from the maybe-something-doesn't-always-happen dept.
The past and the future are tightly linked in conventional quantum mechanics. Perhaps too tightly. A tweak to the theory could let quantum possibilities increase as space expands.
A jarring divide cleaves modern physics. On one side lies quantum theory, which portrays subatomic particles as probabilistic waves. On the other lies general relativity, Einstein's theory that space and time can bend, causing gravity. For 90 years, physicists have sought a reconciliation, a more fundamental description of reality that encompasses both quantum mechanics and gravity. But the quest has run up against thorny paradoxes.
Hints are mounting that at least part of the problem lies with a principle at the center of quantum mechanics, an assumption about how the world works that seems so obvious it's barely worth stating, much less questioning.
Unitarity, as the principle is called, says that something always happens. When particles interact, the probability of all possible outcomes must sum to 100 percent. Unitarity severely limits how atoms and subatomic particles might evolve from moment to moment. It also ensures that change is a two-way street: Any imaginable event at the quantum scale can be undone, at least on paper. These requirements have long guided physicists as they derive valid quantum formulas. "It's a very restrictive condition, even though it might seem a little bit trivial at first glance," said Yonatan Kahn, an assistant professor at the University of Illinois.
[...] The main problem is that the universe is expanding. This expansion is well described by general relativity. But it means that the future of the cosmos looks totally different from its past, while unitarity demands a tidy symmetry between past and future on the quantum level. "There is a tension there, and it's something quite puzzling if you think about it," said Steve Giddings, a quantum gravity theorist at the University of California, Santa Barbara.
Concern over this conflict has been in the air for years. But recently, two quantum gravity theorists may have found a way to loosen unitarity's buckles to better fit our growing cosmos. Andrew Strominger and Jordan Cotler of Harvard University argue that a more relaxed principle called isometry can accommodate an expanding universe while still satisfying the stringent requirements that first made unitary a guiding light.
"You don't need unitarity," said Strominger. "Unitarity is too strong of a condition."
(Score: 3, Funny) by Thexalon on Wednesday December 21 2022, @11:49AM (2 children)
I'm reminded of the Finagle Factor, defined as the numbers you need to multiply your results by to get the answer you should have gotten.
If I had to hazard a wild guess here, what's actually going on that causes the numbers to not add up is some sort of phenomenon that we haven't figured out how to detect. Which could potentially be related to the gravity-related-thingy or thingies we haven't figured out how to detect that we call "dark matter".
"Think of how stupid the average person is. Then realize half of 'em are stupider than that." - George Carlin
(Score: 2) by sjames on Wednesday December 21 2022, @03:12PM (1 child)
OTOH, the circumference of a circle is equal to the diameter. Well, once you use this really strange Finagle factor that can only be approximated.
Sometimes it's hard to know if you have a fundamental constant or a Finagle factor on your hands.
(Score: 2) by Immerman on Wednesday December 21 2022, @03:23PM
Yeah. Especially when dealing with systems that are vastly less intuitive than 2D Euclidean geometry.
(Score: 5, Interesting) by maxwell demon on Wednesday December 21 2022, @09:33PM (5 children)
I already doubt the premise that just because the universe gets larger, the number of possibilities get larger. Yes, the number of spatial positions gets larger. But then, in the past the universe was not just smaller, but also hotter. In other words, it could cover a much larger momentum space.
Also, the past and future being different is not the same as the theory not being time reversible. In particular, general relativity is perfectly reversible. The time reversed expanding universe is a shrinking universe, and that is just as well described by the equations of GR. From GR we can tell that the universe's size isn't constant, but we can't tell from the equations alone whether it is expanding or shrinking. It's only our observations that tell the difference.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by acid andy on Thursday December 22 2022, @12:54AM (4 children)
+1 Insightful
I agree with this. From my limited understanding, our conscious experience of our memories is the only thing that gives an arrow of time that points only into the future. Even then, what evidence do we have that consequences follow causes and not the other way around?
No, the real distinction is the asymmetry between past and future. It doesn't matter whether we, or the universe are moving towards or away from the Big Bang--isn't it simply the fact that the universe is changing size over time that presents the problem for quantum mechanics?
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(Score: 2) by acid andy on Thursday December 22 2022, @01:00AM
Apologies, I think you probably answered me in your first paragraph.
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(Score: 2) by Immerman on Thursday December 22 2022, @06:03PM (2 children)
As I recall there's at least one QM reaction that is not time-symmetric, though I can't remember what it is offhand (P-something maybe?).
And at a larger scale there's also entropy (*very* roughly, the amount of disorder in a system). One of the fundamental laws of thermodynamics, which we've *never* seen violated, is that the total entropy *always* increases over time, creating a very definite way to tell past from future that has nothing to do with our perceptions. As a layman's example, you can't un-scramble an egg.
(Score: 2) by acid andy on Thursday December 22 2022, @07:48PM (1 child)
Thermodynamics ensures what we call the future always has greater entropy (at least on larger / global scales) than what we call the past but I submit that it cannot show us whether we are traveling towards that future or away from it. We could be traveling towards the past, experiencing the consequences of our actions before we instigate them: I think our conscious experience of each instant in time would not be any different.
If it helps, think about the characters in a movie that is being played in reverse. At any given moment, their thoughts, memories and actions are no different than if it were played forwards and it's impossible for them to notice it. Memories of what they call their future would disappear from their brain as time passes, leaving only memories from earlier and earlier in their life.
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(Score: 3, Interesting) by Immerman on Friday December 23 2022, @12:30AM
Or, in a deterministic universe, the entire concept of "flow of time" could be a perceptual illusion, with all moments existing simultaneously as seen by an outside observer. I suppose that doesn't *require* a deterministic universe, but the infinite branching of a multiverse makes visualizing the alternative a nightmare.
My main point was just that thermodynamics imposes a definite time-arrow on the universe, unlike e.g. watching atoms collide in a glass of water, which behave the same in forward or reverse.
(Score: 2) by acid andy on Thursday December 22 2022, @12:57AM (1 child)
We had another article about them wanting to ditch unitarity, posted some time ago. I can't search for it for obvious reasons. The language in TFS they're using to explain it looks awfully familiar.
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(Score: 2) by Immerman on Thursday December 22 2022, @06:09PM
Might be the same people - such arguments for changing the accepted scientific truths tend to get refined over decades before eventually either accumulating enough supporting evidence that they get adopted by the expert consensus, or abandoned for lack of supporting evidence (or because the primary proponents gave up or passed away)