Do the virtual particles in quantum mechanics really exist? —J. Fleming, Madison, Wis.

Gordon Kane, director of the Michigan Center for Theoretical Physics at the University of Michigan at Ann Arbor, answers:

Virtual particles are indeed real—they have observable effects that physicists have devised ways of measuring. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy. So one particle can become a pair of heavier “virtual” particles, which quickly rejoin into the original particle, almost as if they had never existed.

While the virtual particles are briefly part of our world, they can interact with other, less exotic particles, and researchers can study these interactions to test predictions about virtual particles. In a hydrogen atom, photons (elementary particles of electromagnetic radiation) bind together a negatively charged electron and a positively charged proton. Every photon will spend some time as a virtual electron plus its antiparticle, the virtual positron, as described above. The hydrogen atom can be in more than one state; in one of those states, the atom interacts a little differently with the virtual electron and positron than when it is in another, so quantum theory predicts the two states' properties (which would otherwise be identical) to diverge slightly as a result of those interactions. That divergence was measured in 1947 by Willis Lamb, who later received a Nobel Prize in Physics for this work.

Another phenomenon involves elementary particles known as quarks—specifically the “top” quark, the heaviest of the six varieties. In the early 1990s the European laboratory CERN produced millions of particles called Z bosons and measured their mass very accurately. The measured value deviated a little from the mass apparently predicted by the Standard Model of particle physics, but the difference could be explained by the time the Z spent as a virtual top quark if such a quark had a certain mass. The mass of the top quark, directly measured a few years later at Fermi National Accelerator Laboratory in Batavia, Ill., agreed with that obtained from the CERN analysis, providing another dramatic confirmation of our understanding of virtual particles.

When you lose weight, where does it go?

Lora A. Sporny, adjunct associate professor of nutrition education at Columbia University, explains:

When you lose weight, the fat that disappears has been broken down into usable fuel for bodily activities.

Fats exist in chemical form as triglycerides—roughly E-shaped macromolecules with a glycerol molecule linked to three fatty acid chains. When trimming calories or increasing exercise, hormone-sensitive lipase, an enzyme within fat cells, responds to hormonal messages and disassembles triglycerides into their component parts, which then slip into the bloodstream. The liver preferentially absorbs the glycerol and some of the fatty acids; muscle takes in the remainder.

Inside the muscle and liver cells, the triglyceride pieces are further taken apart, eventually yielding large quantities of a compound called acetyl-CoA. Within mitochondria—the powerhouses of the cells—the acetyl-CoA combines with the compound oxaloacetate to form citric acid. This synthesis kicks off the citric acid cycle, or Krebs cycle, a set of chemical reactions that creates usable energy from fat, protein and carbohydrates.

These mitochondrial activities produce numerous products and by-products: carbon dioxide, which the lungs discharge during exhalation; water, which is expelled in urine or perspiration; heat, which helps to maintain a comfortable body temperature; and the energy-carrying molecule adenosine triphosphate (ATP). The ATP powers cellular activities—moving muscles, maintaining the heart's 100,000-plus daily beats, digesting food and processing nutrients into bodily tissues.

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