by Chris Peterson » Fri Oct 14, 2011 2:59 pm
starstruck wrote:Okay, 'stupid question time', from someone who is not an astro-scientist . . is it possible to actually see with our own eyes these optical flashes of Cherenkov radiation? I ask this because, on quite a few occasions over the years, when I have been out giving myself a crick in the neck staring up at a clear night sky on a moonless night, I have observed almost imperceptible flashes across the sky that seem to have no point of origin.
No, the Cherenkov radiation produced by gamma rays in the atmosphere is too dim to see with the eyes. Flashes like you describe are normally a physiological phenomenon (called photopsia), which can be caused by various mechanical stresses on the retina, or by statistical noise in the retina, nerves, or brain. It is also possible, although rare, for a cosmic ray to penetrate the eye and produce a shower of visible photons which will be seen as a flash, or to directly strike the visual cortex, which can also produce a flash. But most of the time you're probably just seeing a spurious nerve signal unrelated to anything outside your body. (And yes, if you see such flashes with any frequency, you should see an ophthalmologist, as it can be a symptom of some serious eye or brain problems.)
Depending on your location and the time of year, it is also possible that distant lightning will create unlocalized flashes above your perception threshold. As a rule, I'd say that's the most likely explanation for flashes with no apparent origin, and the previously mentioned photopsia is the most likely explanation for flashes that appear to come from some point.
[quote="starstruck"]Okay, 'stupid question time', from someone who is not an astro-scientist . . is it possible to actually see with our own eyes these optical flashes of Cherenkov radiation? I ask this because, on quite a few occasions over the years, when I have been out giving myself a crick in the neck staring up at a clear night sky on a moonless night, I have observed almost imperceptible flashes across the sky that seem to have no point of origin.[/quote]
No, the Cherenkov radiation produced by gamma rays in the atmosphere is too dim to see with the eyes. Flashes like you describe are normally a physiological phenomenon (called photopsia), which can be caused by various mechanical stresses on the retina, or by statistical noise in the retina, nerves, or brain. It is also possible, although rare, for a cosmic ray to penetrate the eye and produce a shower of visible photons which will be seen as a flash, or to directly strike the visual cortex, which can also produce a flash. But most of the time you're probably just seeing a spurious nerve signal unrelated to anything outside your body. (And yes, if you see such flashes with any frequency, you should see an ophthalmologist, as it can be a symptom of some serious eye or brain problems.)
Depending on your location and the time of year, it is also possible that distant lightning will create unlocalized flashes above your perception threshold. As a rule, I'd say that's the most likely explanation for flashes with no apparent origin, and the previously mentioned photopsia is the most likely explanation for flashes that appear to come from some point.