hysicists have a history of finding
natural laws that fit elegantly into the language of mathematics but that
become seeming paradoxes when expressed with ordinary words. Now, along
with particles that behave as waves and vice versa, they have a new paradox
to entertain them: noise that makes certain systems in nature quieter.
In a paper titled "Noise Suppression by Noise," which was published
yesterday in the journal Physical Review Letters, two physicists have found
that the noise, or random fluctuations, generated by particular types of
microscopic systems can actually be quieted when more noise is added from
the outside.
The practical consequence of the work may be to show that some systems
in nature, like biological cells, may already be using the effect to operate
more efficiently and smoothly, since noise of various types is abundant
in natural environments. The work could also someday help scientists understand
certain types of advanced circuitry in which noise, or static, is unavoidable.
The finding's immediate effect, however, will be to pull back the veil
a little further on the strange workings of the microscopic world.
In everyday experience, said Dr. Jose M. G. Vilar of the molecular biology
department at Princeton, who is the paper's lead author, "We put more noise
in and we get more noise out."
But the systems he studied with his colleague in the research, Prof.
Miguel Rubí, of the department of fundamental physics at the University
of Barcelona in Spain, have what mathematicians call "nonlinear" behavior,
scrambling that direct relation. Some of those systems, Dr. Vilar and Dr.
Rubí found, reverse ordinary intuition.
"You put noise in the system and it displays less noise," Dr. Vilar
said.
Prof. Charles Doering, of the mathematics department of the University
of Michigan, said the findings added to a growing recognition that noise
in many biological and physical systems could actually make them more sensitive
and efficient, rather than being only a source of confusion.
"The bottom line is that noise can be extremely beneficial," Professor
Doering said. "It can act as a lubricant to make things work better and
smoother."
The first collision of mathematics and ordinary language comes in defining
the "noise" considered in the research. The noise in question mostly involves
random fluctuations in the flow of particles and electrical current in
microscopic systems like cell membranes and advanced circuitry.
Physicists call those fluctuations noise because, like static in a stereo
system, they add to some steady signal and confuse it. In a cell, the noise
may consist of fluctuations in the flow of particles through so- called
ion channels in a membrane, while in advanced circuit elements called quantum
dots it consists of jumps and dips in an otherwise steady and predictable
current.
Each of those systems produces its own intrinsic or internal noise.
Ion channels, for example, open and close to regulate the flow of electrically
charged atoms called ions, and because the process is not perfectly regular,
the steady flow is complicated by noise. The work by Dr. Vilar and Dr.
Rubí shows that, strangely enough, any additional noise - caused,
say, by the jostling of other molecules or by externally applied electrical
fields - can actually quiet the flow of ions, making it smoother and more
regular.
Although the findings are largely theoretical, they could have practical
consequences, since systems like the ion channel often operate in configurations
that produce large amounts of internal noise - exactly the situation in
which the new calculations say external noise could quiet the flow. Similar
conclusions may hold for quantum circuits and other related systems.
Dr. Vilar said that the effect could allow those systems to operate
more smoothly in configurations with just the right amount of average or
steady flow, but also with lots of internal noise. Adding external noise
would not change the average value but would quiet the fluctuations.
"You have a system which in some places displays a lot of noise, but
you like those places," Dr. Vilar said. "And other places which don't display
much noise, but you don't like those places."
By adding external noise, he said, "basically, you get the best of both
parts."
"You get the noise of one and the average of the other," he added.
As always with the verbal "paradoxes" of physics, the hard part is explaining
in common language how and why the phenomenon works.
It helps to visualize how an ion channel or a quantum dot operates normally.
Because nature is grainy on the microscopic level, with everything made
up of particles and lumps of charge, both systems operate like a door that
lets people through. Since the door cannot open partly and let "half a
person" through, the flow rate depends only on how frequently the door
is opened to let whole people through.
That means flow in the ion channel, for instance, can be regulated from
zero to intermediate values to some maximum depending on whether the "door"
is always closed, open as often as closed or always open. Not surprisingly,
the intrinsic noise, or random fluctuations in the flow, occur for those
intermediate values, where slight variations in the door's behavior have
the most effect. Constantly closed or open doors permit no variation.
But it is exactly in those intermediate regimes that the channel is
likely to operate most often as it adjusts to the needs of the cell (as,
for example, when neuron cells take on or expel charge as part of their
electrical firing). An external jitter of some kind can in a sense knock
the system away from its noisiest point of operation without affecting
the average flow, the calculations showed. In a sense, the door becomes
pinned against a doorjamb briefly enough to still the intrinsic noise and
not affect the passage of the people.
