# Why does light move?

hockeyguy
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Topic 194678

Why does light move, and where does it get the energy required to move at such speeds?

tullio
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### Why does light move?

Amor che muove il Sole e le altre stelle (Dante)

ErichZann
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### RE: Why does light move,

Quote:
Why does light move, and where does it get the energy required to move at such speeds?

Well, since Photons dont have any mass their speed does not have anything to do with their energy.

Bikeman (Heinz-...
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### ...but with the wavelength

...but with the wavelength (color).

The energy of a single "quantum of light", a photon, is

E= h*c/lambda

where h is the the Planck Constant, c the speed of light and lambda is the wave length of the light in question.

So where does the energy come from? It depends on what caused the light to be radiated. E.g. if you are looking at the light that is created by hot bodies, that energy that is "in the light" is taken away from the hot body: it cools down by radiating away light. If you are looking at the light from a neon light, the energy comes from the electricity that is ionizing the gas inside the tube, and so on.

CU
Bikeman

Fred J. Verster
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### RE: ...but with the

Message 96042 in response to message 96041

Quote:

...but with the wavelength (color).

The energy of a single "quantum of light", a photon, is

E= h*c/lambda

where h is the the Planck Constant, c the speed of light and lambda is the wave length of the light in question.

So where does the energy come from? It depends on what caused the light to be radiated. E.g. if you are looking at the light that is created by hot bodies, that energy that is "in the light" is taken away from the hot body: it cools down by radiating away light. If you are looking at the light from a neon light, the energy comes from the electricity that is ionizing the gas inside the tube, and so on.

CU
Bikeman

Hi, an other example is Light Amplification by Stimulated Emission of Radiation, L.A.S.E.R.
In this 'case' , the 'light' has an 'amplitude', in only one direction, in stead of multiple.
To make it 'clear', you'll have to make a drawing, an axe (point: you look along the axe), normal light 'swings all over the place' , with LASER it's swinging in 1 direction.

Fred \|/

Mike Hewson
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### RE: Why does light move,

Quote:
Why does light move, and where does it get the energy required to move at such speeds?

I guess it just is that way. The universe has these qualities :

- you can't stand perfectly still ( quantum mechanics )

- if you have no rest mass you can't go slower than light ( relativity )

- but if you have a rest mass then your speed can vary. But it requires energy exchanges to achieve that. Energy in -> the speed goes up, energy out -> the speed goes down.

Quote:

NB. If you can't stop how can you have a rest mass? Well you study the behaviour - it's inertia or resistance to momentum change - over a range of velocities for a particle then you can extrapolate to zero velocity even if you don't achieve that. The rest mass is given by the intercept on the energy axis at zero momentum ( as shown M > m ) :

light is represented on this graph as the red straight lines from the origin to the left and right below ( at any given energy, particles with mass go slower than light speed ) the mass curves shown ( E = pc for photons, so the slope of the E vs p graph is c. However the E = 0 and p = 0 point is excluded ).

For particles with rest mass m :

E^2 = p^2 * c^2 + m^2 * c^4

which is always greater than that given by E = pc ( or E^2 = p^2 * c^2 if you like ) if m > 0. For neutrinos where m is really small then the curve snuggles up really close to the light cone - that is, they travel just under the speed of light. So we expect them to arrive very shortly after light ( or a gravity wave even ) if emitted simultaneously from a supernovae, say.

{ image adapted from Martinus Veltman's Facts and Mysteries in Elementary Particle Physics - a great book you all should buy! :-) }

For photons then, I like to think of them as being created and destroyed. So a photon doesn't start from zero velocity and then rev up to light speed. It just doesn't exist, then it suddenly does and scooting along at light speed. When captured by something it just ceases to exist. But it takes energy to make one, in proportion to it's frequency, and that energy is returned upon destruction to whatever it hit.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

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### RE: For photons then, I

Message 96044 in response to message 96043

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For photons then, I like to think of them as being created and destroyed. So a photon doesn't start from zero velocity and then rev up to light speed. It just doesn't exist, then it suddenly does and scooting along at light speed. [â€¦]

What about when light is apparently slowed or refracted in a non-vacuum? Is the former effect caused by the cumulative â€˜latencyâ€™ in successive absorption & re-emission events? How is the latter explained in the particle model?

Mike Hewson
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### RE: RE: For photons then,

Message 96045 in response to message 96044

Quote:
Quote:
For photons then, I like to think of them as being created and destroyed. So a photon doesn't start from zero velocity and then rev up to light speed. It just doesn't exist, then it suddenly does and scooting along at light speed. [â€¦]

What about when light is apparently slowed or refracted in a non-vacuum? Is the former effect caused by the cumulative â€˜latencyâ€™ in successive absorption & re-emission events?

Yup.

Quote:
How is the latter explained in the particle model?

Poorly, which is why early models ( say before Huygen's ) had the wavefront/beam moving away from the normal to the interface, when going into a denser material.

It's only a melding of 'particle' + 'wave' ideas that gives the good answers. My preference is to call light by neither label. Just call it light and accept that historically it was described with disparate models demonstrating different aspects. It's quantum mechanics, of course, that unites the behaviours consistently. However that requires enjoining probability, uncertainty and whatnot ..... so light arrives in lumps but with a distribution in time/space determined by ( statistical ) summations of ( complex number valued ) functions.

Cheers, Mike.

( edit ) I probably ought give a better answer than 'yup', but beware it can be hard to describe accurately in words without the math, or some neat diagram ..... :-)

In vacuum - with the quantum electrodynamic ( QED ) summation of possible paths between point A and point B - the paths off the 'straight line' b/w A and B give contributions that readily negate ( in the summation ) other off-straight-line paths. So the most probable actual path is that straight line.

Now in the presence of a material the interaction with the material causes 'amplification' of phase contributions from off the straight line axis, relative to on-axis. In effect the most probable path is still a straight line but with a greater input into the integration from more delayed phase components - as the more off-straight-line you are the longer the path and the greater the phase change you will roll through by going that way. So you still get to point B, just later than you would have done in the absence of the material. And generally the more material there is density wise ( electrons specifically ), and the more it interacts, then the greater delay is induced.

NB : the integration I speak of is using complex numbers ( square root of minus one and all that ), so the result/integrand yields both a real number amplitude and a phase value.

At an interface b/w material types, say going from vacuum into glass - then with point A in vacuum and point B in the glass - the paths to be integrated that have a greater within-glass passage will have greater phase delay than those with more-in-vacuo portions. The effect over all point B's with the same phase values ( a wavefront ) is that those in the glass are closer to the interface than those in vacuo. This turns the wavefront towards the perpendicular to the interface.

The paths to be considered through a material can be modeled as 'in effect' successive absorptions/re-emissions OR a single uninterrupted photon that just took longer to turn up. To a certain extent the cognitive difficulty here is the difference between an 'actual' photon ( that was/were detected ) vs virtual ones : but this explanation is already deep enough I think.

( edit ) In this thread I go on a fair deal more about light propagation, phases etc ....

( edit ) Here is some archived streamable lectures by the one and only Richard Feynman who gives a terrific, entertaining and explicable tour of the highlights of QED. View them in their listed order though ....

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal