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By Root 757 0

the circumstance where the first experimenter measures ‘yes’ and the complementary case where they measure ‘no’. Only after summing over the two do we get the correct answer for the chances of measuring a ‘click’ in the second experiment. Is that really right? Do we really have to entertain the notion that, even after the outcome of some measurement, we should maintain the coherence of the world? Or is it the case that once we measure ‘yes’ or ‘no’ in the first experiment then the future is dependent only upon that measurement? For example, in our second experiment it would mean that if the first experimenter measures ‘yes’ then the probability that the second experiment goes ‘click’ should be computed not from a coherent sum over the ‘yes’ and ‘no’ possibilities but instead by considering only the ways in which the world can evolve from ‘first experimenter measures yes’ to ‘second experiment goes click’. This will clearly give a different answer from the case where we are to sum over both the ‘yes’ and ‘no’ outcomes and we need to know which is the right thing to do if we are to claim a full understanding.

The way to check which is right is to determine whether there is anything at all special about the measurement process itself. Does it change the world and stop us from adding together quantum amplitudes or rather is measurement part of a vast complex web of possibilities that remain forever in coherent superposition? As human beings we might be tempted to think that measuring something now (‘yes’ or ‘no’ say) irrevocably changes the future and if that were true then no future measurement could occur via both the ‘yes’ and ‘no’ routes. But it is far from clear that this is the case because it seems that there is always a chance to find the Universe in a future state which can be arrived at via either the ‘yes’ or ‘no’ routes. For those states, the laws of quantum physics, taken literally, leave us with no option but to compute the probability of their manifestation by summing over both the ‘yes’ and ‘no’ routes. Weird though this may seem, it is no more weird than the summing over histories that we have been performing throughout this book. All that is happening is that we are taking the idea so seriously that we are prepared to do it even at the level of human beings and their actions. From this point of view there is no ‘measurement problem’. It is only when we insist that the act of measuring ‘yes’ or ‘no’ really changes the nature of things that we run into a problem, because it is then incumbent upon us to explain what it is that triggers the change and breaks the quantum coherence.

The approach to quantum mechanics that we have been discussing, which rejects the idea that Nature goes about choosing a particular version of reality every time someone (or something) ‘makes a measurement’, forms the basis of what is often referred to as the ‘many worlds’ interpretation. It is very appealing because it is the logical consequence of taking the laws that govern the behaviour of elementary particles seriously enough to use them to describe all phenomena. But the implications are striking, for we are to imagine that the Universe is really a coherent superposition of all of the possible things that can happen and the world as we perceive it (with its apparently concrete reality) arises only because we are fooled into thinking that coherence is lost every time we ‘measure’ something. In other words, my conscious perception of the world is fashioned because the alternative (potentially interfering) histories are highly unlikely to lead to the same ‘now’ and that means quantum interference is negligible.

If measurement is not really destroying quantum coherence then, in a sense, we live out our lives inside one giant Feynman diagram and our predisposition to think that definite things are happening is really a consequence of our crude perceptions of the world. It really is conceivable that, at some time in our future, something can happen to us which requires that, in the past, we did two mutually opposite things. Clearly,