# The Naked Scientists Forum

### Author Topic: Speed of light at the speed of light?!?!!!  (Read 9582 times)

#### lightarrow

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##### Speed of light at the speed of light?!?!!!
« Reply #25 on: 12/09/2007 17:42:58 »
Whoa!, hold on...I thought that light from the lamp consisted of more than one wave-packet.  Each atom in the tungsten filament (assuming of course that it's an incandescent bulb) emitted a seperate wave packet.  Each one of those is due to an electron losing energy (which then gets converted into a wave-packet).  Those electrons don't take 1.5 seconds to lose that energy so how can one wave-packet be that long?  I can see the stream of wave-packets being that long but each individual wave-packet (which is moving at light speed) should be contracted to a point then as a consequence of SR.
No. The electron generates an EM wave that already travels at the speed of light. Read again my previous post: it's not an object of finite lenght which is then accelerated to light's speed, so you  cannot talk about Lorentz contraction, here.
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To clarify your question lightarrow, I mean, is a photon simply a wave-packet length-contracted to a point as a result of SR?  This makes sense to me but I do not know all there is to know about E&M and as I'm only in highschool, you certainly know more about it than me.  I want to understand the modern viewpoint, which I'm assuming you hold as a modern scientist, and reconcile it with what I see in the equations and explanations I am given in school.  Once I understand physics as it is now, I can begin to understand it in a new way, thus making the jump from scientist to Research Scientist.
Apart your mistake of lenght contraction that I've showed you, there is another, more subtle, mistake: you still want to see a photon as a wave packet. A photon IS NOT a wave packet, nor a particle travelling from source to detector. This is common knowledge among physicists.

Now I add something more, but this is only my idea of a photon; I tell you it, to make you understand why photons are not obvious objects as we usually think:
my idea of a photon is "the quantized interaction between electromagnetic field and detector". Where do you see a moving particle (or wave packet) here?

#### Mr Andrew

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• God was primitive man's attempt at Physics.
##### Speed of light at the speed of light?!?!!!
« Reply #26 on: 12/09/2007 21:51:54 »
Ok, I must have been taught incorrectly in school because I always thought a photon represented the particle nature of light.  So, a photon isn't really a particle, or a wave, it's just the symbolic name for the interaction between light and an electron (the detector) which can only occur if the light has an energy equal to any number of quantized values (energy levels).  Understandable.  And now the big question...Why does light behave both as a particle and a wave?  That's what I've been trying to answer by applying the lorentz contraction to light (which I now understand is inapplicable to light because it would involve changing the wavelength to zero and thus making the energy infinite).

Ok, on to another question...since an oscillating electric charge creates an electric wave, shouldn't an oscillating mass create a gravitic wave?  How does this wave interact with mass?  Is it's absorbtion quantized, similar to a photon's.

#### lightarrow

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##### Speed of light at the speed of light?!?!!!
« Reply #27 on: 13/09/2007 12:14:19 »
Ok, I must have been taught incorrectly in school because I always thought a photon represented the particle nature of light.
But this is indeed true, it's only that "the particle nature of light" has this meaning: when you detect light, its energy comes in packets, as if there was a travelling particle with that discrete energy. What we are sure is this discretization of energy, but it's not correct, in my opinion, to associate it to a real travelling particle; this hasn't been proved yet.
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So, a photon isn't really a particle, or a wave, it's just the symbolic name for the interaction between light and an electron (the detector) which can only occur if the light has an energy equal to any number of quantized values (energy levels).  Understandable.  And now the big question...Why does light behave both as a particle and a wave?  That's what I've been trying to answer by applying the lorentz contraction to light (which I now understand is inapplicable to light because it would involve changing the wavelength to zero and thus making the energy infinite).
Light behaves both as particle and wave depending on the kind of the apparatus you use to measure its properties: if you look at its particle-like properties, it behaves as particle; if you look at its wave-like properties, it behaves as wave. It seems curious, but it's true! Also, shouldn't this fact make you think that light properties are not really objective, but depend on how do you look at them?
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Ok, on to another question...since an oscillating electric charge creates an electric wave, shouldn't an oscillating mass create a gravitic wave?
Yes, only that quadrupole oscillations are needed in that case.
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How does this wave interact with mass?  Is it's absorbtion quantized, similar to a photon's.
It should be (according to logic and intuition), but no one can answer clearly to this question yet, since a quantistic theory of gravitation doesn't exist yet.

#### Mr Andrew

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• God was primitive man's attempt at Physics.
##### Speed of light at the speed of light?!?!!!
« Reply #28 on: 13/09/2007 17:06:42 »
Ah, ok.  Now, suppose that atoms did not absorb light in quantized amounts (had continuous energy bands) then would the light hitting these atoms be percieved as having particle-like properties?

What do you mean by 'quadrupole oscillations?'

#### lightarrow

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##### Speed of light at the speed of light?!?!!!
« Reply #29 on: 13/09/2007 19:56:00 »
Ah, ok.  Now, suppose that atoms did not absorb light in quantized amounts (had continuous energy bands) then would the light hitting these atoms be percieved as having particle-like properties?
Yes, if the light's frequency is high enough to produce Compton scattering. In that effect, light interacts with electrons bumping them off the atom and computations shows that everything can be explained as if light were made of particles. The key-point however is still in the interaction between light and electrons (free or bound in atoms or inside a metal etc.)
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What do you mean by 'quadrupole oscillations?'
Let's make the example with electrostatics, which is more simple. You have many point charges, of both polarities, in a specific region of space. If you want to compute the electric field generated by this group of charges away from it, you can add these terms:
1) the total charge of the group (algebric sum of all the charges)
2) the total dipole moment; it's the fact that a sub-total + charge can be slightly displaced from a sub-total - charge (example: only two charges, one + and one -)
3) the total quadrupole moment; it's a more complex unbalancing of charges
...
etc.

A system of charges can have total charge = 0 but total dipole moment ≠ 0; or charge = dipole moment = 0 but quadrupole moment ≠ 0, and so on.
When we compute the electromagnetic radiation generated by a system of moving charges, we see that we can sum the dipole moment variation, then the quadrupole moment variation...and so on.

The vantage of this procedure is that every term we add to the sum is less than the preceding one, so we can stop our sum up to the desired precision of the result.

General Relativity is much more complex than electrodynamics; it turns out that gravitational radiation (gravitational waves) cannot be produced by dipole moment variations (oscillations) only, as it is in electrodynamics, but they need at least quadrupole oscillations. Don't ask me why, however.
« Last Edit: 13/09/2007 20:01:57 by lightarrow »

#### The Naked Scientists Forum

##### Speed of light at the speed of light?!?!!!
« Reply #29 on: 13/09/2007 19:56:00 »