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why does light move?

Struders

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Struders

Struders

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Why does light move? or should that be how does light move?

What I mean is when I turn my desk lamp on why do the light photons move away from the wire element then through the glass of bulb and not just build up round the wire element?

I think I'm being dense here but I've been grappling with this one for the last few nights.

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ian_d

ian_d

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Great question! To answer it fully requires an understanding of quantum mechanics, but basically the way it works is this.

Photons are the fundamental packets of electromagnetic energy, and because they obey quantum mechanics, they exhibit particle-wave duality....in other words, they have equivalent descriptions as particles or as waves. When you work out the wave equations of motion, you find that photons (light) always propagate as waves with a fixed speed (in vacuum, at least). It's a fundamental property of electromagnetic energy that it propagates through space, so any source of such energy (like a lightbulb) necessarily produces photons that move away at the speed of light.

A more complete answer to this question gets you into a discussion of how fundamental symmetries in physical laws lead to quantum gauge field theories, which in turn predict the specific properties of photons (including their motion through space). But that gets proper complicated!!

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acey

acey

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The classical (non-quantum) answer is that light is an electromagnetic wave. James Clerk Maxwell (great Scottish physicist) showed in the nineteenth century that you can have a situation where electric and magnetic fields make a self-sustaining wave. But the only way it can self-sustain (and not just disappear) is for it to move through space. Maxwell worked out the speed it would travel at - and found it was the speed of light. So he'd proved that light is an electromagnetic wave.

Einstein wondered what would happen if you flew beside a light-beam at the speed of light. You'd expect to see a stationary electromagnetic wave - but Maxwell had already proved that the wave can't be stationary. That's how Einstein discovered relativity: he realised that no matter how fast you move, light beams always seem to have the same speed, the one that Maxwell predicted. This has implications for how we measure time, length, mass, etc etc., which have been verified by countless experiments and observations.

So Einstein gave the definitive answer to why light moves, why it always moves, and always at the same speed: it's because of the relativity principle. This remains true in the quantum version.

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themos

themos

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I think the question was what makes them not buzz around near the filament for a long time before they make it "out". It takes light 40,000 years to get out of the Sun's core even though it could travel that distance in vacuum in a second.

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brianb

brianb

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It takes light 40,000 years to get out of the Sun's core even though it could travel that distance in vacuum in a second.

I thought it was more like 1/4 million years - but it's not the same light; the photons generated when the nuclei fuse are almost instantly absorbed by other nuclei, which then transfer their energy to other nuclei by colliding with them or radiating photons, etc, etc, in a very long chain before the photons finally arrive at the photosphere and escape into space. The "original" photons were gamma rays & most of what comes off is visible; the long chain is what takes a long, long time to complete.

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Nexus 6

Nexus 6

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The classical (non-quantum) answer is that light is an electromagnetic wave. James Clerk Maxwell (great Scottish physicist) showed in the nineteenth century that you can have a situation where electric and magnetic fields make a self-sustaining wave. But the only way it can self-sustain (and not just disappear) is for it to move through space. Maxwell worked out the speed it would travel at - and found it was the speed of light. So he'd proved that light is an electromagnetic wave.

Einstein wondered what would happen if you flew beside a light-beam at the speed of light. You'd expect to see a stationary electromagnetic wave - but Maxwell had already proved that the wave can't be stationary. That's how Einstein discovered relativity: he realised that no matter how fast you move, light beams always seem to have the same speed, the one that Maxwell predicted. This has implications for how we measure time, length, mass, etc etc., which have been verified by countless experiments and observations.

So Einstein gave the definitive answer to why light moves, why it always moves, and always at the same speed: it's because of the relativity principle. This remains true in the quantum version.

excellent answer :)

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Struders

Struders

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Ahhhh.. I always got hung up on light being waves or particles but actually they are both depending how/what Theory that you are using. Now knowing that my next question would be.

Why is light effected by gravity if light has no mass... This is where my knowledge of spacetime breaks down. I think what I'm asking is; does everything get effected by the curving of spacetime? With mass or not?

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Zakalwe

Zakalwe

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Light has to travel - at the speed of light - it's the only way it can keep its energy, and the energy just has to be conserved.

But only in a vacuum. Light travelling through matter (gas, liquid or solid) can and does move at different speeds* (this is called refraction). I am glad of this, otherwise telescopes wouldn't work:icon_salut:

*The maximum speed that it can travel at is c 186,000 miles per second

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Zakalwe

Zakalwe

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The matter locally modifies the speed of light. Light still travels at the speed of light.

Agreed. The speed of light is variable depending on the medium through which it is travelling.

When people say "the speed of light" generally they are referring to the speed of light in a vacuum, and very often do not realise that light can and will travel at different speeds.

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ollypenrice

ollypenrice

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Ahhhh.. I always got hung up on light being waves or particles but actually they are both depending how/what Theory that you are using. Now knowing that my next question would be.

Why is light effected by gravity if light has no mass... This is where my knowledge of spacetime breaks down. I think what I'm asking is; does everything get effected by the curving of spacetime? With mass or not?

This is the key distinction between Newtonian and Relativistic gravity. Under Newton, massless light would be unaffected by a gravitating body. In relativistic gravity the gravitating body curves spacetime around it and the photons fly into that curvature and are affected by it.

This was what Eddington discovered when he observed an apparent change in stellar positions during an eclipse, when the incoming starlight was deflected by the sun's curved spacetime. Had Newton been right the star position would not have seemed to move.

Your original question was indeed a beauty!

Olly

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Flyer2o11

Flyer2o11

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I like to visualize the classical description of light as being none other than ripples in the electric and magnetic fields.

Just like gravity waves ripple out in from a moving object in the spacetime fabric, so too light emanates out in the electric and magnetic fields of the cosmos.

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dph1nm

dph1nm

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This is the key distinction between Newtonian and Relativistic gravity. Under Newton, massless light would be unaffected by a gravitating body. In relativistic gravity the gravitating body curves spacetime around it and the photons fly into that curvature and are affected by it.
Actually this isn't quite true. Newtonian gravity does predict a deflection (photons have mass), but only half that predicted by GR. Eddington was able to show that the GR version was correct.

NigelM

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