Can Time Run Backwards? Introducing Retrocausality

July 12, 2017 – Getting the quantum world to align with Einstein’s theory of relativity has always caused headaches for physicists. Where quantum theory explains how the infinitely small subatomic world works, Einstein’s theory describes the larger Universe. The conundrum is something called quantum entanglement, a central pillar of quantum physics. In quantum entanglement particles’ characteristics are correlated instantaneously even though they can be billions of kilometers apart. When the quantum state of one is measured it somehow instantaneously influences its entangled remote partner which exhibits the same quantum state. No time elapses. There is no movement. The speed of light plays no part as a limiting factor.


   Photo credit: Mark Garlick/Science Photo Library/Getty Images.


If you ever have watched “The Big Bang Theory,” the CBS situation comedy, you often hear Leonard and Sheldon referencing the thought experiment known as Schrödinger’s Cat. where an unobserved cat’s fate remains unknown when placed in a box with a Geiger counter, a vial of poison, a hammer, and a radioactive substance. The radioactive decay of the substance which is random at a point triggers a process releasing the poison which then kills the cat. But until the box is opened because of the randomness of the radioactive decay process an observer has no knowledge about the cat’s state of existence. Erwin Schrödinger described this unknown state as being both alive and dead simultaneously. And although his paradox about the cat was an attempt to explain quantum entanglement, it seems bizarre. Yet that is how subatomic particles behave with properties such as energy, momentum, and position far different from our Newtonian view of the laws of physics, or even Einstein’s theory of relativity.

The state of quantum entanglement was recently demonstrated in the NASA Lunar Atmosphere Dust and Environment Explorer (LADEE) mission to the Moon in 2013 and 14. LADEE was equipped with onboard laser communication technology that generated instantaneous uploads and downloads from the Moon to NASA’s ground-based receivers with no time lag. In other words, the characteristic behaviour of the data bits on LADEE entangled with those on Earth instantaneously.

So that brings us to the latest quantum physics conundrum, something called retrocausality. In a paper appearing on June 21, 2017, in the journal, Proceedings of the Royal Society A, the authors, Matthew S. Leifer, and Matthew F. Pusey, propose that the future influences the past or the present. In other words, time doesn’t run in one direction but can go backward and forwards. They call this retrocausality and it has implications about entanglement that go beyond past, present, and future. If correct, retrocausality would mean an experimenter measuring a particle in the present could influence the properties of a particle in the past even before that person chose to make the measurement.

In writing about retrocausality on the TrendinTech website, Samuel Bavor uses the following analogy. “This is essentially the equivalent of you getting a stomach ache today because of sandwich you will eat tomorrow.”

Leifer and Pusey argue that retrocausality removes the boundary conditions of the arrow of time moving forward only. Is this any more confusing than observed quantum entanglement influencing the quantum state of a distant particle from one being locally observed?

For science fiction aficionados, retrocausality opens the floodgates to time travel. But what eager writers and readers of the genre don’t consider is that the observations happen at sub-atomic levels, not in the larger physical world where theoretically we could go back in time and visit ourselves. But could retrocausality if harnessed allow humans to move vast distances in spacetime without worrying about Warp engines exceeding the speed of light?

A last thought experiment:

Apply retrocausality to the current U.S. political situation. Could voters in the present influence and reverse the vote for President of the United States in the past? If only quantum entanglement could work this way!


In the 1966 television series “The Time Tunnel,” two scientists get trapped in their own form of entanglement whisking from one time period to another. Image credit: ABC




Len Rosen lives in Toronto, Ontario, Canada. He is a researcher and writer who has a fascination with science and technology. He is married with a daughter who works in radio, and a miniature red poodle who is his daily companion on walks of discovery.