Can lasers really cause explosions? Let's use a silly hypothetical to find out!

[Ravioliman]

The context behind my question is quite silly, but I've encountered some people who believe lasers cannot produce explosions, even at insane levels of energy. Now, I'm not big on physics nor will I pretend I am, I think they could but I came here to get a more knowledgable opinion(using a silly hypothetical)so maybe I could learn how to explain it better, assuming I'm not wrong.

Silly hypothetical:
Let's say we have a hypothetical laser capable of delivering the equivalent of 1 petatons of TNT within a single, short pulse and we shoot it at a George Washington's nose on Mt. Rushmore(I chose him for no particular reason) - what would be the reaction? Would it simply eat through and create a hole, leaving the body mostly intact or would it heat it up incredibly fast and create a massive explosion?

 

[DaveE]

Could you define what you mean by explosion; chemical reaction? shockwave? rapid energy dispersion?
When an asteroid hits the earth is that an explosion? When a laser starts to drill a hole in a metal sheet do the first molecules removed "explode"?

Anyway, I suspect the answer is yes, but I'm not sure what the real question is.

For laser based material processing, this question can be quite complex and the actual material removal process depends on a lot of parameters, primarily power/energy, pulse width, repetition rate, wavelength as well as the target material composition. Sometimes it's heat based, like melting, other times it's directly breaking molecular bonds.

 

[Frabjous]

Generically, I would say that they can create surface explosions with a area roughly the size of the beam. Once you get away from that point, things are governed by thermal conduction which will not in general cause an explosion. Another issue is the creation of a plasma which will decrease the coupling to the underlying material. The fundamental question is how transparent is the target, i.e., how deep will laser energy be deposited. The deeper it is, the better a chance for a bigger explosion.
I have no experience with unrealistic energy lasers.

 

[davenn]

but I've encountered some people who believe lasers cannot produce explosions, even at insane levels of energy

I would also tend to side with "those people"

what would be the reaction? Would it simply eat through and create a hole, leaving the body mostly intact or would it heat it up incredibly fast and create a massive explosion?

As DaveE said, you need to define your meaning of explosion. Your choice of target for your example is important as it will fracture and shatter explosively and rapidly with the application of energy (heat). This is not a chemical explosion in the way you may be thinking that is occurring.
You can get the same effect with rock by applying energy (heat) by other methods ... oven, gas cutting torch etc.

Pick a different material, a panel of steel, aluminium, ceramic, glass etc and your results will be completely different in the way the laser interacts with that material.

Now if the laser beam hits a material that in of itself has explosive qualities ... tank of petrol, gas etc, then there will probably be
the type of explosion, that I think, you are imagining.

Maybe you are being swayed too much by what you see in scifi TV and movies ? regards
Dave

 

[Ravioliman]

Could you define what you mean by explosion; chemical reaction? shockwave? rapid energy dispersion?
When an asteroid hits the earth is that an explosion? When a laser starts to drill a hole in a metal sheet do the first molecules removed "explode"?

Anyway, I suspect the answer is yes, but I'm not sure what the real question is.

For laser based material processing, this question can be quite complex and the actual material removal process depends on a lot of parameters, primarily power/energy, pulse width, repetition rate, wavelength as well as the target material composition. Sometimes it's heat based, like melting, other times it's directly breaking molecular bonds.

Shockwave. A focal point of the discussion on one end is that if it can cause a large scale explosive reaction followed by a shockwave without the target being a form of fuel or other highly-combustible material - then it isn't really a laser, because photons alone can't incite such a reaction.
I'm talking about the overall possibility of inducing a large scale explosion as the average person knows it - a violent burst of heat followed by a shockwave by heating up objects like boulders, trees, concrete and such with a single, relatively short pulse. I assume it'd take different conditions to achieve the same result with each of the objects stated.
Of course, I'm talking about a hypothetical laser containing incredible levels of energy without having to worry about a medium capable of powering and/or enduring them.

Generically, I would say that they can create surface explosions with a area roughly the size of the beam. Once you get away from that point, things are governed by thermal conduction which will not in general cause an explosion. Another issue is the creation of a plasma which will decrease the coupling to the underlying material. The fundamental question is how transparent is the target, i.e., how deep will laser energy be deposited. The deeper it is, the better a chance for a bigger explosion.
I have no experience with unrealistic energy lasers.

I would also tend to side with "those people"

As DaveE said, you need to define your meaning of explosion. Your choice of target for your example is important as it will fracture and shatter explosively and rapidly with the application of energy (heat). This is not a chemical explosion in the way you may be thinking that is occurring.
You can get the same effect with rock by applying energy (heat) by other methods ... oven, gas cutting torch etc.

Pick a different material, a panel of steel, aluminium, ceramic, glass etc and your results will be completely different in the way the laser interacts with that material.

Now if the laser beam hits a material that in of itself has explosive qualities ... tank of petrol, gas etc, then there will probably be
the type of explosion, that I think, you are imagining.

Maybe you are being swayed too much by what you see in scifi TV and movies ?regards
Dave

I'm referring to a generic explosion as most people know it - a burst of extreme heat followed by a shockwave by heating up materials as those I mentioned above: boulders, concrete, trees-- nothing in particular. And as I mentioned - assuming the laser contains incredible amount of energy. Not something that is feasible for us, at least not anytime soon, I assume.

As for fictional influence - I've always taken fiction with a grain of salt when it comes to real life physics, but at the end it was a discussion regarding fictional shenanigans(I could elaborate if you think this might help understand me better)that made me think a bit harder and actually explore these concepts. It helped pique my interest and so I'm here, after also having visited Quora with a similar question. Of course, I do plan on diving deeper than just asking questions online.

I'm aware the subject at hand is fairly complex and my question was probably a bit too general. My apologies. Also, I figure I should have probably phrased it better, because "I've encountered some people who think..." does sound insulting in a hindsight. I'm aware this is a complex topic and I figure I was a bit too confident in my approach.

 

[Frabjous]

Keeping things simple, the question is how much material is vaporized and if it is done quickly enough. For normal lasers, the penetration depth is small, so only a small region near the surface under the beam has the potential for an explosion. Once you destroy the this layer, the debris begins to interfere with the coupling so the laser become less efficient. Your are postulating a strongly overdriven system, and I am unsure where the limits go in this case.
As a side note, a laser that strong is going to do bad things to the air.

 

[Vanadium 50]

Shockwave.

OK, that's a reasonable distinction: detonation vs. deflagration.

You absolutely can have a shockwave.

 

[DaveE]

Shockwave.

OK, then yes. I've seen (OK, heard) it. A real high power ultrafast laser, when focused, has enough power density to break down air molecules and create a "snap" sound, which of course is the resulting shock wave hitting your eardrum. Granted it's smaller than you had in mind, but it's real.

It's actually a fairly common issue in high power pulsed lasers. Note that it has more to do with spatial and temporal concentration of the laser energy than just "petatons of TNT" energy levels. You can easily* do it with 1 watt if you can make a good laser.

*edit: OK, maybe it's not THAT easy to make, but you can buy those lasers off the shelf.

 

[Ravioliman]

Keeping things simple, the question is how much material is vaporized and if it is done quickly enough. For normal lasers, the penetration depth is small, so only a small region near the surface under the beam has the potential for an explosion. Once you destroy the this layer, the debris begins to interfere with the coupling so the laser become less efficient. Your are postulating a strongly overdriven system, and I am unsure where the limits go in this case.
As a side note, a laser that strong is going to do bad things to the air.

OK, that's a reasonable distinction: detonation vs. deflagration.

You absolutely can have a shockwave.

OK, then yes. I've seen (OK, heard) it. A real high power ultrafast laser, when focused, has enough power density to break down air molecules and create a "snap" sound, which of course is the resulting shock wave hitting your eardrum. Granted it's smaller than you had in mind, but it's real.

It's actually a fairly common issue in high power pulsed lasers. Note that it has more to do with spatial and temporal concentration of the laser energy than just "petatons of TNT" energy levels. You can easily* do it with 1 watt if you can make a good laser.

*edit: OK, maybe it's not THAT easy to make, but you can buy those lasers off the shelf.

What I'm trying to figure out now is if it could theoretically induce the type of explosion I'm talking about, but on a larger scale within a single short pulse given it contains enough energy close to the level of anywhere between a Tomahawk cruise missile(2 GJ) to a MOAB(46 GJ)by simply coming into contact with any solid surface? Could it genuinely blow up boulders, buildings and such?

 

[Vanadium 50]

The goalposts are moving.
Your new requirement is multiple GJ delivered in one pulse. When you did a Google search did you find any lasers that do this? If there aren't any, don't you have your answer?

 

[Ravioliman]

The goalposts are moving.
Your new requirement is multiple GJ delivered in one pulse. When you did a Google search did you find any lasers that do this? If there aren't any, don't you have your answer?

I'm referring to a hypothetical scenario, assuming you could achieve such level of energy with a laser without having to worry about the medium delivering it. I've already stated this. I know that there are no lasers capable of it in real life.

Also, probably should've said this in my previous comment - thank you folks for your answers!

 

[DaveE]

Yes, your mountain would look like it exploded.

 

[Vanadium 50]

So you are putting a lot of energy into a small space in a short time. And yes, that can lead to an explosion. Must it? That's a function of how much, how small and how short.