What is Ultimately Possible in Physics?
That, of course, is the subject of FQXi's current essay contest. FQXi has now approved the first 10 essays, so you should feel free to go to the essay discussion page to check them out, rate, and discuss. Hopefully the discussion going live will encourage essayists to start to get their submissions in: experience from the first contest seems to indicate that early entries got some advantage in voting, and certainly more attention.
A few of my thoughts on what is ultimately possible are below. But first, few notes on the rating and discussion:
1) We've switched from a voting to a rating system (1-10). An advantage of this is that it should provide much more information, as you need not read many essays before rating one. A disadvantage is that it could lead to a judgmental atmosphere, rather than a supportive one. Hopefully the community will make the most of the first, and try to minimize the second.
2) While there will still be an expert panel, more than last time, the results of the contest will be determined by the ratings by the community (see the detailed official rules here. We'll see how this works out!
3) There have been several improvements to the forum system you might want to try out:
(a) LaTeX equations can now be included (and a preview window is available).
[equation]\int e e ?[/equation]
(b) Inline subscripts and superscripts are supported.
(c) Very long posts will now be 'stubbified' into expandable stubs, and authors will (soon) be empowered to stubbify (but stubbify only) comments on their own essay.
So what do I think is ultimately (im)possible? I'd say what intrigues me most is the apparent relation between various types of impossibility. Most readers are familiar with the second law of thermodynamics, and its accompanying 'impossibility' statements, e.g. entropy should never (well, almost never) decrease in a closed system, perpetual motion machines are impossible, etc.
But other 'impossibilities' seem to be closely related. For example, faster-than-light or time travel in general relativity would appear to be possible if one had access to a significant supply of negative energy; this could be used, for instance, to hold open the 'throat' of a wormhole, or create a warp drive, etc. But negative energy is very bad (or at least seemingly forbidden) in large quantities. For example, suppose I have some stuff with entropy S. Now turn the stuff into a black hole of mass M. This increases the entropy (in fact, from Bekenstein and Hawking we have a pretty good idea exactly how much entropy should be associated with a black hole, and it's a lot). Now feed a chunk of negative-energy stuff, say of mass (-M), into the black hole. Voila, no more black hole, no more entropy, second law violated.
With negative mass one can also, as far as I can see, easily violate cosmic censorship (the negative-mass Schwartzschild solution has a naked singularity). And cosmic censorship is used in proving the theorem that the total area of black hole event horizons increases, which in turn is used to make sure that the (generalize) second law is upheld.
So there seem to be very deep ties between various things that Nature appears very unwilling to allow us to do. These ties are often somewhat obscure, and rarely worked out in any rigorous detail. But it might it be interesting to try to do so, and I'll leave that as a challenge.