Slashdot Mirror
Time Dimension To Become Space-like
KentuckyFC writes "The Universe is about to flip from having three dimensions of space and one of time to having four dimensions of space. That's the conclusion of a group of Spanish astrophysicists who have calculated that observers inside such a Universe would see it expanding and accelerating away from them just before the flip (abstract, full paper pdf on the physics arXiv). 'We show that regular changes of signature on brane-worlds in AdS bulks may account for some types of the recently fashionable sudden singularities. Therefore, the fact that the Universe seems to approach a future sudden singularity at an accelerated rate of expansion might simply be an indication that our braneworld is about to change from Lorentzian to Euclidean signature. Both the brane and the bulk remain fully regular everywhere.'" Update: 10/09 16:06 GMT by Z : A few readers have written in to point out that the article is not peer-reviewed; your mileage may vary.
In "Slaughterhouse Five", Vonnegut wrote about creatures who perceived time as a geometric dimension. They could perceive their entire lives as a wide landscape, stretching from past to present to future... and they could move freely within it, to relive the better moments and fast-forward over the unpleasant ones.
One of the implications that these creatures could see, but which we could not, is that the universe can only play out one way. Whatever happens, has always happened, and always will happen, it is unavoidable. The creatures could see their future with absolute certainty, and so they knew that choice is an illusion (or, in my understanding, a mis-connotated word that belongs in the realm of epistemology rather than of metaphysics).
In any case, if the universe experiences this sort of "signature change", then we'll never know it. Consciousness will abruptly cease, like a paused DVD player or a saved Diablo game, waiting forever for time to resume. But, a new sort of consciousness could arise, to which physical movement is the equivalent of temporal progression. Somehow, if it could gather information and then ruminate upon it, by means of movement rather than time, it could become self-aware.
FATMOUSE + YOU = FATMOUSE
Lets say I told you we had two spatial dimensions. You would imagine a plane with perpendicular x-y axes everyone knows and loves. If I asked you to draw the set of points that were equidistant from the origin, you would probably assume the geometry was Euclidian and would probably instinctively draw a circle (a good guess!). It is commonplace to hear "time is the fourth dimension." As first pass to visualize this, you might try to draw a two dimensional space-time plot: an x axis and a perpendicular time axis in a plane. If I then asked you to draw all the points equidistant from the origin, you would probably again draw a circle in this x-t plane. It seems to make sense, but is only true of time is a "space-like" dimension like "y" in the x-y plane. This is way Newton thought of things and it seems to be what the authors of the paper are advocating. But, unbeknownst to some people who cite "time as the fourth dimension," according to the theory of relativity, the set of equidistant points from the origin on a x-t graph would actually be hyperbole, not circles. This is because in relativity space-time is a Minkowski geometry, not Euclidian. All the weird stuff in special relativity like time dilation and length contraction come about because of this weird geometry. In fancier language, time has an opposite sign than space in the metric. The metric determines how distances are calculated in a given geometry. If time has the same sign as space in the metric, then space-time becomes Euclidian and one would say that time was a space-like. The article is probably extra confusing to non-physics people because most probably didn't know time wasn't space-like to begin with.
i\hbar\dot{\psi}=\hat{H}\psi
I knew 3 answers to the Fermi Paradox. Either intelligence quickly builds into quiet-looking shells like in Charlie Stross' Accelerando, or by virtue of being conscious we humans have somehow carved out a light cone or domain excluding other intelligences, some wierd cocoon: it is impossible physically to communicate because other domains have other physics. There's a neat scifi story about that too. The third is a land mine in physics, waiting for young civilizations to liberate enough power to fry them. Heinlein did that one, it's a nasty one. Now a fourth: the universe really is out to get us. Not just out to get aggressive monkeys that want to learn high-energy physics, but even to the point of making a state flip ever so often. I think this last one (today's news) is pretty unlikely to happen any time soon but nevertheless it is a future killer, something harder to understand than the burning out of the stars in the far future. None of these are very nice ideas but I hope some physicists will step up to answering what the latest theory says about when it might happen and whether it could operate on a patchwork basis, killing other civilizations while our planet was still cooling.
It appears to me that this is a purely mathematical result. They are basically saying that an anti-de sitter bulk, the interior of anti de Sitter (AdS) space which is a constant negatively curved (or constant positive cosmological constant) with one time-like dimension (Lorentzian space) can be glued to a euclidean space smoothly along the boundary of the two spaces. Classically, this is of little relevance since time-based trajectories would stop at the boundary (either take infinite time to arrive or the system would "rip" itself apart at the boundary). Instead there could be (though not addressed in the paper) observable quantum effects from having something past the boundary even if it is purely spatial. Space-time states might extend over the boudary into this other space. So you might end up with the strange situation where parts of the universe are interacting beyond the end of time.
This paper doesn't tell you whether that occurs or not. But it does indicate that it is possible for quantum systems to have both Lorentzian and Euclidean space components seamlessly connected.The existence of 3 dimensions of time is one of the suggestions of quantum gravity.
Also, why does time seem to flow into a single direction? Most of the equations of physics work fine both ways, but time only appears to flow in a single direction, only its "pace" changes. The best explanation is that there's a breaking of symmetry, a process which for some reason only occurs in one direction of the time dimension(s). The only such process we can observe at this time is entropy. In a closed system, it always increases, it can never decrease. So entropy seems to be linked to time in some intricate way, or maybe it's actually an extra time dimension linked to the first in some way. So what happens if time changes into a space dimension? What does that even MEAN? The only significant difference between time and space is that single direction in which time flows, so does it mean the second law of thermodynamics will stop applying? The flow of entropy will reverse or break its link to the time dimension? This would not necessarily be so "bad" but it would completely break down most of the laws of physics that depend on this phenomenon, thus destroying the universe, no?
If he explores all forms and substances Straight homeward to their symbol-essences; He shall not die.
When I first heard that the rate of the universe's expansion was actually accelerating, I came up with a weird hypothesis after a few days...
Time in our frame of reference is slowing down.
The only way that seemed possible was if we were traveling at speeds close to c, but that didn't sound feasible since we were observing objects that were moving away from us, in all directions. Then another weird thought occurred to me...
Our observed universe is self-contained within the event horizon of a giant black hole.
We're closer to the singularity, and accelerating towards it faster than objects closer to the edge of the event horizon. Time will move slower for us, and far away objects will appear to speed up. An outside observer (if such a thing could possibly exist) would perceive our universe as shrinking, but in our current frame of reference, we still think of it as expanding.
One other observation that lends to this possibility is the fact that we have not seen evidence of other "Big Bangs" or other "Universes". If the Big Bang happened once, shouldn't it be a repeatable occurrence in the limitless void of space?
Okay, that's my rant. You can slap the straitjacket on me now and ship me off to the funny farm.
Solomon Chang
"Twice half-assed makes an ass whole." --Solomon K. Chang
In a closed system entropy can and does decrease from time to time. It is simply much more likely to increase, due to there being more possible states with high entropy than there are states with low entropy in known physical systems, and the likelihood of it decreasing in a given period decreases sharply as the complexity of the system grows. It never goes to zero, thought.
A classical example is a box with two separate gasses, initially separated by a dividing wall. If the wall is removed, the gasses will mix, eventually spreading equally to every part of the box. However, suppose that the box only contains a single molecule of both gasses. It is certainly possible, and even likely, that both molecules happen to be at their initial side of the box, and both gassed therefore separated back to their own sides, at some future point. Add another molecule to both gassed, and you'll have to wait a bit longer for all four to be at their initial sides, but still not too long. A third molecule, and it takes longer still, then fourth, fifth and so on.
The more molecules you add, the longer you'll have to wait. However, no matter how many molecules there are in the box, given a long enough time, the gasses will separate, simply due to random motion of the molecules happenign to take all the molecules of one gas to one side of the box at the same time, and all the molecules of the other gas to the other side at the same time.
It will take almost, but not quite, forever, but that's a far cry from "never".
Forget magic. Any technology distinguishable from divine power is insufficiently advanced.