Starship Asterisk*

So; in the end; light is light. So why is space filled with so much darkness?
Orin

orin stepanek wrote: The spped of light or the potential of the photon to go at the speed of light does not decrease. Once the photons go through what ever condition, it will resume back to the speed of light. At a different frequency.

That's how I read it, harry. The fact that the light is emitted at a longer wavelength is consistent with the conservation of energy.

I am waiting for Nereid, astro_uk, or one of the other "big guns" to chime in if I am off track.

Orin's article said:

Albert Einstein theorized that light cannot travel faster than 186,282 miles per second. No one has proved him wrong, but he never said that it couldn't go slower.

But as far as I understand it, Einstein’s theory says that light in a vacuum always travels at exactly c...no less, no more.

orin stepanek wrote:So; in the end; light is light. So why is space filled with so much darkness?
Orin

Well, space is mostly empty and dark because there's just so much of it. 8)

Hi all

This section from wikipedia answers the question I think

It is sometimes claimed that light is slowed on its passage through a block of media by being absorbed and re-emitted by the atoms, only travelling at full speed through the vacuum between atoms. This explanation is incorrect and runs into problems if you try to use it to explain the details of refraction beyond the simple slowing of the signal.

Classically, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.

The full quantum-mechanical explanation is essentially the same, but has to cope with the discrete particle nature (see Photons in matter): The E-field creates phonons in the media, and the photons mix with the phonons. The resulting mixture, called a polariton, travels with a speed slower than light. [1][2]

http://en.wikipedia.org/wiki/Speed_of_light

In other words its damn complicated. I never understood phonons properly during my undergrad days, so I'm afraid I won't be able to add very much to this.

Phonons? Polaritons?? Yikes! Well, it is not surprising that the classical view is insufficient to explain the way light behaves in unusual situations...but non-technical articles simplify a topic so much that you assume a classical explanation will be sufficient.

Though I have casually read about GR/SR over the years, just out of interest, I haven't paid quite enough attention to QM in my reading. But then, QM and particle physics expand all the time...

I think the reason it is so dark in space is this. I may be wrong; but, I don't believe we can really see light. We can see the source of light; like the sun; a light bulb; a fire. We can see what light is shining on [reflecting off of]. The light beam or wave; we can't see. We can see where it passes through dust or other media. But we cannot see light itself. In space there is very little for light to illuminate; hence it looks dark.
Orin

The reason the sky is dark is actually one of the successes of the BBT, its because the Universe has
a finite age, there just hasn't been enough time for enough stars to have existed for there always to be a star on any given line of sight. Also the expanding nature of space tends to spread out photons (and the galaxies that emit them).

The actual paradox is called Olbers paradox. http://en.wikipedia.org/wiki/Olbers_paradox

astro_uk wrote:The reason the sky is dark is actually one of the successes of the BBT, its because the Universe has
a finite age, there just hasn't been enough time for enough stars to have existed for there always to be a star on any given line of sight. Also the expanding nature of space tends to spread out photons (and the galaxies that emit them).

The actual paradox is called Olbers paradox. http://en.wikipedia.org/wiki/Olbers_paradox

OK I agree! I still believe you can't really see light. What we see is whatever light illuminates. For instance; if you take a flashlight and shine it into the darkness; you see nothing unless the beam passes through some media or strikes something and illuminates it.
Orin

Hi Orin, the human eyes are light detector. We can see from red to blue wavelenght without problem. So, if someone was in front of the flash light we would see the light coming out of it. Since a star is round we see the light where ever we are in the univers. We also see the light if it hit something and is re emit in another wavelenght. There isnt any flash light in the univers except here on earth. So if the sky is dark its because the univers is not static an infinitely vast. The olbers paradox seem to be the right answer to explain why the univers is not fill with light in every direction.

Doum wrote:Hi Orin, the human eyes are light detector. We can see from red to blue wavelenght without problem. So, if someone was in front of the flash light we would see the light coming out of it. Since a star is round we see the light where ever we are in the univers. We also see the light if it hit something and is re emit in another wavelenght. There isnt any flash light in the univers except here on earth. So if the sky is dark its because the univers is not static an infinitely vast. The olbers paradox seem to be the right answer to explain why the univers is not fill with light in every direction.

Hi Doum! I said I agreed with astro_uk on that. My point was that we see what light illuminates and not light itself. We can see the source of light because the sun and stars are illuminated; as is the bulb in a flashlight. but we cannot see the beam of light itself. We can see where the beam is if it passes through a medium like through dust or in a tube or the like. If you could stand in space with your back to the sun and there weren't any stars and looked out you would see nothing even though there was light all around you.
Orin

I think Orin means that we can only see light that directly passes into our eyes, so a thin beam of light passing us by is only visible if some light gets scattered away from the beam and into our eyes.

Like in a laser pointer, where generally the beam appears to be invisble except where it strikes something that can reflect it back into our eyes.

astro_uk wrote:I think Orin means that we can only see light that directly passes into our eyes, so a thin beam of light passing us by is only visible if some light gets scattered away from the beam and into our eyes.

Like in a laser pointer, where generally the beam appears to be invisble except where it strikes something that can reflect it back into our eyes.

Thanks astro_uk I think that is a good way to put it.
Orin

OK i misunderstood what you were saying. Sorry and thanks for the enlightement.

No apology needed Doum. I enjoy this forum because I learned a lot about the universe and I still am learning. Have a great day.
Orin

astro_uk wrote: Like in a laser pointer, where generally the beam appears to be invisble except where it strikes something that can reflect it back into our eyes.

Exactly the analogy I was thinking of...shine a red laser beam through a cloud of particles such as smoke or ash...and you'll see shape of the beam itself, because the particles are about the same size as the wavelength of the light and the light will scatter. You "see" the scattered light that happens to go directly into your eye.

Hello All

The reason why the sky is dark is because our eyes cannot recieve enough light to see it. Through the aid of long exposure we can see the sky blasting with sunlight.

It has nothing to do with the Big Bang theory and the expanding universe.

Yes, the night sky is dark because the universe is finite , in what we can see of it. Also light scatters and we only get a small amount of it, and the further away the less we get from the source. Spacetime is so big we are separated so much we receive very little, and like Harry says our eyes are not sensitive enough. We do not see the nebulas like the APOD photos show!

Hello All

G'day kovil, good on you mate.

You are but a few who have lateral thinking.

Yes, the night sky is dark because the universe is finite , in what we can see of it.

I'm not sure if you mean that the Universe is infinite (in time and extent, like Harry believes) but that we can only see a finite part of it. Because in an infinite eternal Universe we should be able to see everything.

For an eternal Universe there would have been an infinite amount of time for the light to have travelled so it could reach us from everywhere. In the BBT there has only been a finite amount of time (about 14Gyr) for the light to travel, so we can only see the Universe within a sphere of radius 14Gyr x speed of light (in fact its bigger than this because of the expansion, but this is good enough for this discussion).

Of course things futher away look fainter, but the further away you go the more of them there are. In other words the surface density increases with the square of the distance, exactly balancing the fact that the individual sources are decreasing in brightness with the inverse of the square of the distance.

Olbers point was that in an infinte eternal Universe that is relatively homogeneous (like what we see) every where you look your line of sight will end up on a star. So that overall the night sky should be as bright as the sun is. The fact that the night sky is dark is fundamentally one of the best pieces of evidence that the Universe is finite in age (or more correcly non-static).

You can't use scattering to get rid of any of the light either, because equal amounts will scatter towards you as away.

astro-uk,

That is the classic argument, and very clearly said may I add !

I see it that there is an inefficiency between infinity and here, and the light falls off faster than the distance adds sources. The universe is not a perfect medium, even the level of entropy will soak up something, and reduce the perfect mathematical equivalence between distance surface area and the reverse percentage of surface area from the emitter.

And that is the main reason the sky is dark.

Additional reasons are our eyes are not a sensitive as CCD's and photo film. We cannot take a long exposure with our eyes, and so are limited in our light gathering abilities. 1/20 of a second? is our refresh rate? That's why movies are 24 and 25 frames per second?

And the theoretical premise of there is a limit from which we get no more information of a speed of light nature. So that effectively cuts off the infinite time to reach us and infinite size of the background for stars to cover, and we are left with only as far as the awareable universe extends. That is theory and not proven tho.

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Yes the universe is infinite, it must be; but just as must be, we can only see a finite part of it. So either way, either way; the most we can ever see or get information from, is finite.

Hello All


In addition to what kovil said

Deeper we see although more we should see.

What happens the clusters of super galaxies look like stars and our ability to see them is limited by our tools.

Prime example is when a telescope focuses longer we see more.

Man has always thought that they are it and at the centre of all.

We know that the universe is infinite.

What we don't know are the parts that make it up.

So we study the parts and try to put the puzzel together.

As for time and space,,,,,,,,,,,,,,,,,,,,what do people know or even understnd of it.

I have come across some wild ideas and theories. Who knows some maybe right and maybe I'm on the wrong side of the fence.

Hi Kovil

Well the efficiency of our eyes is actually irrlevant in this argument. You can follow through the maths and prove that if stars are homogeneously distributed (on average) the surface brightness is equal to that of the Sun, so our eyes will see something as bright as the Sun. All the trillions of stars add up to produce something as bright as the Sun, all that the efficiency of our eyes does is change the scaling, either making the Sun appear brighter or fainter, but the sky will always look exactly as bright as the Sun, regardless of how sensitive our eyes are.

You also can't have light being absorbed by intervening material that remains dark, the second law of thermodynamics prohibits that, the material would simply heat up and begin to radiate itself, the energy of the light would get shifted in wavelength but we could still see it dominating the sky at some wavelength. Many people have tried to use just this approach to explain the CMB as scattered or reradiated starlight, but this can't be done because of the blackbody nature of the CMB.

The Universe may well be infinite in size, it just can't be infinite in age.

Sometimes I forget what initials stand for; so I looked them up. CMB
http://lambda.gsfc.nasa.gov/ In case anyone else is interested.
I don't believe that there are enough stars in the sky to fill all the gaps between them. Even though there are billions of galaxies with billions of stars in each galaxy; space is so vast that there isn't enough stars to fill all the gaps between them. The expansion of space will only make these gaps larger.
Orin

Thats the point.

In an infinite (in extent and time) Universe there would be no gaps. There are only gaps because we can only see so far, and we can only see so far because the Universe is not infinite in age.

Hello All

Astro said

In an infinite (in extent and time) Universe there would be no gaps. There are only gaps because we can only see so far, and we can only see so far because the Universe is not infinite in age.

The above quote is not correct, limitations on light travel are finite and therefore in an infinite universe, you could not see ALL. Even if you use the top telsecopes and focus for a year or so or more.

Its just amazing that every time I come across a big bang person they put their guards up to protect the big Bang theory.


You could say that according to the Big Bang the universe is not infinite, but the Big Bang occured everywhere at the same time. How far is finite? To what we can see or what we are able to see in ten years time or 20 yrs time and so on.

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Orin said

The expansion of space will only make these gaps larger.
Orin

What do you mean by expansion of space?

Time or distance or both.

There is no evidence to indicate expansion of the universe. Although we know that parts do expand and some parts contract. Observations of the superclusters of clusters of galaxies tells us its more contraction than expansion.

Harry: I really don't need to argue, but you might read this. I know you won't believe it; and you don't need to; but here it is. http://www.wonderquest.com/ExpandingUniverse.htm
any way I don't see how it affects us as our moment in time is very fleeting compared to the life of the universe.
Orin