Greetings from Paris! Just checking in to do a bit of self-promotion, from which no blog-vacation could possibly keep me. I’ve written an article in this month’s Scientific American about the arrow of time and cosmology. It’s available for free online; the given title is “Does Time Run Backward in Other Universes?”, which wasn’t my choice, but these happenings are team events.
As a teaser, here is a timeline of the history of the universe according to the standard cosmology:
- Space is empty, featuring nothing but a tiny amount of vacuum energy and an occasional long-wavelength particle formed via fluctuations of the quantum fields that suffuse space.
- High-intensity radiation suddenly sweeps in from across the universe, in a spherical pattern focused on a point in space. When the radiation collects at that point, a “white hole” is formed.
- The white hole gradually grows to billions of times the mass of the sun, through accretion of additional radiation of ever decreasing temperature.
- Other white holes begin to approach from billions of light-years away. They form a homogeneous distribution, all slowly moving toward one another.
- The white holes begin to lose mass by ejecting gas, dust and radiation into the surrounding environment.
- The gas and dust occasionally implode to form stars, which spread themselves into galaxies surrounding the white holes.
- Like the white holes before them, these stars receive inwardly directed radiation. They use the energy from this radiation to convert heavy elements into lighter ones.
- Stars disperse into gas, which gradually smooths itself out through space; matter as a whole continues to move together and grow more dense.
- The universe becomes ever hotter and denser, eventually contracting all the way to a big crunch.
Despite appearances, this really is just the standard cosmology, not some fairy tale. I just chose to tell it from the point of view of a time coordinate that is oriented in the opposite direction from the one we usually use. Given that the laws of physics are reversible, this choice is just as legitimate as the usual one; nevertheless, one must admit that the story told this way seems rather unlikely. So why does the universe evolve this way? That’s the big mystery, of course.
James, there are certainly good questions to be asked about how we divide the many microstates of the world into a smaller number of macrostates. I believe that some ways of doing the division are naturally picked out by the laws of physics, but it’s still an open question.
However, it doesn’t really matter. In terms of what we now call “broken” and “unbroken” eggs, there are certainly many more ways to re-arrange the molecules within the set of broken eggs than within the set of unbroken eggs. That’s an objective fact about the world, so you can’t just choose to insist that broken eggs have a lower entropy. You could choose some very specific form of broken eggs to define a particular kind of macrostate, and if you do it carefully there will be very few microstates that correspond to that choice; but in that case, unbroken eggs will almost never evolve into that specific form, so it will still be true that entropy almost always increases.
Excuse my ignorance, is entropy behaviour a classical or non classical result?
In a small particle system ( < 1000 newtonians simulated on a computer) it seems obvious that “chaos” is the “norm” without recourse to quantum mechanics.
At what number of particles does it become necessarry to “invoke” the existence of quantum microstates?
Sean’s article is just one of many which fails to even address this simple question.
So many messy problems with the idea of reversing time. First of all and most importantly you would reverse the second law of thermodynamics. This law is not reversible as all the other laws of physics are. The Universe would not start going backward in time if it did happen to start to contract – do you get younger when you drive in reverse? Peter Lynds has done a fine job of solving the problems of the Big Bang. Also, has Lynds points out, if the Universe was created in a big bang, why was there an infinite amount of time before it decided to do so? Seems absurd. Why now? The only rational solution to Kant’s thoughts is the Universe is cyclic. Gravity is a force that can cause the Universe to return to the primordial soup once again. Lynd’s theory that singularities are not allowed, along with the second law of thermodynmics forces the Universe to Bang yet again as it has forever.
And yes, a cylic Universe would imply that eventually in some distant cycle I will be typing these exact words under my own free will again. Cheers!
Re “I will be typing these exact words under my own free will again,” one question to ponder is whether the “I” typing those words will be the same “I” typing your post, although certainly in no case aware of prior “I”s. In Nietzsche’s inconclusive consideration of “perpetual recurrence” he thought the same “I” would experience the identical same life all over again but he found the idea “horrifying.”
As a latecomer to these discussions, I wonder if the ideas of Julina Barbour (“The End of Time”) have come up?
Basically, he posits that there is no time–only NOW.
My analogy is that of film in a motion picture projector. As per Barbour (my interpretation), only the frame actually in the projector window is “reality”.
P.S. Instead of the “Arrow of Time”, does Barbour suggest THE ARROW OF ENTROPY?
Following the motion picture projector analogy, the film spool that collects frames that have already passed the projection port are “expired QUANTUM EVENTS” (history?). The film spool that contains frames remaining to pass through the projection port are QUANTUM POSSIBILITIES. The sytem is reversible.
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