# Homework Help: Do photons have to accelerate to the speed of light

1. Oct 27, 2007

### indigojoker

i have just a question that came to my mind. do photons have to accelerate to the speed of light every time a light bulb is turned on?

2. Oct 27, 2007

### saket

good question!

1. when you say, "do photons have to accelerate to the speed of light", what are you assuming the initial speed of the photon?
2. do u know that rest mass of a photon is zero?
3. try to calculate, if you know relevant formula, mass of a photon at any speed other than the speed of light.. and you will surely get the answer.

3. Oct 28, 2007

### indigojoker

1. i dont know the initial speed of the photon, that is part of my question
2. yes
3. photons always has to travel a the speed of light due to special reletivity

basically, how do photons accelerate instantaneously to the speed of light? as in, you are in a dark room... thus no photos (in the visible spectrum). Then you turn on the light... electricity excites the filaments in a bulb and photons are produces, are you saying they are produced at the speed of light?

4. Oct 28, 2007

### saket

well... i am a student only, and i havent done a full-fledged course on relativity and/or quantum physics. i think, if u want to discuss it thoroughly, u shud check in higher levels.. this introductory physics wont help u.
still.. what i think is.. consider here the wave nature of the photon.. it will move with the same speed as soon as it is created! (like sound waves emanating from any source: they move with a constant wave speed, they do not accelerate to that speed.)
and, look at the third part of ur answer.. photons always have to travel at the speed of light! so where comes the question of, "accelerating".

5. Oct 28, 2007

### indigojoker

well, how did they originally get to the speed of light if they did not accelerate to it? or maybe it's instantaneous acceleration... maybe i shouldnt post this in the HW section since this was just a thought...

6. Oct 28, 2007

### saket

think: what was being accelerated? (if u even think that there was an instantaneous acceleration!)
There can be no photon below the speed of light! It's created with the speed of light only.