How far will a ray of light travel in space before it disappears, given that it has no obstructions?

Question

I guess, we're lucky the universe is expanding, otherwise, we'd be living in an infrared oven. By the way, how can infinity expand, or what's at the end of the universe, and is it the universe pushing out or is it being pulled?

Some great answers, I've given votes to the best ones, and also one accidentally hmm which one? I'll leave to the experts.

Ok, does it get smaller the farther it goes or what?

Ok, does it get smaller the farther it goes or what?

OK, all these answers are making me conjure up more questions so i better stop.

Answer 1

light will travel forever... the only things that can stop it are black holes, or actual objects in the way... but even then it simply gets reflected back. Considering the vastness of space though, that can be a very rare thing to happen.

It's hard to imagine then, especially as when you look into space or the night sky, you see some points of light but mostly black... if the universe is infinite and light from all the stars rarely gets stopped or fades, then shouldn't the night sky be mostly white?

Here's an interestnig thing... The universe is expanding... incredibly fast. It stands to reason that the stars on the outer edge are moving fatser than those closer to the middle. Basically, every star in the universe is moving away from one another, some faster than others...

Now imagine a fire truck siren... when it approaches you, it seems higher pitched, then as it passes you and moves away, it changes to a lower pitch... It's the same siren, but the frequency (or pitch) you here depends on whether it is stationary or moving... If it moves away from you, the sound waves are further apart, so you here a lower frequency. It's called "Doppler Shift".

Doppler Shift affects ANY wave though... not just sound. It works exactly the same for light waves. We already know that the stars are all moving away from each other, so the light they give off gets "shifted" to a lower frequency, excatly as a siren moving away from you does... In the case of light, this means that you move toward the RED end of the visual light spectrum... this effect is known by astronomers as "RED SHIFT" and the degree of shift can be used to calculate exactly how fast the star is moving away.

Now for the reason why space is black and not white... some (in fact most) of the stars are moving away at such high speed that they not only get shifted to the red end of the visual spectrum, but they go further still... past visible red into INFRA-RED. Infra-red is not visble to the human eye, so you simply can't see it. In other words, there are actually stars completely filling the night sky, but we only physically see those that are not moving away as fast as the majority... so we only see a few white points in an otherwise black sky.

Thats's why radio telescopes can be used to find stars that you simply can't see with lenses and the naked eye... Radio telescopes can be used to "see" parts of the radio spectrum that are beyond our eye's ability.

Ok... I've gone on enough, but as I think you can probably see, I love this kind of stuff... I hope it helps to answer your question, but better yet give you some more food for thought!

Answer 2

"I guess, we're lucky the universe is expanding, otherwise, we'd be living in an infrared oven. By the way, how can infinity expand, or what's at the end of the universe, and is it the universe pushing out or is it being pulled?"

The universe is expanding. We know this because no matter where we look, in every direction, whatever we look at is moving away from us, and the only way for this to be the case is if everything is expanding outward, with the stuff furthest from the center moving the fastest. One of the funny consequences of that is we're unable to determine where the center is.

The universe is infinite because there's no way to reach the end. It would take longer than the lifetime of the universe even if you were travelling AT the speed of light. That's a functional definition of infinity physicists use to describe the universe which could also be phrased as "It can never ever affect us in any way so who cares?"

"Ok, does it get smaller the farther it goes or what?"

No. If you're speaking about one single beam of light you can describe them as a single photon, (the beam would contain multiple photons, all following pretty much the same path in 3-d space,) and a single photon does not get smaller, or weaker, or anything like that.

However: In the real world, it's impossible to create a PERFECTLY collimated (beamlike) ray of light. Quantum tells us we can never have a perfectly parallel beam of light because that would require that our uncertainty in the positions of the photons in the beam would have to be 0 while the uncertainty in the momentum, (and therefore the velocity,) of ALSO being 0. That can never happen.

The light beam will eventually spread out. How does that become relevant when we're only talking about a single photon? Well, a single photon is not a pure particle, right? That's what quantum tells us. So the wave function that describes the photon, (it actually does more than describe the photon, it IS the photon,) will eventually spread out, and when you finally measure the photon at some very very large distance there will be odds of it being detected in a variety of different places.

Answer 3

Chimp: It actually does continue on. It is called the process of diffusion where the original light moves from a higher concentration gradient to a lower concentration gradient. In theory there is a limited distance over which visible light can travel that is governed by the redshift of the light - the light is still there, but it may be in the infrared region as is the case with cosmic background radiation.

Answer 4

A ray of light is made up of many photons all travelling in roughly the same direction, but the further away from the source, the more spread apart they become.
Space is not empty, it's full of dust & stuff, so there is a finite limit to the distance that photons can travel before they become absorbed.
The farthest thing detected so far is a "quasar" about 10 billion lightyears away. See this link.
http://www.newton.dep.anl.gov/newton/askasci/1995/astron/AST165.HTM
Farthest star from Earth

Answer 5

A ray of light will travel forever in space and it will never disappear.

"But," you may ask, "what if I turn off the light source? If the light source is turned off won't the beam of light just stop?"

The answer to that question depends on where you are. If you are at the light source and you turn off the light after 1 minute, you will no longer see the light. If there is someone a couple of feet away they won't see the light anymore either.

But! What if another person was so far away that it took a year for the light to reach them? (That would be about 6 trillion miles away!) That person would not see the light go off for, well, for a year. They would see the light go on a year after you turned it on and then see it go off a year and a minute later.

What if someone were a million light years away, that is, so far away that it would take a million years for the light to reach them? They would not see the light go on until a million years after you turned it on and then see it go off one minute later.

Cool huh!? How would you like to be the one who is a million light years away from the light source?

Well, you are! Look at the stars at night. The light you see from any particular star left the star thousands or millions or even tens of millions of years ago! In fact, some of the stars you are looking at may not even be there anymore!

Back in 1987 an astronomer, actually, a graduate student in astronomy, saw a star blow up right before his eyes! Yet, the star actually blew up over 60 million years ago! People have been looking at the star for 60 million years and it wasn't even there!

So, light goes on forever. But that's not all! All electromagnetic waves go on forever. Radio and TV waves are electromagnetic waves like light and they go on forever too. The radio station you listen to is sending radio signals into outer space and they will go on and on forever. Who knows, some far away person much like you may pick up those waves some day.

Oh, and guess what else is electromagnetic waves? Yep! Your cell phone! Some day far, far, far into a future long after you are dead and gone some 'person' might be able to hear what you said about your brother's best friend!

Answer 6

The light will travel an infinite distance. Whether or not you can see a beam of light that is travelling with "no obstructions" will depend entirely on how good is the instrument that you use to look for this light. You WON'T see it at infinity, however- at this distance, the source of the light would appear infinitessimally small, so no instrument could magnify it enough to be seen!

Answer 7

light will travel forever. Many people have said this but i thought i would add an interseting piece of information. as you can imagine, the earth gives light into the universe. The light does not stop unless there is obstructions. In theory if you travelled faster than the speed of light, you could travel backwards in time as you could see light from hundreds of years ago if you put up a screen to capture it on. If there is life on other planets and they had powerful enough telescopes, they could see images from the earth from hundreds of years ago, depending how far away from earth they are. There are many impossible elements to this theory but it is, in my opinion, still fascinating

Answer 8

A ray of light is composed of photons and will not disappear. They will travel to the edge of the universe, and since the universe is curved will continue to travel around and around forever EXCEPT if the light finds its way into a black hole. Then the photons (the ray of light) will be lost and disappear forever into the dimensions of the black hole (and I don't mean Oprah)!

Answer 9

It certainly doesn't go forever like some of the answers are suggesting. If it did then the universe would get brighter and brighter as all the light produced would never disappear.

Answer 10

A photon of light travels forever in space, until it is forced to interact with a sub-atomic particle carrying an electric charge.

BTW, Larry H, photons have no rest mass and and are not considered "matter."