Delayed Choice Quantum Eraser | Future Affects Past

An enhanced version of the famous double slit experiment, called the delayed choice quantum eraser implies exactly that mind blowing scenario – that future events can influence past results. 

  

What exactly is a delayed choice quantum eraser, and how can it possibly show that the future is affecting the past? In 1978, a physicist by the name of John Archibald Wheeler proposed a thought experiment, called delayed choice. Wheeler’s idea was to imagine light from a distant quasar being gravitationally lensed by a closer galaxy. Wheeler noted that this light could be observed on earth in two different ways. This is called a delayed choice because the observer’s choice of selecting how to measure the particle is being done billions of years from the time that the particle left the quasar. 

But how could this be?...the light began its journey billions of years ago, long before we decided on which experiment to perform. It would seem as if the quasar light “knew” whether it would be seen as a particle or wave billions of years before the experiment was even devised on earth. Does this prove that somehow the particle’s measurement of its current state has influenced its state in the past? The act of measurement gives reality to the quantum particle. So in the delayed-choice experiment, this means the quantum doesn’t become “real” until you measure it. So this experiment does not prove that the present has influenced the past because the light could have been a wave and particle at the same time, and only become real when it was measured.

However, another more recent experiment set up used a more complicated method to determine this idea of the future influencing a past. It introduced something called the quantum eraser to the delayed choice. So it is called the Delayed Choice Quantum Eraser designed by Kim, Kulik, Shih and Scully in 1999. 

It is a complicated construction that introduced entangled pairs of photons to Wheeler’s delayed choice experiment. 

I am going to show you a much simpler set up that will illustrate this concept in easier-to-understand terms. The results of this experiment are pretty amazing - because Here’s what happens. It tells us that when the which way information is known, that is, when the detector can ascertain which slit the photon came from, it always presents as a particle. But when the detector cannot ascertain which slit the photon came from, that is, when the which way information is erased, then the photon acts like a wave.

I left out a tiny but crucial detail that makes these results shocking. And that detail is that the path to detector 1 is much shorter than the paths to detectors 2, 3, and 4. So the photon is always detected at detector 1 BEFORE it is detected on any other detectors.

An interference pattern forms on detector 1 even if the which-path information pertinent to the photons that form it are only erased until much later in time at detector 4. And a double line pattern forms at detector 1 even before its which path information is ascertained at detectors 2 or 3. This shows us that somehow the photon at detector 1 knows ahead of time which detector its entangled pair will end up at in the future.

How is this possible?  

To be perfectly honest, I don’t have an answer. No one has a good answer. If you have an answer, let me know – because there is a Nobel prize waiting for you. But maybe our sense of time flowing in one direction is not something that quantum particles are constrained by. The entangled particles seem to be immune to the direction of time, or are somehow connected in time even when not connected in space.

Maybe our causal experience is just that - an experience, but not the true nature of reality.  

cause and effect seem to be temporally separated in the quantum world.

But let’s extrapolate these results to their most absurd extremes. There can be no communication between the future to the past. This would violate all kinds of laws in physics – like conservation of information, energy, etc. 

But Does backward causation mean that a future cause is changing something in the past? No. What came about in the past already happened, when the past was the present.  

The past cannot be changed. The past is fixed. 

...Or Maybe the past can be changed.