### Staff: Mentor

Let's not create more confusion than we have to. Your description here...

...is perfectly fine, and I, for one, have no objection to it. The simple answer to the questions RiddlerA was asking about "the 4-D stopping" is that he was starting with an incorrect premise--as you and others said, *all* entities are 4-D entities in the sense that matters here. So RiddlerA's further questions were based on a premise (that some entities were "only 3-D entities") that was invalid to begin with; that's all the response that's needed.