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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

Ivan
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  • You'll need to decide how the lightsaber works. For example, is it like any of the ideas given here? – HDE 226868 Jan 30 '16 at 23:55
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    @HDE226868 Hmm i found something interesting on this blog http://scienceblogs.com/dotphysics/2010/02/02/power-source-for-a-lightsaber/ – Ivan Jan 31 '16 at 00:52
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    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. – Cort Ammon Jan 31 '16 at 01:03
  • I think this might belong on the SciFi website as well. – Xandar The Zenon Jan 31 '16 at 02:44
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    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! – user6760 Jan 31 '16 at 02:50
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    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. – Aify Feb 02 '16 at 06:37

2 Answers2

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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.

Monty Wild
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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.

WhatRoughBeast
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    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. – Monty Wild Feb 01 '16 at 00:55
  • 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. – WhatRoughBeast Feb 01 '16 at 01:11
  • 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. – Monty Wild Feb 01 '16 at 01:39
  • @MontyWild Actually, assuming the saber landed on its side, the heat might eventually melt enough rock to create a small ditch like line - the heat near the hilt where the blade starts protruding would also melt, resulting in the blade tipping forwards, until eventually, it is actually pointing straight down. – Aify Feb 02 '16 at 03:14
  • @Aify, with this power level, I wouldn't expect this to happen in water. Sure, it'd become boiling hot, but not much more than that. Water is a very good heat sink. – Monty Wild Feb 02 '16 at 03:58
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    @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. – Aify Feb 02 '16 at 06:38
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    @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 – Monty Wild Feb 02 '16 at 22:01
  • @MontyWild yes, but my point isn't that the lightsaber will boil the water. As evidenced by basically every other scene where the lightsaber blades come into contact with a physical material, it heats up so quickly and to a temperature so high that it typically glows white-red. My point is that the lightsaber blade contact with the stone that the saber is lying on will be melted, ending with the same result as what the answerer said. – Aify Feb 03 '16 at 00:44
  • @Aify, the blade should be effectively weightless. The sabre body should only rotate if the surface it is sitting on is such that it would ordinarily make the body rotate regardless of whether it was on or off. – Monty Wild Feb 03 '16 at 01:27
  • @MontyWild the fact that the blade is weightless makes no difference at all to the end result. The process may not be exactly as described by WhatRoughBeast, but the end result is almost certain to be the same. Besides, depending on the lightsaber design, it could be weighted towards the front of the hilt instead of the back/middle (as it should be, to make for easier use anyways), so the blade may still tilt forwards as it melts the rock. Don't forget that you're also tossing the blade into the lake, meaning there's a 50% chance that the lightsaber lands with blade pointing (somewhat) down – Aify Feb 03 '16 at 07:43