# Does a laser have a recoil

ssope
Does a laser have a recoil since photons have momentum. Would a moving laser be slowed down (even a fraction) if was locked in the on position and then thrown at the ground

## Answers and Replies

Science Advisor
Gold Member
The laser does indeed have recoil; however, the momentum of the particles p=h/lambda isn't very much (h is a very small number).

The laser would be slowed a very small fraction. (very small!)

Frame Dragger
The laser does indeed have recoil; however, the momentum of the particles p=h/lambda isn't very much (h is a very small number).

The laser would be slowed a very small fraction. (very small!)

I think the only time I've really seen the concept of laser recoil explored outside of curiosity, was by Larry Niven who used the concept of humans employing stardrives that are essentially lasers (probably in practice ion drives, but that's sci fi for yah!). Even in that framework they were devestatingly powerful to create thrust with low accelleration.

Science Advisor
Gold Member
I mean, we can work out some math.

p=h/lambda=E/c

This laser I have at home has <30 miliwatts of power. So, each second, my laser's momentum flow is only about 1*10^-10 kgm/s. Which isn't enough to make much of a difference. To get me, a 60kg person, moving at 10 m/s, I would have to be in perfectly frictionless space and be pushed by that laser for 190,000 years.

Now, there are much larger lasers in the world. I am only talking about those hand held lasers.

Frame Dragger
I mean, we can work out some math.

p=h/lambda=E/c

This laser I have at home has <30 miliwatts of power. So, each second, my laser's momentum flow is only about 1*10^-10 kgm/s. Which isn't enough to make much of a difference. To get me, a 60kg person, moving at 10 m/s, I would have to be in perfectly frictionless space and be pushed by that laser for 190,000 years.

Now, there are much larger lasers in the world. I am only talking about those hand held lasers.

Of course, in that perfect frictionless space the 'drive plume' would be a MASSIVELY powerful and destrutive laser with a scary range. "Pardon me, you seem to have been roasted alive by my exhaust! How shockingly impolite." Not the best way to make an impression on any potential aliens or distant (Differentially Aged) ancestors.

Larry Niven employed the drive concept so that a future human civilization which had achieved near-perfect peace, could have devestating weapons as an inherent part of their means of interstellar transportation without considering those ramifications. Ahhh... I remember when I first read good sci-fi... so interesting.

Science Advisor
Gold Member
By my calculations, it would take a 14.7 trillion watt laser to accelerate a 5 ton payload at 1g...

At 1g it would take about half a year to go .5c (I didn't want to mess with relativity, so I keep gamma small.)

Frame Dragger
By my calculations, it would take a 14.7 trillion watt laser to accelerate a 5 ton payload at 1g...

At 1g it would take about half a year to go .5c (I didn't want to mess with relativity, so I keep gamma small.)

Just to be totally clear, this is science fiction I was talking about, and not hard scifi either. I was just appreciating how I viewed the universe at the time of reading that novel, and now.

14.7 trillion watts. Hmmm... this same author posited a laser using the coronal envelope of a sun and a fine manipulation of magnetic field lines to produce a massive weapon on a stellar scale. Not a guy concerned with practicality in his fiction, but with an interesting vision of social order over a massive inhabited surface area limited to sub-FTL travel and coms. Ringworld really was an interesting study in scale.

EDIT: Of course, given the use of fusion and the size of these ships in the novels, maybe tera or petawatt scale laser from multiple fusion plants wouldn't be beyond belief. I did a bit of checking... in the context of the novel in which this concept was introduced, it's a "photon drive". So, not a laser in its usual configuration, but made into one by an ingenious crewmember as a last-ditch means of self defense.

Now, if you took that kind of power and used it to for simple ion drives of current design imagine the possiblities (massive particles being a better reaction mass than the momentum of photons). I'm still shocked that we haven't slapped a gen-III reactor on a probe and really started to explore our solar system like we mean it. Ah well.

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Science Advisor
Gold Member
It's an interesting concept. They have also concepts involving mirrors that would allow the light to do more "work" as it bounced bank and forth between the ship and a stationary mirror.

The strongest laser that I know of is at the NIF. From 192 laser beams, they want to beam 500terrawatts of power over a few picoseconds to create fusion. (In total releasing several million joules of energy, I believe.)This requires, at current, football fields worth of laser generators. XD

But at least the power is there...they would just have figure out a way to keep it on for longer than several picoseconds! hehe

hamster143
Thrust to power ratio is inversely proportional to exhaust speed. Any rocket scientist would tell you that. Lasers have huge exhaust speed (300,000 m/s), therefore very low thrust per unit of energy. Turboprops have modest exhaust speeds, therefore they are quite useful for propulsion. The real reason to go for lasers appears when you're already moving extremely fast to begin with, say, if you want to accelerate beyond 0.5c ... then lasers, and, more generally, photon engines are the way to go.

Frame Dragger
Thrust to power ratio is inversely proportional to exhaust speed. Any rocket scientist would tell you that. Lasers have huge exhaust speed (300,000 m/s), therefore very low thrust per unit of energy. Turboprops have modest exhaust speeds, therefore they are quite useful for propulsion. The real reason to go for lasers appears when you're already moving extremely fast to begin with, say, if you want to accelerate beyond 0.5c ... then lasers, and, more generally, photon engines are the way to go.

Ahhhh, well in the storyline of these novels, at this time the starships in question were sublight, but could be subject to Differential Aging and such... I may be forgetting a primary form of thruster, after which a photon drive would take over.

As interesting as all of these concepts are however, when it comes down to it who wants to ever be subject to (large scale) DA? If the nature of the universe is ultimately and finally that there are no FTL methods of travel or communication (as it seems to be), then a near-c vessle would have to be for research for a civilization taking the long view, but more likely as a means of colonization. It's hard to imagine that pockets of humanity seperated by distance and time would maintain any kind of cohesion.

Nope... I'll take my lasers in my pointers, interferometers, computer mice, etc... No worries about recoil or relativistic effects... unless you count frying your retinas in some self-destructive fit.

EDIT: I added 'computer' to 'mice' just in case some crazed MIT grad is planning to create laser-(animal)mice. I don't need that on my head. :tongue2:

ssope
Is the recoil of a laser precisely planck's constant divided by wavelength?

Science Advisor
Gold Member
Is the recoil of a laser precisely planck's constant divided by wavelength?

The momentum of a photon is planck's constant divided by wavelength. You can see that from: E=pc=hf which gives p=hf/c=h/lambda.

I don't know how you want to define "recoil" tho...

LostConjugate
The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?

Frame Dragger
The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?

So... by your estimation a 50 Watt laser pointer experinces a 'kick' of about 5 Kg per second? I'm just throwing out a guess that such can't be correct, for reasons discussed earlier in the thread.

Science Advisor
a 50 Watt laser pointer

WOW ... where can I buy one of those?

Science Advisor
The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?

Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.

Science Advisor
The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?

Ummm ... not sure how you got there.

1) *IF* the total energy in the beam were carried as kinetic energy by massive bodies, then Newton's third law would apply and you could analyze the problem in such terms. But as has already been posted, the momentum carried by a photon is given by p=h/lambda, while the energy is given by E=pc = h x frequency. So the beam carries a lot of energy per unit momentum.

2) How did you arrive at those units? Power is energy/time, energy (work) is force x distance ... how does the distance factor in to your estimate?

Frame Dragger
WOW ... where can I buy one of those?

I could have said 'CO2 laser in a lab', or 'CNC Machine', but I didn't think that someone who imagined a laser that could toss you across a room (imagine lasers at the NIF, or MW Chemical Lasers! Why that Boeing would explode! :rofl: ) was going to appreciate that.

Science Advisor
Gold Member
Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.

Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.

Frame Dragger
Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.

Most pen lasers and diode lasers for use in read/write applications perhaps. If you want engraving you're talking about 60-100 Watts, and a good CO2 laser definitely can pump 100 Watts.

Then again, the fact that these lasers ablate metals, engrave wood, plastic, etc... makes the case for a laser of 1watt or more being quite strong!

If I remember correctly, a 3mW pointer can pop a balloon, which is near if the not THE upper limit on green laser pointers. I never quite understood the need for a handheld device that can 'point' to the bloody horizon, and fry someone's retinas at the same time. *shrug*.

I would love to see some fellow in a pinstripe suit pull out an uncased CO2 laser to 'point'. "Um, yes folks, as you can see from the trail of burning projector-backing, and WALL... we're trending up this quarter!" :rofl:

....And of course, if that was a 1 newton/s/watt recoil, you could use a 100watt CO2 laser like baseball bat of light. VERY sci-fi, emphasis on the FICTION.

I still want to know how he came to 1N/s...

Science Advisor
Gold Member
Yes, I'm talking about those laser pointers. Sorry if "conventional" wasn't the correct word to use. I have a 30mW laser, and it will point (at night of course) pretty much across town...

In order to go from Watts to N/s one would have to divide by meters...there is to reason to divide the power by 1 meter tho...

Frame Dragger
Yes, I'm talking about those laser pointers. Sorry if "conventional" wasn't the correct word to use. I have a 30mW laser, and it will point (at night of course) pretty much across town...

In order to go from Watts to N/s one would have to divide by meters...there is to reason to divide the power by 1 meter tho...

30mW POINTER?! Heh... You have good toys. So... has it ever had any noticable 'kick' when you use it to perform home LASIK surgery? :rofl:

Just give in and buy a 60 watt laser engraver, and take out the laser assembly...

There was a great comic called Bloom County; in it a young boy (scientist) name Oliver shows up at school for his science project. His professor asks him what he made, and Oliver points to a large object and says, "A thermonuclear bomb". The professor asks, "And where did you get the fissile material?" Oliver replies, "I scraped the glowing stuff off a million Micky Mouse watch hands." -A pause...- the professor turns to the room and claps, "Ok, fire drill everyone!!!"

:rofl:

Ok, so micky mouse watches don't have tritium, and that's not fissile material, but sometimes these ideas are too fun to leave alone. Laser pointers that can spot weld are clearly the next logical concept.

LostConjugate
Ok I guess the correct term is 1 Watt is 1 Newton Meters per second.

So if you pointed a 1-Watt laser at a mirror the mirror should experience a force of 1 Newton Meter per second, so by conservation of energy the kick in the laser would be the same.

Or how about this. 1 Watt laser would be 1 Joule per second kick

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Frame Dragger
Ok I guess the correct term is 1 Watt is 1 Newton Meters per second.

So if you pointed a 1-Watt laser at a mirror the mirror should experience a force of 1 Newton Meter per second, so by conservation of energy the kick in the laser would be the same.

Or how about this. 1 Watt laser would be 1 Joule per second kick

Did you just not read SpectraCat's post?:

SpectraCat said:
LostConjugate said:
The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?

Ummm ... not sure how you got there.

1) *IF* the total energy in the beam were carried as kinetic energy by massive bodies, then Newton's third law would apply and you could analyze the problem in such terms. But as has already been posted, the momentum carried by a photon is given by p=h/lambda, while the energy is given by E=pc = h x frequency. So the beam carries a lot of energy per unit momentum.

2) How did you arrive at those units? Power is energy/time, energy (work) is force x distance ... how does the distance factor in to your estimate?

Then, you probably just didn't read Matterwave's post where he actually GIVES THE UNITS in NEWTONS. Here, I'll refresh your memory.

Matterwave said:
Sylas said:
Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.

Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.

You're (LostConjugate) just throwing out words and concepts with no regard at all, and then you don't bother to read the answers.

LostConjugate
It does not make sense to have the 1/c term in there, c is imbedded in the laser being 1 Watt. If you shine 1 Watt of light off a reflective disc in space it should gain 1 joule per second of kinetic energy, which means you would get a 1 joule kick per second from shining the laser.

Cat was right, I had the wrong units, I meant joules.

Science Advisor
It does not make sense to have the 1/c term in there, c is imbedded in the laser being 1 Watt. If you shine 1 Watt of light off a reflective disc in space it should gain 1 joule per second of kinetic energy, which means you would get a 1 joule kick per second from shining the laser.

Why on earth do you think that? What is the kinetic energy of a photon? I already told you that you can not think of photons as massive bodies that transfer kinetic energy to their targets ... do you think that I am wrong? Photons have momentum ... I gave you the formula for that too .. p=E/c ... that is why matterwave correctly said you needed to divide by c.

Also, if the object is reflective, then most of the energy will propagate in a different direction after the reflection ... so all of the energy is not absorbed by the target. How do you account for that?

Frame Dragger
Why on earth do you think that? What is the kinetic energy of a photon? I already told you that you can not think of photons as massive bodies that transfer kinetic energy to their targets ... do you think that I am wrong? Photons have momentum ... I gave you the formula for that too .. p=E/c ... that is why matterwave correctly said you needed to divide by c.

Also, if the object is reflective, then most of the energy will propagate in a different direction after the reflection ... so all of the energy is not absorbed by the target. How do you account for that?

I think it's time to step back and redirect this fellow to the general physics forum and not this one.

LostConjugate
Why on earth do you think that? What is the kinetic energy of a photon? I already told you that you can not think of photons as massive bodies that transfer kinetic energy to their targets ... do you think that I am wrong? Photons have momentum ... I gave you the formula for that too .. p=E/c ... that is why matterwave correctly said you needed to divide by c.

Also, if the object is reflective, then most of the energy will propagate in a different direction after the reflection ... so all of the energy is not absorbed by the target. How do you account for that?

If you shine the laser at a mirror without any angle it reflects perfect. I don't care about the speed of light. There is 1 Joule of force per second coming from the light, traveling at speed c, hence, 1 Watt Laser.

Science Advisor
Gold Member
30mW POINTER?! Heh... You have good toys. So... has it ever had any noticable 'kick' when you use it to perform home LASIK surgery? :rofl:

Just give in and buy a 60 watt laser engraver, and take out the laser assembly...

There was a great comic called Bloom County; in it a young boy (scientist) name Oliver shows up at school for his science project. His professor asks him what he made, and Oliver points to a large object and says, "A thermonuclear bomb". The professor asks, "And where did you get the fissile material?" Oliver replies, "I scraped the glowing stuff off a million Micky Mouse watch hands." -A pause...- the professor turns to the room and claps, "Ok, fire drill everyone!!!"

:rofl:

Ok, so micky mouse watches don't have tritium, and that's not fissile material, but sometimes these ideas are too fun to leave alone. Laser pointers that can spot weld are clearly the next logical concept.

lol!

I think some of those old glow in the dark stuff actually had Uranium in it...although, it wouldn't be weapons grade uranium but still...maybe the kid was real smart and found a way to enrich it XD. The fusion-able material needed (Deuterium - .015% abundance, and tritium - unstable) is easier to get than the fissile material...not easy by any means, but if the kid was able to enrich the uranium from glow in the dark watch hands, maybe he had something up his sleeve!

Anyways, @Lost Conjugate, the "kick" should be a force (which is measured in Newtons, not Joules!) and not some energy/time deal you have going on. What you are saying though is that a 1 Watt laser will get a 1kg block to 1 m/s in half a second (K=.5mv^2)...which is obviously not true. If you hold this view, my 30mW (.03W) laser should be able to accelerate a paperclip (1g) to something like 8m/s in 1 second! I've never witnessed my laser pointer moving anything like that haha. Although having such a laser pointer would be the epitome of cool...coolness does not equal fact.

Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.

LostConjugate
Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.

However if you measured the energy from a light beam over 1 second and it came out to be 1 Watt, all that energy would be measured in force.

Also p =E/c but the light is traveling at c, you not measuring the force of a single photon, your measuring over a crap load of photons traveling at c for 1 second.

Frame Dragger
lol!

I think some of those old glow in the dark stuff actually had Uranium in it...although, it wouldn't be weapons grade uranium but still...maybe the kid was real smart and found a way to enrich it XD. The fusion-able material needed (Deuterium - .015% abundance, and tritium - unstable) is easier to get than the fissile material...not easy by any means, but if the kid was able to enrich the uranium from glow in the dark watch hands, maybe he had something up his sleeve!

Anyways, @Lost Conjugate, the "kick" should be a force (which is measured in Newtons, not Joules!) and not some energy/time deal you have going on. What you are saying though is that a 1 Watt laser will get a 1kg block to 1 m/s in half a second (K=.5mv^2)...which is obviously not true. If you hold this view, my 30mW (.03W) laser should be able to accelerate a paperclip (1g) to something like 8m/s in 1 second! I've never witnessed my laser pointer moving anything like that haha. Although having such a laser pointer would be the epitome of cool...coolness does not equal fact.

Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.

Actually I think the joke was in reference to the tritium that is used on high end watches (still used), not to mention ACOG and other high-end gun sights. Of course, mickey mouse watches do NOT use tritium. :rofl: The idea was that he was going to make a real mutliple stage weapon a la Teller-Ulam! Good stuff. The tritium would fuse with deuterium... of course now this role is filled with the single Lithium-Deuteride. *shrug* a mushroom cloud is a mushroom cloud.

As for the laser pointer, it would be a coherent BEAM! So... if you swept it across a room it should act like a giant bat in that scenario. I think LostConjugate might have been thinking of the machines the Ghostbusters use.

Frame Dragger
However if you measured the energy from a light beam over 1 second and it came out to be 1 Watt, all that energy would be measured in force.

Also p =E/c but the light is traveling at c, you not measuring the force of a single photon, your measuring over a crap load of photons traveling at c for 1 second.

This problem isn't as simple as you seem to believe it is.

EDIT: I take that back, it IS simple, but you're adding needless complications with this magical kinetic quanta.

LostConjugate
well I might be wrong, but I that was just my opinion seems like a laser measured at 1Watt would give a return kick of 1Watt unless the energy was somehow dispersed as heat which I dont think it is.

Science Advisor
Gold Member
Actually I think the joke was in reference to the tritium that is used on high end watches (still used), not to mention ACOG and other high-end gun sights. Of course, mickey mouse watches do NOT use tritium. :rofl: The idea was that he was going to make a real mutliple stage weapon a la Teller-Ulam! Good stuff. The tritium would fuse with deuterium... of course now this role is filled with the single Lithium-Deuteride. *shrug* a mushroom cloud is a mushroom cloud.

As for the laser pointer, it would be a coherent BEAM! So... if you swept it across a room it should act like a giant bat in that scenario. I think LostConjugate might have been thinking of the machines the Ghostbusters use.

Ah...I didn't know some watches used tritium. I just know that old school glow in the dark used to be Uranium. My chem teacher once brought in some plates with special paint and moved a Geiger counter over them and showed us they were radioactive. XD

@LostConjugate:
Force is measured in Newtons, Energy is measured in Joules, and Power is measured in Watts which is Joules/second. You can't express force in units of Power!

Indeed most of the energy of the laser beam is scattered off, and what isn't scattered is absorbed as heat (heating up the material!). Only a tiny fraction of the energy goes to movement of the material itself (rather than movement of constituent electrons) This is why some lasers are used to cut stuff by melting through them (a la Goldfinger and that infamous moment "No, Mr. Bond, I expect you to DIE!" - one of my faves)

Frame Dragger
well I might be wrong, but I that was just my opinion seems like a laser measured at 1Watt would give a return kick of 1Watt unless the energy was somehow dispersed as heat which I dont think it is.

This isn't really something that's open to opinion. We're trying to tell you that if you were right, industrial and laboratory lasers would have phenomenal recoil.

In fact, for a non-refractive surface it IS dissipated as heat, and likewise during the diffraction process. You can melt your mirrors, but you won't shatter them from an impact.