Seeing Light That Travels Away From Us

[herme3]

Why are we only able to see light that is traveling towards us? For example, if you look into a flashlight, you will see the light that is coming from it. However, if you look in the direction the light is going, you will not see the light going away from you. The only way you will see the light is if it comes back to you by reflecting off of another surface.

 

In the same way, we are only able to see light coming towards Earth from distant objects. We are unable to see the light that is coming from Earth and going out into space. Would it be possible to somehow capture light that is leaving Earth and look at it? Could we create some type of telescope that can see the light as it travels away from us? This would be very helpful in solving crimes. What if a crime happened one year ago, and we could somehow look at a reflection of Earth that is one light-year away? Couldn't this help solve the crime?

[ecoli]

you seemed to be confused...

 

http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/redshf.html

 

edit: oops, I misread the question. Sorry.

[ydoaPs]

why can't we see light that is travelling away from us? simple: it is travelling away from us.

 

you could use a mirror.

[herme3]

why can't we see light that is travelling away from us? simple: it is travelling away from us.

 

If a person is traveling away from you, you can still see them. Even though you won't see their face, you will still see the back of them. You will know someone is there, and you might be able to tell who they are.

 

Even though light has different properties than matter, isn't there anyway that you could see the "back" of light? Even if it isn't visible light, couldn't there be some type of energy that is left behind? If we can detect this energy, couldn't we discover a way to piece it together and tell what is in the light?

[JustStuit]

The light reflects off them and back toward you. The light does not (usually) come from them, rather it is redirected off them.

[insane_alien]

can you feel a bullet hitting you as it travels away from you? Thats a pretty close analogy since the eyes detect the photon(bullet) hitting the receptor cells in your eyes. the only real difference is that the bullet would kill you.

 

The beam of light does not itself emit light and nor does it reflect light. therefore we can not observe it unless that light enters our eyes or any other instrument we may may be using to detect photons.

[swansont]

"see" = "detect photons"

[Klaynos]

Can you catch a ball that is moving away from you?

 

The answer is only if you bounce it off of something so that it's now moving towards you.

 

Could we catch light that is moving away from the earth? Yes if we put a mirror in the way and directed it at us

[alt_f13]

I think what herme3's kindof asking, not in so many words, is "how could we see something happen a long time ago on earth, the way we can see distant past events due to the finite speed of light?"

 

Find a wormhole. Jump through it. Look at Earth.

 

With any luck, you'll have circumvented a large portion of "regular" space and be able to see past events on Earth.

 

As for seeing the "back" of light.... you can see the "back" of laser pointers and bright lights because they bounce off of particles in the atmosphere... but they are focused beams, so unless you had some sort of giant lense and a situation I'm far too tired to try and comprehend right now, there's no way you could extrapolate an image of something reflecting off miniscule, over-excited particles of air.

 

Water, steam and other somehow dense, uniform patterns of liquid matter (mirages... I've seen my own reflection in one) however, are another story. Just don't expect to see all that far into the past, unless we find some uber-cool mirror nebula in the depths of space (good luck finding a reflection of stars among stars themselves too).

[Severian]

Actually, this is a very good question.

 

It is not like a bullet because you could see a bullet moving away from you in principle (or in practice if you had a video camera with a fast enough frame rate). You could see the bullet because light (from a source like the sun or maybe a lightbulb) bounces off the bullet, and the travels to your eye. What we really mean by 'bounces' is that the light interacts with the charged particles in the bullet, which causes it to change direction.

 

This doesn't happen for light because the photons themselves are not charged. Light only reacts with charged particles so one photon cannot interact with another. Therefore any light falling on the light you want to see passes straight through and you can't see it. (This is not quite true - there is a contribution to light-by-light scattering from a quantum effect where the light splits into a virtual electron-positron pair, then the electron-positrons which are charged interact with the incident light, causing a scatering. But this is a very small effect.)

[Edtharan]

It was once believe that the eye projected rays out and when those rays hit an object it allowed us to see. We now know that this is wrong.

 

Light that is emitted from a source (the sun, a light bulb, etc) travels out from the source. Individually these are called photons. If one of these photons reaches out eye tiny receptors in the eye, call rods and cones, have chemicals that react when a photon of light (if the case of the cones certain frequencies of light) hits one of these chemicals it triggers a nearby neuron to fire sending a signal into the brain via the optic nerve.

 

So the reason that we can't see light moveing away from us is that, if the light is moveing away from us it will not hit these light sensitive chemicals in the eye. The reason we can see someone moveing away from us is that the light reflects of them (or is emitted by them if the are holding a torch) and then heads in our direction and it then eventually hits one of the receptors in the eye allowing us to detect that photon.

 

Now the light is moving at a rate of around 300,000 kilometres a second, it does not move instanly. So if we look out at the night sky and see the moon. The light reflected off of it (the light originally comes from the sun) takes about 1 second to reach the earth (after being reflected from the moon). The light from the sun takes about 8 minutes to reach us. The light from the neares star (besides our sun) takes aporximately 4 years to reach us.

[sunspot]

One way to see light traveling away from you is to use fog. The scattering of the light will reflect back so we can see the light moving away. I guess what we actually see is light coming towards us that scattered from light moving away. This limits how far the light can go before the signal is gone.

 

This raises an interesting point. Light from very distance galaxies either never went through any intergalactic fog or if it did what we see is only a fraction of the original signal. In other words, if we had a ten foot thick fog wall, the light existing would only be a fraction of its original self, with the lost fraction scattered, refracted or made diffuse.

[JustStuit]

You can never see light traveling away from you. You can redirect it back toward you though.

[JesuBungle]

Your idea of looking at reflections of past light from the earth is interesting. I think theoretically you could do this, but only if there was a way to amplify the reflection enough to get it back to its original strength. Now making a device to capture light going away from you might be a problem, way over my head. I think if there was the technology and funding, NYPD could put a giant mirror a few light hours away and take a telescope and watch the crime unfold as it did the night before.

[swansont]

Your idea of looking at reflections of past light from the earth is interesting. I think theoretically you could do this, but only if there was a way to amplify the reflection enough to get it back to its original strength. Now making a device to capture light going away from you might be a problem, way over my head. I think if there was the technology and funding, NYPD could put a giant mirror a few light hours away and take a telescope and watch the crime unfold as it did the night before.

 

Or you could just put the video cameras in place with a recording device...

[JesuBungle]

Or you could just put the video cameras in place with a recording device...

 

Now what fun would that be? lol

[Ragib]

The Thing is, to catch up with the light, you would have to violate Special Relativity. As for finding a Wormhole..fat chance..one close to earth? we would have the detected it.