The Beginning of Infinity: Explanations That Transform the World (48 page)

If an object is unentangled, it can be made to undergo interference by something acting on it alone.

(The arrows ‘
’ and ‘
’ represent the action of the transporter.) Once the object is entangled with the rest of the world in regard to the values
X
and
Y
, no operation on the object alone can create interference between those values. Instead, the histories are merely split further, in the usual way:

In entangled objects, further splitting happens instead of interference.

When two or more values of a physical variable have differently affected something in the rest of the world, knock-on effects typically continue indefinitely, as I have described, with a wave of differentiation entangling more and more objects. If the differential effects can all be undone, then interference between those original values becomes possible again; but the laws of quantum mechanics dictate that undoing them requires fine control of
all
the affected objects, and that rapidly becomes infeasible. The process of its becoming infeasible is known as
decoherence
. In most situations, decoherence is very rapid, which is why splitting typically predominates over interference, and why interference – though ubiquitous on microscopic scales – is quite hard to demonstrate unambiguously in the laboratory.

Nevertheless, it can be done, and quantum interference phenomena constitute our main evidence of the existence of the multiverse, and of what its laws are. A real-life analogue of the above experiment is standard in quantum optics laboratories. Instead of experimenting on voltmeters (whose many interactions with their environment quickly cause decoherence), one uses individual photons, and the variable being acted upon is not voltage but which of two possible paths the photon is on. Instead of the transporter, one uses a simple device called a semi-silvered mirror (represented by the grey sloping bars in the diagrams below). When a photon strikes such a mirror, it bounces off in half the universes, and passes straight through in the other half, as shown on next page:

Semi-silvered mirror

The attributes of travelling in the
X
or
Y
directions behave analogously to the two voltages
X
and
Y
in our fictitious multiverse. So passing through the semi-silvered mirror is the analogue of the transformation
above. And when the two instances of a single photon, travelling in directions
X
and
Y
, strike the second semi-silvered mirror at the same time, they undergo the transformation
, which means that both instances emerge in the direction
X
: the two histories rejoin. To demonstrate this, one can use a set-up known as a ‘Mach–Zehnder interferometer’, which performs those two transformations (splitting and interference) in quick succession:

Mach–Zehnder interferometer

The two ordinary mirrors (the black sloping bars) are merely there to steer the photon from the first to the second semi-silvered mirror.

If a photon is introduced travelling rightwards (
X
)
after
the first mirror instead of before as shown, then it appears to emerge randomly, rightwards or downwards, from the last mirror (because then,
happens there). The same is true of a photon introduced travelling downwards (
Y
) after the first mirror. But a photon introduced as shown in the diagram invariably emerges rightwards, never downwards. By doing the experiment repeatedly with and without detectors on the paths, one can verify that only one photon is ever present per history, because only one of those detectors is ever observed to fire during such an experiment. Then, the fact that the intermediate histories
X
and
Y both
contribute to the deterministic final outcome
X
makes it inescapable that both are happening at the intermediate time.

In the real multiverse, there is no need for the transporter or any other special apparatus to cause histories to differentiate and to rejoin. Under the laws of quantum physics, elementary particles are undergoing such processes of their own accord, all the time. Moreover, histories may split into more than two – often into many trillions – each characterized by a slightly different direction of motion or difference in other physical variables of the elementary particle concerned. Also, in general the resulting histories have unequal measures. So let us now dispense with the transporter in the fictional multiverse too.

The rate of growth in the number of distinct histories is quite mind-boggling – even though, thanks to interference, there is now a certain amount of spontaneous rejoining as well. Because of this rejoining, the flow of information in the real multiverse is not divided into strictly autonomous subflows – branching, autonomous histories. Although there is still no communication between histories (in the sense of message-sending), they are intimately affecting each other, because the effect of interference on a history depends on what other histories are present.

Not only is the multiverse no longer perfectly partitioned into histories, individual particles are not perfectly partitioned into instances. For example, consider the following interference phenomenon,
where
X
and
Y
now represent different values of the position of a single particle: