We all know that lightsabers are used for cutting/melting through lots of types of materials (with few exceptions).

Now what will happen if lightsaber is dropped into a lake and stays on?
What are long term implications on environment?

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Edit: lightsaber power source and output
Edit 2: Leidenfrost effect (mentioned in comments) and mythbusters demonstration video

  • $\begingroup$ You'll need to decide how the lightsaber works. For example, is it like any of the ideas given here? $\endgroup$
    – HDE 226868
    Jan 30, 2016 at 23:55
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    $\begingroup$ @HDE226868 Hmm i found something interesting on this blog scienceblogs.com/dotphysics/2010/02/02/… $\endgroup$
    – Ivan
    Jan 31, 2016 at 0:52
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    $\begingroup$ I'm torn between agreeing with HDE that you need to define how your particular lightsaber rules work, and recognizing that there is a large body of literature on the topic, some of which is actually canon! I wonder if this might be a better question for SciFi.SE, where questions based on canon are standard faire, rather than WorldBuilding.SE, where we typically dabble in the more non-canon worlds where you have to define everything. $\endgroup$
    – Cort Ammon
    Jan 31, 2016 at 1:03
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    $\begingroup$ That's nothing! Do you still remember a scene from Spiderman 2(2004) when a supervillain tossed a miniature star into a river... they mentioned the fusion is self sustained it needs to be put out then I lol! $\endgroup$
    – user6760
    Jan 31, 2016 at 2:50
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    $\begingroup$ Related - Kit Fisto's Lightsaber - Actually, if you were to use a lightsaber that was canon that worked underwater (which exists, see above link), it would not do anything other than stay on and be like a normal sword - there seems to be no evidence of any heating of the water or anything like that while it's turned on underwater. $\endgroup$
    – Aify
    Feb 2, 2016 at 6:37

2 Answers 2


The answer depends a little on the "energy cell" that powers the lightsaber. If it is simply a high-energy-density battery, then we have a limited run-time and energy output. If it is actually a miniature fusion reactor that distils deuterium or something from the environment, then it could run indefinitely.

Assuming that the lightsaber is using a battery and not a reactor, and that it has been dropped into a small lake of about 0.01 cubic kilometre volume (1e10 liters):

Using the online calculator here, and the power estimates the OP referenced in the link: Lightsaber power source and output (28500 watts output), it would take 2075963434 minutes (65 years 286 days 2 hours or so) to raise the temperature of a 15 degree Celsius lake of the aforementioned size to boiling point. Since that is more than the estimated run time of a lightsaber (by the OP's quoted source) by some 65 years 286 days, then the power cell would be exhausted before there was any appreciable increase in the lake's overall temperature. A little water would boil, but there would probably be only a few bubbles visible on the surface at best, and there'd be some convective currents for a couple of hours.

If the lightsaber was to have a reactor as its power source (that just happened to be called an "energy cell") that was environmentally fuelled, then in 65 years or so, the 0.01 cubic kilometre lake would begin to boil, assuming that the water lost no energy to the surrounding environment in the mean time. Since the water will radiate its excess heat from its surface at a rate increasing as its temperature increases, it will probably never reach boiling point in its entirety. Effectively, such a lightsaber would also cause no significant environmental effects. There'd be a continuous thermal current in the lake, and aquatic organisms would stay away if they could.

If the entire volume of this small lake was flashed into steam instantly, then its 0.01 cubic kilometre volume would be a drop in the bucket of an earthlike world's estimated atmospheric water content of 12,900 cubic kilometres. It'd take a truly huge amount of water flashed to steam to make any significant environmental difference, and a lightsaber as described just can't do that, regardless of what's powering it.

TL, DR: the answer is: Locally - Only a small effect. Globally - Nothing noticeable at all.


That's pretty straightforward: no long-term effects. The lightsword will sink to the bottom of the lake. There, it will melt the rock under the blade, and the sword will pivot at the guard until the blade is pointing straight down. Then the melting of the rock under the hilt will allow the sword to sink into the earth, slowly melting its way to the core. The melted rock left in its wake will cool off and solidify. If the sword continues to operate indefinitely it will end up at the center of the core, but its total energy output is so low (on a planetary scale) the it will have no noticeable effect.

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    $\begingroup$ You're neglecting the point that the blade is likely weightless or very nearly so: It will not pivot to point downwards due to the weight of the blade. The lightsaber will probably remain in whatever orientation it takes when it lands on the bottom, most likely lying on its side. $\endgroup$
    – Monty Wild
    Feb 1, 2016 at 0:55
  • $\begingroup$ If so, and it has the power needed to melt a sufficiently large volume, the handle will be included in the melted pocket and sink anyways. $\endgroup$ Feb 1, 2016 at 1:11
  • $\begingroup$ At the OP's (indirectly, via link) stated 28500W output, and in water, I wouldn't expect that it would sink into the substrate at all. $\endgroup$
    – Monty Wild
    Feb 1, 2016 at 1:39
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    $\begingroup$ @MontyWild yes, but it's a lightsaber - lightsabers are so hot that they can cut through metal with ease (Proof being Qui gon cutting through blast doors in Episode 1). Not to mention, there would only be a thin layer of water (if not none due to evaporation of water when touching the blade), meaning direct contact with the rock beneath, resulting in part of the heat spreading through the rock and not the water. $\endgroup$
    – Aify
    Feb 2, 2016 at 6:38
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    $\begingroup$ @Aify, Lightsabers have pretty good energy containment, as evidenced by Luke Skywalker's lack of flash burns on his face in Episode 4, despite waving his new lightsaber in front of his face. He obviously wasn't a Jedi then, so had no way of magically avoiding this if it would have happened without Jedi powers. So, I find it unlikely that a lightsaber will do much more than heat the water that comes into contact with its blade. Additionally, the Leidenfrost effect will slow heat transfer, and the heat in the water will decrease proportionally to the square of the distance from the blade $\endgroup$
    – Monty Wild
    Feb 2, 2016 at 22:01

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