Saturday, July 19, 2008

Black holes are politically incorrect

Benjamin has brought my attention to a hilarious story.
John Wiley Price defends his opinion that "black hole" is a racist term.
See Google News. This black commissioner is the sort of folks who are as dumb as a hoe handle and who recently began to influence the public life.

An interview...

John Wiley Price thinks that "that type of language is unacceptable". We are apparently approaching the point at which black aßholes like him will prevent us from using scientific terms such as the "black hole". (If needed, I am ready to call him a "white aßhole" because the focus is on the word "aßhole", not the word "black"!)

In fact, I predicted this story years ago which is why I have often used the term "African American hole" instead. ;-)

He asks why wasn't the term "white hole" used for such a nasty object instead of the "black hole". Well,
  1. the "black hole" was chosen for a good reason that will be explained below
  2. there is nothing nasty about black holes
  3. the term "white hole" is also used, for a similar object, and the relations with black holes will also be explained in detail.
The last, third portion of my explanation will be (relatively speaking) the deepest one and I don't expect people of John Wiley Price's caliber to understand more than 5% of the ideas.

Let me start with a simple comment. "Black" is the (non)color of objects that absorb (nearly) all incoming radiation in the visible part of the spectrum. It just happens that the skin of the people of African descent is closer to black, as defined in the previous sentence, than the skin of the people of European descent.

That has a very good reason. The stronger pigmentation has evolved to protect the deeper layers of the skin from the damaging ultraviolet rays - and this protection is more important in Africa where the solar radiation is more intense. Well, these are the reasons why the people of African descent are called by words related to "black" in most languages. There exist similar rational reasons why "blackberries" and other objects have names incorporating the same adjective.

The history of the term

In 1967, John Wheeler coined the term "black hole" to describe objects that were kind of believed to exist from the 1916 paper about Einstein's general relativity written by Karl Schwarzschild and especially the 1939 paper by Robert Oppenheimer. Their gravitational field is so strong that if you fall into them, it is like if you fall into a hole and you will never escape again. In fact, not even light is fast enough to escape from their powerful gravitational grip. Because light cannot escape, the objects don't emit any radiation and they are black.
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The term "black hole" is therefore exactly what we need. It sounds good, too. And a warning for ill-informed FoxNews journalists: a singularity is surely not the same thing as a black hole! ;-)

Yes, the black holes have the same adjective "black" in them as African Americans, for a good reason. They absorb most of the incoming radiation and don't reflect (or emit) it. If John Wiley Price doesn't want to share this feature with the black holes, he should ask Michael Jackson how to proceed.

Stephen Hawking realized in 1974 that black holes emit some radiation, after all, but it is extremely weak, especially if the black holes are large. I will be mostly neglecting the Hawking radiation in the rest of this note.

There is nothing bad about black holes

Black holes in physics are vaguely analogous to various situations in reality - for example, bureaucracy has become a black hole for lost paperwork (and wasted time of humans). But in pure physics, objects such as black holes or stars don't carry any moral characteristics. There is nothing (morally) good and nothing (morally) bad about the black holes.

In fact, black holes are beautiful and important from a scientific viewpoint. Unlike certain Texan black commissioners, black holes carry the maximum information (or entropy) that one can in principle store in a given volume of space. Their properties completely govern the very high-energy, trans-Planckian scattering of other particles. They are the most natural objects that can be used to verify the consistency of theories of quantum gravity. They became one of the huge success stories of string theory.

Valleyfair attacks: a stunning story how the media have been dishonestly hiding a nasty double hate crime because the racist criminals were black, not white

Black holes and white holes

OK, so why didn't scientists use the term "white holes"? Well, white holes should be objects that tend to emit a lot of light, much like the skin of the Caucasian people, but they don't absorb it. Do they exist? In the real world, they don't. The reason is the second law of thermodynamics, the basic law of macroscopic physics that Sean Carroll completely misunderstands.

There is an inherent difference between the past and the future. If we're thinking about a thought experiment, we may begin with pretty much arbitrary initial conditions in the past. But we are not allowed to decide about the future. The future is, and has to be, whatever follows from the past by the laws of physics. There may exist objects that are "black": they are expected to emit no radiation. But there can't exist objects that are "white" in the sense that there is no radiation coming to their surface. Why? You are simply not allowed to prevent radiation from going anywhere. Some photons are always free to travel in certain directions and hit an object that you wanted to become a "white hole".

This may sound confusing to you. Can't we just apply the time reversal and switch the role of the past and the future? Well, you can do it with spacetime diagrams but it doesn't mean that such reverted histories may occur in reality. In fact, if a plausible history involves an increasing entropy, the time reverted history makes the entropy decrease which is not physically allowed.

Black holes are the highest-entropy objects we can have so this restriction should be the most important in their context: the more entropy a system produces, the more dramatic difference between the past and the future it creates. OK, so how do the rules of thermodynamics work in the presence of black holes?

Black holes and increasing entropy

Let me assume that the reader believes me that the entropy of a large black hole is proportional to the event horizon area, namely "A/4" in Planck units. Jacob Bekenstein was able to guess this relationship by general arguments applying thermodynamics to black holes; Stephen Hawking calculated it by the methods of thermodynamics from the known temperature of the Hawking radiation. Moreover, the relationship can now be calculated and confirmed directly - by counting the microstates in string theory.

Because the total entropy should never decrease, the total area of event horizons should never decrease either.

Indeed, that's correct. And in fact, this law of "increasing event horizon areas" can be derived (and was derived, by Stephen Hawking in 1970) from Einstein's equations of general relativity themselves. Imagine a typical situation. Start with two neutral black holes whose masses are M and M. The radii of the event horizons are R=2M in Planck units so the total event horizon area is twice 4.pi.(2M)^2, i.e. 32.pi.M^2.

These two black holes may merge into one object, one black hole. After some time, it stabilizes. Its mass will be 2M, the radius will be 4M, and the horizon area is 4.pi.(4M)^2 = 64.pi.M^2, more than the initial total area of the event horizons. Indeed, the total area has increased. The important thing is that this process can never occur in reverse: a neutral black hole cannot spontaneously decay into two black holes.

More precisely, such a process is possible in the context of the Hawking radiation but such "huge Hawking particles" are extremely (exponentially) unlikely. For practical purposes, it is impossible for a black hole to split into two.

The important message here is that the classical general relativity including black holes gives a geometric interpretation to the concept of entropy. And the law of increasing entropy can be proved from Einstein's equations. The reason why we can prove this law and not the inverse law is the assumption that the black hole interiors and singularities always occur in the future light cones of normal observers, not in the past light cones.

Do the white holes exist in the Hilbert space?

When you imagine a causal diagram for a star that is collapsing into a black hole and you time-revert it, to obtain a new "object" (really, it's a history) that is naturally called the "white hole", it looks completely different: the singularity is in the past while the star is in the future. The black hole is associated with some microstates in the Hilbert space. Because the causal diagram of a white hole is so different, you might think that the white hole will have to be represented by completely different microstates - because it is so "macroscopically" different.

But this conclusion is incorrect. If you try to find the time-reverted states that are associated with a white hole, you will find the very same states as those that you linked to a black hole. There are simply no other massive microstates and all massive microstates can be used as black hole microstates. It is enough for one slice through the spacetime to look similar in the black hole and white hole case to see that the microstates are "shared". We are led to a clear conclusion first articulated by Stephen Hawking in 1974:
Quantum mechanically, black holes and white holes are the same thing.
When they're the same thing, John Wiley Price could wake up and ask his question "why don't we call them white holes?" again. ;-)

Well, they correspond to the same microstates but if you actually study how they evolve in time, their history includes objects that absorb a lot of radiation but don't emit much. There is indeed an asymmetry between black holes and white holes as long as we define them by the classical causal diagrams. Black holes can exist but white holes can't. The entropy is always increasing because the past is always given by some data carrying finite information while the future is always "derived" and thus more chaotic.

In the context of black holes, this law means that the black hole singularities appear in the future, not in the past, large black holes merge and eat objects around, but they don't decay or emit objects, and this comment applies to light, too. That's why the white/black hole microstates always manifest themselves as black holes in the real world. After all, if they were white holes instead, black morons would also think it is a matter of discrimination because the famous white holes would be everywhere, they would be the key part of the Second Superstring Revolution, and black holes would be underrepresented. ;-)

Because I am not sure that this explanation will convince anyone in this crazy world, Mike Lazaridis should better rename his "BlackBerry" before it is too late. What about a "StrawBerry"? It's not perfect either. While BlackBerry is a nasty attack on a black man, StrawBerry could be an attack on a straw man. ;-)

Meanwhile, "black sheep of the family" should be renamed to a "white wolf of the family" and the "black swan" should become the "white blackbird". A subtle problem with this new notation is that the mostly black wolves are aggressive while the mostly white sheep are peaceful. (The color of devils and angels was simply copied from wolves and sheep.) It seems that Nature is racist Herself. Is it the time to destroy Her? Or is it enough to ban any talk about the color of sheep and wolves in public? :-)

Well, let's not joking about these matters too much because they are pretty serious. I agree with Mr Mayfield that anyone who is offended by the term "black hole" needs a serious psychiatric treatment. Poor Andy Strominger and Cumrun Vafa. They've been voting for junk left-wing politicians like Gore and Kerry throughout their lives and after decades, they find out that 90% of their famous papers are racist crap! ;-)

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