According to the complementarity principle, a photon can exhibit both particle and wave natures, but not both at the same time. This manifestation depends on whether an observer-participator uses a device to observe either particles or waves. In other words, the detector forces the photon to behave as a particle or as a wave. In the double-slit experiment a photon exhibits its wave nature when it passes through both the slits simultaneously while exhibiting its particle nature when it goes through only one slit. In the former case, the observer may conclude that the photon has “decided” to travel as a wave and in the latter case, it “decided” to travel as a particle after it is emitted.

Figure 4: Wheeler’s delayed choice experiment. Source:

But when does the photon decide whether it is going to travel as a wave or as a particle? John Archibald Wheeler proposed as a thought experiment (gedanken experiment) to answer this question.

Wheeler’s original, conceptual version of the delayed choice experiment is shown in Fig. 4. The upper diagram shows a wave packet of light, 1, consisting of a single photon. It passes through a half-silvered mirror and gets split into two wave packets, 2a and 2b, that reflect off two mirrors and then intersect. After the packet is split into two, an observer-participator decides to insert another half-silvered mirror at the intersection point. Before the second half-silvered mirror is inserted (lower left panel), the photon behaves like a particle-—it goes into either the upper or the lower photodetector, revealing which path it took. But as soon as the second mirror is introduced (lower right panel), the two wave packets interfere at the splitter—the photon behaves like a wave-—with destructive interference toward the upper photodetector and constructive toward the lower (wave packet 5).

Thus the observer’s decision, made after the wave packet is split into two, determines whether the photon will behave like a particle or a wave. In other words, the observer decides whether to put in the half-silvered mirror or take it out at the very last minute. This means, she decides whether the photon “shall have come by one route, or by both routes” after it has “already done its travel.” This is the time paradox.

Wheeler also noted that similar arguments can be applied to an astronomical object such as a quasar which is gravitationally lensed by another galaxy. To an observer on earth, light from a quasar would appear as arriving from two slightly different sources (locations), allowing it to be observed in two different ways: (1) by aiming a detector at each lensed image, thus making a particle measurement, or (2) by combining light from these two images in an interferometer, thus making a wave measurement—as proposed in the delayed choice experiment. But the light began its journey billions of years ago, long before the observer decided as to which experiment to launch. It would seem as though the light from the quasar “knew” whether it would be seen as a particle or a wave, billions of years before the experiment was actually performed.

Experimental groups who subsequently performed delayed such choice experiments have confirmed the apparent ability of performing measurements on photons “now” so as to alter events occurring in the past (Jacques, V. et al. 2007). But such interpretation is a non-standard. The time paradox does not exist if the Copenhagen interpretation is adhered to: the wave function of the photon in flight is in a superposition of states. When an observation is made, the wave function collapses and manifests itself as a particle or a wave depending on the detection mode, but while in flight, it is neither.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s