Where does the light go?

[sir drinks-a-lot]

If I shine a flashlight up into the night sky, where does the light go?

I am guessing that it will keep going forever, but will become diffused since it will spread out from the source in a conical shape.

Is this correct?

 

[Buckaroo]

Most of it will be absorbed or scattered by the atmosphere. What little is left will, as you say, continue, at least until the photons collide with something.

 

[RenaissanceBiker]

If I shine a flashlight up into the night sky, where does the light go?

Up into the night sky.

I am guessing that it will keep going forever, but will become diffused since it will spread out from the source in a conical shape.

Is this correct?

Yes. Now think about the Earth rotating on its' axis (a different inertial reference from what you first described). What does your beam of light look like now?

 

[Dustin Kesselberg]

Can't light get absorbed by other light? What happens when I shine one light into another light? Don't the Photons crash into eachother?

Shouldn't that produce some sort of..I don't know..Light show? Why doesn't it?

 

[JayT]

Light - Faster Than A Speeding Bullet

If I shine a flashlight up into the night sky, where does the light go?

I am guessing that it will keep going forever, but will become diffused since it will spread out from the source in a conical shape.

Is this correct?

Light will continue to travel forever until something finally absorbs it.

But the light from a flashlight will mostly be absorbed and scattered by the hundreds of miles of atmosphere it passes through as stated by Buckaroo.

Even a laser would be seriously diminished by 300+ miles of atmosphere.

 

[pchams]

Down the drain, into a big black hole.

 

[Zombified]

Can't light get absorbed by other light? What happens when I shine one light into another light? Don't the Photons crash into eachother?

Shouldn't that produce some sort of..I don't know..Light show? Why doesn't it?

In classical electrodynamics, electromagnetic waves add linearly, which is equivalent to saying photons don't interact with each other. Waves pass over each other without affecting each other.

You can have photon-photon interactions in quantum field theory, but the cross section is very, very small. This is because the interaction is mediated either by a loop involving two electrons or one Z boson (weak force). Since these are heavy particles the range will be very short, and the more intermediate particles involved the less likely the process is. Nonetheless, the latter process limits the spectrum of gamma rays in the universe (because the highest energy rays have enough energy to make the Z process significant), and thus has cosmological consequences.

The significance of photon-photon interaction to conventional light sources is basically so infinitesimal its irrelevent.

 

[Orangutan]

Can't light get absorbed by other light? What happens when I shine one light into another light? Don't the Photons crash into eachother?

Shouldn't that produce some sort of..I don't know..Light show? Why doesn't it?

I think you are thinking of Ghostbusters. "Don't cross the beams Ray!"

It's often better to think of photons as a little electromagnetic ripples. And in the case you mention the photons would intefere with each others when then meet, either re-inforcing or reducing eachother at the meeting point but then they would go along thier way as if nothing had happened like ripples in a pond.

 

[Dustin Kesselberg]

In classical electrodynamics, electromagnetic waves add linearly, which is equivalent to saying photons don't interact with each other. Waves pass over each other without affecting each other.

You can have photon-photon interactions in quantum field theory, but the cross section is very, very small. This is because the interaction is mediated either by a loop involving two electrons or one Z boson (weak force). Since these are heavy particles the range will be very short, and the more intermediate particles involved the less likely the process is. Nonetheless, the latter process limits the spectrum of gamma rays in the universe (because the highest energy rays have enough energy to make the Z process significant), and thus has cosmological consequences.

The significance of photon-photon interaction to conventional light sources is basically so infinitesimal its irrelevent.

Can you translate that into laymen for those of us unfamiliar with quantum dynamics? What do you mean 'add linearly'?

 

[TonyL]

Can't light get absorbed by other light?

Light can interact with light, but one photon cannot "absorb" another photon, per se.

What happens when I shine one light into another light?

That depends on the properties of light and the medium in which they are travelling. Remember, light is a wave, so it will act similar to waves you observe on the top of the water. If you have two sources of waves, and the waves overlap, you will get interference. In some places they will add together and create a more intense wave, while in other areas they will cancel each other out. The same thing happens with light. As the two beams interact, you will get areas where more photons exist and areas where less photons exist. (I'm oversimplifying a little here)
Depending on the medium and the intensity of the light, you could get other effects, like two photons of different wavelengths (colors) combining to form a single photon of a new wavelength.

Don't the Photons crash into eachother?

Not in the way cars crash into each other. Photons have energy and momentum, but no rest mass. It really is easier to think of them as waves that have quantized amounts of energy than to think of them as particles that have wavelike properties since they move and interact like waves.

 

[Zombified]

Can you translate that into laymen for those of us unfamiliar with quantum dynamics? What do you mean 'add linearly'?

Well, that part wasn't quantum mechanics. It just means if you treat the waves in a system as consisting of several distinct waves, you're allowed to get the "total" wave just by adding them together.

Imagine waves in a pond, one going left and one going right. When they meet, the peak will be a little bit higher, but the waves will keep going left and right respectively, and if you look after they've passed each other, it'll look like they never contacted each other.

For some types of waves there's such a thing as a "non-linear medium" where waves don't add linearly. When this happens, you get extra waves after adding together the originals. Some types of distortion in audio gear, for example, acts this way on the sound signals, because of limitations in electronic parts.

In the pond example, if the peak when the waves meet is so high the wave "breaks" the medium is no longer linear, and you get extra waves splashing out from the break.

Normally the waves associated with photons, in both classical and quantum descriptions, just add together. The small effects of field theory that I mentioned are very tiny exceptions.

 

[Dustin Kesselberg]

I see what you're saying.

 

[Ziggurat]

Light will continue to travel forever until something finally absorbs it.

But the light from a flashlight will mostly be absorbed and scattered by the hundreds of miles of atmosphere it passes through as stated by Buckaroo.

Even a laser would be seriously diminished by 300+ miles of atmosphere.

I don't think this is correct for visible light. If it were, most of the visible light from the sun would not reach the ground, but would heat the atmosphere instead. I'm pretty sure that's not what happens.

The 300+ miles of atmosphere directly above you is considerably less gass than 300 miles of atmoshpere at ground level looking sideways.

 

[ImaginalDisc]

.

The 300+ miles of atmosphere directly above you is considerably less gass than 300 miles of atmoshpere at ground level looking sideways.

That is why sunsets and sunrises are such pretty, non-blue, colors.

 

[Buckaroo]

I don't think this is correct for visible light. If it were, most of the visible light from the sun would not reach the ground, but would heat the atmosphere instead. I'm pretty sure that's not what happens.

The 300+ miles of atmosphere directly above you is considerably less gass than 300 miles of atmoshpere at ground level looking sideways.

An awful lot of the sunlight does get scattered, especially in the shorter wavelengths. This is why the sky is blue -- Rayleigh scattering of photons off of oxygen and nitrogen molecules. Longer wavelengths, into the red, are scattered by larger bits of dust and particulates. All in visible wavelengths. This isn't absorption, which is a different process.

I don't have the numbers (it's been 10 years since my last radiative transfer class), but I suspect the intensity of the visible sunlight is considerably less at the surface of the earth than at the top of the atmosphere.

 

[Ziggurat]

An awful lot of the sunlight does get scattered, especially in the shorter wavelengths. This is why the sky is blue -- Rayleigh scattering of photons off of oxygen and nitrogen molecules. Longer wavelengths, into the red, are scattered by larger bits of dust and particulates. All in visible wavelengths. This isn't absorption, which is a different process.

A lot of light gets scattered only because there's a whole lot of light TO scatter: that says nothing about the fraction of light being scattered. I contend that fraction is small. We ONLY notice it with out naked eye with the sun, and never notice it with the moon or stars, for example.

 

[Iamme]

Light will continue to travel forever until something finally absorbs it.

Hmmm. It is said energy cannot be lost. Yet, if we were to suppose that the universe is "open"... that the light could escape into the void of space (where some people think God is)...then it be like...energy got lost?

 

[Zombified]

Hmmm. It is said energy cannot be lost. Yet, if we were to suppose that the universe is "open"... that the light could escape into the void of space (where some people think God is)...then it be like...energy got lost?

Energy is conserved in the sense that it is not created or destroyed. It only changes forms and moved around. Energy that goes flying off into space is lost only in the sense that we're not entirely sure how to get it back down here.

It's like when you lose your cars keys. It doesn't mean they were taken home by God, they gotta be somewhere.

 

[JayT]

QED - Light Edition

Can you translate that into laymen for those of us unfamiliar with quantum dynamics? What do you mean 'add linearly'?

It's hard to explain anything about quantum electrodynamics (QED) without math and even harder to explain with math.

Basically 'adding linearly' is analogous to the photons passing through each other like ghosts.

Photons don't absorb other photons or bounce off each other like billiard balls colliding. Although, I'm sure it would produce some very interesting effects if they did behave that way.

A small cross-section is analogous to how close the photons approach each other or how small the circular cross-section is into which their energy is concentrated, essentially meaning that for photons to interact they must be extremely close to each other. At the microcosmic quantum level, the distance between the photons in a flashlight beam is like light years. The chances of any form of interaction between them is microscopically remote.

Photon-photon interaction generally requires a very, very high density of photons, like an extremely intense laser beam. They must also be of very, very high energy (e.g. like gamma rays). These two factors govern the probability that photon-photon interactions will occur.

Usually a moving particle, perhaps an electron, is involved where the photons interacting with it obtain even more energy which can mediate the photon-photon interaction. The W and Z particles connect the nuclear weak force to the electromagnetic force. They are involved in the beta decay of neutrons in the atomic nucleus.

The photons coming from two flashlights facing each other would have next to nil probability of interacting due to their extremely low density at the microcosmic level and their extremely low energy.

However, gamma ray photons colliding with intense, high-density laser photons could cause interactions between the photons which could generate particles of matter and antimatter from the pure light energy (e=mc² in reverse). The particles created would be electron-positron pairs. If the particles collided, then they would be converted back into light again! Now that's a light show!

You would need some pretty powerful flashlights to do this!

Certain wavelengths of coincident light can produce some interesting interference patterns on a screen, but this is not exactly photons interacting with each other in an intimate sense.



This is interesting:

Light has no mass itself, but can it interact with other light to create particles with mass! Conversely, particles with mass can be converted into light without mass. This is because e=mc² works both ways.

Since the world of quantum mechanics is so counterintuitive, explaining quantum interactions is difficult in terms of everyday experiences and observations, so analogies are hard to establish in many cases.

I'm not sure if I'm getting anywhere with this, but I tried.

LOL

QED is weird, to put it mildly.

 

[Hamradioguy]

Can you translate that into laymen for those of us unfamiliar with quantum dynamics? What do you mean 'add linearly'?

You might want to read "Mr. ompkins in Wonderland" by George Gamow. Parts are a tad dated-it was written over fifty years ago. But it's a neat explaination of both cosmology and quantum machanics. A game of "quantum billiards" and a town where the speed of light is a mere ten miles per hour makes for an interesting read.