Chapter 17 Questions

String theory is probably the currently most favored hypothesis, but it is probably fair to say that it is regarded as a promising start and not as a full Theory of Everything (including quantum gravity).

Well, just what do you mean by "practical application"? You admit an "intrinsic interest" in the answer. Is indulging this interest practical? I think so, but would recognize that some might want to debate the issue. (Are sports practical? Cosmology is exercise for the mind.) Perhaps practical means the immediate business of keeping you (and all humans) alive? Then practical means food, water, and air (maybe shelter). That's a pretty limited definition. Let's expand it to keeping human society alive. Well, the study of cosmology has had unquestionable impact on society. Is a complete and accurate understanding of humanity's place in the vast cosmos going to have a practical impact on the survival of humanity? Hard to say. Crocodiles have existed a lot longer than humans without understanding the cosmos. But then, crocodiles can't do anything that could affect the whole planet. We humans have minds, and we are going to use them. We must try to use them well.

Without a theory of quantum gravity, we cannot say that we are anywhere near understanding some of the fundamentals of the universe. It means that we do not completely understand force unification. It prevents us from understanding the meaning and behavior of singularities. It might also be inhibiting our comprehension of the arrow of time.

It is difficult to arrive at such a theory because we are still trying to understand matter at very high energies. (We still do not have a successful theory of unification of strong, weak, and electromagnetic forces yet.) Furthermore, unlike the other three forces, it is not immediately obvious how to cast the theory. The others are independent of space and time and so we may set up coordinates in the usual way, up to certain known transformations. But gravity seems to be involved in the existence of space and time themselves, and quantum gravity must apply at scales at which space and time as we know them break down. One stumbling block to quantum gravity has been just trying to understand which coordinates might be appropriate. The apparent geometrical nature of gravity is also a factor, since that does not fit into our current understanding of particle physics. It seems likely that the correct theory will recover this geometrical appearance for large scales, but it is unclear how to go from the geometrical theory to a more complete theory; it is quite unlike going from (for example) classical electromagnetics to quantum electromagnetics.

One thing that seems likely is that the eventual theory of quantum gravity may turn out to be very complicated in its details, but will be quite simple conceptually.

Superstring theories and other attempts at grand unification involve higher dimensions than just the usual 3 space dimensions. These other dimensions are "compact" so we don't see them in our present universe.

There are no tests today, although any version of string theory must be compatible with all currently known physics and experimental results, and that is itself a stringent constraint. It is difficult to know whether superstring theory will prove correct ultimately. Are the difficulties physicists encounter with it because they will ultimately find it to be on the wrong track, or simply because it is fundamentally complicated, or because the true, final, beautiful, simplifying principle is not yet uncovered?

The mathematics is pretty much the same as in other areas of advanced physics, including general relativity. It includes tensor analysis (a tensor is a quantity with many components that transform in a certain way under changes of coordinates), partial differential equations, the theory of "generalized functions," and so forth.

Quantum mechanics prohibits infinite subdivisions. We believe that time and space themselves are "quantized," i.e. exist in finite, fixed quantities, at the level of the Planck scale of approximately 10^-31 cm or 10^-43seconds. Below this scale it is probably not possible to describe the universe in terms that we would find familiar.

Alan Guth once speculated (in a jocular vein) that our universe may have begun in somebody's garage as an experiment. There has been a recent suggestion that the singularities in black holes are the spawning ground of new universes that share properties of the universe that created them. If that were the case the new universe is cut off from our universe by a horizon, and presumably our universe is inside some horizon from its parent universe. These are interesting, although completely untestable, speculations.

Since the many worlds interpretation is neither testable nor falsifiable, shouldn't we, by Occam's Razor, just eliminate it altogether?

Is the end of the universe a worry? Should you be worried about this experiment? Probably not, for the first, and No for the second.

Seriously folks, there are cosmic rays (ultra high energy particles) colliding with the Earth all the time which have more energy than being contemplated in the lab. There is no evidence that mini-black holes exist in the cosmos. Apparently nature doesn't create them. And the experiment won't do anything that nature doesn't already do.

Note that the cause of concern (given in the article unreferenced here) is that "it has been theorized by Steven Hawkings (sic) that from this quark-gluon plasma other forms of matter are also formed." Hawking hypothesized that the only place in the universe where miniblack holes might form would be in the very beginning of the big bang itself, at fabulously higher energies that mankind can even contemplate creating. The hypothesis has been examined since it was proposed and seems unlikely. In any case its irrelevant to this experiment. (Citing a famous scientist always lends an air of credibility.)

Let's consider an experiment that really was dangerous. 50 years ago people were worried that atomic testing would blow a hole in the ocean floor and drain the water out, knock the Earth off of its axis, set fire to the atmosphere over the whole Earth, shatter the Earth into pieces.... All concerns that are easy to show are absurd with simple physics calculations. Of course what they should have been worrying about was that these bombs obliterate cities and kill millions of humans, release fallout that increases the likelihood of cancer in humans, but that wasn't enough for some people. Our meddling had to have cosmic consequences.

You want some things to keep you up at night? Try overpopulation, starvation, plague, warfare, pollution.... These are things that are real threats, and things we could do something about if we get our collective act together, species-wise.

However, I sense an opportunity here. Anyone for black hole insurance?

NB: the question was about an article in 1999. As of today, we are still here. I checked.