‘This is the heart of why Einstein referred to entanglement as “spooky action at a distance.” It’s spooky because entangled objects have a quantum connection, even if they are light-years apart’
(Related A State of Everythingness )
Life is full of choices. Do we have a cookie or go to the gym? Do we binge watch our favorite show on Netflix or go to bed at a reasonable time? Our choices have consequences, and we make them of our own free will. Or do we?
The nature of free will has long inspired philosophical debates, but it also raises a central question about the fundamental nature of the universe. Is the cosmos governed by strict physical laws that determine its fate from the big bang until the end of time? Or do the laws of nature sometimes allow for things to happen at random? A century-old series of physics experiments still hasn’t been able to settle the question, but a new experiment has tilted the odds toward the latter by performing a quantum experiment across billions of light-years.
The laws of classical physics are deterministic. Newton’s mathematical cosmos is a clockwork universe, where each cause has a unique effect and we are governed not by our choices but by the rigid laws of nature. Quantum physics, on the other hand, has a property of fuzzy randomness, which some scientists feel could open the door to free will. Since quantum physics lies at the heart of reality, it would seem that randomness wins the day.
But some scientists have argued that quantum randomness isn’t truly random. If I roll a die the outcome seems random, but it isn’t really. All of its bumps and turns are caused by the forces of gravity and the table in a complex dance, but that dance is deterministic. The moment the die leaves my hand, its fate is sealed, even though I don’t know the outcome until it happens. Perhaps quantum objects behave in the same way. They seem to act in random ways, but they are really governed by some deterministic hidden variables.
It is a question that has fascinated me since graduate school. My dissertation focused on aspects of quantum gravity, a subject that we still don’t fully understand. One of the reasons for this is that we don’t know how Einstein’s deterministic theory of gravity can fit together with the randomness of quantum mechanics. The question fascinated Einstein as well, and being much smarter than me, he came up with an experiment that could test the idea. Together with Boris Podolsky and Nathan Rosen he presented a thought experiment now known as the Einstein-Podolsky-Rosen experiment, or EPR experiment for short.
To understand the experiment, suppose we have a mischievous mutual friend named Jane. Whenever Jane wears out a pair of running shoes, she loves to prank us by sending one shoe to each of us. So, whenever you get a shoe in the mail from Jane, you know I’ve gotten one too. One of us gets the right shoe, the other the left. But until either of us open our respective box, neither of us know which shoe we have. Once the box arrives at your door, you open it up, and find you have the left shoe. At that moment, you know I must have the right shoe.
This is the basic idea of the EPR experiment. It’s nothing more than a silly prank in our everyday world, but for quantum objects it gets really strange. You may have heard of Schrödinger’s cat, where a quantum cat is neither alive nor dead until observed in a definite state. Like classical cats, quantum cats like quantum boxes. In the quantum realm things can be in an indefinite state until you observe them. It would be as if our boxes contained a pair of something (gloves, shoes, salt and pepper shakers, etc.) but it is impossible to know what specific something until one of us opens their box. Even stranger, how we measure quantum objects determines what the outcome can be. It would be as if opening the box on the side forces it to be a glove, while opening it from the top forces it to be a shoe. How I open my box affects your box miles away. In quantum theory, we say that our two boxes are entangled, so that observing the content of one box also tells us something about the other.
We can’t do this experiment for gloves and shoes, but we can do it with light. Two entangled photons can be sent in opposite directions. I measure the orientation of one photon at random, you measure the other, and then we compare our results. There are lots of different orientations we would measure, so we can each choose the orientation we want. When this experiment is done in the lab, it actually works. And if our measurements are random, there is no way for the photons to know ahead of time which orientation will be measured. So, there can’t be any hidden variable to determine the outcome. Whether we get the left or right shoe, or the left or right glove, the result is truly random.