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Lightsabers are controlled beams of energy according to the Wookieepedia lightsaber article. Obviously they’re held by their handles, which means a user's hands are only a few inches away from the blade itself. Then they would have to be not very hot in order for the user's hands to not be burned; even if they never actually touch the blade, the energy could superheat the metal of the handle.

Additionally, any time a lightsaber is used to cut something (metal, flesh, etc) it burns and melts like it's being cut by something very hot. The Wookieepedia article even mentions that lightsaber wounds are instantly cauterized, which is demonstrated any time someone has a limb cut off in both the original films and the prequels.

Lightsabers are supposed to be plasma weapons, which means they would be really hot. And they seem to burn when they contact solid matter, so they should also affect the air around them.

So why don't lightsabers boil the air and burn the hands of the people wielding them?

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    One CGI plot point that has continued to confound me is that after cutting an escape hole through a one foot thick blast door, the escapees seem to be able to clamber through the hole using handholds and stepping on the glowing hot metal without harming themselves. I've worked in shops that routinely burned 4in, 6in and even 8in steel plate to size. Believe me, you don't want to be going anywhere near the pieces for quite a long time without protection. With respect to your question though, welders and fabricators routinely use handheld cutting torches with comparatively little protective gear – user62584 Apr 7 at 6:15
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    I doubt Lucas used real physics when writing Star Wars. – RichS Apr 7 at 6:31
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    @RichS Don't try to frighten us with your physicist's ways, Lord RichS. Your sad devotion to that ancient religion has not helped you... – Falyna Apr 7 at 7:28
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    @Jeeped It's a lightsaber, not a heatsaber. Obviously it makes things it cuts glow, not burn. /s – Chronocidal Apr 7 at 15:30
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    They're not hot, but they're Luke warm. – HemiPoweredDrone Apr 8 at 0:58
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For your title question "are lightsabers hot" we can take a look at the novel: A New Hope: Star Wars: Episode IV. On page 77 when Luke is shown Anakin's lightsaber for the first time we read the following passage:

Luke felt no heat from it, though he was very careful not to touch it. He knew what a lightsaber could do, though he had never seen one before. It could drill a hole right through the rock wall of Kenobi's cave - or through a human being.
A New Hope: Star Wars: Episode IV - P.77 (Google Books Preview)

So we can see that lightsabers, although they can cauterize limbs and get through metal, do not give off heat, and so are not hot.

Now as to why they don't give off heat we can have a look at the (now Legends) novel Star Wars: Shadows of the Empire, where we see Luke build a new lightsaber:

He nodded and relaxed his fighting stance. He held his left hand near the blade. No sensation of heat; that was good; it meant the superconductors were working.
Star Wars: Shadows of the Empire - P.116 (Google Books Preview)

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    I gotta say, that "why" explanation isn't much of a "why". It just leads to the obvious next question, why does a superconductor have anything to do with preventing it from being hot? – JMac Apr 7 at 19:58
  • @Kai I initially thought that's what they were going for. But for one thing, a superconductor is for electricity, and for another thing, being superconductive to heat would make it easier to heat up. If "super conductance" were meant in the same way that it works for electricity, a heat superconductor would literally all be at the maximum temperature, which is the opposite of what you would want. Unless it's like one specific part that's "superconducting" the heat away from the lightsaber (but then where the hell is that going?). – JMac Apr 7 at 20:13
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    @Kadima But super conductors literally have no resistance. That's the point of them. So an analogous super heat conductor would all just be raised to the temperature of whatever it's in thermal contact with. It would be the opposite of keeping it from warming up; the whole thing would become plasma temperature (if they actually meant it was superconducting the heat). – JMac Apr 7 at 20:17
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    An interesting discussion about superconductors and their properties has been moved to chat - please continue there. – Rand al'Thor Apr 8 at 10:04
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    Further discussion of real-world physics (responding to the initial comment of @JMac) has been moved to chat - please continue the discussion there. I left the first few comments in the original move to chat, but if it keeps inspiring further discussion in comments I'll have to delete them all. There's a chatroom for discussion, feel free to use that and continue as long as you like - all the existing comments are there :-) – Rand al'Thor Apr 10 at 19:04
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A more "real world" answer. A couple of years(?) ago, I saw a program in which the presenter-scientist described how we could make a lightsaber blade with real-world, current-day technology (noting that his design was more of a stationary saw than a sword).

But basically, the blade was superheated plasma, contained by a magnetic field. The magnetic field prevents contact between the plasma and the surrounding air, so there is no heat transfer. Thus the user is not burned by convection.

When the blade "cuts", say a metal bar, the bar is passing through the magnetic field until it reaches the plasma, which melts/sublimates/burns the bar.

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    OP wasn't asking about someone's real-world attempt to build something a bit like a lightsaber – Valorum Apr 7 at 16:52
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    If that plasma is dense enough to cut a metal bar, I strongly suppose the radiation emitted by it will burn you. Magnetic fields don't stop light. – David Tonhofer Apr 7 at 16:53
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    @sharur Simply because you can have a very "hot" plasma but if there are not many ions hitting you at any given time, you will not even really notice. It's just "number of particles" x "energy of particle" = "energy that hits you". There are also limits on how hot you can make it if you don't have access to the Large Hadron Collider (of the course the plasma in the LHC tube has a solid energy content even if its just a teensy bit of ex-hydrogen) – David Tonhofer Apr 7 at 17:21
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    @DavidTonhofer I can think of a science-ish workaround. If whatever contains the plasma is somehow only letting visible light through, blocking the rest of the radiation; then you might be able to make the case that the visible part of the spectrum isn't carrying that much of the heat. Magnetic fields don't stop light, but the SW universe wouldn't be restricted to that as much as someone trying to actually build one IRL. – JMac Apr 7 at 19:53
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    @JMac Hmmm, my physics is long gone but the light spectrum will not be concentrated, in fact it will be blackbody-ish. If you get hit by visible light, you are already in danger. Watch out for skin blisters! Now you know why Vader wears full body cover.. – David Tonhofer Apr 7 at 20:28
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I can't get sources because I'm at work, but in Legends canon I remember lightsabers included energy converters that would convert the heat generated by the plasma blade back into energy to charge the battery or be recycled back into the blade to help it become more powerful. I read a book that included a scene of Luke building his lightsaber between ESB and RotJ, and it included a passage where he held up his hand to the blade to check if it was emitting heat, and finding that it wasn't so the energy converters were working properly. This would imply that it's only hot to objects that are passing through the blade, but outside that it has little to no effect on temperature.

I do not know if this is maintained in Disney canon.

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  • This is a nice answer but would obviously be better if you find and edit the sources in later. – TheLethalCarrot Apr 7 at 14:18
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    @TheLethalCarrot - I looks like fez may have done that for them. – T.E.D. Apr 7 at 16:28
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    Re, "...convert the heat generated by the plasma blade back into energy to charge the battery..." Basically saying that the laws of thermodynamics either do not apply in the Star Wars universe, or else there's a loophole in them that we haven't found yet. – Solomon Slow Apr 7 at 19:45
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    @SolomonSlow The loophole seems to be that the second law can be reduced to trivial by increasing emphasis on the first law, through finding a way to isolate the lightsaber system (such as a magnetic field to contain the plasma blade and prevent interaction with air) so much that the energy cannot be dissipated as heat and is transformed into some other form. – Kadima Apr 7 at 20:11
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    Y'all, in Legends canon at least, the lightsaber crystal had to be imbued with the Force to work. This means magic is involved and there cannot be a wholly scientific answer. – called2voyage Apr 7 at 20:43
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I'm going to answer this from my experience with laser cutters. This probably means I'll get down votes, as being outside of the Star Wars universe, but whatever.

Lasers

Laser beams in and of themselves aren't hot. It's the interaction between the laser and other materials that makes heat. In a typical laser cutter (specifically non-fiber optic lasers), there are mirrors that redirect the laser beam from the laser emitter to the intended target. These mirrors take the whole brunt of the laser beam without any cooling. There's also a focusing lens that the beam passes through to go from a "thick" laser beam to one that looks like an hourglass, with the focus being the skinny part of the hourglass and where you generally try to get that point at the top of your material to cut or etch. This lens does have some cooling, since often there's usually a gas being directed at the piece being cut for various possible reasons, which I won't get into here. This gas stream isn't intended for cooling, though.

The lens and the mirrors can get hot, but only when they get dirty or are used with a beam that's beyond their intended strength. This is because they are intended to either reflect or refract the light, not absorb it. It's in the best interest of the cutter operator to keep these clean, so they don't get damaged or reduce the effect of the laser.

The laser source itself gets hot and needs cooling, yes, but it also has large amounts of electricity flowing through it to excite the particles to create the laser beam. It also interacts with the light differently, since it's trying to contain the light and get it to exit in only a single direction. Because of this, I'd wonder how a light saber user could hold than handle, rather than worry about the blade. Creating enough laser to cut through a massive steel door is going to create an equally massive amount of heat. It takes over 150 watts for a laser to cut through even thin sheet steel.

I've seen YouTube videos of 4000+ watt lasers cut through 1"-4" of steel, which is impressive enough, and that's a beam the size of a pencil lead, or thinner. You need much more power (and heat generated) to have a 1"-2" laser to cut through a blast door, like we see Qui-Gon and others do.

Laser interactions and cooling

Then there's also the casing around the focusing lens. To direct the gases in the correct direction for the cut, there's a cone shaped nozzle that also contains the laser beam. The only time I've noticed these getting hot is when the beam isn't focused correctly and hits the side of this cone. If it passes by without touching, no heat is transferred to the cone. And this heating happens regardless of the gases blowing through it, so the cooling effect of that gas is very minimal with respect to the heat generated by the laser beam actually interacting with a material.

If you look at the example image below, the laser enters the opening at the top left, then it immediately hits a mirror contained in the 45 degree angle section directing it straight down (if focused properly). Right at the knurled section is where the lens is situated, and it's knurled so you can take this assembly apart and either replace or clean the lens. The barbed tube near the bottom right is the gas (generally air) inlet. The bottom is another opening where the gas and the laser exit. If there wasn't a lens in this, you could blow through it without any problems, so there are no interactions between it and the laser beam except for the mirror and lens. (All the knobs are simply adjustment screws.)

Laser cutting head

Air

Also, air is a really poor conductor of heat. It's so poor, that it's often used as an insulator. How else can you cook over a hot stove and not get burned, except when in accidental direct contact with the stove, pan, or a cooking utensil. You can hold your hand only a few inches over a hot burner and not have a problem, but it'll burn your food with no problems, due to direct contact between good heat conductors and your food.

Air is a collection of gases, and it is not a good conductor or radiator. Air is excellent at convection, but the amount of heat that can be transferred is minimal because the low mass of the substance cannot store a great deal of heat. Air is used as an insulator in coolers and building walls.

https://www.hunker.com/12323257/how-is-air-an-insulator

Materials

Also, I've done a variety of organic materials in the laser cutter, which takes very little laser power to cut. An 80 watt laser cuts through a single slice of bread almost as if it wasn't there. It also etches graham crackers, marshmallows, and chocolate pretty easily. (Yes, I etched the components of a S'mores.) I've also talked with someone who etched hamburgers. In fact, I've been meaning to do a video on how dangerous laser cutter are, by using hot-dogs as a substitute for fingers.

On the flip side, I've etched and cut paper and corrugated cardboard without setting it on fire. All this is done by adjusting the power and speed of the machine, so you put in only the amount of laser into the material you need to vaporize the material, and not heat up the surrounding material.

Relevance

So what does any of this mean to a light saber? Well, it shows that a tool that can cut through a thick steel door can easily pass through even the torso of a human with little to no resistance. It also shows that a cauterizing effect can be possible, due to only "minor" heating of the material surrounding the cut, rather than just setting everything on fire.

It also shows that the laser beam itself isn't really much of a problem, but the hand grip should be a massive heat problem. Of course, getting a laser strong enough to cut through thick steel out of a tiny tool like that can be hand-waved by "alien/advanced engineering", so the heat sink/dissipation problem could be solved the same way.

So far, I've only dealt with laser cutters that are 100 watt or less, so it would be interesting to hear from someone experienced in larger industrial lasers.

Edit:

For those of you who don't think a light saber is a laser weapon, Wookieepedia states them as also being called laser swords, and that it was designed after learning how to "freeze" a laser. The article states that people unfamiliar with the weapon called it a laser sword, but Luke Skywalker called it that in "The Last Jedi". You can say he was being sarcastic about it's use, but he still did say it.

The lightsaber, also referred to as a laser sword by those who were unfamiliar with it, was a distinctive weapon, the very image of which was inextricably bound with the mythos of the Jedi Order and their polar opposites, the Sith.
[...]
The first lightsabers came into being when the precursor Je'daii Order combined advanced offworld technology with a forging ritual, learning how to "freeze" a laser beam.[8]

https://starwars.fandom.com/wiki/Lightsaber/Legends

Wikipedia states that it uses the same crystals used to create the superlaser for the Deathstar.

The source of a lightsaber's power is a kyber crystal.[21] These crystals are also the power source of the Death Star's superlaser.[22][23]

https://en.wikipedia.org/wiki/Lightsaber

Edit 2:

Laser can be used to generate plasma, and high power enough lasers do generate plasma.

Filamentation also refers to the self-focusing of a high power laser pulse. At high powers, the nonlinear part of the index of refraction becomes important and causes a higher index of refraction in the center of the laser beam, where the laser is brighter than at the edges, causing a feedback that focuses the laser even more. The tighter focused laser has a higher peak brightness (irradiance) that forms a plasma. The plasma has an index of refraction lower than one, and causes a defocusing of the laser beam. The interplay of the focusing index of refraction, and the defocusing plasma makes the formation of a long filament of plasma that can be micrometers to kilometers in length.[57] One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons.[58]

https://en.wikipedia.org/wiki/Plasma_(physics)

Other publications also support plasma being created by lasers, and lots of research has been done on the topic.

The interaction between a pulsed laser beam and any substance is extremely complex [3]. It is a non-linear process, dependent upon laser characteristics (fluence, pulse rise-time and duration, wavelength, beam quality), substrate composition and surface character, and the environment in which the plasma forms (pressure and composition). We counted more than 1500 publications in the last 5 years on laser-induced plasmas related to LIBS, many having to do with studies of the influence of laser wavelength, [...]

https://www.sciencedirect.com/topics/chemistry/laser-induced-plasma

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    I've downvoted because, as you say, this has little or nothing to do with Star Wars. These aren't laser swords, they're super-heated plasma generated by a kyber crystal, contained in a magnetically-based forcefield. – Valorum Apr 8 at 18:51
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    @Valorum, then I'd have to ask why they are called "light sabers" and not "plasma sabers"? Being a beam of light would qualify as a laser. Also, other Answers focused on them being lasers, not plasma weapons. Also, I said it was outside of the Star Wars Universe, not that it had nothing to do with Star Wars. – computercarguy Apr 8 at 19:03
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    Your answer competently addresses why a device that bears no similarity to a lightsaber doesn't burn your hands. – Valorum Apr 8 at 19:11
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    @Valorum, unfortunately that doesn't answer any of the questions I asked. Also, Wookieepedia (and Luke) calls it a "laser sword" and describes it as "freez[ing] a laser beam". Wikipedia says the crystals you mention are the source of the superlaser in the Deathstar. And lasers are created using crystals, plasma isn't. Plasma is superheated materials, which a laser would work really well to create. – computercarguy Apr 8 at 19:20
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Here's a semi-scientific explanation: Imagine that the lightsaber blade is like a live electric wire with ultra low resistance, almost a superconductor. As long as the blade doesn't touch anything, nothing happens: no current flows and no energy is converted into heat. When you touch something with the blade, there will be a huge flow of energy through the blade (burning or melting the thing you touch), but since its resistance is so low, the blade itself still doesn't produce much heat. If the flow of energy would be equivalent to 1000 amps at 1000 volts (i.e. 1MW), it would melt a kilogram of metal in a couple of seconds.

Of course in the Star Wars universe the energy is something else than eletricity, but the principle is the same.

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  • But what happens when the blade touches the atmosphere? That's the reason behind most of these questions about lightsabers, if they transfer heat energy to things they touch, why don't they burn the atmosphere, and everyone in it? – Muuski Apr 8 at 22:12
  • Nothing happens when the blade touches air, because air conducts energy very poorly. The same reason let's you put your finger a tenth of an inch from a live 1000V wire and not get electrocuted. There is simply no energy flow. – rijnswind Apr 9 at 5:18
  • Arcing is a thing that exists and air conducts heat very well from a bonfire to your face ten feet away, and a bonfire is a very cold thing compared to plasma that can melt metal doors. – Muuski Apr 9 at 16:12
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Yes, lightsabers are hot. That is clearly evident from The Phantom Menace, in which Qui-Gon Jinn uses the heat of his blade to melt the metal around it. This clearly contradicts any source that suggests the lightsaber blade does not emit heat.

As to why the heat does not burn the user? I don't know, except that this is such an obvious potential problem with a hot-bladed weapon that nobody would manufacture or use lightsabers unless it was possible to protect the handle (and the wielder's hands) from the heat of the blade.

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    Except that it's maybe that the middle part is hot. Or possibly it was heating the metal with something other than heat such as magnetic induction or radiation – Valorum Apr 7 at 17:06
  • @Valorum Radiant heat is heat like any other. Induction heating is indeed different, but would not match the observed characteristics of a lightsaber either (although it would be closer). However, maybe this just means the question should be closed as asking for a real scientific explanation. – Buzz Apr 7 at 17:11
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    Well, given that we know that the exterior of the lightsaber is magnetic, the meltiness could well be magnetic as well as heat from the plasma core. OP is, however, asking about the heat that emanates from the lightsaber, not the temperature of the plasma inside it – Valorum Apr 7 at 17:15
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    @Buzz Radiant heat isn't quite like any other... and that would be part of the danger of a plasma blade. Even if you contained the material, the fact that you can see light being emitted from it means it's definitely emitting some radiation, which would transfer heat. Though you could probably science around it by saying that whatever contains the plasma only allows visible light to pass, and that the visible part of the spectrum doesn't carry much of it's heat. – JMac Apr 7 at 19:47

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