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Planck Time

  1. Jan 20, 2010 #1
    In the most recent theory I have heard, just in the first moments after the Big Bang strange things happened. I heard that there was not four fundemental forces, just one, or superforce. I also heard that the laws of physics did not apply.

    How long was this time. As from research I have seen that the particals reached a lightyear in a second. According to Einstein, nothing can travel faster than light, so that law of physics did not apply at this point. But from other reading I have read that this "Planck time" only existed for perhaps a millionth of a yontosecond.

    Can someone clear out the cobwebs for me.
  2. jcsd
  3. Jan 21, 2010 #2


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    Correct, the forces of the universe were combined at the instant of the BB. Gravity is believed the first to separate. It is unclear when this happened. Apparently a fair amount of inflation occured before gravity jumped in.
  4. Jan 21, 2010 #3


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    Thanks for your question Harry, I think I can answer it.

    First off, the Planck time (around 10-44 sec) is the moment at which a particle's Compton wavelength (the scale at which quantum effects become important) becomes equivalent to the particle's Schwarzschild radius (the scale at which gravity becomes important). Therefore, it defines this moment when two important kinds of physics are both relevant, namely, quantum theory and general relativity. There is currently no known theory that successfully incorporates both quantum theory and general relativity, and so that's why you might hear people say that the "laws of physics do not apply here." However, it is widely believed that some theory applies here, it has just yet to be found.

    However, current physical theories are capable of describing the universe after the Planck time. These theories are consistent with a universe in which 3 of the fundamental forces (electromagnetic, strong & weak nuclear) were unified into a single "superforce". It is conjectured that if we were to rewind the tape all the way back to the Planck time, gravity would join this "superforce" (and I suppose make it a super-superforce). However, whether this does (or even should) occur is not known.

    Lastly -- your question about the expansion. Since we don't know the physics of the Planck era, we don't know how the universe was behaving -- we have no idea if it was expanding, contracting, or just sitting idly. However, after the Planck time, the universe was expanding. At this time, Einstein's special relativity is certainly applicable, and so we expect all particles to obey the speed limit v < c. Particles can't move a light year per second, so perhaps there was more to that story. What can happen, is that two particles can have relative velocities that surpass that of light.This is of course still troubling from the perspective of special relativity. However, everything is OK, and here's why: the particles themselves are at rest in space -- it is the space itself that is expanding, causing these particles to separate. There are no rules limiting the speed at which space itself can expand. In cosmology, we often see particles zipping apart from each other at speeds greater than that of light.
  5. Jan 26, 2010 #4
    But I thought light was not relative.
    Didn't Einstein say that if an object was going even a mps off the speed of light the speed of light would remain c distance away from the velocity of the second body. Light is not relative top anything, and is always C, regardless of the motion of the observer. I thought that was why it was called the universal constant as if it was relative it would never actually be at the speed C if the observers frame was important.
  6. Jan 28, 2010 #5
    I hope they won't call it a super-superforce :-)

    Anyway, there is quite a bit of speculation involved when extrapolating the Standard Model from TeV scales up to the proposed GUT scale (not to mention the Planck scale). Much new physics can occur in that range that is not taken into account in this extrapolation.

    I for one do not trust cosmological statements about how the universe evolved when its energy density was above the energies that we are currently able to probe. I'm not sure what time that was, though.

  7. Jan 28, 2010 #6


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    You're absolutely right to be skeptical. Personally, I'm moved by the MSSM's unification of the coupling constants at around 1016 GeV. From a symmetry restoration perspective, I'm inclined to think that some form of unification occurred for the gauge forces. I'm not sold on gravity though...
  8. Jan 28, 2010 #7


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    HarryDaniels: in your original post you write: But from other reading I have read that this "Planck time" only existed for perhaps a millionth of a yontosecond.

    I think you meant to say “yoctosecond”, which equals 10^-24 second.
  9. Jan 29, 2010 #8


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    The biggest part of expansion most likely occured between t=0 and t= 10^-24
  10. Jan 29, 2010 #9
    It's likely that some set of laws of physics did apply. We just aren't sure what they were.

    Einstein never said that nothing can travel faster than light. This rule is a simplification of what he actually did say.
  11. Jan 29, 2010 #10
    Yes. That's the basic rule. But that rule doesn't translate into "you can't have objects move away from each other at faster than the speed of light."
  12. Jan 29, 2010 #11
    One thing important point here is that the massive expansion of the universe (a.k.a inflation) happened quite a bit later than planck's time. Planck's time is 10^-44 seconds, whereas inflation happened before 10^-32.
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