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."

Comments

Arrow of Time...

[Anonymous Coward]

Arrow of Time confirmed... Wheel of Time fans disappointed.

Re:Arrow of Time...

[fredprado]

Hardly. Recognizing that there are differences between the sexes is common sense. Judging people's values and defining their roles solely by their sexes is sexism.

Women and men are not equal in everything. Trying to see equality in everything because it fits your notion of symmetry, fairness or whatever is self-delusion.

I Wish

[NEDHead]

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?

Re:I Wish

[girlintraining]

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. -_-

Re:I Wish

[TexVex]

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. In one direction of time, entropy in the universe always increases; in the other, it always decreases. The question is, why? If everything at the quantum level always worked the same way forwards as it does backwards, then entropy would be constant; the universe would be in some kind of steady state and nothing would matter because we wouldn't be here.

I think at this stage of research, it's more about finding clues than it is about trying to put them together into a coherent explanation. But if that's not true, I'd love to hear from someone who really knows this stuff..

Re:I Wish

[Anonymous Coward]

Take a video of some physical process and run it in reverse. At a macro level we humans can generally tell that the video is in reverse - tea cups break apart when hitting a floor, they don't spontaneously assemble and then fly off the floor. However, if you analyze such situations using the physics you learned in high school, there is no way to tell that the course of events has been reversed - statistically it's very unlikely that a tea cup would do that, but there is nothing physically impossible about it. So it appear that the laws of physics are the same if time was running in reverse yet to us humans it does not appear that things would be the same if time was running in reverse - because of entropy.

This is the problem of the arrow of time - how can we tell in a physical way which way time is running? Is there any way to distinguish going forward in time to going backward in time using just physical laws? You could say that entropy increases with time (the basis of how we humans can tell on a video whether it is running forward or backward), but that is only a statistical observation and it only holds because it just happens to be that our past has a very small amount of entropy compared to the high entropy situation that the universe will eventually reach. Increasing entropy is a consequence of an accident of what our past looks like and it is not a physical law in the strict sense we are looking for here. So entropy is not a good candidate. This research shows a way that you actually CAN tell if time is running in reverse. Though physicists still believe that there is a CPT symmetry, indicating that if you reverse time and also two other things, then there is no way to tell from physical laws that you did that.

Re:I Wish

[FrangoAssado]

If everything at the quantum level always worked the same way forwards as it does backwards, then entropy would be constant; the universe would be in some kind of steady state and nothing would matter because we wouldn't be here.

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 (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

[ceoyoyo]

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:I Wish

[Artifakt]

From a Doctor's perspective, it may point to the right - from THE Doctor's perspective, time points anywhere he wants (unless it goes wibbly-wobbly).

Dear Slashdot:

[Anonymous Coward]

Bet you wish you had unicode now, eh?

arXiv link

[Baron Eekman]

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

Re:Reading the Article Backwards...

[avgjoe62]

Funny - I read the article backward and I got that Paul is dead.

What this means

[SoftwareArtist]

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?

[Baloroth]

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.

The Ugly Details

[Required Snark]

If your follow the links far enough you get here http://physics.aps.org/articles/v5/129 where they have a detailed non-mathematical description of the experiment.

After detecting and identifying the mesons, the experimenters determined the proper time difference between the decay of the two B states by determining the energy of each meson and measuring the separation of the two meson decay vertices along the e-e+ beam axis. When time-reversed pairs were compared, the BaBar collaboration found discrepancies in the decay rates. The asymmetry, which could only come from a T transformation and not a CP violation, was significant, being fourteen standard deviations away from time invariance. Thus the long wait for an unequivocal time-reversal violation in particle physics is finally over.

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.

Entropy vs. T-symmetry

[Roger W Moore]

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.

CPLEAR

[Roger W Moore]

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.

Re:CPLEAR

[AchilleTalon]

From TFA: "Measurements indicating time reversal was likely violated had already been made in kaons at Fermilab in Batavia, Illinois and at CERN near Geneva, but in those experiments, according to Anulli, the measurement of time reversal were not disentangled from violations of charge-parity that were also present."

Re:Reading the Article Backwards...

[History's Coming To]

Here's my take on it:

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

[arth1]

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.

tsop tsirF

[waynemcdougall]

emit fo worra diputS