Susskind replies: Tarallo provides a succinct account of how classical relativists described matter falling into a black hole before the early 1970s. The problem with that view dates back to Stephen Hawking’s discovery that the combination of quantum mechanics and general relativity implies that black holes evaporate. As Hawking emphasized, if bits of matter “permanently disappear from view,” then that evaporation leads to a contradiction with those rules. His solution was to give up the standard rules of quantum mechanics, but after two decades of confusion a consensus emerged that Hawking was wrong. Today the highly unintuitive black hole complementarity and holographic principles are central pillars of the quantum theory of gravity.
The event is indeed observed in a different way depending on the observer’s frame of reference. That is how two apparently contradictory things can both occur.
I would like to clarify that “reality may forever be beyond reach of our understanding” is a stronger statement than I intended. What I wanted to convey is that the hardwired concepts that evolution equipped us with are not suitable for visualizing the strange and unintuitive behavior of the quantum world, let alone the quantum-gravity world. Still, physicists have been very good at rewiring their circuits by means of abstract mathematics, which must replace old ways of visualizing the world each time we encounter something radically new.
USING YOUR BRAIN
In “The Limits of Intelligence,” Douglas Fox points out that human intelligence is limited by communication among neurons in the brain, which is limited in turn by the size of our neurons. “The human mind, however,” Fox writes, “may have better ways of expanding without the need for further biological evolution.” He goes on to suggest social interactions as a means to pool our intelligence with others. What Fox forgets to point out, however, is that as a species we have not yet learned to use our individual brains to full capacity. In fact, a typical person uses only about 10 percent of his or her brain. Rather than dwelling on the constraints imposed on the human mind by nature, wouldn’t it be more useful—as well as smarter—to figure out ways to boost and strengthen existing neuronal connections in our brains, thereby making the most of what we already possess?
Great Neck, N.Y.
Fox replies: It has been estimated that only 1 to 15 percent of neurons in the human brain are firing at any given instant. But it does not necessarily follow that we could use the other 90 percent or so of our neurons and suddenly be smarter. Letting most of our neurons lie idle most of the time is a design principle that has evolved into our brain. Having neurons lie idle uses a lot less energy than having them spike—and so having lots of neurons that you do not use all that often actually maximizes the ratio of information processed to energy spent.
For example, the more neurons you have, the more pathways any particular nerve spike can travel. So each nerve spike inherently contains more information—and your brain can get away with firing fewer of those energy-expensive spikes. Even if you discounted all of the above and obstinately started firing every neuron in your brain every second, you would still have to pay for all those extra energy-hungry spikes, and it could easily double or quadruple the calories your brain consumes. In other words, nothing is free. The brain we have is almost certainly evolved for maximum information per energy spent.