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Science

Particle Physicists Confirm Arrow of Time Using B Meson Measurements 259

Posted by Unknown Lamer
from the one-way-trip dept.
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."
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Particle Physicists Confirm Arrow of Time Using B Meson Measurements

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  • by bcrowell (177657) on Monday November 19, 2012 @07:18PM (#42034103) Homepage

    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.

  • Re:I Wish (Score:5, Interesting)

    by TexVex (669445) on Monday November 19, 2012 @07:29PM (#42034227)
    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..
  • The Ugly Details (Score:5, Interesting)

    by Required Snark (1702878) on Monday November 19, 2012 @08:42PM (#42035039)
    If your follow the links far enough you get here http://physics.aps.org/articles/v5/129 [aps.org] 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.

  • Re:I Wish (Score:5, Interesting)

    by Anonymous Coward on Monday November 19, 2012 @08:44PM (#42035067)

    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 (Score:2, Interesting)

    by Anonymous Coward on Monday November 19, 2012 @08:51PM (#42035137)

    I don't really know this stuff that well. I did well in quantum computing, which covers the philosophy of the quantum postulates, but never really gets past baby level physics. That said, I've been thinking that maybe there's a bit more to flip than C, P, and T to get a better picture.

    If you think of an explosion starting at time 0 and centered at [0,0,0], you start with density concentrated around that point and over time it spreads out from that point. Alternatively, starting at time t and going backwards to time 0, you see the dispersed mass flowing to concentrate at that point.

    However, another picture is that starting at time 0, a lack of matter begins flowing into the area around the point [0,0,0]. Essentially this view looks at it like the negative space in a painting.

    These two pictures aren't quite mirror images of each other, however with a coordinate transformation it might be possible to make them mirror images of each other. Something along the lines of mapping [0,0,0] to the point at infinity and continuously map the points surrounding the origin such that [1/d,1/d,1/d] goes to [d,d,d], basically turning space inside out around your origin point.

You see but you do not observe. Sir Arthur Conan Doyle, in "The Memoirs of Sherlock Holmes"

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