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Does a laser have a recoil

  1. Feb 19, 2010 #1
    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
  2. jcsd
  3. Feb 19, 2010 #2


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    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!)
  4. Feb 20, 2010 #3
    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.
  5. Feb 20, 2010 #4


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    I mean, we can work out some math.


    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.
  6. Feb 20, 2010 #5
    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. :wink:

    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. :smile:
  7. Feb 20, 2010 #6


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    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.)
  8. Feb 20, 2010 #7
    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.
    Last edited: Feb 20, 2010
  9. Feb 20, 2010 #8


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    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
  10. Feb 20, 2010 #9
    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.
  11. Feb 20, 2010 #10
    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:
  12. Feb 21, 2010 #11
    Is the recoil of a laser precisely planck's constant divided by wavelength?
  13. Feb 21, 2010 #12


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    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...
  14. Feb 22, 2010 #13
    The recoil is the force on the laser device. Wouldn't a laser at 1 Watt experience a force of 1 Newton per second?
  15. Feb 22, 2010 #14
    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.
  16. Feb 22, 2010 #15


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    WOW ... where can I buy one of those?
  17. Feb 22, 2010 #16


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    Force is in Newtons, not Newtons per second. The force from a 1 Watt laser is 1/c Newtons.
  18. Feb 22, 2010 #17


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    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?
  19. Feb 22, 2010 #18
    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.
  20. Feb 22, 2010 #19


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    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.
  21. Feb 23, 2010 #20
    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. :smile:

    I still want to know how he came to 1N/s...
  22. Feb 23, 2010 #21


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    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...
  23. Feb 23, 2010 #22
    30mW POINTER?! Heh... You have good toys. :wink: 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... :smile:

    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!!!"


    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. :smile:
  24. Feb 23, 2010 #23
    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
    Last edited: Feb 23, 2010
  25. Feb 23, 2010 #24
    Did you just not read SpectraCat's post?:

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

    You're (LostConjugate) just throwing out words and concepts with no regard at all, and then you don't bother to read the answers.
  26. Feb 23, 2010 #25
    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.
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