| Quantum Mechanics | |
| the study of the nature of things at the atomic level | |
| Physicists seek to put one thing
in two places Physicists say they have made an object move just by watching it. This is inspiring them to a still bolder project: putting a small, ordinary thing into two places at once. Sept. 25, 2006 Special to World Science It may be a “fantasy,” admits Keith Schwab of Cornell University in Ithaca, N.Y., one of the researchers. Then again, the first effect seemed that way not long ago, and the second is related. The research comes from the edge of quantum mechanics, the submicroscopic realm of fundamental particles. There, things behave with total disregard for our common sense. They can show signs of being in two places at once; of being both waves and particles; of taking on some characteristics only at the moment these are measured; and of acting synchronously while far apart, with no apparent way to communicate. Although these tiny building blocks of our universe do this, the relatively huge things we see every day don’t. The uncanny behavior fades the bigger a thing becomes. This is because when quantum entities are combined to make ordinary objects, the rules governing each component’s behavior add up to produce new rules. These increasingly resemble the laws of our familiar reality as more additions take place. But just how big can something be and still show signs of slipping back into its quantum-mechanical nature? Schwab and his colleagues decided to find out. In work described in the Sept. 14 issue of the research journal Nature, they built a device colossal by quantum standards: about nine thousandths of a millimeter long, containing some 10 trillion atoms. The object was a sliver of aluminum and a type of ceramic, fixed at both ends but free to vibrate like a guitar string in between. To measure its movements, the scientists set nearby a tiny detector called a superconducting single electron transistor. They found that random motions of charge-carrying particles, electrons, in the detector emanated forces that affected the metallic sliver. When the detector was tuned for maximum sensitivity, these forces slowed down the sliver’s shaking, cooling it as a result. This effect, Schwab said, is a basically quantum-mechanical phenomenon called back-action, in which the act of observing something actually gives it a nudge. Back-action in quantum mechanics also makes it impossible to know a particle’s exact location and speed simultaneously. This limitation is called the uncertainty principle. A common example: measuring place and speed requires some detector that can “see” the particle. But this involves bouncing a light wave off it, which gives it a random push. “We made measurements of position that are so intense—so strongly coupled—that by looking at it we can make it move,” said Schwab. Normally, such motion wouldn’t cool an object. But the motion can be such as to oppose ongoing movements and slow them down. This reduces an object’s heat, which is just the jiggling of particles in it. If back-action applies such a large item, Schwab reasons, maybe that can also be true of other quantum-mechanical rules. Particularly intriguing, he said, is the superposition principle, which holds that a particle can be in two places at once. A classic example is the shooting of light particles, called photons, through two slits in a barrier. Past the slits, they will behave as if they were waves. This alone is no surprise: it’s a well-known quantum mechanical phenomenon that particles can paradoxically act like waves in some situations. The photons’ waviness then makes them “interfere” with each other. In other words, they make patterns like those seen when you toss two pebbles in a pond, and the ripples they make overlap. When the waves passing the two slits mutually interfere, the pattern becomes visible if you set up another wall where the photons can land. There, alternating bright and dark stripes appear. But bizarrely, this works even if you fire just one photon at a time through the slits. You can see the effect then by putting photographic film on the landing wall, so each photon leaves a lasting mark. Keep firing photons, and the marks gradually add up to make the stripes again. It’s as if each photon is interfering with itself—that is, going through both slits simultaneously. This also works for bigger particles, up to a point. But what point? Schwab wants to know. “We’re trying to make a mechanical device be in two places at one time. What’s really neat is it looks like we should be able to do it,” he said. “The hope, the dream, the fantasy is that we get that superposition and start making bigger devices and find the breakdown.” In a commentary in the same issue of Nature, Michael Roukes of the California Institute of Technology in Pasadena, Calif., wrote that Schwab’s work with the cooling is part of an emerging field, quantum electromechanics. This, he added, focuses on submicroscopic devices called nanomechanical systems, “poised midway between two seemingly antithetic domains” of size: fundamental particles at one end, the objects of everyday life at the other. |
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Extracts from the Carl Sagan Mailing List and ScienceWeek: "Hello Anne, I hope that the following (which amazingly enough is still to be found in the Deja archives written by one William somebody :-) will provide a very simple adumbrative map of the quickest way out of this strange labyrinth of particle physics to the ultimate exit of better understanding the notion of "Quantum Weirdness". Your original question was:- What is Quantum Weirdness and what simple macro analogy can help in better familiarizing oneself with its main founding principles? Quantum weirdness refers to the fact that merely observing a phenomena can change a result. The usual experiment to show this is as follows: An electron is directed at a screen with 2 holes in it. A photographic plate is behind the screen. This electron behaves like a wave and goes through both holes at the same time. The waves from each hole then interfere with each other and create an interference pattern (only waves do this) on the photographic plate. Now here's the strange part. If a detector is set up in the holes, it will detect the electron going through one or the other of the holes. And the electron will make a dot on the paper--not a wave pattern. So by having been observed the electron now appears as a particle--not a wave. How is this explained? Physicists state that the electron's wave function is just a wave of "probabilities" This gives the key to an answer. Imagine that you are blind-folded and told to flip a coin and call it in the air. You can also change your mind at any time before removing the blindfold. What are the probabilities while the coin is in the air? 50-50 right? What are the probabilities after the coin has landed, but before you take off your blindfold? I believe they are still 50-50. It might be argued that the odds are either 100% or 0%. But this changing of the odds only occurs when you take off your blindfold and observe---then the odds collapse to 0% or 100%. The same thing happens with an electron. It is a wave of probabilities until an observation is made. Then the probability turns out to be 100% in one place, and 0% in the rest of the (former) wave. Two objections to this theory. First: The electrons position was undetermined before the observation--that is why an unobserved electron forms a wave of probabilities. How then can the observation later change the electron without some physical interaction? Answer. The misconception in the above objection has to do with the words "before" and "after" which refer to time relationships. The electron is travelling at the speed of light and so time stands still for it (or doesn't exist--Einstein). Once it is observed, it's position is no longer a wave of probabilities--but is determined (w/in limits set by Heisenberg), and this determination applies to the whole life of the electron. Remember there are no time distinctions for a " fast as light" electron. What applies to an electron at any moment must apply to it's entire "life" Second objection: Really a question. What type of "consciousness' needs to make the observation. A machine? A gnat? A human? Ans. All of the above are sufficient. The world we live in is a "sub-light speed" realm. The world of the electron is a "speed of light realm". The rules are different. Specifically the rule, "Reality determined at one instant must be reality forever, (and always in the past), (Amen) applies only to the speed of light realm" The realm of the electron. This has been verified by experiments which show that a measurement at this instant will cause an electron to exhibit a past history (either wave or particle) consistent with the present measurement. (and remember the experimenter can choose the present experiment to make it come out either wave or particle). So all that is needed is one moment of reality in this sub-light speed realm and the electron is fixed for life--"past and future". Remember that the probabilities before the observations are just that "probabilities" ie they are nothing. The observation transforms them from nothing to something real about the electron and then it must be fixed in this observed state forever, past and future... ~George ---------------------------------------------------------------------
For a long time quantum physics was synonymous in my mind with witchcraft.
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