On Physics: Black holes and quantum spacetime

The quantum revolution in physics — whose 100th anniversary we have just celebrated — taught us that at the most basic level the world is bizarrely different than it seems. But, we appear to only have a taste of this weird behavior, and expect an even more radical transformation of our knowledge when we extend quantum physics to encompass space and time. 

We can begin to understand the weirdness of quantum mechanics by comparing it with familiar classical physics. Throw a ball to a friend, and have them throw it back — it follows a particular path. Or, do the same with another nearby friend. The ball takes one path, or the other. But, in an analogous quantum experiment, a ball aimed between them goes to both of them, and returns — it simultaneously takes both paths!

This phenomenon is observed in a famous experiment where particles, for example electrons, are aimed at a screen with two slits, and collected on a screen behind that. They create a pattern like that from water waves, clearly indicating the electrons follow paths through both slits. This can be explained by saying that the basic description of electrons is not as particles or waves, but as a new thing, the electron quantum field, which goes through both slits like a water wave but behaves like an electron when measured at the second screen. This behavior has now been observed for particles as large as 100,000 times the mass of an oxygen atom. 

We are used to thinking of space and time as fixed structures through which we move. But Einstein taught us that they are part of a combined entity, spacetime, and that this entity is not fixed, but bends and ripples. The Earth creates bending of spacetime, and that is what explains its gravity — you can’t see it directly, but you are feeling the effect of spacetime curvature creating the pressure between you and your chair, or your feet on the ground. Then, if you have a moving mass, it creates bending that changes with time, and that radiates out in ripples of bent spacetime — gravitational waves, which have now been measured by ultrasensitive detectors. Spacetime is not fixed, but is dynamical.

Quantum behavior is expected to extend to all dynamical systems, at the most basic level. If so, then what is the new quantum entity replacing classical spacetime, and how do we describe it? It is likely to be as weirdly different from spacetime as quantum fields are from classical particles. This is possibly the most profound problem confronting current physics, as spacetime ultimately provides the stage for all other physical phenomena. How can we find clues to its resolution? 

One promising approach is to confront spacetime in the extreme, through black holes. Throw a ball up in the air and watch it return. Throw it faster, and it goes higher. Throw it fast enough, and it escapes to distant space; the needed speed is the escape velocity. If we could compress the Earth to a smaller size, the escape velocity goes up, due to its concentrated gravity. If compressed enough, the escape velocity reaches the speed of light. Einstein also taught us that things can’t move faster than light, so at that point, nothing could escape from Earth’s surface — it would be a black hole. 

Gravity near a black hole is such an extreme distortion of spacetime that it tears apart quantum fields, producing outgoing quantum particles from near the black hole. These outgoing particles — one of famed physicist Stephen Hawking’s greatest discoveries — carry away energy from the black hole, and eventually cause it to “evaporate.” 

This ultimately yields a conflict with quantum mechanics. If you imagine dropping some information into the black hole — say your smartphone — the outgoing particles carry very little of that information back to the outside world. This violates one of the principles of quantum mechanics, that at the fundamental level, information is never truly lost. This clash of spacetime principles with the principles of quantum mechanics ultimately results in what has been called the black hole information paradox. 

There are numerous hints that the resolution to this paradox involves taking into account the quantum properties of spacetime itself, changing how black holes interact with their surroundings. Better understanding of this may provide us with critical hints about the quantum nature of spacetime.

This also hints at the possibility of guidance from astronomical observations. Black holes can now be observed in the cosmos, both by measuring light from supermassive black holes, and gravitational waves from coalescing black holes. The new interactions due to quantum spacetime may alter the observable signals compared to those predicted by Einstein’s gravitational theory. If so, observation of such alterations could possibly even provide key clues about the mysterious fabric of quantum spacetime.

Stay tuned!