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Does the universe need an escape velocity?

  1. Jan 26, 2006 #1
    The question is, Does the universe (the conglomeration of all energy in existence) logically require an escape velocity?

    My reasoning is thus. If the universe is at least partially composed of massive objects that exert gravitational pull, wouldn't the sum of every massive object in the universe and it's corresponding gravity factor into some finite (although perhaps unknowable) force from which we could calculate a theoretical escape velocity - what we'd escape into, I'm not sure :eek: ?

    If the answer turns out to be beyond the speed of light, wouldn't that mean that we are in some type of a black hole?

    This hardly seems logical but I can't locate the break.

  2. jcsd
  3. Jan 26, 2006 #2


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    It's an interesting question. Keep in mind that the notion of an "escape velocity" necessarily involves some gravity well with finite extent. You start out with some minimum gravitational potential energy, and continue to gain gravitational potential energy forever. It implies that you can move away from the object from which you are escaping.

    On the other hand, the universe's geometry is "closed" in the sense that you cannot leave it, no matter if it's finite or infinite in extent. Also, one of our tacit assumptions is that the universe is homogenous at large scales -- that is, the matter density is roughly the same in every reasonably large volume of space.

    This means that, while you can move away from one galaxy and "escape" it, you're necessarily moving toward some other galaxy, and will end up bound to it. There's no way to get away from ALL the matter in the universe at the same time, because the matter density is believed to be constant throughout its entire extent.

    - Warren
  4. Jan 26, 2006 #3
    Okay but, all that being said, wouldn't there at least be some sort of rim to this gravitational well (even if we could never reach it), which, provided we knew the mass of everything in the universe, would be calculable?

    A similar question would be, if for whatever reason 1/3 of all the mass in the universe was simultaneously converted entirely into energy, wouldn't this produce some sort of gravitational wave or waves?

    If so, where would this gravitational wave expand into?

  5. Jan 26, 2006 #4


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    No, the universe does not have a 'rim.' If it's finite in bound and curved, like the surface of an apple, there's no place you can go to be "away" from the rest of the matter in the universe.

    If the universe is infinite, and homogenous everywhere, you still cannot go anywhere to get "away" from the matter.

    - Warren
  6. Jan 26, 2006 #5
    ...And that's the stumper.

    Isn't there necessarily a fundamental disconnect somewhere then, between our understanding of local gravity, which is intrinsically connected to finite distances, and the universe's macro-gravity behavior?

    The closest conceptualization I can think of is the inside of a black hole. Hmm.
  7. Jan 27, 2006 #6


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    It does not 'matter' if matter is converted to energy. Energy exerts the same gravitational influence on the universe as does its matter equivalent. It would have a local effect [e.g., if the sun converted a significant portion of its mass to energy, planetary orbits would change], but it would not make any difference on cosmological scales.
    Last edited: Jan 27, 2006
  8. Jan 27, 2006 #7
    "Energy exerts the same gravitational influence on the universe as does its matter equivalent"

    Cool, I've been wondering about this. I guess this means that an individual photon has gravitational influence (albeit small). Okay but bear with me hear: this means that the total gravitational force of the entire universe since the Big Bang has not changed (except locally), it's just converted from mass to light and vice versa?

    One last, seemingly unrelated, question - is the force or amount of dark energy changing or has it changed OR has it kept the same ratio (as we factor now) since the BB?

    Sorry, sometimes I feel like I'm spamming this site with questions. :confused:
  9. Feb 12, 2006 #8
    Only a theory because I haven't been able to exact a value to the mass and velocity of all particals durring the first few moments post big bang, but, escape velocity from falling back into singularity may have been achieved by "the draw of the nothing on the something." Do you remember the jr high experiment with a single drop of oil in a vacume, the mass tends to expand to fill the space available.
  10. Oct 16, 2010 #9
    well, plugging in some rough values, this is really an order of magnitude back of the envelope thing. mass of the universe is something like 1.6e60 kg and just for entertainment instead of setting it equal to 1 or 1000 the radius of the earth is 6e3 meters. but if you had the means to make such speed, would it really matter?
    (2*1.6e60*6.7e-11/6e3)^.5; the escape velocity goes to something like.. 1.9E23 m/s (really just something something e23) a good idea of what it might take to 'escape' the universe. but judging from the guys above i dont really see a point in trying. so i guess if you wanted to know the kind of speed you would need to wiz by galaxies without fear of them manhandling your spacecraft, i would use that speed. btw. such speed far exceeds the speed of light
    3E8 m/s.
    if you performed E=mc2 on the seawise giant, the biggest ship in the world, you would have about the right kind of energy to make voyager 1 attain that speed (einstien's nagging not withstanding). interestingly that kind of energy is only 5 orders of magnitude short of being able to smash one hemisphere off of earth. but its still 2 orders higher than the Chicxulub event. we'd all be dead thanks to voyager and his big sea giant friend. But if it ran into the sun. no problems at all. pft! where'd it go?
    Last edited: Oct 16, 2010
  11. Oct 18, 2010 #10


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    severnIJKGU's answer implies a finite mass to the universe--such a univese would have an escape velocity.

    In the assumption that if you move away from one object you must be moving toward another, this assumes the amount of matter in the universe is infinite. In this case, there is no possible escape velocity.

    In what way does energy exert gravitational influence? Does this include radiant energy, thermal energy, kinetic energy, spring potential energy, electrical potential energy?
  12. Oct 18, 2010 #11
    pardon my back of the envelope, im an engineering student. if you are a professor or a high level student, my hat goes off to ya.
  13. Oct 21, 2010 #12
    This makes me ponder about the idea of parallel brane universes where gravity seeps from one universe to the other. It makes me wonder if there would be an escape velocity needed to prevent a collision of two brane universes.
  14. Oct 21, 2010 #13
    Pardon my foolery, but could someone point out how a photon, for instance, exerts a gravitational force on another particle? As far as I was aware, the particle could experience a gravitational pull but not exert one. I was under the impression that mass is a requirement to exert any gravitational force because higher mass warps spacetime with greater effect. I'd appreciate if someone could clarify this.
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