The big accelerator in the sky

I used to fly quite a bit – for a decade I flew at least 50,000 miles a year, and was at more than 75,000 for nearly half that time.  Every time I fly this means that cosmic rays are falling on me like rain, and continue to do so until we land.  In fact, most of these years, I was exposed to far more radiation from flying than from my job – as much as radiation from flying as from every other source of natural radiation in most of those years according to the dosimeter I bring with me on each flight. And lest you wonder, I’m really not concerned about this radiation – I just don’t consider it a high risk, but since that’s not the point of this posting, I’ll leave it at that!

What fascinates me is where these cosmic rays come from.  Some, of course, come from the sun; protons and neutrons that fly through space at 400 km/sec and more before encountering our planet.  The protons are most likely to be deflected by our magnetic field before they reach us, and the neutrons are by and large too weak to even reach the surface.  But, as with all radiation shields, some small amount of solar radiation leaks through, and this amount grows larger as I fly higher.

The sun, however, is not very exciting when we get right down to it. It is, by and large, a fairly ordinary star that emits fairly predictable amounts of radiation.  When we look at other stars, and when we examine lunar rocks, we find that, with rare exceptions, our local star is just not likely to have any serious impact at all on sea-level radiation levels.  Those rare exceptions arrive about once every few million years, when solar “superflares” might deliver a radiation dose of as much as 100 rem (300 years’ worth of natural background radiation) in a few hours – enough to cause radiation sickness in any person who might be exposed.  But this happens so rarely that, while each species is likely to experience one of these events, they’re not likely to affect any individual.

Far more interesting are the galactic cosmic rays that are striking our atmosphere (and many air travelers) at any moment.  Most of these are born in the death of massive stars, and are accelerated to extraordinarily high energies by processes that we still don’t fully understand.  “Garden-variety” GCRs are fairly straightforward – they simply carry with them the kinetic energy imparted by the supernova in which they were born.  But the highest-energy cosmic rays (a single atom may have as much kinetic energy as a fast-thrown rock) are formed under conditions that we can only guess at.  Some may even originate outside our galaxy – possibly born before our solar system even existed.  I find it sobering to think that a cosmic ray may have been born in a distant explosion billions of years ago, making its way through the universe as our sun formed, as the earth cooled, and as life arose.  And how ignominious for it to meet its end colliding with the aluminum skin of the airplane in which I am riding, after all that travel!

As I mentioned before, the highest-energy cosmic rays are quite inexplicable, and this is for two reasons.  The first is that physicists are hard-pressed to think of mechanisms that can accelerate particles to such high energies to begin with.  We have measured cosmic rays with energies that are staggeringly high – a single atom carrying nearly as much energy as a pitcher’s fastball.  There’s a tremendous amount of speculation about how one can accelerate a particle to so high an energy, but speculation is easy when we don’t know very much.  One possibility is that some supernovae may emit their energy in jets, rather than spraying it randomly into space, while others have suggested that magnetic fields near black holes, pulsars, or highly magnetized stars (magnetars) may be responsible.  But, frankly, we just don’t know for sure.  And the mystery deepens even more, because it shouldn’t be possible for these cosmic rays to retain such high energies for very long – certainly not for the tens or hundreds of millions (or even billions) of years that they are thought to travel through space.  The reason is that, while in transit, these particles are constantly colliding into microwave-energy photons, the cosmic microwave background that is the echo of the Big Bang in which our universe began its life (as an aside, those of us who still have a TV hooked up to an antenna should know that some of the “snow” on-screen when we tune to unused channels is from these Big-Bang produced photons).  These constant impacts are thought to whittle away at the energy of the cosmic rays, never bringing them to a halt, but also placing a putative upper limit on their speed.  So there is no known way to produce these highest-energy cosmic rays and no known way for them to retain their energy – so there is obviously something we don’t know because we see these “impossible” rays on a regular basis.

I have read that, before the first run of any new, higher-energy particle accelerator, physicists very carefully calculate the possibility that they might accidentally form new and dangerous states of matter, or new and dangerous energy densities.  One of the concerns I have read about (admittedly in a popular-level science magazine) is the concern that the fate of our planet may be at stake.  Each time, of course, the reassuring answer is that there’s nothing to worry about, and so far our planet and our universe have survived quite nicely.  And, with each article that I read, the author happily reassures us that, of course, we don’t have to worry.  Our best accelerators cannot even begin to reach energies comparable to the cosmic rays that are, as I sit and read and sip my wine, ending their long journeys in the airplane.  If, the thinking goes, nature’s own accelerator has not managed to produce an end to the universe (or to local parts of it), how can our meager contraptions hope to do so?

So I am comforted that our quest for a better understanding of the universe will not endanger us.  At the same time, I am simultaneously proud that we are capable of producing devices powerful enough to even raise this question, and sobered by the fact that we are so far from reaching the standard set by dying stars in far-off parts of our galaxy.  And, I feel a somewhat wistful for the cosmic rays that, having traveled so far and so long at such incomprehensible speeds, are destined to end their travels being absorbed by me, my dosimeter, and my fellow passengers.  Surely they deserve a more noble end!

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