Ask Ethan: Why Is The Black Hole Information Loss Paradox A Problem?
“Why do physicists all seem to agree that the information loss paradox is a real problem? It seems to depend on determinism, which seems incompatible with QM.”
There are a few puzzles in the Universe that we don’t yet know the answer to, and they almost certainly are the harbingers of the next great advances. Solving the mysteries of why there’s more matter than antimatter, what dark matter and dark energy are, or why the fundamental particles have the masses they do will surely bring physics to the next level when we figure them out. One much less obvious puzzle, though, is the black hole information loss paradox. It’s true that we don’t yet have a theory of quantum gravity, but we don’t need one to see why this is a problem. When matter falls into a black hole, something ought to happen to keep it from simply losing its information; entropy must not go down. Similarly, when black holes evaporate, a la Hawking radiation, that information can’t just disappear, either.
We Still Don’t Understand Why Time Only Flows Forward
“It’s true that entropy does explain the arrow of time for a number of phenomena, including why coffee and milk mix but don’t unmix, why ice melts into a warm drink but never spontaneously arises along with a warm beverage from a cool drink, and why a cooked scrambled egg never resolves back into an uncooked, separated albumen and yolk. In all of these cases, an initially lower-entropy state (with more available, capable-of-doing-work energy) has moved into a higher-entropy (and lower available energy) state as time has moved forwards. There are plenty of examples of this in nature, including of a room filled with molecules: one side full of cold, slow-moving molecules and the other full of hot, fast-moving ones. Simply give it time, and the room will be fully mixed with intermediate-energy particles, representing a large increase in entropy and an irreversible reaction.”
Why does time flow forwards and not backwards, in 100% of cases, if the laws of physics are completely time-symmetric? From Newton’s laws to Einstein’s relativity, from Maxwell’s equations to the Schrödinger equation, the laws of physics don’t have a preferred direction. Except, that is, for one: the second law of thermodynamics. Any closed system that we look at sees its entropy only increase, never decrease.
Could this thermodynamic arrow of time be responsible for what we perceive as the forward motion of time? Interestingly enough, there’s an experiment we can perform: isolate a system and perform enough external work on it to force the entropy inside to *decrease*, an “unnatural” progression of entropy. What happens to time, then? Does it still run forward?
If you have explored the interior of your CPU then you might have noticed that there are these horizontal metal plates (called fins) many a times with a fan on top of the central or graphic processors.
They are called heat sinks/heat exchangers and are used to dissipate the heat generated by the processor to the surrounding.
The reason why they work is that according to the Fourier’s law, the
heat dissipated is directly proportional to the cross sectional area.
And adding protrusions to the surface increases the net cross section area for exchanging the heat with the surrounding.
Cooking with a computer
In order to demonstrate the extent to which the processor would heat up, let’s remove the heat sink and place a piece of meat on it.
At such high temperatures where cooking a piece of meat becomes possible on a processor, you can be damn sure that the probability of the survival of a computer running without a heat sink is just 0. **
If you’ve ever popped open a chilled bottle of champagne, you’ve probably witnessed the gray-white cloud of mist that forms as the cork flies. Opening the bottle releases a spurt of high-pressure carbon dioxide gas, although that’s not what you see in the cloud. The cloud consists of water droplets from the ambient air, driven to condense by a sudden drop in temperature caused by the expansion of the escaping carbon dioxide. Scientifically speaking, this is known as adiabatic expansion; when a gas expands in volume, it drops in temperature. This is why cans of compressed air feel cold after you’ve released a few bursts of air.
If your champagne bottle is cold (a) or cool (b), the gray-white water droplet cloud is what you see. But if your champagne is near room temperature ( c ), something very different happens: a blue fog forms inside the bottle and shoots out behind the cork. To understand why, we have to consider what’s going on in the bottle before and after the cork pops.
A room temperature bottle of champagne is at substantially higher pressure than one that’s chilled. That means that opening the bottle makes the gas inside undergo a bigger drop in pressure, which, in turn, means stronger adiabatic expansion. Counterintuitively, the gas escaping the warm champagne actually gets colder than the gas escaping the chilled champagne because there’s a bigger pressure drop driving it. That whoosh of carbon dioxide is cold enough, in fact, for some of the gas to freeze in that rushed escape. The blue fog is the result of tiny dry ice crystals scattering light inside the bottleneck.
That flash of blue is only momentary, though, and the extra drop in temperature won’t cool your champagne at all. Liquids retain heat better than gases do. For more, on champagne physics check out these previous posts. (Image and research credit: G. Liger-Belair et al.; submitted by David H.)
“Sound waves emerge from molecular interactions; atoms emerge from quarks, gluons and electrons and the strong and electromagnetic interactions; planetary systems emerge from gravitation in General Relativity. But in the idea of entropic gravity — as well as some other scenarios (like qbits) — gravitation or even space and time themselves might emerge from other entities in a similar fashion. There are well-known, close relationships between the equations that govern thermodynamics and the ones that govern gravitation. It’s known that the laws of thermodynamics emerge from the more fundamental field of statistical mechanics, but is there something out there more fundamental from which gravity emerges? That’s the idea of entropic gravity.”
There are many attempts out there to reconcile the quantum field theories that describe the electromagnetic and nuclear forces with general relativity, which describes the gravitational force. Certain questions, about gravitational properties in strong fields and on small scales, will never be answered otherwise. In order to make that happen, we’d need a quantum theory of gravity. While string theory is the most popular idea, there are others, such as asymptotic safety, loop quantum gravity, and causal dynamical triangulations. But perhaps the most radical idea came from Erik Verlinde in 2009: the idea that gravity itself is not fundamental, but rather arises from a truly fundamental entity: the entropy of quantum bits of information. Verlinde’s work has been intriguing and especially controversial, and I myself have spotted a number of problem areas with his results so far, but it’s certainly an idea worth exploring further. At 7 PM ET / 4 PM PT tonight, he delivers the Perimeter Institute’s inaugural public lecture of their 2017-2018 series.