Q: Can Spiders swim? (Claire, England)
A small water spider swimming in an English pond. Photo courtesy of
Canterbury Environmental Education Centre.
A: Some spiders can swim. The half-inch (13 mm) water spider, in
fact, actually lives underwater (the only spider to do so), and swims so well he
can catch fish (although he doesn't usually),
along with more standard fare: midge larvae, water mites and mayfly
nymphs. This innovative spider makes his own undersea haven
— a diving bell.
Image, courtesy of ARKive. He spins an underwater
bell-shaped nest, descends with bubbles of air on his legs and under his belly
and places the bubbles under the platform and, thereby, bubble by bubble, 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 forays out of the shelter to hunt,
and brings back the kill to eat inside the bell. The male builds his
diving bell next to the female, and tunnels over to mate. The female lays eggs in
her partitioned-off upper part of the 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
Musuem 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.
A swimming tarantula, rowing to shore by using her first three pairs of
legs like paddles. Drawing courtesy of Jason Dunlop and
the
American Tarantula Society. Copyright, used with
permission.
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
rapid "flurry" of legs, row to shore. A few, though, 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 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.
Benjamin Franklin, courtesy of Wikipedia.
What's more: the carrier photon is not a 'real' photon, but
rather a virtual one that winks into and out of existence (from space vacuum)
solely to do the job: 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 emailed 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
Particle physics, Wikipedia
(Answered Nov. 27, 2006)
