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Do we observe fast moving objects in the universe?

  1. May 5, 2007 #1
    Do we observe anything in the universe that is traveling at relativistic speeds?

    If something actually does travel at relativistic speeds, would we be able to observe it?

    I can imagine that a light source that´s (more or less) traveling in our direction at say, half the speed of light, would be invisible to the naked eye. The frequency we measure would be double compared to it being stationary compared to us, wouldn't it?

    I´m confused about that everything we observe seems to be more or less in our speed range. Is there anything we observe traveling at for example 0.1 c? (not counting the expansion of space)
  2. jcsd
  3. May 5, 2007 #2
    Subatomic particles, such as those produced in the atmosphere by cosmic rays, or in accelerator labs. And photons, of course.
  4. May 5, 2007 #3
    Ah yes, the small particles, but is there anything at our size or planetary or solar size that travels at relativistic speeds?
  5. May 5, 2007 #4
    Distant stars, galaxies, etc, travel away from us (in a loose kind of way) at relativistic speeds (hence, astronomers speak of redshift).
  6. May 6, 2007 #5
    The brightness would be greater as well. In principle, we can observe things going very fast, but this is rare, from which we can conclude that most things move slowly compared to the stuff around them.

    This is trivial in some sense. If something were going very fast compared to everything else, it would collide hard into lots of everything else, and get slowed down, until everything had about the same average velocity. This is basically thermal equilibrium (see inflation theory). It's a reason why the planets in the solar system all orbit in the same direction. Likewise, if you start with a mixture of matter and antimatter, you're likely to end up with only one kind.

    Perhaps it seems profound in another sense. Since the big bang is like a kind of quantum fluctuation associated with no specific location, where does a preferred direction arise from? Has it anything to do with Mach's principle? Will we eventually find the direction symmetry breaks down, just like the antimatter symmetry seems to? Are these the kinds of thing you're asking about?
  7. May 6, 2007 #6
    But that is because of the expansion of space, isn't it? They´re not really going that much faster than we are (i.e. if space didn't expand, we wouldn't observe the redshift.)
  8. May 6, 2007 #7
    That makes sense, but the universe is so empty. Light reaches us from the edge of the observable universe, so I could imagine that some objects of reasonable size still would be able to travel without bumping into anything as large or larger than it self. Or maybe bumping into small particles is enough to slow it down?

    is there some relation between size and speed? When 2 galaxies colide they do so because of gravity pulling them together, but why doesn't that happen at a relativistic speed? billions of years can pass before they actually reach each other, so why haven't they reached relativistic speeds after so much time
  9. May 6, 2007 #8
    Some quasars (galaxies) emit gigantic jets of relativistic gas.

    Pulsars, rapidly rotating dying stars, are relativistic.

    The primary confirmation of SR comes particle physics.
  10. May 6, 2007 #9
    Don't know what you mean by "faster than we are". Distant stars are moving extremely fast relative to us. I also don't know what you mean by "expansion of space". Everything in the universe is flying apart at high velocity because they were ejected in the big bang and still have momentum left over, and also because of dark energy.

    You are realizing that these objects have been being pulled together for astronomically long periods of time, but you are apparently not realizing that they are also astronomically far apart. The further apart two objects are, the less gravitational acceleration they feel. That's why you don't feel pulled toward the Earth when you're on the moon.

    By my math, the Andromeda galaxy is pulled toward the Milky Way at:

    a = F / m2 = G m1 / r^2 =6.67x10^-11 x 1.15x10^42 / (2.37x10^22)^2 = 1.36x10^-13 m/s/s

    That's not very freakin' fast. A billion years is only 3.15x10^16 s, so assuming that the above acceleration holds constant for a billion years (which I'm guessing it doesn't), the andromeda galaxy would gain only about 4284 m/s in that time. Right now, it's coming toward us at 300000 m/s.

    Use the math, it's more reliable than intuition :smile:

    EDIT: could someone check my numbers and my math? Those numbers look a little fishy for some reason.
    Last edited: May 6, 2007
  11. May 6, 2007 #10
    There are two interpretations to the observable red-shift:

    1) Galaxies, stars, etc are actually moving away from us and the light is being doppler shifted.

    2) The expansion of space during the time that the light traveled causes the wavelength to stretch.

    Xezlec, you believe (1), and Peter believes (2). In fact, all currently active astrophysicists agree that the answer is (2). Unfortunately, (1) is just a fiction that was invented to talk to beginners about the red shift.

    To reiterate, this is complete fiction. Unfortunately you will see this statement all over the web, and in popular level books, but you can't find it in textbooks beyond the intermediate college level or in respectable research journals.
  12. May 6, 2007 #11
    First of all, if I can't trust intermediate college level texts, what can I trust? That's my entire education.

    Second, why wouldn't the galaxies have leftover momentum?

    Third, what is "expansion of space" if not objects flying apart? Is there some other possible interpretation of that phrase? If a baloon expands, the patches of rubber on opposite sides are certainly flying away from one another. Hence the word "expand".

    And fourth, how can distant stars not be moving relative to us? That seems amazingly improbable. What force is holding them apart, at fixed locations?

    Fifth, since "popular media" say that the andromeda galaxy is heading toward us, does that mean what's really happening is that the space between us and them is "shrinking"?
    Last edited: May 6, 2007
  13. May 6, 2007 #12
    how fast?
    you mean like the cars in f1?
  14. Jan 19, 2008 #13
    As I understand it, space expansion only moves matter with it on huge distances. Locally the expansion of space, basically expands through us. So the andromeda galaxy and the milky way are getting closer to each other because of gravitation. Distant galaxies are moving away from us because the space between them and us is expanding. I don't really understand why space would be taking matter with it on those huge distances while it doesn't do that on smaller distances. But maybe it does actually do that on smaller distances but it's so small it can be ignored.

    But if it would do that too on smaller distances, then aparently not on particles. I mean, atoms don't grow with the expansion of space, otherwise we wouldn't be aware of any expansion and expansion would be irrelevant or non existing.

    Which raises the question of course: Why doesn't the expansion of space take matter with it on atomic scales while it does on cosmological scales?
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