Turning to gravity for a solution to the quantum mystery is in many ways a natural strategy, at least from Penrose’s perspective. There are four fundamental forces in the universe: electromagnetism; the strong force, which binds atomic nuclei together; the weak force, which is responsible for radioactive decay; and gravity. Gravity is the only one of the forces that physicists have been unable to explain in quantum terms. Albert Einstein spent more than 30 years in fruitless attempts to harmonize his theories of gravity with quantum mechanics, and his successors are still stumped.
To Penrose, the failures are a clue that physicists are on the wrong path. Most believe that quantum theory is fundamentally sound but that our understanding of gravity must change. Penrose says that rather than seeking to change Einstein’s theory of gravity, we should study how gravity affects an object small enough to exist in the borderland between the quantum world of atoms and the human world of visible objects.
An object the size of a speck of dust would provide the perfect test. At this scale, an object is small enough to be strongly affected by the rules of quantum mechanics but large enough to observe directly. Current theory predicts that such an object could exist in more than one location and could remain in that split state almost indefinitely. If there were a way to observe the speck without disturbing it, we would see quantum strangeness laid bare: a macroscopic thing sitting in two places at the same time, confounding reality as we know it.
Penrose is convinced that conventional quantum theory seems absurd because it is incomplete. Specifically, it ignores the effects of gravity. On atomic or subatomic scales, gravity is so weak compared with the other forces that most physicists see no problem with leaving it out of the picture. But in Penrose’s view, the only way to understand the quantum world is to consider all the forces that act on it. To do that, he is combining Einstein’s relativity with quantum physics in a way nobody has considered before.
In Einstein’s theory, any object that has mass causes a warp in the structure of space and time around it. This warping produces the effect we experience as gravity. Penrose points out that tiny objects—dust specks, atoms, electrons—produce space-time warps as well. Ignoring these warps is where most physicists go awry, he believes.
If a dust speck is in two locations at the same time, each one should create its own distortions in space-time, yielding two superposed gravitational fields. According to Penrose’s theory, it takes energy to sustain these dual fields. The stability of a system depends on the amount of energy involved: The higher the energy required to sustain a system, the less stable it is. Over time, an unstable system tends to settle back to its simplest, lowest-energy state—in this case, one object in one location producing one gravitational field. If Penrose is right, gravity yanks objects back into a single location, without any need to invoke observers or parallel universes.
How long the process takes depends on the degree of instability. Electrons, atoms, and molecules are so small that their gravity, and hence the amount of energy needed to keep them in duplicate states, is negligible. According to Penrose, they can persist that way essentially forever, as standard quantum theory predicts. Large objects, on the other hand, create such significant gravitational fields that the duplicate states vanish almost at once. Penrose calculates that a person collapses to one location in a trillion-trillionth of a second. For a dust speck, the process takes nearly a second—long enough that it might be possible to measure.
Growing excited, he hoists himself to a more upright position on his sofa. “Here is the scale where you should start to see differences between what quantum mechanics says and what reality does,” he says. “The superposition that is part of quantum mechanics is unstable for large objects; an object will assume one or the other position on a timescale of about a second. Is this true? Well, we have to do an experiment.”
A few years ago, Penrose figured out how to perform that experiment. Instead of a speck of dust he would use a tiny mirror, which would allow him to bounce radiation off it to see if it was in one or two places at the same time. If traditional quantumtheory is right, the doubled state could remain stable for a long time. If Penrose is right, the mirror would maintain a dual existence for no more than a second before gravity chains it to a single location.
Penrose initially envisioned putting his theory to the test using an X-ray laser mounted on a platform in outer space. The laser would shoot photons toward a tiny target mirror tens of thousands of miles away. Here is where quantum weirdness comes into play. A half-reflective mirror, called a beam splitter, would separate each photon into two states so that it follows two paths (that is, it goes in two directions at the same time). On one path, the photon strikes the tiny mirror, moving it slightly; on the other, it is reflected away from the target mirror, so the mirror does not move.
In the prevailing view of physics, both events occur simultaneously: The mirror moves and remains in place at the same time because the mirror—like the photon—can remain in two states at once. On its return path, the duplicate photon that struck the tiny mirror hits the same mirror again, returning it to its initial position. The whole system then returns exactly to its initial state, and there is fundamentally no way to tell which path the photon took. As a result, the two versions of the photon interfere with each other and recombine into a single photon that is always reflected along a path back toward the laser. No X-ray photons can ever follow the path that leads them to a detector.
Other Penrose Questions #4
What is consciousness?
Penrose argues that it is a byproduct of quantum mechanical processes operating in the brain. Some intriguing recent research supports his contention that microtubules—tiny structures in brain cells—can allow quantum phenomena to influence how neurons behave.
Three Different Views of Quantum Weirdness (and What It Means)
A: According to the orthodox view of quantum mechanics, called the Copenhagen interpretation, a system (represented here by a child’s block) does not occupy a definite state or location until it is measured. Before then it is just a blur of overlapping possibilities.
B: The many worlds interpretation insists that the system occupies all its possible states but that every one of them exists in its own alternate universe. Each universe sees one state only, which is why we never observe the block in two states at once.