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Does something have to be within Superman's line of sight to hit it? Or can he just look out into the cosmos and shoot heat vision beams that cross the universe?

According to one source, the answer is

about 6 panels. give or take

I'm wondering if there is a unit of measurement more commonly used by humans.

Note:

This question How does Superman's heat vision work exactly? doesn't seem to be asking the same thing.

I read "how far can he expand his heat vision?" to mean an area of effect, rather than a question of distance.

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    Worth noting: line of sight has nothing to do with distance. Even if he was able to shoot other galaxies, they would have to be in his line of sight, or else he would hit whatever was in the way instead. – Nerrolken May 28 '15 at 23:14
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    @nerrolken umm, gravity, time, distance. Those three can cause light or the laser to bend, so he could hit something behind or away from his line of sight target – user16696 May 29 '15 at 1:45
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    "Can Superman ...?" "Yes" – Paul Draper May 29 '15 at 4:59
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    @cde: But the light from the object and the laser should "bend" equally, so whatever it is he's hitting, he can still see it, even if it's not in a straight-line. – BlueRaja - Danny Pflughoeft May 29 '15 at 5:27
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    If the object he aims at is 100 light years away, that's a round trip of 200 light years. The object would have moved as we are looking into the past. He would have to account for that. Now make it a million light years. As objects move the gravity in the path will change. A millimeter of difference in one spot along the path will mean million of miles difference further down. – user16696 May 29 '15 at 11:42
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Probably not.

We can safely assume that Superman's heat vision moves at the speed of light, meaning that shooting across the cosmos would involve some delay for the light to get there.

But otherwise, it's just a matter of how focused the beam is: perfectly focused and it will continue forever. But if it's even slightly unfocused, it will gradually get dimmer as the distance increases, the way the brightness of a flashlight diminishes over distance, because the light cone has gotten wider.

So how focused is Superman's heat vision? Not very, under most circumstances. Most often, it's depicted as a rapidly widening cone, starting at eye-width but growing to several feet across by the time it reaches the opponent, even at close range.

enter image description here

That being said, Superman seems to be able to deliberately control how focused the beam is at any given time. He's routinely shown making fine, precision cuts with his heat vision, such as during emergency surgeries, and perhaps most famously when he lobotomizes supervillains during one of his many "dark Superman" arcs.

In those moments, the scars are often even smaller than Superman's eyeballs, indicating that the heat vision beam had narrowed during its path before reaching their foreheads.

enter image description here

If he's capable of a widening cone and a narrowing cone, odds are he's capable of a focused beam. It might take a lot of concentration to focus it so perfectly that it could remain focused across light-years...

...but this is Superman we're talking about. He can do it.

  • It's not just about focus; particles in the way will diffuse the beam, and there's not much you can do about that at the source! – Lightness Races in Orbit May 28 '15 at 23:51
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    @LightnessRacesinOrbit True, but that's just a given for anything. A thrown object in space will go forever, unless there's something in the way. I was assuming a straight, clean shot of hypothetically infinite distance. – Nerrolken May 28 '15 at 23:56
  • It's a given yes but that's what the question asks :P There is no such thing as a straight, clean shot of hypothetically infinite distance (in any universe I'm aware of) so that's relevant. – Lightness Races in Orbit May 28 '15 at 23:56
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    If we assume that Superman's heat vision is subject to the normal laws of physics (which of course is not necessarily true) then there is a limit to how well-focused it can be. The narrower the beam, the more it diverges. en.wikipedia.org/wiki/Gaussian_beam#Beam_divergence – Harry Johnston May 29 '15 at 3:59
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    @LightnessRacesinOrbit There's no such thing as superheroes, either. We're taking a certain amount on faith here, hence the word "hypothetically." – Nerrolken May 29 '15 at 23:08
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Effectively infinite (see below)

In Superman Man of Steel #112, Superman and Krypto bounce "low intensity" eye-beams off the surface of the moon, some 384,000km from Earth.

enter image description here

Superman's heat-vision functions as a line-of-sight beam (like a large powerful laser). That being the case, I can't see any obvious reason why it would have a maximum range once it left the Earth's atmosphere, aside from any interaction with interstellar dust.

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    The idea of a dog with heat vision really scares me. I love dogs, but, seriously... – Lightness Races in Orbit May 28 '15 at 23:51
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    Woof, woof, squirrel! I mean barbecue! – user16696 May 29 '15 at 1:50
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    High-luminosity and low-intensity? At the same time? O.o – Ajedi32 May 29 '15 at 3:29
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    @Ajedi32 - Bright, but not hot. – Valorum May 29 '15 at 5:42
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    @VGR Why? Just because he instantly understands English phrases like "Let's try bouncing high-luminosity, low-intensity beams off the moon!"? And acts on them with perfect accuracy and understanding? Well, OK, sure, that does make him slightly smarter than Jimmy Olsen ... – RBarryYoung May 29 '15 at 13:39

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