Einstein, like no other physicist before or after him, demonstrated how the power of human thought alone, used skillfully, can make up for the lack of real experiments. He showed we little humans have the power to deduce equations that govern the natural world by logical conclusion. Thought experiments are common in theoretical physics today. Physicists use them to examine the consequences of a theory beyond that what is measureable with existing technology, but still within the realm of that what is in principle measureable. A thought experiments pushes a theory to its limit and thereby can reveal inconsistencies or novel effects. The rules of the game are that a) relevant is only that what is measureable and b) do not fool yourself. This isn’t as easy as it sounds.The famous Einstein-Podolsky-Rosen experiment was such an exploration of the consequences of a theory, in this case quantum mechanics. In a seminal paper from 1935 the three physicists showed that the standard Copenhagen interpretation of quantum mechanics has a peculiar consequence: It allows for the existence of “entangled” particles.
Entangled particles have measureable properties, for example spin, that are correlated between two particles even though the value for each single particle is not determined as long as the particles were not measured. You can know for example that if one particle has spin up the other one has spin down or vice versa, but not know which is which. The consequence is that if one of these particles is measured, the state of the other one changes – instantaneously. The moment you measure one particle having spin up, the other one must have spin down, even though it did, according to the Copenhagen interpretation, not previously have any specific spin value.
Einstein believed this ‘spooky’ action at a distance to be nonsense and decades of discussion followed. John Steward Bell later quantified exactly how entangled particles are stronger correlated than classical particles could ever be. According to Bell’s theorem, quantum entanglement can violate an inequality that bounds classical correlations.
When I was a student, tests of Bell’s theorem were still thought experiments. Today they are real experiments, and we know beyond doubt that quantum entanglement exists. It is at the basis of quantum information, quantum computation, and chances are all technologies of the coming generations will build upon Einstein, Podolsky and Rosen’s thought experiment.
Another famous thought experiment is Einstein’s elevator being pulled up by an angel. Einstein argued that inside the elevator one cannot tell, by any possible measurement, whether the elevator is in rest in a gravitational field or is being pulled up with constant acceleration. This principle of equivalence means that locally (in the elevator) the effects of gravitation are the same as that of acceleration in the absence of gravity. Converted into mathematical equations, it becomes the basis for General Relativity.
Einstein also liked to imagine chasing after photons and he seems to have spent a lot of time thinking about trains and mirrors and so on, but let us look at some other physicists’ thoughts.
Before Einstein and the advent of quantum mechanics, Laplace imagined an omniscient being able to measure the positions and velocities of all particles in the universe. He concluded, correctly, that based on Newtonian mechanics this being, named “Laplace’s demon”, would be able to predict the future perfectly for all times. Laplace did not know back then of Heisenberg’s uncertainty principle and neither did he know of chaos, both of which spoil predictability. However, his thoughts on determinism were hugely influential and lead to the idea of a clockwork universe, and our understanding of science a prediction tool in general.
Laplace’s is not the only famous demon in physics. Maxwell also imagined a demon, one that was able to sort particles of a gas into compartments depending on the particles’ velocities. The task of Maxwell’s demon was to open and close a door connecting two boxes that contain gas which initially has the same temperature on both sides. Every time a fast particle approaches from the right, the demon lets it through to the left. Every time a slow particle arrives from the right, the demon closes the door and keeps it right. This way, the average energy of particles and thus the temperature in the left box increases, and entropy of the whole system decreases. Maxwell’s demon thus seemed to violate the second law of thermodynamics!
Mawell’s demon gave headaches to physicists for many decades until it was finally understood that the demon itself must increase its entropy or use energy while it measures, stores, and eventually erases information. It has not been until a few years ago that Maxwell’s demon was in fact realized in the laboratory.
A thought experiment that still gives headaches to theoretical physicists today is the black hole information loss paradox. If you combine general relativity and quantum field theory, each of which is an extremely well established theory, then you find that black holes evaporate. You also find however that this process is not reversible; it destroys information for good. This however cannot happen in quantum field theory and thus we face a logical inconsistency when combining the two theories. This cannot be how nature works, so we must be making a mistake. But which? There are many proposed solutions to the black hole information loss problem. Most of my colleagues believe that we need a quantum theory of gravity to resolve this problem and that the inconsistency comes about by using general relativity in a regime where it should no longer be used. The thought experiments designed to resolve the problem typically use an imagined pair of observers, Bob and Alice, one of which is unfortunate to have to jump into the black hole while the other one remains outside.
One of the presently most popular solution attempts is black hole complementarity. Proposed in 1993 by Susskind and Thorlacius, black hole complementarity rests on the Gedankenexperiment main rules: That what matters is only what can be measured, and you should not fool yourself. One can avoid information loss in black holes by copying information and let it both fall into the black hole and go out. One copy remains with Bob, one goes with Alice. Copying quantum information however is itself inconsistent with quantum theory. Susskind and Thorlacius pointed out that these disagreements would not be measureable by neither Bob nor Alice, and thus no inconsistency could ever arise.
Black hole complementarity was proposed before the AdS/CFT duality was conjectured, and its popularity sparked when it was found that the non-locally doubled presence of information seemed to fit nicely with the duality that arose in string theory.
As of recently though, it has become clear that this solution has its own problems because it seems to violate the equivalence principle. The observer who crosses the horizon should not be able to notice anything unusual there. It should be like sitting in that elevator being pulled by an angel. Alas, black hole complementarity seems to imply the presence of a “firewall” that would roast the unsuspecting observer in his elevator. Is this for real or are we making a mistake again? Since the solution to this problem holds the promise of understanding the quantum nature of space and time much effort has focused on solving it.
Yes, Einstein’s legacy of thought experiments weighs heavily on theoretical physicists today – maybe too heavy for sometimes we forget that Einstein’s thoughts were based on real experiments. He had Michelson-Morley’s experiments that disproved the aether, he had the perihelion precession of mercury, he had the measurements of Planck’s radiation law. Thought alone only gets one so far. In the end, it is still data that decides whether a thought can become reality or remain fantasy.
|[Cartoon: Abstruse Goose, Missed Calling]|
This post first appeared on "Starts with a Bang".