QR3.8.4 Non-physical Effects

Quantum theory lets physicists detect an object without physically touching it, which in a purely physical world should be impossible, using a Mach-Zehnder interferometer (Figure 3.22). 

This device works as follows. First, it splits light into two paths that go to the two detectors, where the mirrors make the paths cross. The result is that each detector fires half the time, as expected. Then a second light splitter is added where the paths cross to split the light again. If half the light shining on the first splitter goes down path 1, and half goes down path 2, then adding a second splitter splits the light again, half to each detector, so there are four paths to the two detectors. Light going down path 1 goes to both detectors, as does light going down path 2, so how do they respond?

The result is that detector 1 still fires but detector 2 never does! There is no physical explanation for this, but quantum theory explains it, based on quantum waves, as follows:

As photon waves evolve down the paths, each mirror or splitter turn delays its phase by half. Both paths to detector 1 have two turns, so they add because they are in phase. In contrast, path 1 to detector 2 has three turns while path 2 has two, so they cancel out because they are out of phase. Detector 2 then never fires because the waves from the two paths to it always cancel.

This setup allows a very unusual result. If a light sensitive object is put on path 2, the previously silent detector 2 sometimes fires, even when the object didn’t detect any light. This never happens if path 2 is clear, so this proves there is an object on path 2, yet no light went that way. The results (Kwiat et al, 1995) are unequivocal:

1. With two clear paths, only detector 1 fires.

2. If an object blocks path 2, detector 2 sometimes fires, even when no light touched the object.

Quantum theory then explains what materialism can’t (Audretsch, 2004), p29, as follows:

Light waves evolve down both paths, so they hit the path 2 object half the time. The other half of the time they go down path 1, but if path 2 is blocked, the waves to detector 2 no longer cancel out, so it fires sometimes, even when the path 2 object registers no light. Detector 2 then only fires if there is an obstacle on path 2.

To illustrate how strange this is, suppose a light-sensitive bomb blocks path 2 but we don’t know its there. With luck, sending one photon down the system will trigger detector 2, proving the bomb is there without exploding it. This is bad bomb detection because half the time it sets the bomb off, but it can detect a bomb on a path no light took. Table 3.2 shows the four paths, their probability, and the detector results. Half the time the bomb goes off, or detector 1 fires, but if detector 2 fires without triggering the bomb, there must be a path 2 obstacle blocking the quantum wave that stops detector 2 firing.

Non-physical detection proves that in our world light can detect an object without touching it! It follows that quantum waves must exist because physical particles can’t do this.

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