Scientists often use the concept of a “phase space“, which is basically a representation of all the possible states that a system may be in. For the trajectory of a thrown stone for instance, the phase space would be a four-dimensional space, comprising the three dimensions of space, which define where the stone is, and one of time, which defines when the stone is in a particular position.
The trajectory of the stone is a line in this 4-d space, as the location and time information about the stone is known exactly. However, the stone is not a point and maybe be spinning at the same time that the whole object is flying through the air. This means that the trajectory would actually be a complex four-dimensional worm in phase space.
What if we were to introduce a probability factor into the experiment? Maybe we would set up the projectile to be triggered by an atomic decay or something similar. We would get a different worm depending on how long the atom takes to decay.
Clearly, if we want to show the all of the possible versions of the worm, the worm now becomes a sort of 4 dimensional sheet. Well, more like a 4-d duvet really, as the stone is not a point object.
Within the 4-d duvet, each worm represents a case where the atom has decayed, and each of these cases has a probability associated with it. The probability can be expressed as the probability that the atom has decayed by that time or not, and can run from one to zero.
Actually the probability starts from zero and approaches one but doesn’t quite reach it. In practise in a group of atoms some will decay quickly and others will take longer. If there are a finite number of them, then the chances of any one lasting a long, long time are quite small, and all of the atoms are likely to decay in a moderately short time, a few multiples of the half-life anyway. However there will be a finite but microscopic in the extreme possibility, that an atom will survive for as long as you may consider.
We can add another dimension to the phase space, one of probability. This gives us a five dimensional phase space, and the duvet becomes five dimensional. However, an atom decays at a certain time, and there is a single five dimensional worm in the phase space going forward. The space is no longer a phase space though, as a phase space, by definition, describes all possible states of the rock/launcher/atomic trigger, and doesn’t change.
According to the Copenhagen interpretation of quantum physics the state of a quantum system is described by a set of probabilities. When a measurement of the system is made the state becomes certain, and it is said that the waveform described by the probability function has “collapsed”.
The famous thought experiment of Schrodinger’s Cat is a description of the difficulties of such a case. The cat is enclosed in a box equipped with a mechanism which will release a poison and kill the cat if triggered by the decay of an atom. At some time after the experiment starts the atom may or may not have decayed so the quantum states “decayed” and “not decayed” are superimposed, and therefore so are the states “dead” and “not dead” of the cat.
How do we know if the stone has been fired yet? Well, we go and look to see, and we either see the stone in its launcher or we don’t. Quantum physics says that the stone exists in a superposition of states – launcher and not launched. The question this raises is, if this is so, how does looking at the stone “collapse” the superposition when we look?
That quantum superposition is real is indicated by any number of experiments, even though many physicists working in the field (including Schrodinger himself) have expressed discomfort at the idea.
In quantum physics the evolution of everything is defined by the Universal Wave Function. This can be used to predict the future of any quantum physical system (and all physical systems are fundamentally quantum physical systems). Unfortunately for easy understanding, interpretation leads to the superposition problem mentioned above.
Many people have tried to resolve this issue, and the best success has been achieved by the exponents of the Many Worlds Interpretation (MWI), as described by Everett and championed by Bryce DeWitt and David Deutsch. The view of the MWI exponents is that the Universal Wave Function is fundamental and expresses a true picture of all reality. All of it, that is. Not just a physical system and its observer.
Everett’s view, as described in his thesis, is that an observer, as well as the object that he is observing is a subsystem of the system described by the Universal Wave Function. The wave function of these two subsystems does not describe a single state for each of these subsystems, but the states of the two subsystems are superposed, or in Everett’s term, correlated.
When a particle is observed it may appear to be in state A 70% of the time (correlated with a state A for the observer). Similarly it may appear to be in state B 30% of the time (correlated with a state B for the observer). This led Everett to postulate a ‘split’ of the universe into a state A and a state B. (The term ‘split’ appears to come from DeWitt’s interpretation of Everett’s work).
The probabilities don’t seem to have a function in this model, and this is odd. The probability that the cat is dead when you open the depends on how long you wait until you open the box. If you wait a long time the cat will more likely be dead than if you opened it earlier.
This means that the world splits when the cat is put in the box, as from any moment it can be alive or dead, but you do not find out which branch you are in until you open the box sometime later.
I’m ambivalent about the MWI. On the one hand it is a good explanation of what happens when a measurement is made or the cat’s box is opened, and it does away with the need for a waveform collapse, which Everett argued against in his paper. However it is profligate in terms of world creation.
Another issue is that the split is decidedly binary. The cat is alive in this world and dead in that one. However most other physical processes are, at the macro level anyway, continuous. When a scientist takes a measurement he writes down, for example, 2.5, but this is only inaccurate value as it is impossible to measure something exactly and it may be wrong by up to 0.05 on either side of 2.5 (given the one decimal point value shown).
Consequently, I’d prefer an interpretation where there is no split, but instead a continuum of possibilities as part of a single world. Maybe the single path that we tread through life is an illusion and across the Universe, by virtue of the Universal Wave Function, we experience all possibilities, though to us it feels like we are only experiencing the one.