ioned
Typeset in
November 2002
at the start of quantum theory that life cannot be described by classical physics. Life is a
quantum effect. Let us show this.
Living beings can be described as matter objects showing metabolism, information pro-
cessing, information exchange, reproduction and motion. Obviously, all these properties
follow from a single one, to which the others are enabling means:
£ Living beings are objects able to reproduce. This denition implies several consequences. In order to reproduce, living beings must be
able to move in self-directed ways. An object able to perform self-directed motion is called
a machine. All self-reproducing beings are machines.
Since reproduction is simpler the smaller the system, most living beings are extremely
small machines for the tasks they perform, especially when compared to human made ma-
chines. Astonishingly, this is the case even though the design of human machines has con-
siderably fewer requirements: human-built machines do not need to be able to reproduce;
as a result, they do not need to be made of a single piece of matter, as all living beings have
to. Despite the restrictions nature has to live with, living beings hold many miniaturization
world records:
The brain has the highest processing power per volume of any calculating device so far.
Just look at the size of chess champion Gary Kasparov and the size of the computer against
which he played.
The brain has the densest and fastest memory of any device so far. The set of compact
disks (CDs) or digital versatile disks (DVDs) that compare with the brain is many thousand
times larger.
See page
438
Motors in living beings are many orders of magnitude smaller than human-built ones.
Just think about the muscles in the legs of an ant.
The motion of living beings beats the acceleration of any human-built machine by orders
of magnitude. No machine moves like a grasshopper.
Living beings sensor performance, such as that of the eye or the ear, has been surpassed
by human machines only recently. For the nose this feat is still extremely far away. Also the
sensor sizes developed by evolution think about the ears or eyes of a common y is still
unbeaten.
However, there are examples of objects which reproduce and which nobody would call living. Can you nd
some examples, together with an clearer denition?
Challenge 993 n
Motion Mountain www.motionmountain.net Copyright c November 1997 November 2002 Christoph Schiller 584
Flying living beings such as a fruit y are still thousands of times smaller than
anything built by humans.
Can you spot more examples?
Challenge 994 n
The superior miniaturization of living beings is due to their continuous strife for efcient
construction. In their structure, everything is connected to everything: each part inuences
many others. Indeed, the four basic processes in life, namely metabolic, mechanical, hor-
monal and electrical, are intertwined in space and time. For example, breathing helps di-
gestion; head movements pump liquid through the spine; a single hormone inuences many
chemical processes. Furthermore, all parts in living systems have more than one function.
For example, bones provide structure and produce blood; nails are tools and shed chemical
waste.
The miniaturization, the reproduction, the construction and the functioning of living be-
ings all rely on the quantum of action. Let us see how.
Reproduction
All the astonishing complexity of life is geared towards reproduction. Reproduction is the
ability of an object to build other objects similar to itself. Since reproduction requires mass increase, reproducing objects show both metabolism
and growth. In order that growth leads to a similar object as the original one, a construction
plan is necessary. This plan must be similar to the plan used by the previous generation.
Organizing growth with a construction plan is only possible if nature is made of smallest
entities which can be assembled following that plan.
We can thus deduce that reproduction implies that matter is made of smallest entities. If
matter was not made of smallest entities, there would be no way to realize reproduction.
Reproduction thus requires quantum theory. Indeed, without the quantum of action there
would be no DNA molecules and there would be no way to inherit our own properties our
own construction plan to children.
Passing on a plan implies that living beings must have ways to store information. Living
beings must have some built-in memory. We know already that a system with memory must
be made of many particles. There is no other way to store information. The large number of
particles is mainly necessary to protect the information from the inuences of the outside
world.
Our own construction plan, made of what biologist call genes, is stored in DNA
molecules. Reproduction is thus rst of all a transfer of parents genes to the next gener-
ation. We will come back to the details below. We rst have a look on how our body moves
itself and its genes around.
Quantum machines
Living beings are machines. How do these machines work? From a physical point of view,
we need only a few sections of our walk so far to describe them: universal gravity and
Only a similar object is possible, as an exact copy would contradict the properties of nature, in particular those
See page
549
of quantum theory itself, as we found out above.
Motion Mountain www.motionmountain.net Copyright c November 1997 November 2002 Christoph Schiller 23. Applications of quantum mechanics life, pleasure and their tools
585
QED. Simply stated, life is an electromagnetic process taking place in weak gravity. But
the details of this statement are tricky and interesting. Table
491
gives an overview of motion
processes in living beings. Interestingly, all motion in living beings can be summarized in a
few classes by asking for the motor driving it.
Nature has developed a few small but powerful devices to realize all motion types used
by living beings. Given the long time that living systems have been around, these devices
are extremely efcient. In fact, both the ion pumps and chemical pumps found in cell mem-
branes are specialized molecular motors. Even though there is still a lot to be learned about
them, what is known is already spectacular enough.
How do we move? Molecular motors
How do our muscles work? What is the underlying motor? One of the beautiful results of
modern biology, helped by chemistry and physics, is the elucidation of this issue. It turns
out that muscles work because they contain molecules which change shape when supplied
with energy. This shape change is repeatable. A clever combination and repetition of these
molecular shape changes is then used to generate macroscopic motion. There are three basic
classes of molecular motors: linear motors, rotational motors, and pumps.
Linear motors are at the basis of muscle motion; other linear motors separate genes during
cell division. They also move organelles inside cells and displace cells through the body
during embryo growth, when wounds heal, or in other examples of cell motility.
We encountered rotational motors already above. Nature uses them to rotate the cilia of
See page
57
many bacteria as well as sperm tails. Researchers have also discovered that evolution also
Ref.
617
produced molecular motors which turn around DNA helices like a motorized bolt would
turn around a screw. Such motors are attached at the end of some viruses and insert the
DNA into virus bodies when they are being built by infected cells, or extract the DNA from
the virus after it has infected a cell. Another rotational motor, the smallest known so far
Ref.
614
10 nm across and 8 nm high is ATP synthase, a protein that synthesizes most ATP in cells.
The ways molecules produce movement in linear motors was uncovered during the 1990s.
Ref.
615
This started a wave of research on all other molecular motors. There are no temperature
gradients involved, as in car engines, no electrical currents, as in electrical motors, and no
concentration gradients as in chemically induced motion. The central part of linear molec-
ular motors is a combination of two protein molecules, namely myosin and actin. Myosin
changes between two shapes and literally walks along actin. It moves in regular small steps.
The motion step size has been measured with beautiful experiments to be some multiple
of 5
.5 nm. A step, usually forward, but sometimes backwards, results whenever an ATP
Ref.
615
(adenosine triphosphate) molecule, the standard biological fuel, hydrolyses to ADP (adeno-
In fact, also the nuclear interactions play some role for life: cosmic radiation is one source for random mu-
tations, which are so important in evolution. Plant growers often use radioactive sources to increase mutation
rates. But obviously, radioactivity can also terminate life.
Ref.
613
The nuclear interactions are also implicitly involved in several other ways. They were necessary to form the
materials carbon, oxygen, etc. required for life. Nuclear interactions are behind the main mechanism for the
burning of the sun, which provides the energy for plants, for humans and for all other living beings (except a
few bacteria in inaccessible places).
Motion Mountain www.motionmountain.net Copyright c November 1997 November 2002 Christoph Schiller 586
Motion type
examples
main involved devices
Growth
collective molecular processes in
cell growth
ion pumps
gene turn-on and turn-off
linear molecular motors
aging
linear molecular motors
Construction
material
types
and
properties
(polysaccharides, lipids, proteins,
nucleic acids, others)
material transport through
muscles
forces and interactions