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Universal Acceleration and Boltzmann Brains

  1. Dec 22, 2007 #1
    Something has recently occurred to me which I think I should mention here. Boltzmann brains have a definite probability of "popping up" anywhere in the universe as fully formed, sentient beings. Although the odds against such a thing happening are astronomical, there is a probability which can be calculated as to their arrival. Thinking on this vein, it occurred to me that as the solar system accelerates due to universal expansion, the time relative to some stationary reality at the center of the universe would become slower for those within it. If it is the case that Earth will eventually hurtle through space at close to the speed of light, time would pass extremely slowly relative to the center mentioned previously, and the chances of weird quantum anomalies would increase. Does anyone know of calculations/work done concerning this topic? How about speculations as to the possible effects?
  2. jcsd
  3. Dec 23, 2007 #2
    I believe that relative to other definable existing frames of references in distant parts of the universe, the Earth is already hurtling through space at close to the speed of light. For us, we cannot notice anything unusual, but for a hypothetical astronomer (or Boltzmann brain) located at that distant frame of reference, we are moving in slow motion.
    Last edited: Dec 23, 2007
  4. Dec 23, 2007 #3
    Sure, time on Earth would slow relative to something not moving nearly so fast...though my point was not to illustrate the [incorrect] fact that we would notice things slowing down, but to point out that as time dilated on Earth, there would be more of a chance of quantum 'weirdness,' since there would be more time in which strange particle events could take place. There comes a point at which the slow passage of time on Earth is sufficient to counteract the wild improbability of something like a brain's occurring, and anything could happen - even if we here on Earth don't notice anything different by way of time.
  5. Dec 23, 2007 #4
    "Boltzmann Brains" are not a validated principle of physics. However, the hypothetical statistical arguments of an encounter between thermodynamic and quantum mechanic life are fully covered by Oxford philosopher scientist Nick Bostrom in his landmark PhD thesis.
  6. Dec 23, 2007 #5


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    typically more than the speed of light, if you are using standard cosmology and talking about the Hubble expansion recession speed. However in standard cosmology one does not have any time dilation associated with recession speed.
    Signals are stretched out by the redshift though.

    all the galaxies are approximately at rest with respect to the expansion flow, and all have roughly speaking the same time (minor local variations due to depth in gravity well etc. and proper motions of a few hundred km/s, but this is almost negligible)

    BTW I like your handle "Seize the heavens!"

    Also I'm not saying you are wrong. But what you and sysreset are saying is not consistent with the usual model that you get with professional cosmologists. So I don't know what to make of it. You seem to be using a cosmic model that I never heard of and I'm suspicious it would not fit the observational data very well. I don't think it's wrong to use nonstandard models, we just have to be clear about it (and it makes it harder.)

    According to the usual model there isnt any time slowdown. And how would you calculate it anyway, with recession speeds that are 2, 3, and 4 times the speed of light? I don't know any formula.

    According to them, when we look at a galaxy that is like at redshift z = 6, then that galaxy is receding some multiple of the speed of light, and if we could watch events occurring there, like birds flying or kids playing ball, there would be no slow motion except for the signals getting stretched out by redshift. Their clocks would be ticking more or less like ours with minor difference due to proper motion and suchlike trivial adjustments. We are talking proper motion on order 300 km/s which is only a THOUSANDTH of the speed of light so it doesnt cause much time dilation.
    Last edited: Dec 23, 2007
  7. Dec 23, 2007 #6


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    since the talk here is about recession speeds, those who haven't played around with Morgan's recession speed calculator might like to check it out


    some directions are at post #94 of the astronomy links thread

    you need to type in three parameters 0.27 for matter density, 0.73 for cosmological constant (Lambda) and 71 for Hubble (71 km/s per Mpc)
    then if you put 6 in the redshift box it will tell you the recession speed out at z = 6

    I just tried it and it said that a galaxy that we are observing today at z = 6 is receding at 2 c TODAY
    and at the time the light we are now seeing took off from the galaxy on its way to us
    the galaxy was receding at a speed of 2.75 c.

    (However since recession speed is not subject to special relativity, there is no suggestion of any time dilation associated with this. Their clocks presumably go the same speed as ours. there is only the 7-fold slowdown of the NEWS getting to us that is associated with redshift z = 6. Wallace please correct me on ths if i have left something out.)
    Last edited: Dec 23, 2007
  8. Dec 23, 2007 #7
    To clear things up a little; I was referring to a reference point affixed, if you will, to a region of spacetime itself. Weather such a thing is possible I am not sure. If this could be done, however, it is my understanding that time on a rapidly moving object would pass slower than time on that affixed point. I then went on to say that this longer period of time would allow for more quantum effects to take place, increasing the chances of a Boltzmann brain's occuring. Thinking about it further, however, I realized that my original assumption about quantum events is incorrect, since the quantum processes which cause brains to arise are also slowed by the dilation.

    Getting back to the topic of expansion, however, I have a basic question which I hope someone can clear up; how can galaxies have recession speeds of greater then 2c, since in my understanding c is the absolute speed limit?
  9. Dec 23, 2007 #8


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    Simple. c is just NOT an absolute speed limit

    it is a speed limit that applies in very special situations
    where special relativity applies

    most galaxies are receding at speeds greater than c
    the socall speed limit does not apply in general situations
    where general relativity applies

    we deal with this over and over at Cosmology forum and various people have various words they say to help make it all right (Pervect, SpaceTiger, Wallace, George Jones, Chronos they would all say slightly different words to help cushion the shock and help you assimilate).

    I don't know what words are best to say, others might say better, but here goes

    1. just because a very long distance between two galaxies which are sitting still in the space around them, just because this very long distance can increase at a rate that is more than one lightyear per year, does NOT mean that you could make a spaceship that could whiz past the earth faster than c, or that something could ever CATCH UP TO A PHOTON AND PASS IT. those are physical impossibilities. they would involve short, local distances increasing faster than c. Change of short local distances is governed by special.

    2. recession speeds are the rates of expansion of distances between two things each of which is at rest

    3. no teacher, no book that pretends to teach, should ever present the speed of light as an absolute limit on the rate that all distances can change. It gives people the wrong idea and makes it more difficult for them to learn general relativity and cosmology later. If this has happened to someone they should try to get over the misconception. It's all one can do. Someone taught you a wrong thing, so you get over it and move on.

    4. in the world we live in it is natural for distances between stationary things to change----by stationary I mean with respect to the microwave background or (equivalently) the expansion flow. Unless a distance is held constant by atomic forces or metal rod crystal lattice forces, or earth bedrock, or the gravity forces of stable orbit, you EXPECT that distance to change. (the geometry of our world is dynamic, dynamic means it changes according to physical law)

    5. in our world we have a criterion for being at rest, which is at rest with respect to the microwave background. if you are moving then there is a doppler hotspot ahead of you and a doppler coldspot behind you, and then you should COMPENSATE your observations.

    the solar system is moving 380 km/s in a direction marked by the constellation Leo, which creats a doppler hotspot in the sea of microwave around us, a hotspot around constellation Leo and a coldspot in the opposite direction in the sky.
    when we make observations that require our reference be at rest, we COMPENSATE for solar system motion and also we can compensate if necessary for the earth's individual orbit motion which is not so fast, only 30 km/s.

    that means the data is adjusted to make it be from the standpoint of a platform in space which is not moving with respect to the background----in other words not moving with respect to the universe, or the surrounding space.

    for practical purposes, a few hundred km/s is not much, and individual galaxies are almost not moving at all, so for many purposes we just assume that two galaxies are each at rest with respect to background
    then the distance between them will typically be expanding as I said earlier---it is what you expect to be happening between two stationary points----on average something like 1/140 of a percent every million years.

    About Boltzmann brains. the actual idea of a Boltzmann brain is charming and I've thought about them some myself over the years---but very speculative of course.

    However there is no TIME DILATION business that applies, of the sort you are talking
    because recession speed does not cause it. special rel does not apply to recession speed.

    it was a good try to think that time dilation could give more time for brains to pop into existence out of the seething vacuum, but it doesnt work (the time dilation idea doesnt work)

    Hope this has helped.
    Last edited: Dec 23, 2007
  10. Dec 27, 2007 #9
    It really has; thanks for clearing some of the ideas up for me. It has certainly given me a more informed outlook...
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