JWST first light.
https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb-team-begins-aligning-the-telescope/
Does an individual photon strike just one of the mirror segments, or does one photon hit them all? The way they talk about dealing with individual photons, it almost sounds like an 18-slot version of the double-slot experiments.
I read one article somewhere that mentioned how few incoming photons they expect to capture from those most distant depths of the early universe – I don’t remember the number, but it was something line one photon an hour or something like that. So it takes a long time to gather any one image. Does anybody remember reading that, and what the right number was?
Strictly speaking, every photon everywhere hits everything, but some things to a much greater degree than others. In practice, you can draw a circle that contains almost all of where the photon hits, and the size of that circle will be comparable to the wavelength. Infrared has longer wavelengths than visible light, but they’re still far, far smaller than the mirror segments.
So for all practical purposes, each photon hits one mirror segment.
Well, i’m going to need to see pictures of these so-called “photons” before i believe it.
In addition to what Chronos described, it’s worth noting that the answer would be “just one of the mirror segments” even if the entire mirror assembly were arbitrarily small. This is due to the “observer effect” that collapses the photon wavefunction. One of the spooky consequences of the double slit experiment is that the interference pattern caused by the photon going through both slits at once disappears when its path is actually being observed. This effect is so pervasive that it occurs even when a path detector is activated after the photon has already passed through the slits – this is the so-called “delayed choice experiment”. As long as the path is discernible – even after the fact – the photon behaves like a classic particle.
So no matter how small the segments were, as long as it was possible to tell which segment the photon hit, it would be observed to hit only one.
It’s all a plot by “big photon” to steal our daylight and then sell it back to us.
Wouldn’t surprise me !
I don’t think we do know which segment was hit, though. A distant point of light emits a photon, which spreads out over distance, hitting the entire telescope (and then some). However, every point will be reflected back to the same photodetector, which has no angular information (either the photon kicks an electron to the conduction band, giving it a signal, or it doesn’t).
I was thinking of this kind of image which will occur during the first stages of alignment:
The initial alignment steps configure the individual segments to produce sharply focused individual images as shown above. Then they “stack” the images from the A, B, and C mirror segments so they converge on one point, and then conduct coarse and fine phasing to align the segments to less than the wavelength of the light spectrum it’s designed to receive. At what point, if any, during this process they may lose the ability to detect which mirror segment a photon came from is an interesting question. Since they must have information about the performance of each mirror segment in order to periodically repeat the fine phasing alignment, I suspect they will always have this information.
Physics is weird, but at least it has math behind it.
Math is weird, but at least it has philosophy behind it.
Philosophy is weird, but at least it has physics behind it.
This gets into classical versus quantum views of just what exactly the mirror is doing.
If you think of light as classical waves (pre- photoelectric effect and Einstein), what’s happening is that based on the waves’ frequency there’s a limit to how sharply they can be focused. By sampling the wavefront at two separated points (the width of the mirror) you can increase the resolution to a limit based on the ratio of the mirror width to the light wavelength.
In the quantum view, what’s happening is that per the Heisenberg principle we’re discarding position information- exactly where the photons hit the mirror- for momentum information: from what exact direction the photons came from, and therefore how sharply we can image the object that emitted them.
Bottom line in both interpretations is that wider objective equals sharper resolution, but for rather different reasons.
And philosophy has Wikipedia behind it:
Well yes, I was assuming the case where it’s already aligned.
I don’t think they have the information you think in normal operation. They have a number of instruments there to help out with the fine-tuning process, but I’d guess those aren’t in the lightpath normally (since they’d reduce the performance).
It gets even spookier with the delayed choice quantum eraser experiment!
I think those billion-light-year-away distant stars that we are observing as they were a billion years ago will change their behavior a billion years ago as a result of our observing them now. So the observations we make now about how the ancient universe evolved a billion years ago don’t tell us how the ancient universe really evolved a billion years ago had we not looked, except that now that we are looking, the ancient universe evolved a billion years ago differently than it did a billion years ago when we aren’t looking today.
This was perfectly clear to the Tralfamadorians, who understood these things.
I see they’ve deployed some more thermometers on JWST !
5 more temps showing on the “Where is Webb” site, including in oK - which is handy.
Technically Kelvin doesn’t use the degree symbol
23 Kelvin, not 23 degrees Kelvin
Brian
As long as we’re nitpicking, technically the kelvin (K) is a unit like any other SI unit (like the meter or second), so not capitalized (although the symbol for a kelvin is a capital K), and pluralized with an “s” like any other unit.
So 23 kelvins (or 23 K).
Thanks, I wasn’t sure about capitalization and didn’t even think about plural.
Brian