Laser impacts

[ARS]

Laser guns appeared on movies, literatures, novels and many other media. People agree that it's a cool device when a battle scene occurs where both sides literally fires beam of lights on each others. Aside from (mostly) unrealistic slow speed of laser projectile, let's talk about how the laser impacts the surface. Basically, there are 2 kinds of laser categories that appear in most sci-fi works:

The pulse firing laser: this laser is basically the one who goes pew pew pew, aka shooting in short burts or semiautomatic firing. Usually, whenever the laser projectile of this kind hits any surface, it will almost always produce spark explosion, or sometimes a huge explosion and on some cases in huge battleship caliber laser cannon, literally leaves a crater on the ground, as if the gun that's being used is a railgun instead of energy weapon

The persistent firing laser: this is a type of laser gun that's fired in a long stream instead in controlled bursts. Usually used by focusing the beam on target or sweeping multiple targets at once. Sometimes potrayed by being used to (fairly realistically) gradually vaporize the surface of the target, or pierce straight through the target

Considering it takes much more power to create a sustaining laser beam instead of firing it in pulse, the impact of persistent laser should be more pronounced (not only just gradual vaporizing or piercing) than pulsed laser (where usually each shot can create mini explosion on impact)

I'm not a laser scientist, but just asking, is it really possible that laser guns (both types, assuming we can create them) behave like what movies or sci-fi works potrayed? Was it really possible for a pulsed light emission leave a crater on the ground? If it's indeed possible for a pulsed laser to create mini explosions on impact, then should the effect of persistent laser far more destructive than pulsed laser?

Let me know what you think

[Steel]

12 minutes ago, ARS said:

...

I'm not a laser scientist, but just asking, is it really possible that laser guns (both types, assuming we can create them) behave like what movies or sci-fi works potrayed? Was it really possible for a pulsed light emission leave a crater on the ground? If it's indeed possible for a pulsed laser to create mini explosions on impact, then should the effect of persistent laser far more destructive than pulsed laser?

...

No, a laser is ever going to leave a crater (unless you class a hole caused by the laser melting something a crater) or a mini explosion, unless the ground is made of some highly volatile substance that explodes when heated.

Traditional weapons (i.e projectile weapons) use kinetic energy to cause damage, thus the impact craters and people being thrown backwards when they're hit. Lasers are very good at transferring energy to a target, however a laser beam (pulsed or continuous) has very little kinetic energy. This means you wouldn't be thrown backwards by being hit, and you wouldn't get the impact marks that you see from projectile weapons.

Edited August 14, 2017 by Steel

[Shpaget]

44 minutes ago, Steel said:

No, a laser is ever going to leave a crater (unless you class a hole caused by the laser melting something a crater) or a mini explosion, unless the ground is made of some highly volatile substance that explodes when heated.

Like water? If you vaporize it fast enough, it looks a lot like explosion.

Iron evaporates at around 2860°C, which is still within the realm of believable sci-fi weapons.

[AeroGav]

Outside of industry, where a steady beam is preferred for cutting, etching etc,  pulsed lasers are common.   You can make a higher instantaneous power output because of discharging caps and because the emitter has time to cool between firings.    In nuclear fusion research as well as weapon applications, i think the pulses  are between a millionth and a thousandth of  a second.        In weapon applications , you have the problem that vaporising part of the target creates a cloud of plasma that is opaque to further EM radiation.     The plasma cloud from the beam hitting you absorbs the heat of continued firing, so rather than burning a deep hole in you, you get a  superficial flesh burn,  then the little puff of vaporised flesh gets heated to extremely high temperature by the continued firing and expands explosively.    Problem is , the force of the blast goes mostly outward so again not particularly lethal.     This is why the bad intention research is into a short pulses that do the damage before the plasma forms then hit again as soon as it dissipates.

Alternatively,  if we can figure a way to make efficient UV lasers,  the terminal ballistics guys will be happy.     They are used in surgery because they don't burn or create heat.     Each photon has just the right amount of energy to break the chemical bonds of your body, converting organic matter to CO2 and water directly rather than by just heating it up  until it burns or vaporises.

Therefore you get a nice deep non-cauterised wound from which the victim bleeds rapidly.

 

[Scotius]

Energy is energy. Thermal or kinetic - for the target it doesn't matter much in what form scores of joules arrived. If energy input will be big and fast enough, structural integrity of the obiect hit will break - thus, crater.

[kerbiloid]

Depends on its power per area.

If enough much energy is released inside the vaporized volume of substrate during the shot,
before the vaporized substance gets leaked away,
then the hot cloud of the vaporized substance (still filling the burnt hole)
will have density of original material, heat energy of the shot,
and will work as any hot gas cloud created by an explosive or a nuke.

So, if the laser has enough high power per area, this will cause a regular explosion with crater and so on.

Upd.
Say, this is an iron plate. Mr = 56 g/mol. Let the gas temperature be 10000 K.
Heat speed of the vapourized material = sqrt(3 * 8.31441 * 10000 /56*10^-3) = 2 km/s.

So, during the first millisecond after the shot, it will expand only for 2 mm.
And inside the burnt hole you will have a plasma cloud with density 7800 kg/m³ and temperature of 10000 K.
Its energy per volume ~ 7800 * 8.3 * 10000 / 56*10^-3 = 11.5 GJ/m³ ~ 2.5 t of TNT/ m3.
 

Edited August 14, 2017 by kerbiloid

[Hannu2]

Laser may evaporate and even ionize material in which it hits. That hot gas or plasma expands and can make crater in surrounding material like hot gases from explosion or fast projectile. It is utilized in microscopic scale for some laser cutting applications, but such lasers which could make a large crater on ground will not exists in foreseeable future.

Comparison between continuous wave and pulsed lasers are not straightforward. Effect depends on material, power level, time etc. and if we talk about power levels several order of magnitudes above anything which have ever been tested answers will be very hypothetical. Certainly both techniques can be destructive weapons if we can assume arbitrarily large power levels. But I suspect that there may be more effective and practical weapons than laser beams for those who have technical skills to handle such power levels.

[KSK]

It depends what frequency your laser is operating at, what your target is made of and, critically, what sort of power densities the laser can put out

Power density depends how focused the beam is and how much energy you can put into the target in a given time. A tightly focused, fast pulse laser (delivering picosecond or shorter pulses) can give you gigawatts or terawatts of power on target - for the duration of that pulse.

The power density will largely determine the interaction between the laser and the target. For moderate power densities, you might expect to see thermal ablation - basically the target gets hot enough to boil away but is also losing heat fast enough (by conduction to the rest of the target as well as the heat carried away by the vaporised material) that boil is all it does.

However, for higher power densities, you might expect to see photoablation where the target material is ionised and then - literally - blows itself apart due to electrostatic repulsion.

So I have no trouble imagining that a big sci fi grade pulsed laser could make a sizeable crater in something.

 

[kerbiloid]

The next question we should discuss: can some visible craters in the Solar System be identified as mega-laser shots.

Spoiler

 

Edited August 14, 2017 by kerbiloid

[DDE]

Off to Atomic Rockets you go!

http://www.projectrho.com/public_html/rocket/sidearmenergy.php

[Green Baron]

Material science uses laser and plasma cutting/drilling/ablating/converting/blabla for various purposes. What exactly happens when a high energy laser hits a surface can easily be researched there.

To bring this a little bit into a sciency direction ... ;-)

Edited August 14, 2017 by Green Baron

[vger]

Even Star Wars technical stuff admits that blasters aren't actually laser guns. They actually fire blobs of plasma. Simplest way to look at it I guess would be a "ball lightning" cannon. Ball lightning has been known to explode on impact, leaving massive gaping holes. Of course, sometimes it also passes straight through objects, doing no damage at all.

A laser, by comparison, if we were ever able to make them powerful enough, I guess could produce instantaneous explosions. But would using that much power really be worth it vs using it for a slower cutting/melting process. I guess it depends on what kind of fighting you wanted to use it for. But the damage would probably be comparable to a prolonged lightning strike, and would also look and sound like one too if it was hot enough to ionize the surrounding air. Just a straight white light instead of a zig-zaggy path to the target though.

I've always imagined practical military applications of a laser (downing aircraft aside) to be an anti-personnel artillery weapon. Something that could mow down large groups of advancing troops like a ridiculously long chainsaw. But it also has anti-armor applications. It's not particularly humane, but just because you can't tear through a tanks armor, doesn't mean you can't heat it up and cook those inside like a potato wrapped in tin foil.

Edited August 14, 2017 by vger

[tater]

Lasers weapons turn the target surface to plasma, which creates an explosion. 

[DerekL1963]

2 hours ago, tater said:

Lasers weapons turn the target surface to plasma, which creates an explosion. 

Yep.  In the various plans to use lasers for missile defense the damage mechanism (contrary to what people think) isn't "melt the target" or "drill a hole through it" - it's "induce shock by instantaneously vaporizing a portion of the target's skin".  A friend of mine who worked on one or another of the many laser projects the DoD was pursuing back in the 90's likened the effects to taping a hand grenade to the surface of the target and setting it off.

[YNM]

It depends on what is the laser power, wavelength and the object it hit.

For instance, laser cutting creates near-perfect holes on thin materials. Pulsed laser actually can't be recharged fast yet (you still need to produce the energy), so I also assume they make holes. But say, for example, on thick rocks. Laser "impact" on rocks would cause it to melt - but there's more on the behind. Depending on how one applies it, the "melted" material might not "leave" the site (bad citation), so most likely you'll just ablate the surface material.

 

tl;dr it leaves a mess. Just control it.

[tater]

I know people who have worked on them as well (usaf airborne laser program is done here in ABQ). Actually, the navy lasers are also at Sandia.

[Codraroll]

5 hours ago, vger said:

I've always imagined practical military applications of a laser (downing aircraft aside) to be an anti-personnel artillery weapon. Something that could mow down large groups of advancing troops like a ridiculously long chainsaw. But it also has anti-armor applications. It's not particularly humane...

That depends on the level of precision. With sufficient technology, you could make the enemy's weapons useless in their hands, or flash-weld the moving parts of a vehicle-mounted gun together. Just hope they have the precense of mind to drop the weapon before it becomes too hot to touch...

[wumpus]

On 8/14/2017 at 6:56 AM, Hannu2 said:

Laser may evaporate and even ionize material in which it hits. That hot gas or plasma expands and can make crater in surrounding material like hot gases from explosion or fast projectile. It is utilized in microscopic scale for some laser cutting applications, but such lasers which could make a large crater on ground will not exists in foreseeable future.

Remember that each of these things has a high energy cost that is essentially stored in the target as its new phase.  Most military lasers are only interested in blasting an arbitrarily small hole (however well they can focus*) on a fuel tank to destroy a missile.  They have massive power and aren't remotely capable of digging a deep enough hole to cause a crater.

I suspect nearly all craters made military action involved buried explosives (probably a shell going too far and burying itself, then immediately exploding.  Or perhaps exploding sometime in the next century, see the "iron harvest").  Dropping a solid bullet is pretty inefficient, and only done to minimize blasts (I think the US Air Force has dropped guided blocks of concrete from B-52s to minimize nearby destruction).

* make the pulse short enough and you don't need tracking.  And anything in this power range is definitely pulsed.