The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [120]
Everett’s idea was that Schrödinger’s math, the core of quantum mechanics, is compatible with such basic experiences. The source of the supposed ambiguity in device readings and mental impressions is the manner in which we’ve executed that math—the manner, that is, in which we’ve combined the results of the measurements illustrated in Figure 8.10 and Figure 8.11. Let’s think it through.
When you measure a single spiked wave, such as that in Figure 8.9, the device registers the spike’s location. If it’s spiked at Strawberry Fields, that’s what the device reads; if you look at the result, your brain registers that location and you become aware of it. If it’s spiked at Grant’s Tomb, that’s what the device registers; if you look, your brain registers that location and you become aware of it. When you measure the double spiked wave in Figure 8.10, Schrödinger’s math tells you to combine the two results you just found. But, says Everett, be careful and precise when you combine them. The combined result, he argued, does not yield a meter and a mind each simultaneously registering two locations. That’s sloppy thinking.
Instead, proceeding slowly and literally, we find that the combined result is a device and a mind registering Strawberry Fields, and a device and a mind registering Grant’s Tomb. And what does that mean? I’ll use broad strokes in painting the general picture, which I’ll refine shortly. To accommodate Everett’s suggested outcome, the device and you and everything else must split upon measurement, yielding two devices, two yous, and two everything elses—the only difference between the two being that one device and one you registers Strawberry Fields, while the other device and the other you registers Grant’s Tomb. As in Figure 8.12, this implies that we now have two parallel realities, two parallel worlds. To the you occupying each, the measurement and your mental impression of the result are sharp and unique and thus feel like life as usual. The peculiarity, of course, is that there are two of you who feel this way.
To keep the discussion accessible, I’ve focused on the position measurement of a single particle, and one that has a particularly simple probability wave. But Everett’s proposal applies generally. If you measured the position of a particle whose probability wave has any number of spikes, say, five, the result, according to Everett, would be five parallel realities differing only by the location registered on each reality’s device, and within the mind of each reality’s you. If one of these yous then measured the position of another particle whose wave had seven spikes, that you and that world would split again, into seven more, one for each possible outcome. And if you measured a wave like that of Figure 8.11, which can be partitioned into a great many tightly packed spikes, the result would be a great many parallel realities in which each possible particle location would be recorded on a device and read by a copy of you. In Everett’s approach, everything that is possible, quantum-mechanically speaking (that is, all those outcomes to which quantum mechanics assigns a nonzero probability), is realized in its own separate world. These are the “many worlds” of the Many Worlds approach to quantum mechanics.
Figure 8.12 In Everett’s approach, the measurement of a particle whose probability wave has two spikes yields both outcomes. In one world, the particle is found at the first location; in another world, it is found at the second.
If we apply the terminology we’ve been using in earlier chapters, these many worlds would