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When we use the strongest optic telescope available, how far from Earth can a hydrogen bomb explode in space and be visible for this telescope?

Is there a fundamental limit to this distance? Even for the strongest telescope, in the future, will it be impossible to see such an explosion in the most faraway galaxies (assuming they took place long ago)? Say it's a 100 megaton explosion.

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Is there a fundamental limit to this distance? (emphasis added)

Yes.

We'll never see anything beyond the Observable universe, by definition :-)

A very quick and approximate explanation is that there is metric expansion of space due to dark energy as quantified by the Hubble constant. I'll say (without being cautious to get the science and math words right) that beyond some distance, the space itself between us and the detonation is getting bigger at a rate that will outpace the light's attempt to reach us.

One interesting consequence of all this is that we can not know from direct observation how big the universe is!

From Universe; size and regions:

Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite. Estimates suggest that the whole universe, if finite, must be more than 250 times larger than the observable universe.

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  • $\begingroup$ Obviously, I dont agree with space litterally expanding but I see what you mean ( you should check articles that say that the expanding balloon model is misleading). $\endgroup$ Jun 26 at 7:16
  • $\begingroup$ @DescheleSchilder my questions in Physics SE physics.stackexchange.com/search?q=user%3A83380+metric and my "anti cake and balloon" comment $\endgroup$
    – uhoh
    Jun 26 at 7:18
  • $\begingroup$ In your question you ask about the different rates of expansion. If you see the universe (siplified to visualze) as a 2d circle (on which particles are tied by some underlying mechanism) and if this circle moves on a cylinder ( gravity being able to distort the cylinder) then the circle gets distorted. It can grow larger on the static 2d cylinder and it can do so non uniformy, depending on the mass distribution on it. $\endgroup$ Jun 26 at 7:31
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    $\begingroup$ @DescheleSchilder comments are not for parallel discussions. Stack Exchange is a question and answer site. Comments need to try to stick to addressing the posts they are under. So I won't engage in a expanded exploration of cosmology here. I don't mean to be impolite at all, but comments aren't for discussions, or places to raise alternative theories of cosmology. $\endgroup$
    – uhoh
    Jun 26 at 7:44
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    $\begingroup$ I get it! I realy dont think you are unpolite! $\endgroup$ Jun 26 at 7:49
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Any source of light obeys the same law: light intensity diminishes with the square of distance.

Taking the analogy that the first A-bomb “the radiance of a thousand suns,” as reported by Robert Oppenheimer from witnessing its explosion from a distance of 10,000 yards—9.144 km, so let’s round it to 9 km—this means that seen from 18 km, it was four times fainter, so 250 suns; from 27 km, about 110 suns; from 36 km, about 63 suns; etc.

So, no matter what the yield of an explosion is, eventually, one may be too far to detect it.

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  • $\begingroup$ Current "large aperture but minuscule quantum efficiency) neutrino detectors might compete; that might be an interesting new question that requires numbers to answer, but I'm not sure if Astronomy SE or Space SE is the best place for it. $\endgroup$
    – uhoh
    Jun 26 at 6:34
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The current magnitude limit of the Hubble telescope is 31.5. The James Webb telescope magnitude limit is going to be 34. Any magnitude number greater than this will remain optically unobservable at current technology levels. Uhoh mentions neutrinos, but that is a different issue.

https://fas.org/nuke/guide/usa/doctrine/dod/fm8-9/1ch3.htm

This site states that a 35% of the energy from a nuclear explosion is released as thermal radiation, including infra-red, optical, ultraviolet and x-ray. A megaton of TNT releases 4.2 peta-joules of energy. That is 420 peta-joules for the given example, 35% of which would be roughly 147 peta-joules of thermal radiation. it's visibility would decline according to the inverse-square rule.

My back hurts too much to do that calculation right now. But, when, due to dissipation over distance, the magnitude from that equation exceeds 34 the explosion would be too dim for us to perceive it based on current technology. So, Yes, there is a limit to how far away we can observe a nuclear explosion of 100 megatons in space.

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  • $\begingroup$ It's the first time I see a nuke guide! Anyhow, good luck with your back. $\endgroup$ Jun 26 at 21:14

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