Apparently, time goes in one direction. But why?
The answer to why time has a direction takes many forms, usually described as "arrows of time." We've had time defined by how information increases or how entropy increases. But all of the arrows we've considered are a bit unsatisfactory. For instance, an arrow of time derived from entropy starts with the assumption that the Universe had to begin in a highly ordered state. If that assumption fails, so too does our explanation for time.
The thermodynamic arrow of time also conveniently ignores gravity. When gravity dominates, it spontaneously orders stuff—this is why we have galaxies. So although the thermodynamic arrow of time suffices for parts (any part) of the Universe, it doesn't work for the whole Universe or the whole of time.
A new paper ( abstract; pdf ) in Physical Review Letters presents a new arrow of time, which the authors hope might lead to a sort of general description from which all the other arrows can derive their power.
I hope somebody more knowledgeable, at SN, would be able to explain this.
(Score: 0) by Anonymous Coward on Saturday November 01 2014, @03:04PM
Abstract link is broken.
(Score: 3, Funny) by wonkey_monkey on Saturday November 01 2014, @06:43PM
That's what makes it abstract.
Ah! *handwave*
systemd is Roko's Basilisk
(Score: 0) by Anonymous Coward on Saturday November 01 2014, @03:18PM
In restaurants, all theories concerning time slink away in shame.
(Score: 2) by jimshatt on Sunday November 02 2014, @11:39PM
(Score: 2) by sudo rm -rf on Monday November 03 2014, @10:58AM
Bistromathic [wikipedia.org]
(Score: 1, Informative) by Anonymous Coward on Saturday November 01 2014, @03:31PM
The super-intelligent beings from the other multiverse who created our universe as a scientific experiment or amusement don't have to wait to see how things will turn out. They can see everything in 12 dimensions, and time is just another one of those dimensions. So the history of our entire universe is like a Jackson Pollack painting to those people^H^H^H^H^H^H entities.
This is a straightforward consequence of the Lagrange-Jacobi relation and Hamiltonian mechanics, which I taught in college for about six years. Unfortunately, I've forgotten all of it, so don't ask me to explain it.
(Score: 1) by gznork26 on Saturday November 01 2014, @05:24PM
Not knowing why we can move in space but not in time (as in choose to go future or past like we can go left and right) gives us the chance to speculate. I took a stab at this in "The Shoals of Time", which I wrote under the name P. Orin Zack, as are the short stories on my wordpress blog. (The novel is available on Nook and Kindle.) In that fictional universe, I postulated that the dimensions you can travel in are determined by the dimensions that you hold fixed. We experience a world in which you can't travel future or past, but you can travel across alternative futures by the choices you make. If you were to hold fixed the choices you can make, and experience a world with no ability to choose your path, then you would be able to travel past and future along that fixed choice line. The tech that drives the story is some that enables you to choose which dimension to hold fixed, so you can do either thing at will. The implications are troublesome.
Khipu were Turing complete.
(Score: 2) by wonkey_monkey on Saturday November 01 2014, @06:49PM
The implications are troublesome.
And sexy!
systemd is Roko's Basilisk
(Score: 2) by maxwell demon on Sunday November 02 2014, @09:42AM
Actually Relativity gives us a very nice explanation of why we cannot turn around in time. Namely, we actually can turn in spacetime (it's actually quite obvious that we can, if you look at a spacetime diagram, and is actually already true in Galilean spacetimes), that turning simply being acceleration; however (and this is the non-obvious part that comes out of Relativity) already turning to the direction of light would mean turning by an infinite amount (the speed of light is constant in all frames of reference because it is in some sense an "infinite" speed, although it is of course finite in the sense of covered distance over time — the parameter that gets infinite is sometimes called "rapidity" and is basically the equivalent of an angle in space). This infinity is why you cannot reach the speed of light, because of course no matter how much you try, you cannot get closer to infinity. To turn around, you'd have to turn even more than to reach the speed of light; that is, you'd have to turn by an angle that is larger than infinity. This quite obviously is not possible.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by mcgrew on Sunday November 02 2014, @01:48PM
Indeed. If you could instantaneously accelerate to the speed of light toward Proxima Centauri, it would seem to you that you got there instantly. To someone watching from Earth, the trip would seem to take four years.
mcgrewbooks.com mcgrew.info nooze.org
(Score: 1, Interesting) by Anonymous Coward on Saturday November 01 2014, @06:55PM
Have we even proven that time actually exists? e.g. that there is actually a past to go to. If there isn't a past then asking why the arrow of time goes in a particular direction becomes meaningless, like asking why speed is not a vector.
http://discovermagazine.com/2007/jun/in-no-time [discovermagazine.com]
http://www.popsci.com/science/article/2012-09/book-excerpt-there-no-such-thing-time [popsci.com]
http://www.scientificamerican.com/article/is-time-an-illusion/ [scientificamerican.com]
(Score: 2) by maxwell demon on Sunday November 02 2014, @07:47AM
Using Einstein's definition, "Time is what we read on a clock", I prove the existence of time whenever I look on a clock.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 0) by Anonymous Coward on Sunday November 02 2014, @08:57AM
Seriously though if time is more like distance travelled (no matter what direction), it becomes obvious there's no turning the clock backwards in the real world. There's no past, no future. There is only the now.
Is this compatible with other stuff we know about the universe? Perhaps it is.
(Score: 2) by maxwell demon on Sunday November 02 2014, @09:22AM
Elapsed time is indeed very much like distance travelled (that's also why you can have different elapsed times between the same events — known as twin paradox — just like you can get from one point to another travelling over a different distance by simply making a detour.
However just as covered distance needs a space in which you can cover the distance, elapsed time needs a spacetime in which the time can elapse. And the direction of time is about this difference: It is very possible to get back to the place where you started after travelling some distance; that's called a round trip. But there are, as far as we can tell, no round trips in time: You cannot get back to an event a second time.
Whether the past exists? Well, you first have to define what you mean with "exists". Indeed, I suspect that all arguments of whether the past (or the future) "exists" or not ultimately boils down to differing (usually implicit) definitions of the very term "exist".
The Tao of math: The numbers you can count are not the real numbers.
(Score: 0) by Anonymous Coward on Monday November 03 2014, @02:15PM
It's like having a grid with an object that moves from A to B. Once it's at B there is no grid where the object is at A. You can't turn the clock and go back. Our time is merely a measurement of moving stuff compared to other moving stuff (light).
Now whether our universe is really like that is a different matter. But I don't see much conclusive proof that there really is a past (as in terms of this universe's laws).
(Score: 2) by maxwell demon on Monday November 03 2014, @07:25PM
Is distance "just a measurement that humans make"? So you think there's nothing physical about the distance between New York and Los Angeles? And given the analogy of elapsed time and travelled distance, how can you reconcile your claim that elapsed time being "just a measurement humans make" (whatever that is supposed to mean) would already imply there's no going back to the same event with the very easily verified fact that you can very well go back to the same place in space despite the fact that the travelled distance can only grow?
What the analogy does mean is that you cannot "rewind" time (that is, even if you could get back to a past event, you could not see it again "the first time", but you'd see it with the added experience, knowledge etc. which you accumulated in the time which elapsed in between your two visits of the same time. Just like your car will not be in the very same state after a round trip as it was before the round trip. But it does not imply that you cannot get back. That you cannot get back to a past event is some independent principle (which could in principle still be wrong, although I guess very few people actually believe that).
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by MickLinux on Saturday November 01 2014, @10:55PM
I am going to give this my best shot to explain the issue. Here goes nothing:
There are two time dimensions, two space dimensions; because of the cross product, the two space dimensions yield a third space dimension; one might assume the same for the time dimensions as well.
It appears that for electrons in a electron shell, the presence of a very strong electric charge in its vicinity overwhelms other forces, and causes the electron to mistake the three space for one space. As a result, it "sees" only two or three time dimensions, and one space dimension. This is important to all communication between atoms, and therefore is important to our protein structures, our light, and other such things.
On the other hand, at macroscopic scale, the space dimensions become very important to interaction, and especially the motion of light; and the transfer of light between them, locks that time dimensions roughly together. So all matter flows like a river through the time dimension. As a result, we mistake the two (or three) time dimensions for one time dimension. This results in the cross terms for momentum dropping out of our equations, and throws off the calculation of energy when we deal with black holes. As a result, we get energetic discontinuities in our equations when an object passes the S-C radius. Energy appears to be violated.
If, instead, though, you put that cross term back in, then you get back your conservation of energy, and everything works out.
Ref Arxiv paper: Joseph D. Rudmin: an Isotropic metric [arxiv.org]
(Score: 0) by Anonymous Coward on Saturday November 01 2014, @11:40PM
... you what?
(Score: 1, Informative) by Anonymous Coward on Sunday November 02 2014, @12:56AM
Now that is a very bold statement full of assumptions... But one may argue that you did not have time/space to explain it properly.
Next time if you plan to "explain" things using some fringe theory, please note it as such.
Adding a "fudge factor" to explain one problem is easy. Correcting a theory so it is correct for all observations is a different story.
(Score: 2) by Blackmoore on Monday November 03 2014, @05:10PM
Wouldn't it be simpler to assume that the Dimension of time in THIS universe is only one dimensional, but could have more than one in some other multiverse?
(Score: 2) by Gaaark on Sunday November 02 2014, @05:02PM
Julian Barbour explains the arrow of time and why it only appears to us to travel in one direction...
Read "The End of Time", etc. Interesting stuff.
http://www.platonia.com/ [platonia.com]
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 2) by Gaaark on Sunday November 02 2014, @05:17PM
Basically, time doesn't exist: it is the movement of 'particles' through space that creates the 'idea' of time.
Einstein made time and space inseparable basically because the math was easier than in the Machian view that time and space exist separately which was the direction he was originally looking at (according to what Barbour says after reading Einstein's notes in the original German).
Barbour says that if you separate space and time (and time is basically an illusion), a lot of what Einstein says in his theories still works, but without the nonsense of time travel, etc.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---