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Particle Physicists Confirm Arrow of Time Using B Meson Measurements
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."
Arrow of Time confirmed... Wheel of Time fans disappointed.
Well, reading the article backwards still results in WTF?
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?
Time flies like an arrow. Fruit flies like a banana.
That's all I've got to contribute. Carry on.
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I wouldn't really describe this as confirming the arrow of time.
The really powerful arrow of time is the thermodynamic one. The second law of thermodynamics says that entropy always increases. This thermodynamic arrow is essentially the same arrow as the psychological one, which allows us to remember the past but not the future, and all the other ones we see in nature, such as the laws of black hole thermodynamics, which say that the area of a black hole's event horizon always grows with time. This group of time-arrows, which are all essentially the same time-arrow, appear to occur because the big bang was fine-tuned to be extremely low in entropy, with its gravitational-wave degrees of freedom inactive. Nobody knows why we had a low-entropy big bang, when a random choice of initial conditions would be overwhelmingly more likely to produce a maximum-entropy one. (In particular, inflation doesn't explain it. Also, statistical mechanics doesn't explain it, because to produce the second law from statistical mechanics, you need to assume a low-entropy initial state.)
This paper is about an arrow of time that is obscure and completely unrelated to the others. It has to do with the weak nuclear force. Unlike the others, it has essentially no effect on the world we see around us.
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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
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.
"I'm too busy to research this and form an educated opinion, but I do have time to tell everyone my uninformed opinion."
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.
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IANAP, but here is my understanding of the experiment. They knew that two different decay chains occur from some positron/electron collisions. If time is symmetric, there should be equal numbers of both chains. By making the beam energies different between the positron and electron (e-e+) beams, they were able to differentiate the decay order. If time symmetric decay occurred then there would be one spacial pattern in the results, and if time was asymmetric there would be another. The results conclusively show that for this subatomic event time runs in the direction we know as "forward". This is a big deal for subatomic physics.
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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.
Arrow of Time confirmed... Wheel of Time fans disappointed.
Physicists on the CPLEAR experiment will be disappointed as well - they actually discovered this effect (called T-violation) back in the 1990's before Babar was running by looking at kaon oscillations produced in low energy proton/antiproton collisions [Phys. Lett. B 444 43 (1998)]. So Babar was certainly not the first experiment to see the "arrow of time" although it is the first to do so using B mesons.
But what does it all *mean*?
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Actually, it's more like a big blob of wibbley wobbley, timey wimey.... stuff.
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Yes, and not just photons. Any particle will follow the same path backward, as long as you also reverse its charge (which has no effect on a photon, since they're uncharged) and parity (which I think flips the polarization of a photon, but don't quote me on that). What CPT invariance really says is that there are two ways of describing the universe that are exactly equivalent in every way. They predict exactly the same result for any experiment you can ever do. But what one description calls "forward in time", the other one calls "backward in time".
"I'm too busy to research this and form an educated opinion, but I do have time to tell everyone my uninformed opinion."
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.
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...but then I took an arrow of time in the knee.
"Time flies like an arrow. Fruit flies like a banana." Is it still a banana going backwards or do fruit flies now like an ananab? Also, can anyone explain what this paper means in English for an average moron like me?