Does light accelerate to its speed or is it instant?

In summary, the question is whether light accelerates to its top speed or if its acceleration is always at 0, assuming a vacuum environment. The consensus is that photons always move at speed c and there is no acceleration. The concept of a photon and its speed is not clearly defined, but can be established experimentally. This can be done by measuring the speed of a particle in a pulse and timing between two detection events at different places.
  • #1
Magnus Warhol
4
0
Someone actually asked me an intersting question. Does light accelerate to its top speed or is its acceleration at 0 and is always at the top speed. Please assume we are in a vacuum, and you slam two protons together to release light, does that light start of at max speed or does it accelerate to top speed? Is there such a thing as Planck Acceleration?
 
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  • #2
Photons always move at Speed c in a vacuum... there is no acceleration. Newton's law doesn't do well solving for a if m=0

a=F/m
 
  • #3
I'll second that - photons are created traveling at c.
 
  • #4
First of all, the quantized EM field is not constituent of individual systems, photons, although we do measure them as such they do not exist as such prior to our measurement in the interim.

To picture a photon you have to understand the field excitation process that results in a quantized version of it, and when you understand this (which I don't completely) then apparently you can envision how it is always at light speed.
 
  • #5
To be honest, AFAIK there is no clear definition of a "photon" anyway, nor of its "speed". Is it any state of the photon field with number operator eigenvalue 1?
Is it an excitation with a definite wavelength?
If you have a wavepacket, where and how exactly would you measure its group velocity?
 
  • #6
AFAIK there is no clear definition of a "photon" anyway, nor of its "speed".
:eek: Resolve the electromagnetic field into normal modes. A photon represents the first excited state of one of those normal modes. The normal modes aren't unique, you're free to choose to use any set you like. Mosttimes we use the plane waves, which possesses a well-defined wave vector k, and they clearly travel at speed c.

Another common choice is the set of angular momentum states wrt a particular origin. Nuclear gamma decay is usually expressed as the emission of a photon which is E1, M2, etc according to its L and J values. These states don't have a unique wave vector, they are linear superpositions of such.

In any case photons in free space obey the wave equation and have speed c in that sense.
 
  • #7
What Bill_K said. We need to have a clear definition of what "photon" means if we are to be able to say we have detected them.

Thus, what a photon is and how fast it goes can be established experimentally.
For instance, you can measure the speed of a particle in a pulse by timing between two detection events at different places.
What I think you mean is that we don't time a particular photon so what we get at the finish-line detector are different photons from the start-line photons.
We have to find the speed indirectly. Would that be correct?
 
  • #8
@Bill_K
If this is your definition of a photon, first of all a single photon is not normalisable in space and the speed would be the phase velocity, not the group velocity.
This does not fit the original question, I think, because there it is assumed that the photon is created somewhere in space (implying that it is a wave packet).
 

Related to Does light accelerate to its speed or is it instant?

1. Does light have a maximum speed?

Yes, light has a maximum speed which is known as the speed of light. In a vacuum, this speed is approximately 299,792,458 meters per second.

2. Does light accelerate to its maximum speed?

No, light does not accelerate as it already travels at its maximum speed. This is due to the fact that light has no mass and therefore cannot accelerate.

3. Is the speed of light constant?

Yes, the speed of light is constant in a vacuum. This means that it does not change regardless of the observer's frame of reference or the speed of the source emitting the light.

4. Does light take time to reach its maximum speed?

No, light does not take any time to reach its maximum speed. As soon as it is emitted, it is already traveling at the speed of light.

5. How is the speed of light measured?

The speed of light is measured using various methods, such as using a laser interferometer or timing the speed of light pulses in a vacuum. It can also be calculated using the relationship between the electric and magnetic fields in light waves.

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