Particle Physicists Confirm Arrow of Time Using B Meson Measurements 259
ananyo writes with bad news for John Titor. From the article: "Four years after its closure, researchers working with data from the Stanford Linear Accelerator Center's particle physics experiment BaBar have used the data to make the first direct measurement confirming that time does not run the same forwards as backwards — at least for the B mesons that the experiment produced during its heyday. The application of quantum mechanics to fundamental particles rests on a symmetry known as CPT, for charge-parity-time, which states that fundamental processes remain unchanged when particles are replaced by their antimatter counterparts (C), left and right are reversed (P), and time runs in the reverse direction (T). Violations of C and P alone were first seen in radioactive decays in the 1950s, and BaBar was used to confirm violations of CP in B meson decays in 2001. To keep CPT intact, that implies that time reversal is also violated, but finding ways to compare processes running forward and backward in time has proven tricky. Theoretical physicists at the Universityof Valencia in Spain worked with researchers on BaBar to exploit the fact that the experiment had generated entangled quantum states of the meson Bzero and its antimatter counterpart Bzero-bar, which then evolved through several different decay chains. By comparing the rates of decay in chains in which one type of decay happened before another, with others in which the order was reversed, the researchers were able to compare processes that were effectively time reversed version of each other. They report in Physical Review Letters today that they see a violation of time reversal at an extremely high level of statistical significance."
Re:I Wish (Score:5, Informative)
They would be more specific about the arrow of time. I get that they have confirmed it and all, but which direction is it pointing?
As I understand it (greatly simplified), time is a consequence of matter and energy interactions in space; They don't all happen at once though because of separation, and the distance (relative or absolute) between them is what creates time. That's why they call it 'spacetime'; The smallest unit of time then is the fastest change in quantum state possible. As time is a byproduct of matter and energy interactions, and couldn't exist without it, there's still the question of the "arrow of time". We perceive it to be always moving "forward", but there's no reason why the reaction A-B-C shouldn't go C-B-A as well, or instead.
If I understand this experiment correctly, what they're saying is "as well" is bogus. It's not just that it isn't observable, but that it just doesn't happen. No matter which way the reaction chain goes, there's no mirror reaction that goes unobservable. But perhaps someone who actually is a particle physicist could provide a layman explanation better than mine... I'll be honest: Most of what they do is beyond my grasp because they talk mostly in math and seem to eschew visualization or story explanation. -_-
arXiv link (Score:5, Informative)
Come on people, how hard is it to include the arXiv link? Just google the title, it's usually the first hit.
http://arxiv.org/abs/1207.5832 [arxiv.org]
What this means (Score:5, Informative)
The summary is a bit confusing if you don't know what it's talking about. The title is even worse, since it implies the exact opposite of what it actually means. Let me try to explain it.
First: physicists believe that the "arrow of time" isn't a fundamental property of the laws of nature. There's no fundamental difference between "forward in time" and "backward in time". The laws of physics operate identically in both directions. So why do those directions seem so different? Why do objects fall down but not up? Why can you make an egg into an omelet, but not an omelet back into an egg? Why can you remember the past, but not the future? This turns out to be a property of our local region of spacetime. More precisely, we live very close (a mere 13.5 billion years or so) away from a point of incredibly low entropy (known as "the big bang"), and that creates an entropy gradient throughout our region of spacetime. What we call "forward in time" simply means "the direction of increasing entropy", or more simply, "away from the big bang".
A good analogy (not involving a car - sorry!) is the direction "down". It seems obvious to you that one particular direction in space is fundamentally different from all other directions. Objects fall down. They don't fall in any other direction. Yet to person on the other side of the earth, the direction they call "down" is completely different from the direction you call "down". That's because the "arrow of gravity" is not a fundamental property of the laws of nature, just a property of our local region of space. "Down" means "toward the center of the earth." In the same way, "forward in time" means "away from the big bang".
Second: what I just said swept a few details under the rug. You see, the true symmetry is not time reversal (which would imply that simply reversing the direction of time would leave all laws of physics unchanged), but a slightly more complicated symmetry called CPT invariance. That stands for Charge, Parity, and Time. It says that if you multiply the charge of every particle by -1 (so positive charges become negative and negative become positive), flip space as if in a mirror so that your left and right sides are reversed (a "parity inversion"), and reverse the direction of time, then all the laws of physics are left unchanged.
Scientists had previously observed a violation of CP. That is, swapping only charge and parity is not an exact symmetry of the universe. If CPT is an exact symmetry (which scientists generally believe), that implies that T is not - changing only the direction of time without also swapping charge and parity should change the laws of physics. But testing that experimentally turned out to be very hard to do. Well, they've finally done it. And the results are exactly what people expected: it appears that CPT really is an exact symmetry of the universe.
Re:wait what? (Score:5, Informative)
Let me explain. Most reactions are time-reversible. (Sort of) example: oxygen and hydrogen can combine to form water and release energy, but you can put energy back into the system to get hydrogen and oxygen back out again (thermodynamics states you will always lose some energy in this process, however, no matter how efficiently you conduct the H+O->water->H+O process).which indicates time is not perfectly reversible, but doesn't explain why). At the subatomic level, however, some similar (vaguely similar, anyways) reactions cannot be reversed, or don't reverse in the same way. In this case, they studied a meson that spontaneously changes from matter to antimatter and back (don't ask). If time reversibility held true for them, the probability of matter->antimatter would be the same as the probability of antimatter-> matter. It was not, by a very very very very significant margin (14 sigma, or a 1 in 10^43 chance this was seen by accident). Note this may also help to explain why matter is more prevalent than anti-matter in our universe.
Re:I Wish (Score:5, Informative)
That's not true. "Everything at the quantum level always working the same way forwards and backwards" is completely consistent with the second law of thermodynamics ("entropy never decreases"), and completely consistent with the observable universe (barring CP violation). All that's necessary is that the universe started with very low entropy -- like, say, the Big Bang.
See for example this from this Arrow of Time FAQ [preposterousuniverse.com] (from cosmologist Sean Carroll):
The observed macroscopic irreversibility is not a consequence of the fundamental laws of physics, it's a consequence of the particular configuration in which the universe finds itself. In particular, the unusual low-entropy conditions in the very early universe, near the Big Bang. Understanding the arrow of time is a matter of understanding the origin of the universe.
Re:I Wish (Score:3, Informative)
The reason is very simple. Entropy is a measure of the probability of a particular outcome. The statement that "entropy increases" is simply the statement that the most probable thing to do happen is almost always the one that does happen. The "almost always" is a fantastically high probability. For example if I through a 1 cm cube of of aluminium at 26 C into a lake where the water is at a temperature of 25 C, there is something like a 10 ^-(10 ^10^23) chance that heat will from the lake into the aluminium cube and cause it's temperature to rise. If it did this the entropy of the Universe would decrease.
What this experiment observed is profound and extremely interesting. For some reason that isn't known, the Universe prefers that certain microscopic and reversible processes occur with a greater probability if time increases.
In other words there truly is a preferred direction to time which independent of tautology that the Universe is constantly evolving into a more probable state.
Actually this result is was first observed in K-mesons but this new result in B-mesons has much greater significance and confirms the previous observation.
Entropy vs. T-symmetry (Score:5, Informative)
The arrow of time is the reason why random bits of shrapnel and chemicals don't fly together and "un-detonate" to become hand grenades.
No, that is entropy. The reason that balls fall off tables and rarely bounce onto them (when provided with enough heat energy) is because there are many, many more states where the balls atoms vibrate incoherently and only one state (or a tiny handful) where the vibrations are organized enough to cause it to bounce back onto the table.
With mesons you can study a particle oscillating between two states. What you find is that the P(A -> B) is not equal to the P(B -> A) where B is the anti-particle state of A and there is no entropy involved. It's all to do with something called CPT symmetry which is a result of relativity and, since CP together are violated (anti-matter is not exactly the same as matter) we expect that T (time reversal symmetry) is also violated so this is an expected result.
Re:I Wish (Score:5, Informative)
The arrow of time refers to the fact that we perceive a difference between the past and the future: we remember the past, but not the future. That's explained adequately by noting that entropy tends to increase and the universe, for some reason, was in a low entropy state in the past.
What they've found is that, at least for b-mesons, going forward in time is different than going backward in time, presumably in addition to the rest of the universe accumulating entropy. It's as if there was a fundamental difference between moving "north" and moving "south" in empty space.
Re:Reading the Article Backwards... (Score:5, Informative)
In theory the basic mathematics of quantum theory is time-symmetric. You can write equations to describe particles x and y colliding to produce a and b, and those equations work perfectly well to describe particles a and b colliding to produce x and y. It's why Feynman diagrams are so useful, you can just flip the time dimension around and see something else described by the same maths.
The point of what these folks have done is to look very closely at one particular Feynman diagram, that of the B meson decay, and showing that it is not time symmetric in some way. So the flow of time is something extra on top of the basic quantum theory...that's fascinating.
Re:Quick question then (Score:5, Informative)
Since photons do not routinely experience CP violation they also behave the same way forward and backwards in time.
Well, photons don't experience time in either direction. Those who experience photons do, but to a photon, no time can pass because they by definition move through vacuum at c.
It also has no antiparticle (or, you could say, it is its own antiparticle), so there's no way to reverse time even if you managed to prolong the subjective lifespan of a photon beyond instantaneous.
Re:CPLEAR (Score:5, Informative)