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.
Right. Because there clearly isn’t any difference in the macroscopic laws of physics with respect to time asymmetry.
Can you even imagine how one would go about envisioning the laws of physics spontaneously reversing a supernova, for instance?
Or, to take a simpler example, to un-fry an egg?
It might be a useful visual tool, to play with “what would happen if we ran time backwards,” but what do we learn from this? Anything physical seems unlikely.
Going to the lavatory is not going to be a pleasant experience! Shamelessly stolen from Red Dwarf: Backwards, which makes a comedy of this perspective.
Sean, that was a great read. Thanks.
Angular momentum is the strong arrow of time. Feynman’s sprinkler spins from emission but not in time-reversed absorption. The situation is not symmetric. A motion picture film can with an orthogonal port at its broad face’s center and another port tangent to its edge is an absolute direction of time detector. Fill with water then pump water center inflow to edge outflow. No problem. Reverse time – (nearly) no flow! Conservation of angular momentum creates a fluid diode with no internal obstructions.
Ah, but that’s what’s so fun about computational physics – “set Time Scale to -1”
What I really would like to see in any article on multiple universes is how to test the theory.
I tried to read this earlier, but as I have ADD, I find the written format to be a very poor one for data absorption. That is always a source of disappointment for me, because I love science. I thought to myself, “if only someone else could read this and summarize it for me in an illustrative way”. You have actually done just that in your timeline above. Thank you!
Sean,
You failed to mention Peter Lynds’ model – something which does address the initial conditions problem.
“What I really would like to see in any article on multiple universes is how to test the theory.”
Good point, and it’s long past time to refute people who think that it’s obvious that multiverse theories cannot be tested *in principle*. To be brief: this particular instance of the Popperian delusion is even more ludicrous than most. Just to get y’all started, read these:
http://arxiv.org/abs/hep-th/0612242
http://arxiv.org/abs/0704.3473
In reality, the problem with multiverse theories is to explain why we *don’t* see evidence of the other universes, why they don’t mess up our universe completely, etc etc etc. The whole “it isn’t science because other universes are not observable even in principle” industry is a load of rubbish.
Hi Sean
I enjoyed your Teaching Company Course on Dark Energy and Dark Matter.
I was reading this recent article from yahoo, “Missing Matter found in Deep Space” and would love to know your opinion
http://news.yahoo.com/s/nm/20080520/sc_nm/space_matter_dc
regards
Ronan
new baby universes as increasing of calculus? what is computing the multiverse? may be intelligent? why all that?
Boltmann’s Reptilian Brain,
Well, that is one very specific scenario, though, that of eternal inflation. There are other potential multiverse ideas that don’t predict any such effects. With Sean’s concept, the essential way of testing whether the theory makes any sense would be to show that a galaxy (or some other feature of our universe) is more likely to arise from a big bang like our own instead of fluctuating randomly out of the vacuum. Further testing is, of course, a really messy problem, but is in principle possible.
So, if for the sake of argument, no other universe other than our own exists, then what test would falsify the multiverse hypothesis?
Garth
So let me get this straight…. In the past week huge chunks of missing matter have been found and a supernova has been caught exploding. And you’re writing to tell us about an article you wrote?
I can understand the reverse or ‘mirror’ universe
like the reverse image/universe we see in a mirror.
But I can’t understand moving backwards in time
even in the morror one is walking backwards or forward >>> forward in time
@ Sean Carroll,
I want to translate this article into Bengali Language. So I need your permissio. I tried to get permission to translate an article of SciAm earlier and it was not easy. Finally I translated it without permissio but failed to publish anywhere.
I want to publish the Bengali translation in a Bengali Space Sciences Magazine name “Mohakash Barta” or “Space News”.
So, what do you say?
Another way to say it is that all particles move in only one (the one we call forward) direction in time, and this causes thermodynamics. The equations describing our microscopic laws are time-symmetric, but they apply in only the one direction. The laws of nature are not time-symmetric, because they include that constraint. Why is that? Why is alpha 1/137, and why is the Weinberg angle what it is? Because Mother Nature says so.
Khan– you would have to talk directly to the people at Scientific American. I don’t have the rights to the article. You should definitely not translate articles without permission!
“String theory”? How’s that?! Surely you can have quantum gravity, extra dimensions and other exotic phenomena naturally, but with string theory you have to have conciseness or con-science. I really don’t think you made a mistake when you wrote, “string theory” instead of strings.
I really enjoyed reading your article, that the first time I have enjoyed reading something about science since I read Brian Greene The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory.
except for the white holes, it sounds pretty familiar
not looking forward to the big crunch though…
It would be interesting to flesh out this timeline with some narrative describing biological and cultural evolution, and the lives of individuals, where history and memory play crucial roles, if only to emphasize how much time-reversed scenarios conflict with our understanding of our existence. The apparent indifference of the fundamental laws of physics to such considerations is perhaps the central problem.
Boltmann’s Reptilian Brain: “The whole “it isn’t science because other universes are not observable even in principle” industry is a load of rubbish.”
I’m not sure it’s an industry, but I agree with you.
Reptile and Joseph,
There is no industry claiming that “other universes are not observable even in principle”, since it is logically possible to come up with models with such observable effects.
On the other hand, there are a lot of people who want to see specific arguments about a multiverse make some sort of standard, testable (even if only in principle) scientific prediction before they agree that these specific arguments are science. I don’t see how the arguments being sold in Scientific American lead to any possible predictions that would allow one to test them in any conventional sense of scientific testability.
I’ve raised the point before, but; The only thing particles move through is space. Now with a sea of particles, they are creating a series of configurations, called events. Such as the earth and sun rotating and revolving relative to each other create days and years. While the physical reality goes from past to future events, these events go from being in the future to being in the past. So there are two directions of time implicit in this reality. Content going past to future, as form goes future to past. To the hands of the clock, it’s the face going counterclockwise. Our physical brain moves to the future, as our memory of past events fades into the past.
We understand there is no objective perspective, but we assume there is such a thing as an objective past, because it is unchanging. To the extent energy is conserved, information of the past is constantly being incorporated into the present, as the energy is recycled. So the past changes because it is being erased and any knowledge of it is due to information we are consuming and adding to. Time is like a rope being woven out of strands pulled from what was previously woven.
The result is that time has two directions, but no dimension because it is a consequence of motion, not the basis for it.
Xenophage on May 21st, 2008 at 8:01 pm
Angular momentum is the strong arrow of time. Feynman’s sprinkler spins from emission but not in time-reversed absorption. The situation is not symmetric. A motion picture film can with an orthogonal port at its broad face’s center and another port tangent to its edge is an absolute direction of time detector. Fill with water then pump water center inflow to edge outflow. No problem. Reverse time – (nearly) no flow! Conservation of angular momentum creates a fluid diode with no internal obstructions.
I believe the point is more subtle, because simply reversing the pump is not at all equivalent to reversing time. If you were to record the Feynman sprinkler setup and run the tape backwards, nothing unphysical would happen. Like light falling into stars the situation would look very strange but no law of nature is being violated.