Previously, top quarks had only been observed when produced by the strong nuclear force. That interaction leads to the production of pairs of top quarks. The production of single top quarks, which involves the weak nuclear force and happens almost as often as the strong force production, is harder to identify experimentally. Now, scientists working on the CDF and DZero collider experiments at Fermilab achieved this feat, almost 14 years to the day of the top quark discovery at Fermilab in 1995.
Archive for March 9th, 2009
It goes right to the heart of one of the greatest philosophical difficulties of science. All we can do is measure correlation. We can never be assured that we’re not just getting lucky and that in fact the fundamental-seeming physical laws we deduce are just flukes.
And this is true — science is inductive, and we draw conclusions rather than prove. What distinguishes science from superstition is what happens next. Correlation is the start, but can easily be wrong, which is the basis of the logical fallacy post hoc, ergo propter hoc (happened after, therefore was caused by). If one is not cognizant of this, one might notice that the US never used nuclear weapons until after women got the right to vote, and think there’s meaning to the correlation.
So we ask for more. What we can do is set up conditions where if the phenomenon does happen to be a fluke, that the odds of it being so are really, really small. (Flip a coin and get heads 10 times in a row? Doesn’t mean you have special powers. Do that significantly more often than once per thousand attempts and we’ll talk). That’s the power of statistics, and why a single event is not enough to demonstrate causation. But even then, there are potential pitfalls. Two correlated effects might be caused by a common factor. If you don’t consider this possibility, you might conclude that buying a Lexus causes people to vote Republican.
But even that isn’t enough. We also want there to be a plausible mechanism that we can model, and use that to predict other behavior. Then you test — can you turn the effect on and off, and do it in such a way that eliminates other explanations? And the tests must be rigorous, with specific predictions and carefully executed experiments. It’s only after that testing that the suggestive winking of correlation can be reasonably concluded as causation.
In their experiment, they cause a gas of rubidium-87 to form an ultracold state of matter known as a Bose-Einstein condensate. Then, laser light from two opposite directions bathes or “dresses” the rubidium atoms in the gas. The laser light interacts with the atoms, shifting their energy levels in a peculiar momentum-dependent manner. One nifty consequence of this is that the atoms now react to a magnetic field gradient in a way mathematically identical to the reaction of charged particles like electrons to a uniform magnetic field. “We can make our neutral atoms have the same equations of motion as charged particles do in a magnetic field,” says Spielman.
Cross-Dressing? Someone has broken into the liquor cabinet again.
As computer chips shrink ever smaller, the background flicker of electronic noise threatens to undermine the vital precision of digital processing. Unlike ordinary circuits, a newly designed digital circuit element only works properly when the noise level is sufficiently high. The circuit, described in the 13 March Physical Review Letters, is not only well-suited for noisy nanoscale operations, but it can also be changed on the fly to perform different logic functions–a property that could lead to reconfigurable computer processors.