Depending on how much energy they're putting out, according to my high school physics computations, they might...Take, for example, a human's body - composed approximately of what, 70% water? Since water probably has the highest calorific value of all the materials (4200 joules per kilogram * degree (C)) and human body temperature is around 37 degrees Celsius, we can calculate the amount of energy required to increase the temperature to boiling (100 degrees), which should be enough to cause a burn wound. If you want to burn off 10 ml (about a hole the size of 10 quarters stacked one upon the other), you'd need to emit about of 2520 joules. Using electromagnetic radiation (which light is) as a transmission method, the rays emitted would have a total momentum of 8.4 millimetergrams per second. Divide this by the weight of the average laser pistol (500g) and you end up with the astronomically small kickback of the gun moving at 16.8 micrometers per second backwards. However, these lasers are emitting bursts, and they're not just burning small holes in people; they are stunning, and sometimes even killing the target...How much energy would that take? The ratio is right there: For every megajoule of laser energy, you cause a kickback of 6.6 millimeters per second...That's starting to get noticeable.
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Do laser weapons have kickback?
- Thread starter Kimura
- Start date
The idea is that the total momentum of any system remains constant; so, by projecting photons forward, those photons will have momentum (in short, E/c) which must be counterbalanced by the backwards movement of the projector (gun). However, you're right - it would have some effect on the target as well - I hadn't considered that. Anyway, it's more noticeable with bullets because they go at super high velocities (~500m/s was the example question) and have considerable mass compared to a photon (whose mass is virtual, and can only be substantiated by the classic equation E=mc^2).
I believe most laser (and related) systems work by focusing beams or pulses rather than generating them out of thin air in a specific direction. You're more likely to find that energy weapons, and I use the term loosely, are simply magnifying glasses for a random emitter somewhere within the weapon. Various fields may be used to direct and contract beams or pulses into the desired shape or intensity, but the recoil produced by such systems is zero.
<technical mode>
With current technology, lasers work by generating pulses of light energy, which bounce back and forth between two mirrors within a collection chamber. The net momentum of this is 0.
The light energy does eventually leave the weapon, however, and the recoil generated by this would be directly related to the amount of energy released, which could be inferred from the damage done. Bonichi is correct.
However, with the current decriptions of the laws of physics, the mass of a photon is not well defined. It may or may not be zero. However, photons as we know them cannot accelerate or decelerate, and thus, cannot exert a force. Therefore, there would be no 'kickback'.
However, the victim would feel a kickback of sorts if they were seriously burned, from the sudden rush of gas from the vaporized material.
</technical mode>
I'm in college, we talk about things... ^^;
With current technology, lasers work by generating pulses of light energy, which bounce back and forth between two mirrors within a collection chamber. The net momentum of this is 0.
The light energy does eventually leave the weapon, however, and the recoil generated by this would be directly related to the amount of energy released, which could be inferred from the damage done. Bonichi is correct.
However, with the current decriptions of the laws of physics, the mass of a photon is not well defined. It may or may not be zero. However, photons as we know them cannot accelerate or decelerate, and thus, cannot exert a force. Therefore, there would be no 'kickback'.
However, the victim would feel a kickback of sorts if they were seriously burned, from the sudden rush of gas from the vaporized material.
</technical mode>
I'm in college, we talk about things... ^^;
Yangfan
Inactive Member
Ah, finally. A use for my physics minor.
While photons can be considered massless for most purposes, pure energy still carries momentum. The real equation for momentum is:
P = energy / velocity = E/v
For objects with mass:
E = 0.5mv^2
P = 0.5mv
(E = mc^2 is for when you turn the mass into energy via fission or fusion.)
For obects without mass:
P = E/c
Disclaimer: I'm only 80% sure that the energy of a beam can be used in this equation in this manner, but as it turns out, it won't matter much.
Say I have a 10000W energy pistol. I fire off an one-second shot. The energy is:
E = 10000 W * 1 s10000 J/s * 1 s = 10000 J = 10000 kg m^2/s^2
The momentum is:
P = E/c = 10000 kg m^2/s^2 / 3*10^8 m/s = 0.00003333 kg m/s
= 0.03333 g m/s
Which is an order of magnitude or two below a peashooter. So, you won't feel it at all. This is all because c (speed of light) is huge.
While photons can be considered massless for most purposes, pure energy still carries momentum. The real equation for momentum is:
P = energy / velocity = E/v
For objects with mass:
E = 0.5mv^2
P = 0.5mv
(E = mc^2 is for when you turn the mass into energy via fission or fusion.)
For obects without mass:
P = E/c
Disclaimer: I'm only 80% sure that the energy of a beam can be used in this equation in this manner, but as it turns out, it won't matter much.
Say I have a 10000W energy pistol. I fire off an one-second shot. The energy is:
E = 10000 W * 1 s10000 J/s * 1 s = 10000 J = 10000 kg m^2/s^2
The momentum is:
P = E/c = 10000 kg m^2/s^2 / 3*10^8 m/s = 0.00003333 kg m/s
= 0.03333 g m/s
Which is an order of magnitude or two below a peashooter. So, you won't feel it at all. This is all because c (speed of light) is huge.
Solar sails only work in theory, one has never been built to demostrate if it actually produces any thrust. And even so, solar sails concepts are enormous, if there is any thrust from a laser weapon it's going to be very tiny right?
The only time photons can change direction is under the effect of a gravitational field, which, by current physics theories, isn't really changing at all because of how gravity curves space. When light is reflected off a mirror, the photon doesn't change direction; The photon strikes the surface and is absorbed as energy, this energy raises the energy level of the atom struck, if this new energy level is an unstable one, the atom gives off energy in the form of a new photon.
The only time photons can change direction is under the effect of a gravitational field, which, by current physics theories, isn't really changing at all because of how gravity curves space. When light is reflected off a mirror, the photon doesn't change direction; The photon strikes the surface and is absorbed as energy, this energy raises the energy level of the atom struck, if this new energy level is an unstable one, the atom gives off energy in the form of a new photon.
Itkatsu Kiyoko
Inactive Member
Back in highschool science, there was a device that was basically like a wind speed measuring device (you know, those things with the hemispherical cups on the end of spokes that spin around) except with flat surfaces instead of cups with a shiney surface on one side and black on the other, in a low-pressure inside a glass container. Shine a torch at it, and it will slowly start spinning. Basically you will get some recoil, but (as Yanfang pointed out) it will be so tiny you wouldn't notice it.
Replying to Kami, I don't believe mirrors work by photon absorbtion/release, as that would just give you a fairly uniformly glowing surface (like for non-reflective surfaces) rather than a crisp image. To get that crisp image you need precise reflection rather than the random directionality that absorbing/releasing would result in.
Replying to Kami, I don't believe mirrors work by photon absorbtion/release, as that would just give you a fairly uniformly glowing surface (like for non-reflective surfaces) rather than a crisp image. To get that crisp image you need precise reflection rather than the random directionality that absorbing/releasing would result in.
Right, when striking a mirror, light is not acting as a bundle of particles, it is acting as a wave.
That is what makes light, and, especially, lasers so unique and problematic. One can speak of the interaction of photons all day, but light does not always behave as if it were a particle.
This is one of the things makes an actual LASER (capitalized as it is an acronym: Light Amplification by Stimulated Emission of Radiation) not a particularly effective weapon. The only energy that EM radiation (Light as a Wave) or, for that matter, the virtually-massless photon would transfer to its target would be thermal energy. LASERs just burn neat little holes in things. A high powered LASER firing fast pulses of focused light is more effective, but still, it burns not-so-neat little holes in things.
A weaponized "laser" would actually more likely be a small linear particle acclerator, firing heavier particles such as neutrons or hydrogen atoms, exoatmospherically, or charged particles such as positrons endoatmospherically (the latter of which being much more destructive, requiring no dwell time on target and producing ancillary gamma radiation).
Not only does such a device transfer thermal energy to the target in the same manner as a laser, but also a good deal of kinetic energy as well, since the particles have a definable mass (The positron, the lightest likely particle, has a mass of approximately 9.11E-31Kg, while the neutron has a mass of 1.67E-27). Now, the heavier particles of course travel slightly less than the speed of light, but this is of little consequence, as the mass of the particles are orders of magnitude greater than that of a photon (assuming photons transfer kinetic energy at all, pesky dual-natured radiation). Thus, if enough particles were being emitted, such a weapon could have a considerable effect against the target, and, quite possibly, felt recoil, depending on the exact mechanics of emission.
That is what makes light, and, especially, lasers so unique and problematic. One can speak of the interaction of photons all day, but light does not always behave as if it were a particle.
This is one of the things makes an actual LASER (capitalized as it is an acronym: Light Amplification by Stimulated Emission of Radiation) not a particularly effective weapon. The only energy that EM radiation (Light as a Wave) or, for that matter, the virtually-massless photon would transfer to its target would be thermal energy. LASERs just burn neat little holes in things. A high powered LASER firing fast pulses of focused light is more effective, but still, it burns not-so-neat little holes in things.
A weaponized "laser" would actually more likely be a small linear particle acclerator, firing heavier particles such as neutrons or hydrogen atoms, exoatmospherically, or charged particles such as positrons endoatmospherically (the latter of which being much more destructive, requiring no dwell time on target and producing ancillary gamma radiation).
Not only does such a device transfer thermal energy to the target in the same manner as a laser, but also a good deal of kinetic energy as well, since the particles have a definable mass (The positron, the lightest likely particle, has a mass of approximately 9.11E-31Kg, while the neutron has a mass of 1.67E-27). Now, the heavier particles of course travel slightly less than the speed of light, but this is of little consequence, as the mass of the particles are orders of magnitude greater than that of a photon (assuming photons transfer kinetic energy at all, pesky dual-natured radiation). Thus, if enough particles were being emitted, such a weapon could have a considerable effect against the target, and, quite possibly, felt recoil, depending on the exact mechanics of emission.
I know what you're thinking of, it's known as a Radiometer. I actually happen to have one, and it spins quite well in the light, but it spins the wrong way. They spin as if the light is pushing harder on the black side than the shiny side. What's actually happening is the the black side heats up some in the sunlight, causing a pressure difference which causes it to spin. Any 'light recoil' would impede this spinning, because in any possible situation the effect of convection is going to be far far stronger than any pressure light could exert.Itkatsu Kiyoko said:
All atoms reflect light in this way, it's a matter of the materials. Something that is structurally rough, like the surface of a towel, will scatter the photons into just a glow. Something structurally smooth and atomically high-density, like plastic, will reduce the randomness of the scattering. Metals are unique in how the share electrons between atoms, which creates a surface which is more 'atomicly smooth' to the incoming light. ..This is hard to explain, and saying it reduces the randomness isn't really true either, it because of how the light particle acts like a wave at the boundery between the materials. To be honest, I don't really understand it completely myself.Itkatsu Kiyoko said:
Actually, laser (uncapitalized as it is not an acronym) is a word which is defined as:Yuuki said:
(It was originally an acronym though)Dictionary said:
On an atomic level, heat and kinetic energy are the same thing, but that's nit-picky, and it still doesn't result in any net 'impact'.Yuuki said:
But hey, all bullets do is poke neat little holes in things right?Yuuki said:
Ah, but the problem there is that it would significantly increase the complexity of the weapon. Not to mention the fact that accelerating any 'massy' particles to anything near light-speed requires far more energy than a a EM-beam of comparable power. Given a relatively unlimited energy supply, of course, particle accelerators are capable of inflicting far more damage.Yuuki said:
Err, back to the nit-picky thing I mentioned before? At an atomic level, thermal energy and kinetic energy are the same thing. The particle impact would generate some 'net' kinetic energy with would be 'felt' as an impact by the target. (using felt figuratively, in that it would be more of an impact than an EM-beam) Like I mentioned just before, there is actually little problem getting 'massy' particles up near light speed, except, of course, for the energy requirements. With current technology, doing anything close to this requires a facility miles long and as much electricity as a small city.Yuuki said:
And here I think this whole issue was avoided out long ago by having Phasers instead of Lasers, oh well..
When talking about practical things such as weapons, it is the macro effect that is what as important, not what occurs on an atomic level. Thermal and Kinetic energy aren't the same thing when used as a weapon, from the target's point of view. I deliberately used the acronym LASER, meaning something that projects focused light, to distinguish it from its newer usage, which is often used to refer to a device that projects about any sort of beam, such as in the third and fourth paragraphs of that post.
Bullets cause a compression wave in objects. They don't just poke neat little holes in things. When a high-powered bullet enters something, for this example, a body, the kinetic energy transferred to the target causes the soft tissue around the wound channel to compress suddenly, creating a large cavity which crushes the surrounding tissues, greatest at the wave's peak. When the wave has passed, the tissues just as suddenly pop back into place, oftentimes causing the explosive rupture and liquefication of the decompressing tissues.
A stream of heavy particles, if enough of them struck the target at the same time, would have the same effect as any other mass. We are talking Newtownian inertia here, not atomic reactions.
This is most pronounced in a soft, liquid-heavy mass such flesh, but it occurs in everything.
I based my previous post mainly on airforce documents relating to the cancellation of the Strategic Defense Initiative (Star Wars).
The reason that we don't use lasers as weapons now isn't because of any technological limitation, it is because lasers don't make good weapons.
Bullets cause a compression wave in objects. They don't just poke neat little holes in things. When a high-powered bullet enters something, for this example, a body, the kinetic energy transferred to the target causes the soft tissue around the wound channel to compress suddenly, creating a large cavity which crushes the surrounding tissues, greatest at the wave's peak. When the wave has passed, the tissues just as suddenly pop back into place, oftentimes causing the explosive rupture and liquefication of the decompressing tissues.
A stream of heavy particles, if enough of them struck the target at the same time, would have the same effect as any other mass. We are talking Newtownian inertia here, not atomic reactions.
This is most pronounced in a soft, liquid-heavy mass such flesh, but it occurs in everything.
I based my previous post mainly on airforce documents relating to the cancellation of the Strategic Defense Initiative (Star Wars).
The reason that we don't use lasers as weapons now isn't because of any technological limitation, it is because lasers don't make good weapons.
I think burning a nice little hole in someone would be really quite effective at killing them off (and potentially causing cancer, since they will be shaking the person apart on an intracellular level) or at least making them experience pain, followed by fear and/or panic and/or psychological 'knockback' (try focusing a (small) magnifying glass on the back of your hand from the sun in the middle of summer; see how you jump?)
Since, unless you're using an anti-tank-strength laser (ie. vaporizing iron), the kickback wouldn't be noticeable even if it was applicable, and an anti-tank laser would have to be large enough for it not to matter, we can safely exclude kickback from the list of things to consider when firing it.
On that note, someone who has spent a lot of time playing shooting games in arcades would be also very good at aiming and firing (rapidly) a laser weapon, since the effect on the shooter is comparable. This would also make gun training sims for new recruits a heck of a lot easier/cheaper/more efficient to produce and run, since they don't have to actually hit things to practice their shooting - on the other hand, solid ammo weapons would still require a shooting range.
Projecting a beam of particles would likely just cause radiation poisoning rather than anything else; and, though I disagree with the 'solar windmill'/'Radiometer'/thingamajig analogy, I think I'm psychic because I was thinking about that around the same time Tiff posted about it.
In order to verify this 'Radiometer', do you have a laser pointer? What happens when you point it at one side or the other?
Like this:
|--------------------<(LASERPOINTER(TM)))
|White/Black
-----------
|Black/White
|
SPINNY!
Since, unless you're using an anti-tank-strength laser (ie. vaporizing iron), the kickback wouldn't be noticeable even if it was applicable, and an anti-tank laser would have to be large enough for it not to matter, we can safely exclude kickback from the list of things to consider when firing it.
On that note, someone who has spent a lot of time playing shooting games in arcades would be also very good at aiming and firing (rapidly) a laser weapon, since the effect on the shooter is comparable. This would also make gun training sims for new recruits a heck of a lot easier/cheaper/more efficient to produce and run, since they don't have to actually hit things to practice their shooting - on the other hand, solid ammo weapons would still require a shooting range.
Projecting a beam of particles would likely just cause radiation poisoning rather than anything else; and, though I disagree with the 'solar windmill'/'Radiometer'/thingamajig analogy, I think I'm psychic because I was thinking about that around the same time Tiff posted about it.
In order to verify this 'Radiometer', do you have a laser pointer? What happens when you point it at one side or the other?
Like this:
|--------------------<(LASERPOINTER(TM)))
|White/Black
-----------
|Black/White
|
SPINNY!
Vesper
Inactive Member
The equation for the momentum, p, of the photon is given by:
p= h/ gamma
where h is h-bar (which is ~equivalent 1.0546e-34 j*s) and gamma is the wavelength of the photon. With photons within the range of 500-700nm (visible light) this results in a imparted momentum in the magnitude of 1e-31, which means only very, very high power lasers would impart anything noticable by a person. But then, said lasers are probably vehicle mounted anyway, due to power and cooling requirments.
[ If you would like a more indepth explanation of this stuff, see here:
http://math.ucr.edu/home/baez/physics/R ... _mass.html
or
http://scienceworld.wolfram.com/physics/Photon.html ]
Also, contrary to popular believe, lasers would not cause a neat, cauterized hole. A weapons grade laser would vapourize the tissue at the impact site, causing a preassure wave on the surrounding tissue. This would cause a small crater on the surface about a magnitude bigger than the beam (so ~20 times the beams diameter). Ideally, the laser would then wait for the fraction of a second needed for the steam cloud to disipate before firing a subsequent pulse so the steam did not act as a buffer to the target. This process would continue until the taret was penetrated (probalby just a fraction of a second).
Since the damage mechanism is mechanical in origin (the steam's preassure) there would be no cauterization, just a messy hole, pretty close to what a gun would cause, except the gore would go out both ways.
p= h/ gamma
where h is h-bar (which is ~equivalent 1.0546e-34 j*s) and gamma is the wavelength of the photon. With photons within the range of 500-700nm (visible light) this results in a imparted momentum in the magnitude of 1e-31, which means only very, very high power lasers would impart anything noticable by a person. But then, said lasers are probably vehicle mounted anyway, due to power and cooling requirments.
[ If you would like a more indepth explanation of this stuff, see here:
http://math.ucr.edu/home/baez/physics/R ... _mass.html
or
http://scienceworld.wolfram.com/physics/Photon.html ]
Also, contrary to popular believe, lasers would not cause a neat, cauterized hole. A weapons grade laser would vapourize the tissue at the impact site, causing a preassure wave on the surrounding tissue. This would cause a small crater on the surface about a magnitude bigger than the beam (so ~20 times the beams diameter). Ideally, the laser would then wait for the fraction of a second needed for the steam cloud to disipate before firing a subsequent pulse so the steam did not act as a buffer to the target. This process would continue until the taret was penetrated (probalby just a fraction of a second).
Since the damage mechanism is mechanical in origin (the steam's preassure) there would be no cauterization, just a messy hole, pretty close to what a gun would cause, except the gore would go out both ways.
Legix
Well-Known Member
... So do laser weaponry in the setting have recoil? I figure no, but I can see "cinematic effect" being used for if they do. This is one lengthy talk that really doesn't answer the question posed... it just seems a lot like people arguing theories and possible science behind it.
Navian
Inactive Member
I would sooner go with there being two types of weapons. Plasma weapons shoot physical projectiles (hot gas) so they may have recoil, lasers not so much, but they probably look similar (pew pew beams of colour, since invisible lasers are boring in anime.)
Pulse lasers make explosions on the surface of the target, beam lasers carve holes, and the really good weapons can switch modes, so you don't have to be stuck with the version that's less fun that day.
Pulse lasers make explosions on the surface of the target, beam lasers carve holes, and the really good weapons can switch modes, so you don't have to be stuck with the version that's less fun that day.
