QR3.8.4 Non-physical Detection

Using quantum theory, physics has discovered how to detect an object without physically touching it, which in a purely physical world should be impossible. The amazing device that does this is called 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, so now there are four paths to the two detectors. To recap, light shines on the first splitter, to send half down path 1 and half down path 2, then a second splitter splits the light, again half to each detector. Light going down path 1 now 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 always stays silent! This finding has no physical explanation 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 now 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 result 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 the experimenter doesn’t know this. If he is lucky, sending one photon down the system will trigger detector 2, proving the bomb is there, yet it didn’t go off. This isn’t a good bomb detection technique, as half the time it sets the bomb off, but it proves that it is possible to detect a bomb without touching it!

It follows that in our world, light can detect a physical object with no physical contact, but how can light detect a bomb on a path it didn’t take? Table 3.2 shows the four paths that quantum waves can take with their hit probability. As shown, half the time the bomb goes off, or detector 1 can fire, but also detector 2 can fire without triggering the bomb. The latter shows the bomb is there because it blocked the quantum wave that normally prevents detector 2 from firing.

Non-physical detection proves that quantum waves exist because matter can’t do what they do, but physics still denies them. It prefers the ancient myth of particles to the evidence of waves, so quarks we can’t see are accepted but quantum waves we can’t see aren’t. Yet isn’t science supposed to be driven by the evidence, not past tradition?

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