Questtion about Photons - Stars - and why can we see them anywhere?

Question

I really don't know how to phrase this question at all. But i'll try.

Say you observe a star at the maximum possible distance still able to see the light coming from it (i don't know if there is an exact figure or what factors play a part of this, but i'm not worried about that, this is all hypothetical.

Lets say you can view the light from a star 5 billion miles away (random guess). Now, obviously it does not matter *where* you view the star from, it can be 5 billion miles on any plane at any angle and your eyes will still have photons from that star entering them.

I suppose what i'm asking is how it is possible for there to be photons from this star absolutely *anywhere*. Are they anywhere? Or are they only there when we want them to be?

I know - confusing question. Sorry :(

Answer 1

An object like a star emits photons in all directions from its spherical surface.  But the photons travel in basically straight lines (lets keep gravitational fields out of the discussion, it just clutters things up).

The photons we can see are a small "wedge" or "cone" of the total light emitted by the star.

So any instrument or sensor that can detect photons (such as our eyes or a telescope) would see some of those photons.  Not all of them, because the eye or telescope would only detect the small percentage of photons that travelled in the right direction to be seen.

But no matter where your eye or telescope was located, it would still detect some of the photons from that star.

That's why the apparent brightness of a star decreases as the square of the distance.  Twice as far means 1/4 the amount of photons striking the eye or telescope.

So a star 10 light years away is half as bright as one only 5 light years away.

Eventually, the distance is so great that the number of photons that can be detected by the eye or telescope are so few that the star is no longer visible.

Answer 2

KISS - Keep it simple [something]. We see our sun as a ball of light at a specific location at all times of the year as we move around it. We hapen to see a particular star at a specific location at all times of the year as we move in our orbit. The difference lies in the fact that see cannot see the distant star as a ball because it is so far away that our eyes (even the largest telescope) cannot resolve the star as a ball (disc). We see the star as a point of light. This happens because the star is emitting light from its surface outward in all directions and each photon is travelling in a straight line. So, no matter where you are situated around, up, or down from this star (barring obstructions) the star's light will reach you no matter how far it is from your eyes.

Answer 3

I'll try here . . . Photons are light particles that move in a wave form. We measure stars with a "RED SHIFT" in the light spectrum that they emit. The term can be understood literally - the wavelength of the light is stretched, so the light is seen as 'shifted' towards the red part of the light spectrum. Something similar happens to sound waves when a source of sound moves relative to an observer. This effect is called the 'Doppler effect' where that the frequency of sound waves changes if the source of sound and the observer are moving relative to each other. If the two are approaching, then the frequency heard by the observer is higher; if they move away from each other, the frequency heard is lower.  

Light behaves like a wave, so light from a luminous object undergoes a Doppler-like shift if the source is moving relative to us. Ever since 1929, when Edwin Hubble discovered that the Universe is expanding, we have known that most other galaxies are moving away from us. Light from these galaxies is shifted to longer (and this means redder) wavelengths - in other words, it is 'red-shifted'. Since light travels at such a great speed relative to everyday phenomena (a million times faster than sound) we do not experience this red shift in our daily lives. The red shift of a distant galaxy or quasar is easily measured by comparing its spectrum with a reference laboratory spectrum. Atomic emission and absorption lines occur at well-known wavelengths. By measuring the location of these lines in astronomical spectra, we (astronomers) can determine the red shift of the receding sources. However, to be accurate, the red shifts observed in distant objects are not exactly due to the Doppler phenomenon, but are rather a result of the expansion of the Universe. Doppler shifts arise from the relative motion of source and observer through space, whereas astronomical redshifts are 'expansion redshifts' due to the expansion of space itself.

Two objects can actually be stationary in space and still experience a red shift if the intervening space itself is expanding. A convenient analogy for the expansion of the Universe is a loaf of unbaked raisin bread. The raisins are at rest relative to one another in the dough before it is placed in the oven. As the bread rises, it also expands, making the space between the raisins increase. If the raisins could see, they would observe that all the other raisins were moving away from them although they themselves were stationary within the loaf. Only the dough - their 'Universe' - is expanding.

Answer 4

Wow, so many complicated answers, and none of them seem to really answer the question.

Yes, even at 15 billion miles, there are still photons from a star entering your eye.  But there are *very* few of them; even a huge 10-meter telescope might get only one or two every second, which is too dim to detect.  In practical terms, an extremely large bright star is visible only out to about 3 billion light years.

Photons don't know what anybody "wants" -- they go in pretty much straight  lines, with some slight bending in electric or gravity fields, until they hit something and get absorbed.

Answer 5

Remo Aviron has got it right on the nail.  

The photons go in every direction, but they don't move as a "wave" per se.  They move as a probability wave and, like Remo said, sort of get "fixed" once we observe them.  

If you want a longer (but still easy to understand) explanation, check out Brian Greene's book titled, "The Fabric of the Cosmos", pages 77-123 (chapter 4).

Answer 6

Well, understand that light travels at a speed - a very fast speed - and that speed is the speed of light.

Now, lets say a star "ignites" 5 billion years ago tomorrow.

And, it's 5 billion light years away.

The light from that sun will *not* reach earth until tomorrow!  In other words, if someone happens to be looking in that direction, theoretically they would see that star at the moment of its ignition!

SO, you are correct, light isn't "everywhere".  Light *does* spread outward more or less evenly - in other words, it radiates.  However, it takes time for the light to reach any particular location.

Once that star goes out, it will be 5 billion years before we know it!

At least in theory, a star 10 light years away could have gone supernova 5 years ago, and we won't know about it for 5 more years!

Jim

⌘ http://www.life-after-harry-potter.com ⌘

Answer 7

Your treading on Quantum Mechanics and Quantum field theory.

These distant stars produce a lot of photons.  The photons go everywhere and in every direction.  But, by the time they get here, you can literally point your telescope at the star and count the photons one by one as they come it.

Now, if you want the really difficult answer, photons don't really exist until they interact with your eye or the telescope photon detector.  Before they interact, they are part of an electro magnetic wave.  And they could just as easily miss you as hit you.

Answer 8

Think of them like little messengers flying away from the star and moving out in straight lines until they hit something. If they hit your eye, it gathers them in and your brain figures out from them what the thing they came from looks like and you see it, and if they don't hit your eyes, you your brain never knows anything is there and you don't see it.