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A Lazy Layman's Guide to Quantum Physics
What is Quantum Physics?
That's an easy one: it's the science of things so small
that the quantum nature of reality has an effect.
Quantum means 'discrete amount' or 'portion'. Max Planck
discovered in 1900 that you couldn't get smaller than a
certain minimum amount of anything. This minimum amount
is now called the Planck unit.
Why is it weird?
Niels Bohr, the father of the orthodox 'Copenhagen
Interpretation' of quantum physics once said, "Anyone
who is not shocked by quantum theory has not understood
it".
To understand the weirdness completely, you just need to
know about three experiments: Light Bulb, Two Slits,
Schroedinger's Cat.
Two Slits
The simplest experiment to demonstrate quantum weirdness
involves shining a light through two parallel slits and
looking at the screen. It can be shown that a single
photon (particle of light) can interfere with itself, as
if it travelled through both slits at once.
Light Bulb
Imagine a light bulb filament gives out a photon,
seemingly in a random direction. Erwin Schroedinger came
up with a nine-letter-long equation that correctly
predicts the chances of finding that photon at any given
point. He envisaged a kind of wave, like a ripple from a
pebble dropped into a pond, spreading out from the
filament. Once you look at the photon, this 'wavefunction'
collapses into the single point at which the photon
really is.
Schroedinger's Cat
In this experiment, we take your pet cat and put it in a
box with a bottle of cyanide. We rig it up so that a
detector looks at an isolated electron and determines
whether it is 'spin up' or 'spin down' (it can have
either characteristic, seemingly at random). If it is
'spin up', then the bottle is opened and the cat gets
it. Ten minutes later we open the box and see if the cat
is alive or dead. The question is: what state is the cat
in between the detector being activated and you opening
the box. Nobody has actually done this experiment (to my
knowledge) but it does show up a paradox that arises in
certain interpretations.
If you dare to think about it (you're not really
supposed to), you have to believe one of the following
things:
MENU
Your consciousness affects the behaviour of subatomic
particles
- or -
Particles move backwards as well as forwards in time and
appear in all possible places at once
- or -
The universe is splitting, every Planck-time (10 E-43
seconds) into billions of parallel universes
- or -
The universe is interconnected with faster-than-light
transfers of information
----
Full English Breakfast
Coffee or Tea
These are the results of the different interpretations
of quantum physics. The interpretations all compete with
each other. Otherwise respectable physicists can get
quite heated about how sensible their pet interpretation
is and how crazy all the others are. At the moment,
there's about one new interpretation every three months,
but most of them fit into these categories.
What does it mean?
The meaning of quantum physics is a bit of a taboo
subject, but everyone thinks about it. To make it all a
bit more respectable, it is better to say 'ontology'
than 'meaning' -- it's the same thing. There are several
competing interpretations and the one thing they all
have in common is that each of them explains all the
facts and predicts every experiment's outcome correctly.
Copenhagen Interpretation (CI)
This is the granddaddy of interpretations, championed by
the formidable Niels Bohr of Copenhagen university. He
browbeat all dissenters into submission (with the
notable exception of Einstein) at a Brussels conference
sponsored by a man called Solvay in 1927. Bohr thereby
stifled the debate for a generation or two.
The CI has a bit of a cheek calling itself an
interpretation, because it essentially says "thou shalt
not ask what happens before ye look". He pointed out
that the Schroedinger equation worked as a tool for
calculating where the particle would be, except that it
'collapsed' as soon as you took a peek. If anyone asked
why this was, he would say, "shut up and calculate" (or
he might as well have done).
When you do try to take Copenhagen seriously you come to
the conclusion that consciousness and particle physics
are inter-related, and you rush off to write a book
called The Dancing Wu-Li Masters.
More recently, Henry Stapp at the University of
California has written papers such as On Quantum
Theories of the Mind (1997). Stapp's central thesis is
that the synapses in your brain are so small that
quantum effects are significant. This means that there
is quantum uncertainty about whether a neuron will fire
or not - and this degree of freedom that nature has
allows for the interaction of mind and matter.
What happens to the cat? You're not allowed to ask.
Many Worlds Interpretation (MWI)
The various paradoxes that the Copenhagen Interpretation
gave rise to (famously Schroedinger's cat, and
Einstein's dislike of "spooky action at a distance") led
others to keep on trying to find a better
interpretation.
The simplest was put forward by a student, Hugh Everett,
in 1957. He simply said that the Schroedinger equation
does not collapse. Of course, everyone laughed at him,
because they could see that the photon, for example, was
in just one place when they looked, not in all possible
places. But after a couple of decades, this issue was
resolved with the concept of decoherence - the idea that
different universes can very quickly branch apart, so
that there is very little relationship between them
after a tiny fraction of a second.
This has led to what should strictly be called the
'post-Everett' Interpretation, but is still usually
called MWI. It is now one of the most popular
interpretations and has won some impromptu beauty
contests at physics conferences. Unfortunately it means
that billions of you are splitting off every fraction of
a second into discrete universes and it implies that
everything possible exists in one universe or another.
This comes up with its own set of hard-to-digest
concepts, such as the fact that a 500-year-old you
exists in some universes, whereas in others you died at
birth.
In 1997, Max Tegmark at Princeton University proposed an
experiment to prove that MWI was correct. It involved
pointing a loaded gun at your head and pulling the
trigger. Of course, you will only survive in those
universes where the gun, for whatever reason, fails to
go off. If you get a misfire every time, you can satisfy
yourself -- with an arbitrarily high level of confidence
-- that MWI is true. Of course, in most universes your
family will be weeping at your funeral (or possibly just
shaking their heads and muttering).
What happens to the cat? It's dead in half the
subsequent universes and alive in the other half.
Pilot Waves, Hidden Variables and the Implicate Order
David Bohm (1917-1992) was a very brilliant physicist
and that's why people went along with him when he came
up with an elegant but more complicated theory to
explain the same set of phenomena (normally, more
complicated theories are disqualified by the principle
known as Ockham's Razor).
Bohm's theory follows on some original insights by
Prince Louis de Broglie (1892-1987), who first studied
the wave-like properties of the behaviour of particles
in 1924. De Broglie suggested that, in addition to the
normal wavefunction of the Copenhagen Interpretation,
there is a second wave that determines a precise
position for the particle at any particular time. In
this theory, there is some 'hidden variable' that
determines the precise position of the photon.
Sadly, John von Neumann (1903-1957) wrote a paper in
1932 proving that this theory was impossible. Von
Neumann was such a great mathematician that nobody
bothered to check his maths until 1966, when John Bell
(1928-1990) proved he'd bodged it and there could be
hidden variables after all -- but only if particles
could communicate faster than light (this is called 'nonlocality').
In 1982 Alain Aspect demonstrated that this superluminal
signaling did appear to exist, although David Mermin
then showed that you could not actually signal anything.
There is still some argument about whether this means
very much.
Bohm's theory was that the second wave was indeed faster
than light, and moreover it did not get weaker with
distance but instantly permeated the entire universe,
acting as a guide for the movement of the photon. This
is why it is called a 'pilot wave'.
This theory explains the paradoxes of quantum physics
perfectly. But it introduces a new faster-than-light
wave and some hidden mechanism for deciding where it
goes -- to create an 'implicate order'. That's quite a
lot of extra baggage, and scientists like to travel
light. Worse still, Bohm went on to become a mystic,
identifying his 'implicate order' with Eastern
spirituality and spawning books like Fritjof Capra's The
Tao of Physics . That's heretical behaviour in the eyes
of any decent physicist.
What happens to the cat? It's either dead or alive, of
course!
Consistent Histories
The Consistent Histories interpretation, put forward by
Robert Griffiths in 1984, works backwards from the
result of an experiment, arguing that only a few
possible histories are consistent with the rules of
quantum mechanics. It's an interesting idea but not very
popular because it still doesn't explain how a particle
can go through two slits and interfere with itself.
Roland Omns, in The Interpretation of Quantum Mechanics
(1994) wrote down 80 equations in a single chapter and
came to the conclusion that the 'consistent histories'
interpretation was pretty much the same as Copenhagen,
with a few knobs on.
What happens to the Cat? Again, you're not supposed to
ask.
Alternate Histories
The Alternate Histories Interpretation is quite
different, being similar to the Many-Worlds
Interpretation, but with the insistence that only the
actual outcome is the real world and the ones we're not
in don't actually exist. Unfortunately this gets us
right back to their being some kind of 'collapse'.
What happens to the cat? Again, you're not supposed to
ask.
Time Reversibility
Richard Feynman (1918-1988) was a genius who developed a
new approach to quantum mechanics. He formalised its
crowning achievement, Quantum Electrodynamics, which is
the most accurate scientific theory ever devised. He
also developed the Feynman Diagram, which represents the
interaction of two particles as the exchange of a third
particle. This diagram has time on one axis and space on
the other and the interaction can be viewed as happening
both in forward and in reverse time.
An electron, on its way from point A to point B, can
bump into a photon. In the diagram this can be drawn as
sending it backwards not just in space, but also in
time. Then it bumps into another photon, which sends it
forward in time again, but in a different direction in
space. In this way, it can be in two places at once.
There is little doubt that a Feynman diagram offers the
easiest way to predict the results of a subatomic
experiment. Many physicists have seen the power of this
tool and taken the next step, arguing that reverse time
travel is what actually happens in reality. Victor
Stenger of the University of Hawaii argues strongly for
this ontology in his forthcoming book. Of course, for a
layman, it is hard to understand why a photon bounces
around in such a way that it appears in two slits at
once.
What happens to the Cat? It is both dead and alive
simultaneously. We don't see this because of the
macroscopic 'measurement problem'.
Transactional Interpretation
Like Stenger's, John Cramer's Transactional
Interpretation relies on the fundamental time-symmetry
of the universe. He argues that particles perform a kind
of 'handshake' in the course of interacting. One sends
out a wave forward in time, and another sends one out
backwards in time.
What happens to the Cat? Ermm...
Gremlins
A new interpretation, presented for the first time here,
is that there are little green gremlins hovering around,
going backwards and forwards in time, shaking hands and
collapsing with mirth as they poke and prod subatomic
particles in a way they calculate most likely to confuse
us. This explains all of the observed experimental
results, but it does introduce gremlins, and the need
for a further theory about why they should want to
confuse us. Using the principle of Ockham's razor, this
interpretation will probably not find much popularity
among the scientific community although it may be the
basis for a new religion. Watch this space.
What happens to the Cat? Depends on what the gremlins
think will confuse us most.
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