2013 : WHAT *SHOULD* WE BE WORRIED ABOUT?

One might have supposed the fact that we live in relatively quiet suburb of an average galaxy in an apparently uniform universe 14 billion years after the big bang would not be large source of worry. However, lately it has been worrying me.

 Thanks to the two pillars of 20th century physics, relativity and quantum mechanics, we are by now familiar with the fact that the information we can obtain about the universe is limited by our particular circumstances. But these don't really put strict limits on what is ultimately empirically knowable.

There may be, however, new limits looming on our ultimate ability to probe nature—made manifest because of the truly remarkable successes of physical theory and experiment in the past 50 years—due to the accident of the circumstances in which we find ourselves living, which could, at least in principle, change the way fundamental science may progress in the future.

In the first places, current physical theories suggest that our universe is probably not unique. Well beyond our ability to directly probe, there may be an infinity of universes, with differing laws of physics, and perhaps different characteristics of space and time. This is not necessarily a problem if we are interested in understanding the nature of our particular universe.

But it could be that the laws of physics are probabilistic, and there is no fundamental reason why they are what they are in our particular universe. But—just as an epidemiologist studying a single patient can say little about what may be the cause of some condition, because it may be impossible to know what is normal and what is not—if we can study only one universe (our own), then we may never be able to directly empirically determine if fundamental laws are indeed fundamental, or just an accident of our circumstances. It is possible that we might be smart enough to derive a theory that explains precisely how the laws of physics are distributed across universes, and what the probability that our laws are what they are. But it is equally plausible that without access to a larger sample set we may never know.

It gets worse. Because we have discovered that the expansion of the universe is accelerating, the longer we wait, the less we will see of our universe. Eventually all the galaxies we now see will disappear, and even the evidence of the big bang itself will disappear. While I have argued, unsuccessfully, to Congress that this means we should do cosmology while we have a chance, it does mean that we cannot necessarily take comfort in the presumption that if we and our intelligent successors keep working long enough, nature will reveal more of her secrets.

Actually, the very cause of the acceleration of our universe may be forever impossible to pin down. If empty space has energy, then this energy can cause the observed acceleration of the universe. But as there is no known laboratory experiment that can probe this energy, the only way we may be able to probe it is to observe the expansion of the universe over time. A constant rate of acceleration is consistent with a fundamental energy in empty space, but it is also consistent with a host of other possible sources of trapped energy in some otherwise invisible fields. We may have no way of knowing. And if it is really associated with the properties of empty space, we may never know why, because that too may be an accident, with different energies in different universes. With just a single universe to probe, we may never know.

And, as long as one is in the mood for worrying, even the remarkable apparent discovery of the Higgs Boson at the Large Hadron Collider, which validates the most remarkable intellectual journey humans have ever embarked upon, may yield frustration on the horizon. So far, just the Higgs has been seen. But the standard model has a host of weirdnesses and a plethora of physics ideas have been proposed to explain them. Most of these ideas suggest some other new physics that will be seen at the LHC. But what if the Higgs is all that is seen? Then we will have no guidance on where to turn to resolve the underlying puzzles of the standard model. If nature is not kind, it is possible that the resolution of these problems may be at a distance and energy scale that is simply unattainable, either because of practical physical constraints, or the constraints of small-minded politicians.

Perhaps then, at the extremes of scale empirical science will reach its limits, and we will be reduced to arguing about what is plausible, rather than testing our ideas. I hope not, and I wouldn't bet on it either. After all, every time we do open a new window on the universe we are surprised, and there are many windows left to open. But worrying does help prepare the mind.

I should conclude by stressing do not believe that any of these possible limits will lead to the end of science itself, or even the end of physics, as some naysayers have proposed in the past. There are enough remarkable and perplexing aspects of the universe we can measure to keep us going for a very long time.