QR3.8.4 Non-physical Detection

Figure 3.22. The Mach-Zehnder interferometer

Quantum theory allows experimenters to use light to detect an object on a path it didn’t travel. In Figure 3.22, a light source shines on a beam splitter which sends half the light down path 1 and half down path 2. At first, since path 1 goes to detector 1 by a mirror and path 2 goes to detector 2 by another mirror, the light travels both paths equally so each detector fires half the time. Then the experiment adds a second splitter where the paths cross and now detector 1 registers but detector 2 stays silent. Quantum theory explains this as follows:

As photon quantum waves evolve down the paths, each mirror or splitter delays the phase by half. The two paths to detector 1 have two turns so they are in phase but path 1 to detector 2 has three turns and path 2 has only one so they cancel at detector 2. Detector 2 never fires because quantum waves from the two paths to it always cancel out.

Now if a receptor sensitive to any light is put on path 2, the previously silent detector 2 sometimes fires without triggering the receptor. This never happens if path 2 is clear, so it proves that something is blocking path 2. This experiment (Kwiat et al, 1995) verifies that:

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

2. With an object blocking path 2, detector 2 sometimes fires,

3. Yet the path 2 receptor registers nothing.

Using this setup, one can register an object without physically touching it (Audretsch, 2004) p29. Quantum theory explains this odd result as follows (see Table 3.2):

As photon quantum waves evolve down the paths those on path 2 are now blocked by a receptor that registers light half the time. Since the path 1 waves to detector 2 no longer cancel out, it fires a quarter of the time even though no light is registered on path 2. The other quarter of the time the path 1 light registers on detector 1. Detector 2 firing proves there is an obstacle on path 2.

To show how strange this is, suppose path 2 contains a bomb so sensitive that even one photon will set it off but the experimenters don’t know this. Now suppose they set up the system to send one photon and get lucky – detector 2 fires proving something is there. They have detected the bomb without physically touching it in any way! This is a bad bomb detection technique as half the time it sets the bomb off, but even so it is possible.

This non-physical detection supports quantum theory but again physical realism can’t explain it and never will. If the physical world is all there is, it isn’t possible to register a thing with no physical contact whatsoever. How can a photon detect a bomb on a path that it didn’t take?

This result confirms that quantum waves really exist but physical realism has no explanation for it at all.


Table 3.2. Non-physical detection




No Obstacle

Path 2 Obstacle

Path 1 to Detector 1


Detector 1 fires

Detector 1 fires

Path 2 to Detector 1


Detector 1 fires

Path 2 registers light

Path 1 to Detector 2


Detector 2 doesn’t fire

Detector 2 fires but path2 doesn’t register any light

Path 2 to Detector 2


Detector 2 never fires

Path 2 registers light