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Light in our universe.

  1. Jul 22, 2011 #1
    Ok this is a difficult question but i'll try to keep it simple and if I say anything that is wrong, if it doesnt change the basis of my question or the answer then just ignore it.

    My question is related to particles that are given off by stars, and perhaps other places.

    Light is everywhere in space, stars give off light in all directions, everysquare inch of space is filled with millions of particles. If not then there would be points in space that when you were inside of them you would look around and see nothing. Since you can always see stars then there is always light.
    So my question is why doesnt the neverending stream of particles that fill this universe add to its weight or energy, or does it?
    I know that photons and neutrinos have a very very small mass but surely in the numbers that they exist in in the universe the overall weight has to be substantial.
    Like plankton, they are one of the smallest living things on earth but in terms of total weight they are one of the largest.
    Also if you can always see something so there is always light, then the entire universe is filled with light, so can there be a limit on how much light can be crammed into our universe, or does expansion solve that?
    I hope you all get my main point and can provide some insight on the subject.
  2. jcsd
  3. Jul 22, 2011 #2


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    It does add. When people do an inventory of all the forms of mass/energy in the U they include the part that is in the form of photons.

    Here is a sample inventory:

    Here is the same thing posted at arxiv.org.

    It is the 2004 inventory by Fukugita and Peebles. You probably know of the cosmologist James Peebles by reputation.

    Note that as a fraction, dark energy is showing up as around 0.23, that is between 1/5 and 1/4 of the total, but light or electromagnetic radiation in general is showing up as around 1/10,000 or one in 104

    Not very much of the available mass in the stars has been converted to radiation yet. Not all of the total mass has even condensed into stars yet.
    The original CMB radiation is not a large fraction either.

    But yes, cosmologists keep track of it in the inventory.
    Last edited: Jul 22, 2011
  4. Jul 22, 2011 #3


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    Hi YoungDreamer! :smile:
    Yes, photons have energy and therefore mass …

    they exert a (very small) gravitational effect

    (so do neutrinos)

    but I don't know what the proportion is.
    photons are bosons, so no there's no limit (except that if you concentrate enough photons into such a small volume that you raise the temperature to that of the big bang, then presumably something nasty's gonna happen :wink:)
  5. Jul 22, 2011 #4
    But with all the stars that have ever existed with all the light that has been emitted over billions of years then shouldnt the universe already be packed with photons.
    And if it is packed with photons could it be heating up the universe but the universe is turning that heat into momentum and we see it as expansion?
  6. Jul 22, 2011 #5


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    Did you check the inventory?

    the fraction that is CMB photons is given as around 10-4.3

    the fraction that is "poststellar" EM radiation is given as around 10-5.8

    these are negligible amounts compared with the amount of ordinary matter.

    The explanation for why distances in the U are expanding is contained in a very simple equation called the Friedmann. Actually two equations. Google "friedmann equations".

    You will see that positive pressure tends to SLOW expansion, also matter and energy SLOW expansion. So you have it backwards. The light with its heat etc would not tend to contribute, it would slow expansion.

    I don't know how you can begin to get intuitive understanding until you have put full effort into understanding the Friedmann equations. They are a simplified version of the Law of Gravity represented by the Einstein field equation. They are the law of gravity which we use in practice, and which governs both contraction and expansion of distance. They are very simple equations involving simple familiar things like the density of mass energy in the U, and pressure or momentum, and the Hubble expansion rate parameter. My advice would be not to try to find an explanation for why distances currently expand based on your familiarity with gasses fluids metalobjects. It does not work like that. Your intuition will mislead you until you have studied the governing equations. You could get someone to help you understand. They are college freshman calculus level. But a good highschool math student could understand them too.

    Verbal understanding fundamentally does not cut it.
  7. Jul 22, 2011 #6
    Ok I looked at the inventory, for some reason I just figured that the amount of photons had to be so high that they would have significant mass. But I guess the fact that it doesnt shows me that I didnt have a very good grasp on how small of a mass a photon actually had.
  8. Jul 22, 2011 #7
    Does the photon carry the whole spectrum of light or just the visible wavelengths?
    If it only carries the visible spectrum are there other particles for each wavelength?
    If it carries all wavelengths then a photon would be carrying gamma rays and x-rays as well right?
    And if it carries these other wavelengths why arent photons extremely dangerous?
  9. Jul 22, 2011 #8
    A better question to that last part would be if the photon carries all the wavelengths how do we get one wavelength over another.
    What I mean is if we wanted to take an x-ray how do we get the x-ray wavelength to show over the other wavelengths.
  10. Jul 22, 2011 #9


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