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]

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. :D

[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. :boggled:

[JayT]

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.

[Hamradioguy]

The who gave me the book-when I was maybe 10 or 11 wrote in the front, "The world is a screwy place".

[JayT]

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.




I can't entirely agree with that.

Originally I said absorbed by the atmosphere, but I should have more accurately said the dust in the atmosphere. The air is mostly transparent to the visible spectrum.

I see your point, but in this case, it's invalid to compare the sun to a battery-powered flashlight in terms of atmospheric penetration potential. This has a lot to do with the difference between the energy span of their respective spectrums and the way the inverse square law of light intensity works.

The inverse square law of light propagation alone would greatly effect a flashlight beam over a 300 mile span than it would sunlight over the same interval. The flashlight beam would be extremely reduced by the time it reached 300 miles, but sunlight would be far less affected.

The sun produces much more intense light and it covers an entire hemisphere of the world, scattering light and illuminating the entire sky from all directions. The atmosphere isn't nearly strong enough to absorb all of it as readily as a weak flashlight beam consisting of mostly longer wavelengths which are more easily absorbed by dust.

The sunlight reaching the earth's surface is about 800 to 1000 watts per square meter per second, depending on location and other factors. Above the earth's atmosphere it is about 1370 watts per square meter per second.

The photon energy of a flashlight beam is comparably very weak. The number of dust particles it would collide with on its way to space would easily absorb the lion's share of its energy. A much smaller proportion of it would reach space.

Another important consideration is that the energy spectrum of a flashlight beam is nowhere near as wide as the energy spectrum of the sun.

The scattering of light is more confined to shorter wavelengths, such as blue light, which is why the sky is blue.

The light of a flashlight is mostly longer wavelengths and so most of it would get absorbed by gas molecules and dust particles in the air, rather than scattered like sunlight with a much wider spectrum contributing to the whole.

As a result, a far larger proportion of sunlight penetrates the atmosphere than would a flashlight beam.

Suppose there were no atmosphere at all and we are 300 miles above the earth.

If we sent a beam from the flashlight to the earth below, the inverse square law alone would reduce it to near invisibility at 300 miles, however the sunlight reaching the surface would still be almost exactly the same as it was at 300 miles above the surface.

If we added the atmosphere, the flashlight beam would nearly disappear entirely, between the inverse square law and dust absorbing the longer wave lengths and the scattering of the shorter wavelengths, but the sunlight would only be reduced only a few percent comparatively.

[JayT]

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?


That's the first law of thermodynamics.

Energy can neither be created nor destroyed, only converted from one form into another.

Light energy is 100% pure momentum - the energy of pure motion. It can continue forever through space until something stops it by absorbing its energy.

If you were inside a room with mirrors on the floor, ceiling and walls and turned off the light, the room would go dark rather than the existing light continue to bounce off the mirrors endlessly. The mirrors would quickly absorb the photons' momentum and they would all disappear.

If the light did continue to bounce off the mirrors forever after the light was turned off, that would be a form of perpetual motion and let's not go down that dark alley!

LOL

The current understanding of space is that it is not actually empty. Space is something analogous to the lowest state of energy and it is interconnected to all the objects within it. The universe as a whole is treated as a closed system.

This means that space, energy and matter are just different manifestations of the same stuff, just as steam, water and ice are different manifestations of the same stuff.

According to this scenario, light doesn't escape or get 'lost' anywhere, just carries its energy to remote places until it finally encounters something that stops it.

As soon as light stops, it disappears and increases the energy of whatever absorbs it by an amount equal to the energy it carried while moving. This increase in energy could be manifested as an increase in heat or a change in the motion of the absorber or any number of combinations summing up to the original light energy, but no more.

The theory is that the energy content of the universe remains constant at all times and only keeps transforming into different forms of energy and matter according to the various laws of mass-energy transformation.

Energy never disappears. It just keeps moving around in different disguises that sometimes produces the illusion that it disappears.

[bruto]

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?How do you define the universe? If there's space there that light can travel to and through, then isn't it still part of the universe?

[blutoski]

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.

Noticeably less. Astronauts' suits are designed to protect against em/light intensity and ionic radiation. Not just vacuum.

In fact, solar em+ionic radiation is a bit of a show-stopper right now for a manned Mars mission. I've read a few proposals, but there are no solutions yet.

[blutoski]

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?

Another thing that might be of interest: the Apollo missions placed mirrors on the moon for the purpose of reflecting laser pulses fired from observatories. These are used to determine the distance between earth and moon.

I haven't seen the details, but it might be interesting to research such things as the intensity of the laser pulse, and the %loss over the return trip.

[Ziggurat]

The inverse square law of light propagation alone would greatly effect a flashlight beam over a 300 mile span than it would sunlight over the same interval. The flashlight beam would be extremely reduced by the time it reached 300 miles, but sunlight would be far less affected.

Inverse square law has absolutely nothing to do with it. We aren't talking about how diffuse the light will get, but whether or not it travels off into the deep vacuum of space. The inverse square law has to do with how the same amount of light gets spread over more and more area, and it's certainly relevant to how bright the flashlight will look to any given observer. But the INTEGRATED intensity at any given distance over the same solid angle will NOT decrease with distance, because the light isn't actually diminishing, it's only spreading out. So yes, the light from a flashlight will be spread out significantly by the time it reaches space. But most of the light will still reach space.

The sunlight reaching the earth's surface is about 800 to 1000 watts per square meter per second, depending on location and other factors. Above the earth's atmosphere it is about 1370 watts per square meter per second.

In other words, it only loses about 30% of its energy. But of course, it loses all of its UV-C and much of its UV-B energy, whereas the flashlight emits very little UV light to begin with (and most of which would be UV-A anyways). Within the visible spectrum, the flashlight should then lose LESS than 30% of its energy to scattering. Now, it might lose more than 30% overall if you count infra-red. But certainly within the visible part of the spectrum, it should be losing less than 30% of its intensity.

Suppose there were no atmosphere at all and we are 300 miles above the earth.

If we sent a beam from the flashlight to the earth below, the inverse square law alone would reduce it to near invisibility at 300 miles,

Irrelevant. All the light that originated from the flashlight would still hit the earth: it would just be spread over too large an area to make it visible to the naked eye. But we're not talking about how VISIBLE the light from a flashlight pointed at the sky is, we're talking about what happens to it. And most of it will just head off into deep space.

[JayT]

Inverse square law has absolutely nothing to do with it. We aren't talking about how diffuse the light will get, but whether or not it travels off into the deep vacuum of space. The inverse square law has to do with how the same amount of light gets spread over more and more area, and it's certainly relevant to how bright the flashlight will look to any given observer. But the INTEGRATED intensity at any given distance over the same solid angle will NOT decrease with distance, because the light isn't actually diminishing, it's only spreading out. So yes, the light from a flashlight will be spread out significantly by the time it reaches space. But most of the light will still reach space.



In other words, it only loses about 30% of its energy. But of course, it loses all of its UV-C and much of its UV-B energy, whereas the flashlight emits very little UV light to begin with (and most of which would be UV-A anyways). Within the visible spectrum, the flashlight should then lose LESS than 30% of its energy to scattering. Now, it might lose more than 30% overall if you count infra-red. But certainly within the visible part of the spectrum, it should be losing less than 30% of its intensity.



Irrelevant. All the light that originated from the flashlight would still hit the earth: it would just be spread over too large an area to make it visible to the naked eye. But we're not talking about how VISIBLE the light from a flashlight pointed at the sky is, we're talking about what happens to it. And most of it will just head off into deep space.



In the sense you refer to, you are correct.

I was reading more into the problem than was asked.

naughty me.

[HeavyAaron]

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?

The light will eventually hit a something, most likely a star.

In any given direction, travel long enough and you'll hit a star. The analogy I find most help ful is that it's like being in a forest. Go far enough in any direction and you'll hit a tree. The "forest" need not be infinitely large, but could be.

Aaron

[Molinaro]

Light only has one place to go -- from one electron to another electron.

[Ziggurat]

Light only has one place to go -- from one electron to another electron.

Light can interact with protons too.