![Confused :?](./images/smilies/icon_confused.gif)
Orin
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.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.
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.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.
Well, space is mostly empty and dark because there's just so much of it. 8)orin stepanek wrote:So; in the end; light is light. So why is space filled with so much darkness?![]()
Orin
http://en.wikipedia.org/wiki/Speed_of_lightIt 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]
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.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
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.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.
Thanks astro_uk I think that is a good way to put it.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.
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.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.
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.Yes, the night sky is dark because the universe is finite , in what we can see of it.
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.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.
What do you mean by expansion of space?The expansion of space will only make these gaps larger.
Orin