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.
