how does light behave/travel?

Question

heres my scenario: i just walked outside to get the mail, its 10pm here so i look up and see a star. it occurs to me that the light from that star has traveled however many millions of miles just to end up in my eye. its a cool thought to look into the past like that but heres what i don't understand. i was thinking that the light was coming from a direct line from the star to my eye. then i looked out the window to find the star again, wanting to see the past again. it then occurred to me that if i stood outside and looked at it, and my friend stood at the window and looked at it, we would both have the same scenario: light traveling across the universe into our eyes. do all stars (or all light and light sources for that matter) project light in all possible directions? is light from a source omitted everywhere and it travels until its absorbed? why don't stars that we can see but are far away illuminate our skies?

i know part of this question seems dumb, stars being far away thus not illuminating our sky seems obvious but light traveling until its absorbed kinda throws me off

Answer 1

This is really a thoughtful question.

Light has many characteristics. First, most of us agree, light is particle...called a photon.

Second, the photons taken together form transverse waves, like molecules form ripples of water on a pond. Looking end on into the direction of travel, the waves are oriented uniformly 360 deg around the axis of travel. This is why we can filter out all but one orientation with a polaroid filter. And we can filter out even that last wave orientation with a superimposed filter oriented 90 deg from that last wave.

Third, the energy of each photon is proportional to E = hf; where h is Plank's constant of proportionality and f is the frequency of the photon. This energy does not wear out; so until something hits that photon or until that photon runs into something, it will continue on its way with that energy without end.

Fourth, photons travel in a straight line unless something changes the direction (like a mirror or a heavy gravitational field).

So, to answer your question. That photon impacting your retina traveled more or less in a straight line from the star without losing one erg of its energy en route. Since the photon travels at the speed of light, the length of time in years it traveled from the star to your eyeball is the number of light years away the star is from Earth.

The photon that hit your buddy's retina likewise traveled a more or less straight line from that same star. But his photon pathway is a bit offset from yours because you are not both standing in the same spot.

Both you and your buddy are assured of seeing that dim star because photons by the gazillions (depends on the size of the star) are emitted every second from all around the surface of that star. That is, light goes out like a continual explosion from the entire surface of that star.

One question may crop up from reading the above. Why is that star light so dim if the photons don't lose energy enroute?

Light dims as we see it from a distance because the photons thin out (the photon density becomes less per unit volume) acording to the inverse square law. That is, for example, the photon density at 2R will be 1/4 the density at R. As you might imagine, when R ~ 4 or more lightyears, the photon density gets a wee bit thin. So the stars appear dim.

When we use a telescope to look at the stars (or any body), we are gathering in far more photons than the naked eye can gather up. Thus, the dim stars become visible to us where before, with just the naked eye, they could not be seen.

Answer 2

This is due to the fact that light travels at a definite speed.
The universe by our best estimation today is 14 billion years old and expanding faster every second. You are right, in every direction there is a star somewhere in some galaxy, but it is only possible to see back as long as the light has been traveling. A star who emits light 15 billion light years away hasn't had enough time to reach us. Also light is emitted in a perfect sphere from a star into space. If a star emits a certain amount of light in one second, that amount of energy is stretched further and further as the sphere gets bigger and bigger, after a certain amount of stretching the light is no longer visible to the unaided eye. The whole explanation is complex but very interesting and worth researching :o)

Answer 3

Actually, there are stars that we see that illuminate the skies, (e.g. the sun).

anyway, to answer your question, light can and usually does travel in all directions, but can also be polarized, or concentrated into a beam. Since the light from far away stars is far away, it encounters many obstacles that often try to absorb the light, and filter it. so it does not fully come through, however, some light does, which is the light that you see.

Answer 4

Light travels in 3 different ways.
1) In a Straight Line until it is absorbed
2) In waves this can be seen using Polaroid lenses
3) In energy bursts

Stars do illuminate our sky how else do we see them? The reasons they don't have the same effect as our sun is due to the fact that they are further away

Answer 5

the light is projected in all directions from that point... the light is omitted until the source that produces that light is lost... in the case of starts i think the source is usually explosions at that site... with hydrogen and such... the stars are too far away to illuminate our skies... we do have a star that illuminates our skies: the sun... perhaps there are beings on other planets far away who see our sun as a tiny speck of light the way we see other stars...

Answer 6

Lights off.