By Deepak Chopra, MD and Menas Kafatos, Ph.D., Vice Chancellor of Special Projects and Director, Fletcher Jones Endowed Professor of Computational Physics, Chapman University
Out of sight, and for most people out of mind, the physical world has been vanishing. For over a hundred years quantum theory has shown that the solid objects of the physical world are made of invisible energy clouds. Atoms have no fixed physical properties until they are measured; therefore, it remains to be shown why our world of everyday experience feels solid in the first place. At the same time, other properties we take for granted are dissolving. Einstein described time as dependent on frames of observation. Now it seems that in the world of quantum phenomena it can appear to move backwards.
This is a fascinating topic, and one that raises more questions about things we take for granted. Quantum physicists at the University of Vienna were looking at particles of light that are either entangled or separable. These are technical terms going back to the era of Einstein and Schrodinger. If two particles are entangled, they will exhibit synchronized behavior no matter how far apart they are in space. As soon as one particle is measured, its exact counterpart will show up in the entangled twin state, even if they are far, far away from each other. In other words, this "action at a distance" defies the speed of light. Einstein could not accept the consequences of quantum entanglement, and so he added the word "spooky" to action at a distance.
Yet quantum behavior is frequently spooky, and experiments have validated entanglement very soundly. In a recent article a useful analogy was given. Two entangled particles are like a pair of tumbling dice. If you stop one to see which number comes up, the other dice must show the same number; it has no other choice. If the two dice are separable, then the measurement of one doesn't affect the other. Being separable seems normal to us. We never expect two dice to exactly match. If they did, Las Vegas would go out of business, since chance would disappear.
Now on to time. We expect time to move forward, the so-called arrow of time. Past, present, and future constitute the normal progression of events. For the same reason, cause precedes effect. It would be bizarre to bleed before you cut yourself shaving or to hear a car crash before the two vehicles collided. In the quantum world, however, certain phenomena have arisen known as retro causation, and exactly as it sounds, a future measurement appears as if it is affecting a past event. This would be a form of entanglement that reaches backward in time, a new form of spookiness.
Physics has depended for decades on "thought experiments," where a new concept predicts what will happen before a physical experiment proves or disproves the predicted result. In this case, the Viennese team was working to prove "delayed-choice entanglement swaps." As a thought experiment, this has existed for over a decade. Let us follow the team's description closely:
Four photons, made of two entangled pairs, are produced (think of them as four tumbling dice waiting to be measured). One photon from each pair is sent to a physicist named Victor. He will be assigned the task of measuring them. The two remaining photons are put in separate packages, one sent to a physicist named Alice, the other to a physicist named Bob. The three physicists now have their sealed packages of photons that have not been measured yet.
Victor can choose between two kinds of measurements. If he decides to measure his two photons in a way such that they are forced to be in an entangled state, then Alice's and Bob's two photons also become entangled. But if Victor chooses to measure his particles individually, Alice's and Bob's photons end up in a separable state. This is a point that Einstein was stuck on. He couldn't believe the assertion made by Bohr and Heisenberg that the mere act of measurement by an observer determines where a particle will be. But accepted quantum theory has shown that particles have no physical characteristics until they are measured. For a long time this has been true for position in space. Now it seems that where a particle is in time also depends on measurement.
Modern quantum optics allowed the team to delay Victor's choice and measurement with respect to the measurements which Alice and Bob perform on their photons. As the lead author in Vienna describes it, "We found that whether Alice's and Bob's photons are entangled and show quantum correlations or are separable and show classical correlations, can be decided after they have been measured." In layman's terms, what you do today can affect what happened yesterday. Or, perhaps, to put it in better way, the future and the past are entangled, in a way that classical physics could not explain it. The experimenters are working on a quantum scale billions of times smaller than everyday events, and rather than claiming to change the past, they say that their experiment "mimics" the effect of turning time's arrow around.
So no one is saying -- yet -- that present causes can change past effects. The mystery still remains over how entanglement, defying the speed of light and now the arrow of time, actually relates to the "naive classical world," which is to say, the everyday things we take for granted. Our own bias is for expanding the observer effect more and more, until science accepts that awareness is key to everything. We are making reality through our role as conscious agents. But that's an argument for another day -- perhaps yesterday if we get around to it.