Enlarge Courtesy of Jason Dunlop and the American Tarantula Society; Copyright, used with permission A swimming tarantula, rowing to shore by using her first three pairs of legs like paddles. Enlarge Courtesy of Wikipedia Benjamin Franklin Swimming spiders, Freaky photons Q: Can Spiders swim? (Claire, England) A: Some spiders can swim. The half-inch water spider, in fact, actually lives underwater (the only spider who does), and swims so well he can catch fish (although he doesn't usually) and more standard water fare including midge larvae, water mites and mayfly nymphs. This innovative spider makes his own undersea haven — a diving bell. Here's an image of one. He spins the underwater bell-shaped nest, descends with bubbles of air on his legs and under his belly and places the bubbles under the platform. Slowly, bubble by bubble, he builds an air reserve. Once the air pocket is in place, it lasts indefinitely. "Oxygen diffuses from the surrounding water" into the pocket, and waste carbon dioxide diffuses out, says Beth Coleman of the Canterbury Environmental Education Centre. The spider leaves the shelter to hunt and brings back his kill inside the bell. The male builds his diving bell next to the female, and tunnels over to mate. The female lays eggs in a partitioned-off upper part of her bell, but young spiders hatch out into the water. These spiders live in ponds and rivers in Europe, northern Asia and north of the Sahara Desert in Africa. Most spiders, though, don't swim as a routine part of life, but some can to survive. "Tarantulas don't take kindly to large bodies of water," reports zoologist Jason Dunlop of the Museum für Naturkunde in Berlin. They are "clearly able to detect its presence", probably with humidity sensors on their legs, and actively try to avoid the water. But if, for example, a tarantula falls in the water when scampering along an overhanging tree limb, she's likely to float on the water's surface film, buoyed by air trapped in her hairy legs, and then, with a rapid flurry of legs, row to shore. A few merely remain motionless until they sink. Further Reading: Water Spider, Canterbury Environmental Education Centre Swimming in tarantulas by Jason A. Dunlop, University of Manchester American Tarantula Society Headquarters Water Spider images, ARKive Q: Photons carry the electromagnetic force, but don't have a charge. Like charges repel, differing charges attract. So when a photon from an electron approaches another electron, how does it "know" whether to repel or attract, if it is the same kind of photon that would come from a proton? (Someone, World) A: It's a puzzle all right. Photons have no charge so how does a photon know about charges and pass the information on to the charged particles? Let's work our way to the answer using negatively-charged electrons and positively-charged protons as our example. Our results will hold, however, for all charged particles from quarks to omegas. Photons are the carrier particles for electromagnetic interactions. That means charged particles (for example, electrons and protons) interact by exchanging photons. One electron (for instance) radiates a photon; another absorbs the photon. The absorbing electron necessarily absorbs the photon's energy, spin and momentum. That's the tip-off. The photon carries the information in its spin and momentum. We get a clue how this takes place through the calculations of quantum mechanics: the sign of the momentum transferred is determined by the product of the charges. In this case both particles are electrons, so their charges are negative. The product of the charges is a plus, which means they repel each other. Similarly, if the two particles have unlike charges, for example, an electron and a proton, then the product of the charges is a minus, which means they attract. The carrier photon carries information about the interaction in its spin and momentum. The momentum is either positive or negative (we don't know why), depending on the charges of the interacting particles. About the momentum sign (positive or negative): "You might be interested to know that I've talked with the smartest experimentalist I know and the smartest theorist I know (crushingly intelligent people) and they cannot deliver an explanation for the dual nature of electromagnetism (attraction and repulsion) beyond the empirical observation of Ben Franklin that there are two different charges of electricity: positive and negative," physicist Erik Ramberg of Fermilab says. Like charges repel, unlike charges attract. What's more: the carrier photon is not a "real" photon, but rather a virtual one that winks into and out of existence solely to do the job of carrying interaction information. "What does this mean?" Ramberg mulls. It means the interaction takes place in an interval of space and time that "violates" the conservation laws of momentum and spin. The Heisenberg uncertainty principle says such a violation is okay if the charged particles emit and absorb the virtual photon in an "undetected" state. It's a stealth photon that operates in a smaller time interval than we can imagine. "I hope your brain is hurting, because that's what quantum mechanics does to us all!" I smiled as I read Ramberg's e-mailed comment. "In some sense, this is almost a key fact of our existence. Every atom of every cell has electromagnetic interactions with its neighbors. That is what makes our world seem 'solid.' And it's all because of clouds of virtual photons that pop into and out of existence in unfathomably microscopic periods of space and time." Further Reading: The Charm of Strange Quarks by Barnett, Muhry and Quinn In search of the ultimate building blocks, Gerard 't Hooft The electromagnetic interaction by Rod Nave, HyperPhysics The particle adventure, Lawrence Berkeley National Laboratory (Answered Nov. 27, 2006) April Holladay, science journalist for USATODAY.com, lives in Albuquerque, New Mexico. A few years ago Holladay retired early from computer engineering to canoe the flood-swollen Mackenzie, Canada's largest river. Now she writes a column about nature and science, which appears Fridays at USATODAY.com. To read April's past WonderQuest columns, please check out her site. If you have a question for April, visit this informational page. Enlarge Courtesy of Canterbury Environmental Education Centre A small water spider swimming in an English pond.