# The Naked Scientists Forum

### Author Topic: How do photons accelerate to the speed of light?  (Read 3170 times)

#### thedoc

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##### How do photons accelerate to the speed of light?
« on: 02/08/2012 05:30:02 »
Shee Hong  asked the Naked Scientists:

Hi Dr Chris,

When photons are created, how are they accelerated to the speed of light?

You have mentioned that when the light hits a lens, it slows down and through the other side of the lens, it speeds up again. I was just wondering what propels this motion. Doesn't Newton's second law of motion exist here? Is it because photons have no mass?

Appreciate if you could help me answer this one.

Cheers,
Hong Shee

What do you think?
« Last Edit: 02/08/2012 05:30:02 by _system »

#### Soul Surfer

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##### Re: How do photons accelerate to the speed of light?
« Reply #1 on: 02/08/2012 09:38:33 »
like all wave motions, sound, water etc no true accelerations are involved because the "medium" is not actually moving with the wave.  The wave flow is an energy flow through the medium in the form of a periodic disturbance so any motion is purely oscillatory with energy flowing locally backwards and forwards but propagating at the wave velocity as it does so.

In the case of electromagnetic radiation, this included  radio waves, microwaves, infra red, visible light, ultra violet X rays and gamma rays the disturbance is in electromagnetic fields which are normally zero in the absence of anything.

Let me explain.  To create an electric field charge has to flow.   moving charge is an electric current and this creates a magnetic field, a changing magnetic field creates an electric field and so the process is cyclic once some energy is inserted into the system.

Like everything else in the universe it is dynamic it is the changing fields that create the energy flows which are all oscillatory.  It is important to remember that even matter is a wave that is localised, a bit like a vortex in water or a smoke ring in the air.

The behaviour of waves as you say depends on the velocity of the waves through the medium the presence of large numbers of atoms impede the flow of the electromagnetic wave by changing the amounts of charge and field that need to flow to create the fields and charges respectively at propagate the wave so all that happens is the wave morion locally slows down as the radiation enters the medium.

#### Pmb

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##### Re: How do photons accelerate to the speed of light?
« Reply #2 on: 02/08/2012 21:30:20 »
When photons are created, how are they accelerated to the speed of light?
They aren't accelerated. They are created already moving at the speed of light.

You have mentioned that when the light hits a lens, it slows down and through the other side of the lens, it speeds up again.
That's a misleading way to look at it. Photons are constantly being created and destroyed in an optical medium so that it appears as if the speed of light is less than c.

#### Pmb

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• Physicist
##### Re: How do photons accelerate to the speed of light?
« Reply #3 on: 02/08/2012 21:33:02 »
Let me explain.  To create an electric field charge has to flow.
What do you mean by this? Do you mean that for a field to exsist there has to be net charge present and for that to happen the net charge has to come from some place?

#### Soul Surfer

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##### Re: How do photons accelerate to the speed of light?
« Reply #4 on: 03/08/2012 00:46:10 »
Yes, read through the full description it comes from the changing magnetic field.  it is true that for photons to be generated charge has to be moving and this is electrons moving around in an atom for light or alternating voltages being applied to an aerial wire for radio waves.

These classical descriptions are perfectly valid even in the quantum case it is just when quantum rules are applied there is a minimum and a graininess on precisely how much energy is released to propagate.

The problem is that people get these hangups about particles and quanta.  The classical theories about how things behave underlie all quantum theory.   Quantum theory only limits what we can see and measure.

People use Dirac notation to describe quantum field theory,  this is the basic accepted equations of quantum mechanics. It is by far the most precise scientific theory we have of anything.  Dirac notation is only a shorthand way of expressing a series of classical continuous infinite integrals over all of space and time.  Being old I had to do it the old fashioned way and work out the integrals to start with, but most modern text books dive in with their "bras" and "kets" without explaining properly what they actually are at a deep mathematical level and most modern students do not learn about the classical continuous roots of quantum theory.

Let me explain the concept of the background maths in a much more simple way using something that you are all familiar with that is pi the ratio of the diameter to the circumference of a circle.  This number cannot be written down precisely but there are several sorts of infinite series and or integrals that can describe this number absolutely precisely with a modest limited number of terms we could use any of these to express this number but it is far simpler to use a single greek letter to do this.

« Last Edit: 03/08/2012 01:00:19 by Soul Surfer »

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##### Re: How do photons accelerate to the speed of light?
« Reply #4 on: 03/08/2012 00:46:10 »