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Isn't that awesome, the galaxy is huge, our planet is tiny, and yet all the objects in the whole galaxy have photons that reach our little planet, the lens of t

Isn't that awesome, the galaxy is huge, our planet is tiny, and yet all the objects in the whole galaxy have photons that reach our little planet, the lens of that specific telescope even. The photons traveled that far, that long, just to be finally absorbed by that telescope. What are the chances of a photon from an object that far away to hit specifically this location?

pavel_lishin 14y ago
That specific photon? In hindsight, 100%. If you had to guess for any given photon, approximately 0%. But the odds that some photon would hit us is probably not too low.
gus_massa 14y ago
The surface of the sphere of the light that is traveling since an event "soon" after to the big bang is (4 * pi * (15000000000 lightyears)^2) (I don't know how to handle the space expansion, so I will simply ignore it.) The surface of the main mirror in the Hubble Telescope is (pi * (2.5 meters)^2) The gravitational lens gives a 8x magnification, so the telescope picks 8^2 times the light (not sure about this ^2). So, the (approximated) probability that a photon reach the telescope is: (pi * (2.5 meters)^2) * (8^2) / (4 * pi * (15000000000 lightyears)^2)~=5E-51 http://www.google.com/?q=(pi*(2.5%20meters)%5E2)%20*%20(8%5E2)%2F%20(4*pi*(15000000000%20lightyears)%5E2 http://www.google.com/?q=(pi*(2.5%20meters)%5E2)%20*%20(8%5E...) So, it had to be a very bright object! Or saying it in another way http://xkcd.com/811/ http://xkcd.com/811/
- ctdonath 14y ago
  A flippant googling suggests the Sun outputs 10^45 photons per second. The very bright object in question is a galaxy consisting of, we can presume, a few orders of magnitude more than 10^6 stars. Upshot is that the Hubble Telescope should be receiving a few hundred or thousand photons from that galaxy every second. In comparison and for scale, IIRC the human eye has been shown capable of detecting individual photons.
- andrewcooke 14y ago
  [update: i think i was wrong in a previous version of this post]. i think you need an extra 1/(1+z)^2 for expansion. see, for example, http://en.wikipedia.org/wiki/Luminosity_distance http://en.wikipedia.org/wiki/Luminosity_distance where there is a (1+z) correction to D (and you want D^2 for area). that would decrease probabilities by ~1/100. that seems reasonable given ctdonath's comment - tens of photons a second at a telescope sounds like it should be detectable (IR is harder than optical as it has higher backgrounds). am also unsure about the ^2 (i think you're worried whether it's intensity or not? i don't know either).

pavel_lishin 14y ago

That specific photon? In hindsight, 100%. If you had to guess for any given photon, approximately 0%. But the odds that some photon would hit us is probably not too low.

gus_massa 14y ago

The surface of the sphere of the light that is traveling since an event "soon" after to the big bang is (4 * pi * (15000000000 lightyears)^2) (I don't know how to handle the space expansion, so I will simply ignore it.) The surface of the main mirror in the Hubble Telescope is (pi * (2.5 meters)^2) The gravitational lens gives a 8x magnification, so the telescope picks 8^2 times the light (not sure about this ^2). So, the (approximated) probability that a photon reach the telescope is: (pi * (2.5 meters)^2) * (8^2) / (4 * pi * (15000000000 lightyears)^2)~=5E-51 http://www.google.com/?q=(pi*(2.5%20meters)%5E2)%20*%20(8%5E2)%2F%20(4*pi*(15000000000%20lightyears)%5E2 http://www.google.com/?q=(pi*(2.5%20meters)%5E2)%20*%20(8%5E...) So, it had to be a very bright object! Or saying it in another way http://xkcd.com/811/ http://xkcd.com/811/

ctdonath 14y ago
A flippant googling suggests the Sun outputs 10^45 photons per second. The very bright object in question is a galaxy consisting of, we can presume, a few orders of magnitude more than 10^6 stars. Upshot is that the Hubble Telescope should be receiving a few hundred or thousand photons from that galaxy every second. In comparison and for scale, IIRC the human eye has been shown capable of detecting individual photons.
andrewcooke 14y ago
[update: i think i was wrong in a previous version of this post]. i think you need an extra 1/(1+z)^2 for expansion. see, for example, http://en.wikipedia.org/wiki/Luminosity_distance http://en.wikipedia.org/wiki/Luminosity_distance where there is a (1+z) correction to D (and you want D^2 for area). that would decrease probabilities by ~1/100. that seems reasonable given ctdonath's comment - tens of photons a second at a telescope sounds like it should be detectable (IR is harder than optical as it has higher backgrounds). am also unsure about the ^2 (i think you're worried whether it's intensity or not? i don't know either).