Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Where photons go to die

  1. Jul 18, 2004 #1
    I'd like to understand something. Namely the interaction between gravity and light.

    Here's what I know: (or what I think I know -- not a physics wizard. And I hate it when people use the word 'wizard' in such manner. But that's irrelevant.)
    1. Gravity bends the path of a photon because the space around it is curved.
    2. Time slows down inside a gravity field.
    3. The reach of gravity is infinite and it propagates at the speed of light.
    4. Photons spend some of its energy when travelling through a gravitational field. (I'm not so sure about this one, actually. I think it's called a gravitational redshift.)

    Here's what I don't know: (I don't know many things, but still.)
    1. If gravity has effects on time, can time exist without gravity? If there were no particles with mass in our universe, would time still exist? Are mass and gravity needed for space to exist? Or in a massless universe, would time pass... quicker? (As much sense as that makes...)
    2. Does time pass at a different speed when observed by a person in intergalactic space and a person on Earth? Suppose synced clocks were given to two people. One sits on Earth the whole year and the other pops into intergalactic space where there are no bodies of great mass for lightyears and then he would pop back after a year. Would the clocks show any difference?
    3. Could a gravitational redshift explain the observation that everything is moving away from us in space? (Redshifts.)
    4. What happens to a photon when it... (this will sound stupid, hold on to your chair) runs out of energy when flying through a gravity field?
    5. Can the expansion of the universe be tied to the expansion driven by gravity? What I mean is could gravity be creating space at the boundaries of our universe? Related to question 1, I suppose.

    And now for two unrelated ones...

    6. Could the ever-elusive tachyon actually be a particle that acts like the Alcubierre drive, but does it naturally? How would normal matter react when driven through by a spaceship with the Alcubierre drive?

    Yes, well, some questions are probably off-topic and some are probably stupid... Anyway, I'd appreciate any and all replies to these questions. They've been bothering me for quite some time now.
  2. jcsd
  3. Jul 19, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  4. Jul 19, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Ah, I found it.

    it is : "introducing Einstein's Relativity"
    by Ray d'Inverno

    It is readable if you have a good knowledge of high school mathematics (or a little bit beyond).

  5. Jul 19, 2004 #4
    I thank you for your reply. It helped me much. Though I do have a few follow-up questions, if you don't mind.

    How can photons gain and lose energy when they travel? Is the energy lost and gained based on the curvature of spacetime at its location or does the photon use its 'energy' to do 'work' to move through curved spacetime? What I mean is when a photon moves through spacetime that's curved, can it eventually run out of energy? Or can it be compared to potential energy from classical physics? For example, if you took the rubber sheet analogy, then when there are no marble balls making a dent in it and the sheet is completely flat, then the energy of a photon on it would be maximum, but when it falls into the lowest possible dent, it'd be minimum?

    If it's more like potential energy then a photon that falls into a singularity would be frozen in time? The time dialation at a singularity would cause that?

    Does the redshift tell us the difference between 'flat' spacetime and our spacetime or does it tell the difference between the spacetime at where it was created and our spacetime? (By 'our' I mean the observer's.)

    (Well, I probably have it all ass-backwards, heh.)

    You seem to imply that the redshift does not mean that galaxies are moving away from us, but that the light's shift is caused by gravity. I remember watching a short series on cosmology (had Hawkins in it and all) where they said that everything is moving away from us and that it was first measured by a guy named Hubble. So does it mean that galaxies are not moving away from us? That the expansion of the universe could possibly be not true and everything might just be collapsing right now, but we're still measuring gravitational redshifting and thinking that it's related to distance? What I mean is when gravitational redshifting is compensated for in the measurements, then are the galaxies still moving away from us?

    I've heard that the universe could have expanded faster than now after the big bang -- does it mean that the speed of light could have been faster as well? You did say that the expansion of our universe's gravity field is the same as the expansion of the universe. Can bubbles without gravity exist? Would spacetime exist in those bubbles? Or better yet, if humans do ever master faster-than-light travel and go beyond our universe, what is there? I've heard buzzwords like 'n-space' and 'h-space' being used.

    The theory of relativity with its curvatures and the like reminds me of the few things I've heard about aether theories. That is, if you were to think of gravity as the 'density' of space at some location. Density that can slow down time.

    Oh and here's an idea -- if white holes exist (theorethically, they could, right? Or does that fact that black holes emit radiation mean that there's no need for a white hole?) would time slow down or speed up near them compared to the 'flat' background? (Oh and when a black hole has evaporated enough for its mass to fall below the critical mass needed for a singularity to form, does it... explode? Turn into a star? A ball of superheavy elements? Would it emit energy much like a star, but not via fusion, but fission?)

    Okay, that's probably more questions than anyone could answer. I really haven't found a good book on this, but if anyone could recommend something that explained in layman's terms what physics are all about without getting into complex equations and such, then it would be nice. (I've already ordered "Euclid's Window" -- promises to be something in these lines, but I doubt it can answer all my questions.)

    It's just that I want to know, but I don't want to keep bugging you people here and I can't seem to find any good books on this either. So I'm stuck :redface:

    But do look on the bright side -- if someone replies to these questions, then you can in the future point anyone asking similar questions to this thread :)


    Thanks! I'll look into that book. My math skills are alright -- I'm majoring in computer science :)
    Last edited: Jul 19, 2004
  6. Jul 20, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is very close to that idea. If a photon has to "climb out of a potential well", it looses energy (wrt an observer at a distance). If it "falls into it" it gains energy. Just as a marble does.

    There is of course only one spacetime, and the event of creation of the photon and the event of observation are both part of it. So the redshift is a property of that whole chunk of spacetime envelloping both events.

    No. I should have been more careful when I said this. If you insist on a large (but not too large) piece of spacetime around us (say, up to 1 billion light years from here) to be "classically flat", then the redshift should be interpreted as the sources moving away from us ; it is a kind of an approximation, just as newtonian gravity is a good approximation to the local curvature of spacetime around the earth. That's what people usually do when they give popularizing talks, and it works quite well for not too great a distances. However, at very large distances, you get funny results, such as relative receding speeds bigger than lightspeed and so on. This is simply because the flat space+moving approximation becomes badder and badder.
    From a general relativity point of view, it is the space that is stretching between us and them, which is a gravity effect (but not a local one of course, it is the dynamics of spacetime on a cosmological scale that does it).
    So saying that galaxies move away from us in a classical context, or that they are "not moving but space is expanding" in the general relativity context, is _almost_ saying the same thing. The results differ at very great distances, but at relatively small ones, both are comparable.
    If this sounds mysterious, then just pick up a book like I suggested, and look
    at the Friedman model of the universe.

  7. Jul 20, 2004 #6
    It does sound mysterious to me. Suppose you were sitting in the center of our galaxy and suppose that the nearest galaxy to ours had a mirror in its center. If you were to send a photon to that mirror, it'd bounce back and return to you in x years. If you were to repeat that experiment after you receive a reply, you'd get another reply in y years. Which number would be bigger?

    Also, I heard that another galaxy was on a collision course with ours. Does that mean it is moving faster towards us than the space between these two galaxies is expanding?

    And to keep the discussion on-topic...
    ...A photon reaches its maximum possible energy when it enters a singularity? But if the mass there is infinite, then the energy of a photon that falls into it must also become infinite, but then it should be able to climb out of it as well?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook