Wheeler's Delayed-Choice Experiment

Pavitra Kanetkar |

Wheeler's delayed choice experiment is a thought experiment in quantum physics that was proposed by physicist John Archibald Wheeler in 1978. The experiment is designed to explore the nature of wave-particle duality and the role of observation in quantum measurements.

Case : A

The experiment involves a setup where a beam of photons is directed towards a beam splitter at lower left corner. There is 50-50 probability that the photons will be transmitted (pass through straight direction to the bottom right mirror) or reflected (sent to the top left mirror). 

At beam splitter, if photon is transmitted, it is sent to the mirror which is placed at the lower right corner. It will reflect the photon to the top of the apparatus where screen - A is placed

At beam splitter, if photon is reflected to the top left mirror, it will reflect the photon to the top right where screen - B is placed. 

In both cases we will find the path of the photon. Either it follows clockwise route or anti clockwise route. We will not get the interference pattern on any screen.

Case : B


If a second beam splitter is added at top right corner, the interference pattern will be appeared at the right screen constantly. Other screen will not have any pattern as in this case photon will act as a wave. There is no 50-50

In case A photon travels in single path which ends up with particle behaviour but when second beam splitter is placed, `which way` information is hidden that's why single photon travels in both way simultaneously. The result is wave like behaviour.

Photons are changing the behaviour based on the `which-way` path information. If the information is disclosed, they show the particle behaviour otherwise wave behaviour.

While entering the first beam splitter, it appears photons decide to show their particle behaviour if there is no beam splitter or their wave behaviour if there is a beam splitter at the end.

Cosmic Interferometer

Most theoretical physicists argue that somehow photons have the information about the beam splitter is going to be used or not used at the end that's why they choose their final behaviour before entering the first beam splitter.
 
Wheeler proposed, A quasar which is billions of light years far, its light crosses a galaxy cluster which is millions of light years far from us. We see dual image of the quasar due to Gravitational Lensing. The left telescope shows left image due to photons passed through the left side of the cluster and same for right side image.
 
If we detect the photons at either side, we can see the particle behaviour as `which way` path information is exposed but if we use interferometer at the end, we see interference pattern.
 
As photons have crossed the gravitational lens million of years before, it looks they are changing the past decision based on the future outcome but possible explanation can be wave function is collapsed when we take decision that the measurement should be done or not and that's when photon show particle or wave behaviour.
 
But this is not all, another Mind Boggling experiment is 'Delayed Choice Quantum Eraser', where it appears photon travels back in time to change the outcome. We will discuss it in a future post.
 

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