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Why does light travel at the speed of light?


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Moderator Note

petermartin, please note that any discussion of your theory should take place in the Speculations forum, and is subject to its rules. This is a physics board, and is limited to discussion of accepted physics.




There is a speed at which massless particles travel. Photons are massless, so they travel at this speed.
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But why? When that photon, be it massless particle or wave, is liberated from an atom why does it race off at the cosmic speed limit? Is there not a mathematical formula for an electromagnetic wave which can be used to calculate the speed at which it propogates through space?

 

Sorry Swansont, only just noticed my 'telling off'! Can you move this to its correct pleace?

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Is there not a mathematical formula for an electromagnetic wave which can be used to calculate the speed at which it propogates through space?

Sure is. The dispersion relation (the relation between energy E and momentum p) for the free electromagnetic field is [math]E = pc[/math], which is a simplification of the more general dispersion relation for free fields ([math]E^2 = m^2 c^4 + p^2 c^2[/math]) resulting from a mass-like term that is zero (m=0). From that, you directly get a group and phase velocities [math] \frac{\partial E}{\partial p} = \frac Ep = c[/math] for wave packets of the free electromagnetic field.

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I agree, (I've always struggled with massless particles having momentum) but are these equasions not back engineered? They certianly explain the relationships but not the reason, that is, they do not explain why the photons leaving your display are so repulsed by the atoms in the LED and so attracted to the atoms in your retina.

 

Go easy on me!

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I agree, (I've always struggled with massless particles having momentum) but are these equasions not back engineered?

 

No, massless particles having momentum can be derived in special relativity.

 

E2=(mc2)2+p2c2

 

where m is the rest mass, p momentum, E energy and c the speed of light in a vacuum.

 

They certianly explain the relationships but not the reason, that is, they do not explain why the photons leaving your display are so repulsed by the atoms in the LED and so attracted to the atoms in your retina.

 

I think you might be looking for something outside the bounds of science. Science answers how, not why.

 

Maxwell's equations will also pop out a speed for Electromagnetic Radiation through vacuum

Relation between electricity, magnetism, and the speed of light

 

This is the answer I would give.

 

But then you just get the question of why are the free space permittivity and permeability the values that they are. We're back to science not answering why but how.

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Thank you Klaynos, I am looking at understanding something that is outside the current bounds of science, it is something that will change our whole perception of the universe and the way it works. The speed of light question is just the start.

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  • 2 weeks later...

No, massless particles having momentum can be derived in special relativity.

 

E2=(mc2)2+p2c2

 

Please post such a derivation. Typically such derivations are really identities.

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Please post such a derivation. Typically such derivations are really identities.

 

Well, typically [math]p_\mu p^\mu \equiv m^2[/math]. Since [math]p^\mu =(E,p^i)[/math] where [math]p^i[/math] is the three-momentum, it's easy to see for a particle with m=0 that [math]E^2=p_i p^i[/math]. So massless particles have momentum by virtue of their energy.

Edited by elfmotat
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The four-velocity of a photon is undefined.

 

And multiplying by their mass doesn't mean much, either.

 

We aren't discussing photons, specifically. As with a few aspects of physics, they are a special case.

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And multiplying by their mass doesn't mean much, either.

 

We aren't discussing photons, specifically. As with a few aspects of physics, they are a special case.

 

pmb's question was about photons, not 4-momentum in general.

Edited by elfmotat
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pmb's question was about photons, not 4-momentum in general.

 

You're right; I misread it.

 

p= E/c is a classical notion from EM waves. It's established in relativity with a discussion of a photon being emitted in a system (a railroad car). Einstein had already established that this would change the mass of the emitter, but this changes the center of mass of the system, which violates conservation of momentum. p=E/c drops out of that discussion.

 

http://farside.ph.utexas.edu/teaching/em/lectures/node90.html

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The reason that the speed of light is a constant in relativity is because all observers observe the same spacetime interval (s) when:

 

s = 0 = x^2 + y^2 + z^2 - (ct)^2

 

suppressing y and z axes this happens when:

 

x = ct

 

ie when x/t = c , ie: when an object is moving at light speed. The explanations in many of the posts above give an idea of why electromagnetic phenomena propagate along a zero spacetime interval.

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Moderator Note

Wucko's theory approaching the question from a new and speculative angle has been split off to speculations

 

http://www.sciencefo...post__p__675852

 

if i write it again in math or in C++, will you leave it here? :) Can I suggest an option to mark a post as 'a try on a hypothesis', just to give hypothetical reasoning a chance here and with that a chance to answer real questions by asking better ones?

 

for your inspiration:

Edited by wucko
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I agree, (I've always struggled with massless particles having momentum) but are these equasions not back engineered? They certianly explain the relationships but not the reason, that is, they do not explain why the photons leaving your display are so repulsed by the atoms in the LED and so attracted to the atoms in your retina.

 

Go easy on me!

 

One of the pseudoscientific instances is the confusion of correlation with causality. Physics seems to be biased in the dirrection of confusing causality with correlation. Its an ideology as much as a science. Thats the problem also.

Edited by wucko
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