A tool to measure what happens in empty space

<body leftmargin="0" bgcolor="#000060" text="#FFFFFF" link="#FFFF00" vlink="#FF9900"> <table border="0" cellpadding="0" cellspacing="0" width="95%" height="1"> <tr> <center> <td valign="top" width="33%" height="33"> <b> <p class="MsoNormal"><font face="Arial" size="5">A tool to measure what happens in empty space</font></p> </b> <p class="MsoNormal"><font face="Arial" size="1">July 13, 2005<br> Courtesy the California Institute of Technology<br> and World Science staff<br> </font> <font size="2" face="Arial"> <b> <br> </b> </font> <b><font size="2" face="Arial">Physicists have devised a new tool to track what goes on in what we normally call empty space.<br> <br> An “empty” space is never truly empty, physicists believe, even if every atom and particle in it has been removed. This is because particles will continue to appear out of nowhere, and rapidly vanish.<br> <br> In the new research, physicists report having measured this activity using a cloud of atoms that merge to effectively become one giant atom. This bizarre substance, called a Bose-Einstein condensate, was invented a decade ago but has found little practical use since then.<br> <br> The new findings, researchers say, mark the first time a Bose-Einstein condensate has been used to study anything besides its own properties. It was used to investigate something perhaps even stranger: the so-called virtual particles that appear and disappear in the void.<br> <br> Engineers must take virtual particles into account as they design ever-tinier machines and robots, a growing industry. On small scales, virtual particles create unpredictable forces that can throw off these devices.<br> <br> In studying virtual particles, the researchers probed a phenomenon that seems to violate a physical law recognized more than two centuries ago: the law of conservation of energy.<br> <br> The law says energy cannot come into existence out of nowhere, nor can it be completely destroyed. It’s also true of any object, because objects have mass, and mass is convertible to energy. Einstein showed this.<br> <br> Virtual particles get around this law thanks to a subatomic phenomenon called the uncertainty principle. Understanding this principle, as well as Bose-Einstein condensates, requires a digression to explain the nature of subatomic particles.<br> <br> Particles and waves<br> <br> Scientists consider subatomic particles as things with two seemingly contradictory natures: they are both particles and waves. This is because they act like one or the other depending on the experiment one does. <br> <br> One can shoot them into a target like tiny bullets, in which case they act like particles. <br> <br> But they also move like waves: for instance, they create interference patterns. These are the patterns similar to those that appear when one drops two pebbles in a pond. Complex ripple patterns appear where the two sets of circles, each expanding outward from a dropping place, overlap.<br> <br> Physicists have found that subatomic particles can’t have both a precisely defined location and speed, because of their wave nature. This ultimately lets particles briefly appear out of nowhere. <br> <br> The effect is due to certain oddities of particle-waves.<br> <br> One oddity is that in their case, unlike with water waves, there is no physical thing that actually “waves” or oscillates. With particle-waves, what oscillates is the probability that the associated particle will be found in one place or another when an experimenter looks for it. <br> <br> Physicists have no idea why any of this is so, or what it means. They’ve just found that it happens to work this way.<br> <br> This is just the beginning of the strangeness. <br> <br> Another unusual property of a particle-wave is that, unlike water waves, it’s not part of a long series of ripples following each other like a parade. It’s instead a small, localized packet of a few ripples, called a “wave packet.” <br> <br> Mathematically, a wave packet can be represented only as a composite of many waves, lined up so that their peaks and troughs cancel out everywhere except in the area of the wave packet. The resulting wave packet will consist of one bigger central wave, with smaller waves in front of it and behind it, dying down with increasing distance from the central wave. <br> <br> Thus the wave packet has no precise location; it’s a little spread out. By adding more overlapping waves, one can reduce this spread, though never eliminate it completely.<br> <br> Each of the many waves that go into a wave packet has a slightly different speed. Thus the wave packet itself has a range of speeds, which of course makes no sense if you think of it as a particle. But the wave nature of particles is like this. <br> <br> Uncertainties<br> <br> So not only does it have an imprecisely defined location, it also has an imprecisely defined speed. In fact, more precisely you define its location, the less precisely you define its speed—because you’re adding more waves. The more precisely you define its speed, the less precisely you define its location—because you’re subtracting waves and increasing its spread.<br> <br> The idea that there’s no such thing as empty space stems from this finding that a particle can’t have both an exact speed and location. A point of “empty” space is mathematically identical to a weightless particle with a speed of zero and a perfectly defined location, that being the point itself. This isn’t allowed.<br> <br> Therefore, physicists postulate that empty space is actually full of subatomic particles that rapidly appear and vanish. <br> <br> This doesn’t violate energy conservation because it turns out that the uncertainty in speed and position can be translated, mathematically, into uncertainties in energy and time. If a particle is short-lived enough, its energy can be so “fuzzy” that whoever or whatever enforces the conservation of energy law can’t detect a violation.<br> <br> Unfortunately, the fuzziness of virtual particles also makes them impossible to detect. Not directly, anyway. But circumstantial evidence of their existence is obtainable.<br> <br> One way to find this evidence is through an effect called the Casimir-Polder force. If an atom is very close to a flat surface, certain particle-waves can’t fit between the atom and the surface. Waves, in particular, need space. <br> <br> This means there will be a few less virtual particles to one side of the atom than the other.<br> <br> On the side of the atom where there are more virtual particles, the atom will “feel” a slight force pushing it toward the plate. This is because the virtual particles will be occasionally banging into the atom from that side, more often than from the other side. <br> <br> A related effect occurs when two flat plates are close enough together, in which case the plates will be attracted to each other.<br> <br> Physicists have difficulty trouble these forces because they are so slight. But Eric Cornell and his colleagues at the University of Colorado in Boulder, Colo. reported last month they were able to measure the Casimir-Polder force using a Bose-Einstein Condensate. The experiment, they added, may lead to new, more sensitive measurements of these small-range effects.<br> <br> Bose-Einstein Condensates<br> <br> A Bose-Einstein condensate, like the Casmir-Polder force, exists thanks to strange laws of quantum mechanics.<br> <br> Normally, the atoms in a gas are scattered, bouncing around like ping-pong balls. But if the gas is cooled, the atoms slow down. Cooling it more makes their speeds approach zero. But this is a precisely defined number. Since the speed becomes more precisely defined, each atom’s location must become less defined. In technical terms, each atom’s wave packet—the zone in which the particle might be found—grows.<br> <br> Make the gas cold enough, and each wave packet starts to overlap with neighboring ones, growing and growing until it envelops all the rest. Thus, all the wave packets overlap. If all the atoms are identical, the wave packets, and thus the atoms, can merge and become indistinguishable. They are all in the same place, have the same speed, and so on. They are like one atom. <br> <br> This is a Bose-Einstein Condensate.<br> <br> Because a condensate acts like one atom, it feels the Casimir-Polder force. But since it’s much easier to see than an atom, it makes that force easier to measure, said John Obrecht, a member of the University of Colorado team.<br> <br> In a paper published in the Sept. 15 issue of the research journal Physical Review A, Cornell and colleagues created a very flat Bose-Einstein Condensate and, by the use of a magnetic field, made it float in the air a few thousandths of a millimeter from a flat plate made of silica. They then set it gently oscillating.<br> <br> Because the Casimir-Polder force pulled more strongly on the side of this cloud closer to the plate than on the further side, it disrupted the normal oscillations slightly. By comparing the oscillations with and without the force present, Cornell’s team estimated how strongly the force was tugging on the cloud.<br> <br> By this method they measured the force at a distance of 5 thousandths of a millimeter, “significantly farther than has been previously achieved,” the team wrote.<br> <br> Cornell and colleagues said the work could be useful in the design of microelectromechanical systems (MEMS), tiny electronic devices built at this scale or smaller. They have a range of uses in industries such as medicine, automobiles and electronics.<br> <br> “Tremendous experimental progress in both ultracold atomic systems and microelectromechanical systems (MEMS’s), has pushed both fields towards precise work very close to surfaces—regimes where Casimir-type effects become important,” Cornell and colleagues wrote.<br> <br> Maarten DeKievit of the University of Heidelberg in Germany said Cornell’s approach is a good start towards getting more precise measurements of these forces, but needs more work to become useful. This is because the shape of the condensate isn’t yet definite enough to measure the force in a clear way that can then be used to predict its effect in a range of situations.<br> <br> “It’s a very nice experiment,” he said. “What you could dream of is if that they could change the form of the condensate” to get a range of precise shapes, he added. Then they could measure the force “as a function of shape,” information that MEMS designers could use.<br> <br> * * *</font> </p> </b> <p><b><font face="Arial" size="2" color="#FFFFFF">Send us a <a href="mailto:editor@world-science.net">comment</a> on this story, or <a href="mailto:?Body=http://www.world-science.net/othernews/051014_emptyfrm.htm">send it to a friend</a></font></b> </p> <p>  </center> </p> </td> </tr> </table> </body>