Biology Reference
In-Depth Information
to the movement of a raft with which it is about to collide (
Figure 13.2
d). With no active
systems for correlated movement, its own movement will be blocked and, depending on
how 'slippery' its interactions with the raft are, it will either stay where it is, pointlessly
trying to push the heavy raft sideways, or will be carried along by sticking to the raft, still
pushing pointlessly sideways and adding its small inertia to slightly slow the raft's own
progress down (
Figure 13.2
e). If, on the other hand, the object had an active system that
detected collision with the raft and reorientated its own motion to correlate perfectly
with that of its new neighbours (
Figure 13.2
f), its energy would not be wasted and it would
contribute to the joint effort. On the other hand, if this same interaction took place when
rafts collided, what would otherwise be traffic jams would become highly correlated, group
motion.
The power of active correlation to allow efficient movement at densities that would other-
wise be almost frozen is well known in the human-scale world. When people have to cross
a large area crowded with other pedestrians, all trying to achieve a variety of destinations,
they seldom travel in a straight line from A to B but rather correlate their motion to fit in
with a series of existing flows that takes them, in a zig-zag path that covers extra distance
but minimizes conflict, to their goal. The irritation caused by the rare people who refuse
to do this and insist on travelling in a straight line through a crowd, temporarily breaking
up other people's correlated motion, bears witness to our unconscious understanding that
correlated motion is efficient. Even on straight roads, where everyone is moving in the
same direction, density-related phase transitions can be seen, in this case between free
flow, synchronized flow and moving jams.
3
The instability that results in jams is essentially
a result of imperfect active correlation. On a road in which vehicle speeds differ, the driver of
a faster vehicle arriving behind a slow one should reduce speed so that the vehicle matches
that of the one in front while it is still following at a safe braking distance. In reality, reactions
can be too late and leave a driver too close to the vehicle in front: to increase this distance, he
or she therefore travels even more slowly for a period. With the driver behind making the
same mistake, and so on down the line, vehicles will end up travelling very slowly indeed,
thus creating a traffic jam (this is exactly the kind of traffic jam that appears, to the frustrated
motorists who have been delayed by it, to have had no cause). In order to avoid this, many
urban motorways have now been fitted with variable speed limits. The idea behind this is
that, when the roads are busy, the speed limit can be reduced (which allows higher traffic
densities without compromising braking distance). Rigid enforcement of speed, if necessary
a speed so low that even heavy goods vehicles can reach it, confers a very high correlation of
the motions of the vehicles and avoids unnecessary jams. Some car manufacturers are
working to develop 'car-train' technologies that allow vehicles to communicate directly
and to correlate their motions nearly perfectly (
http://www.autocar.co.uk/car-news/
motoring/volvo-road-train-hits-public-motorway
)
. All of this illustrates, at a human scale,
the point that correlated motion is quicker, when densities are high, than individual
sprinting.
How do cells correlate their motions? One of the most powerful mechanisms is contact
inhibition of locomotion,
4
a phenomenon by which a leading edge is shut down when
a cell meets another cell of its own kind. If two cells are making error-prone progress towards
a chemotactic goal and, because of the directional errors, they collide, the parts of their
leading edges that make mutual contact with the other cell will collapse. This will leave
