Pages

Thursday, April 8, 2010

Would a laser gun recoil?

Today I'd like to approach a question near and dear to many a geek heart: do laser guns have recoil?

Let's motivate our question a little bit.  I've wondered about this question since I saw star wars.  Though I'm no firearms expert, the recoil in guns must come from conservation of momentum principles. Momentum is conserved in a system. The gun starts with zero momentum. We fire, give the bullet momentum, and so to keep the system at zero momentum, the gun must gain equal and opposite momentum. That is, the gun will move backwards.

All of that was for conventional guns. Light carries momentum, so if we fire a pulse of light, we expect our laser gun to recoil. So yes, they do have recoil. Satisfied, dear readers? Neither am I. The question we really mean to ask is, does a laser gun have noticeable recoil?



We need to make a few reasonable assumptions. Let's assume that the laser gun fires a pulse with as much energy as a bullet has kinetic energy, KE. The energy, E, of light is related to its momentum, p, by E=pc, where c is the speed of light. This gives a momentum of

\[ E=KE=pc \]
\[ p=\frac{KE}{c}\]

What is the kinetic energy of a bullet? A little searching reveals that a .22 bullet is ~2.5g and fires with a muzzle velocity of ~330m/s. Kinetic energy is given by KE=1/2mv^2, where m is mass and v velocity. So, the momentum of a laser pulse with equal energy would be

\[ p=\frac{mv^2}{2c} \]
\[ p=\frac{.0025kg*(330m/s)^2}{2*3\cdot 10^8m/s} \]
\[ p=4.5\cdot10^{-7}kg \cdot m/s \]

For comparison, the momentum (p=mv) of a .22 bullet is .83 kg*m/s. The momentum of a laser gun is 2 million times less than the momentum of a .22. But is momentum all we should consider?  I suspect the 'kick' we feel on the recoil is directly related to the force that the gun exerts on the holder.  This means that instead of momentum we need to consider impulse, momentum per time.  We estimate the time it takes to fire a .22 is ~.1s, so the force delivered 8.3 N.

Let's estimate the time it takes a laser gun to fire.  Unfortunately, not having a laser gun to fire (feel free to send me one, dear readers), we're more or less going to have to guess at the firing time.  Most movies with laser guns show pulses of light (which, incidentally would move so fast we wouldn't see them) on the order of a meter or two long.  Given the speed of light, this would give a firing time of ~30 nanoseconds.  This would give a force delivered of 15 N.  This is close to what we estimated or a .22.  So, if movies are to be believed (and really, why wouldn't we believe them?), it seems like laser guns may well have recoil.

Note:  It is worth questioning if we need the same energy for a laser as for a bullet.  That could certainly change our estimate.  Maybe we'll return to this question again.

72 comments:

  1. But a force of 15N applied for 30ns would hardly be noticed by the user.

    ReplyDelete
  2. Isn't 30ns about 30 light feet or maybe 8 meters, not one or two meters. The first Star Wars movies actually had shorter pulses, maybe a foot or two long.

    ReplyDelete
  3. Kaleberg:
    You're absolutely right, 30ns gives 9m. As for the pulses . . . I actually did a little research (I'm such a geek!), the hand held blasters in the star wars movies don't shot light, they shoot some kind of directed plasma. Only the starships shoot actual laser beams (along with some of the defensive emplacements, like on Hoth). I don't know the size of the starships, but 10m seemed about right. As the image above shows, the lasers from ships were on the order of the ship size! That's the long answer. I did so much research both for fun, and because I figured someone would call me on the hand-held blasters otherwise.


    Devin,
    I've really got no idea how you would feel a force applied over such a short time scale. It's completely out of my realm of experience! You could well be right. I'll let you know when I find out ;)

    ReplyDelete
  4. I always assumed (because it was convenient to me) that the cannons expelled massy particles and then propelled/continued to elevate their energy level by way of laser. This also allows Star Wars fans to overcome the standard Star Trek defense that craft in Star Trek have mirrored hulls and thus deflect lasers.

    Of course, this creates a requirement that the bore-sighted laser stays perfectly aligned as the gun carriage recoils, less it cause the massive slug to dissipate.

    ReplyDelete
  5. Nato,
    I did a little research on this. Not much. What I determined is that it looks like the hand-held guns expel massive particles, while the ship-to-ship weaponry is actually laser beams. Of course, I make no claim to be an expert in this area.

    ReplyDelete
  6. calculate how far the gun would hve accelerated (s = 1/2at^2) for the duration. this should give a better indication of how noticeable the recoil is

    ReplyDelete
  7. An interesting idea. We'll use Matt's estimate of 5kg for our laser gun mass. This would give a distance traveled of
    \[d=\frac{15N(30ns)^2}{2(5kg)}=1.35\cdot 10^{-15}m\]

    This is a miniscule distance (about a nuclear diameter), so if this estimate is correct, we would certainly not notice the recoil.

    ReplyDelete
  8. addtionally a laser is very very hot so when you shoot your laser gun the air at muzzle break will also get very very hot so the expanse of air could also give the appearance and feel of recoil

    ReplyDelete
  9. I've done some research on realistic man-portable laser weapons and photon rockets. A photon rocket is a rocket whose propellant is photons- light. They have the highest possible exhaust velocity, so they have a very high specific impulse. Specific impulse is the measure of how much change in velocity a rocket gets for every gram of propellant it uses. Since light carries momentum, photon rockets and light sails work. A spaceship can be propelled by light. However, thrust is very low. To obtain one newton of thrust, a photon rocket would need to output 300 freaking megawatts!! The spacecraft would be spewing out a freaking super laser beam and riding the recoil. We don't have such rockets today- for obvious reasons. You could make nuclear fission powered photon rocket, but the thrust would be so miniscule you would not notice it. You might sit around for years in orbit before you even notice a change in your speed. Obviously, there is no way a handheld laser gun would ever output enough power to have noticeable. Ever.

    Real man portable laser weapons won't resemble star wars blasters. The gun will have a lens or mirror in the beam emitter- probably several centimeters across to avoid being damaged by the super- intense light. The heat from the beam will cause the water in flesh to bloom explosively into steam, ripping flesh apart and causing third and second degree burns over much of the surrounding tissue. A pulsed laser action would allow the laser to drill deeply and reach vital organs. A variant in which several beams were fired slightly off- axis of each other would leave a wound much like a knife wound, or possibly cut a human in half. No laser will have noticeable recoil. Lasers move at lightspeed and strike instantly. In a vacuum, lasers are invisible. In air, the beam might leave a visible trail due to atmosphere induced scatter, ionization, and bits of debris burning under the irradiation of the beam.

    -Cristopher Phoenix

    ReplyDelete
  10. I am learning about all sorts of waves now in physics 3, including electromagnetic waves and one of my homework questions is whether or not someone who turns on a flashlight experiences recoil due to the light having momentum and why it is not observed. It would make sense for light to cause a recoil because one knows light has a speed and would therefore have a momentum and with a momentum, p=mv^2 proves it has a mass.

    However, in my opinion, as counter intuitive as it may seem, I believe light, and laser beams in that extent, do not cause recoil. Light is composed of photons, which according to modern physics have no mass. This is because the modern definition of mass depends on the object in question at rest and photons are never at rest and therefore, never have mass (mind boggling for me).

    I am a chemist undergraduate and I do research and experiments with lasers and I know that a laser primarily heats up materials due to excitation of the molecules and breaking down of bonds. This is caused by the nature of the wavelength of lasers. A neat trick is that if you are working with a laser, lets say a red one, and you have a red shirt of the exact same wavelength, then it will not burn. Any other color would become singed almost immediately (depending on the wavelength of light versus shirt and its strength).

    Hence, assuming modern physics is correct, photons have no mass and therefore, no momentum, but they do have velocity, which allows them to move through mediums. As well, they react with their environment due to their wavelength. So, a laser gun would likely have no recoil. Unless of course I have some sort of flaw in my logic, which I most likely do.

    -Johnny boy

    ReplyDelete
    Replies
    1. Sorry Johnny, photons have no REST mass. That photons in motion have mass is proven by the gravitational lens effect that allows astronomers to detect black holes.

      Delete
  11. Johnny,

    Here's the issue with what you said. While it is true that if something has mass and velocity it has momentum, the converse is not true. That is, if something has momentum, it is not required to have mass and velocity. Photons are proof of that. We've shown this experimentally (we being the general physics community).

    However, the momentum of light is quite small (as I mention in the post), so that is why we don't tend to notice it.

    ReplyDelete
  12. By my reckoning, you divided by two where it was not necessary:

    .5(m1)(v1)(v1) = KE = .5(m2)(v2)(v2)

    p2 = (m2)(v2)

    p2 = KE2/v2/.5

    p2 = .5(m2)(v2)(v2)/(v2)/.5

    Or

    p2 = KE1/v2/.5

    p2 = .5(m1)(v1)(v1)/(v2)/.5

    Cancel

    p2 = m1(v1)(v1)/v2

    In other words, you're off by a factor of two.

    Also, you should probably consider that the bullet continues to accelerate even after it exits the barrel of the gun (this varies in distance), which means variable impulse.

    ReplyDelete
  13. Am I the only one who objects with using a 2.5g .22 at 330m/s, when a 9.7g 7.62x51 at 850m/s would be more suitable?

    .22 is a varmint round, noone walks into combat with that caliber if they can help it. 7.62 is considered a full power combat round.

    ReplyDelete
  14. Heya¡­my very first comment on your site. ,I have been reading your blog for a while and thought I would completely pop in and drop a friendly note. . It is great stuff indeed. I also wanted to ask..is there a way to subscribe to your site via email?







    Ghid de lumini laser

    ReplyDelete
  15. The equation seems to be very difficult for a layman

    ReplyDelete
  16. Just one thing: unless I am mistaken, blasters in the Star Wars universe don't fire lasers at all -- they're actually (well, supposedly) firing clumps of charged gas or something like that. Ergo, you get your recoil.

    My source is The Star Wars Sourcebook, published by West End Games in 1987. I'll verify that once I am able, but if someone beats me to it...

    ReplyDelete
  17. I like your post. This post really awesome and very helpful to me. Please keep posting good contents. Thank you.
    laser tag rental in Iowa

    ReplyDelete
  18. This comment has been removed by the author.

    ReplyDelete
  19. question: would the Spartan Laser from Halo have recoil?

    ReplyDelete
  20. when you're propagating over long ranges through atmospheric turbulence..best laser tag sets

    ReplyDelete
  21. Well i dont know what the equations mean but it was an interesting read anyway.

    ReplyDelete
  22. People tired to find a relevant place where they can know real facts and myths about the topic mentioned by author of this blog. I want to help a lot of needy people through this blog to come up at my blog to know the real facts and myths regarding this topic. ป้าย ระยอง

    ReplyDelete

  23. Interesting post. I work out almost everyday too. Really enjoyed this article too.laser cutting service london

    ReplyDelete
  24. Thank you for helping people get the information they need. Great stuff as usual. Keep up the great work!!! Best Laser Tag Guns

    ReplyDelete
  25. A delightful perusing for any individual who adores perusing online journals.
    best-laser-tag-guns

    ReplyDelete
  26. After perusing the blog compositions, an otherworldly connection is made amongst peruser and essayist.
    best-laser-tag-guns

    ReplyDelete
  27. This comment has been removed by the author.

    ReplyDelete
  28. If you want to understand how laser gun can recoil just go to any lasert tag operator and try their
    laser tag equipment.
    You will understand that laser gun itself can't recoil. It can recoil only if it has vibromotor inside it, lol.

    ReplyDelete
  29. When you use a genuine service, you will be able to provide instructions, share materials and choose the formatting style. glock 19 sights

    ReplyDelete
  30. A bullet round moves with a single event. The explosion. The laser is a continuous impulse so the numbers really should not involve 30ns. It should take into account the number of photons being emitted and the amount of time to leave to laser active to the point where the photon kinetic energy equals a bullet discharge. It depends on the grains in the bullet and the power of the laser as well

    ReplyDelete
  31. Thanks for sharing nice post...

    Looking for the best Glock laser, Pistol Laser and Glock accessories? Viridian, the leader in Glock laser and Glock accessories at factory outlet prices. Buy direct and save.


    Glock laser
    Glock Accessories
    Best Glock Accessories
    Pistol laser
    Glock 19 laser
    Glock 26 gen 5 laser sight
    Glock 42 laser
    Glock 43 laser
    Glock light
    Weapon Lights

    ReplyDelete






  32. Thanks for sharing nice post..

    Looking for the best Ruger laser, Pistol Laser and Ruger accessories? Viridian, the leader in Ruger laser and Ruger accessories at factory outlet prices. Buy direct and save.

    Ruger LCP Laser
    Laser for Ruger LCP 2
    Ruger LCP
    Ruger LCP 2
    Ruger LC9 Laser
    Ruger LC9S Laser
    Ruger AR 556 Accessories
    Ruger LC9S Accessories
    Best Laser Sight for Ruger LCP
    Ruger LCP Light

    ReplyDelete
  33. Hello, my name is Huynh and I am a full-time online marketer and more, I help, guide and inform people to make sure they are making the right choice when choosing any product carefully, digital number. The recommendations I make are genuine and the owner and members of this site. I hope this helped you in your decision to find out if this is for you.

    Best Prebuilt Gaming PC Under $1000
    Hello, my name is Huynh and I am a full-time online marketer and more, I help, guide and inform people to make sure they are making the right choice when choosing any product carefully, digital number. The recommendations I make are genuine and the owner and members of this site. I hope this helped you in your decision to find out if this is for you.

    5MP 8Channel

    ReplyDelete
  34. Thanks for nice post...

    Looking for Police camera equipment, Police video surveillance that captures Officer-involved shooting? Gun Cameras, the leader in Police camera equipment and Police video surveillance to capture Officer-involved shooting. Buy direct and save.

    Police Body Camera Cost
    Police Camera Equipment
    BWC Police
    Bodycam Police
    Police Video Surveillance
    Cameras on Police Officers
    Checking Police
    Police Shooting Statistics
    Officer-Involved Shooting
    Police Accountability

    ReplyDelete
  35. If you choose to bet on one place and it is correct, you will be paid up to 11 times. But some of you may be in accordance with the rhythm of multiple bills or predict multiple results in one bet, the money will be less, followed by the fruit game. Easy isc888 สมัคร to play, pay for real, because we are a website that normally is a direct website, does not go through, and is the best online forum in Thailand.

    ReplyDelete
  36. Thanks for sharing this post. It is really informative for me.
    See More

    ReplyDelete
  37. This comment has been removed by the author.

    ReplyDelete
  38. This comment has been removed by the author.

    ReplyDelete
  39. M88 เว็บพนันออนไลน์ ที่มีเกมเดิมพันมากมาย ไม่ว่าจะเป็น บาคาร่าออนไลน์ คาสิโนออนไลน์ สล็อตออนไลน์ หวยล็อตโต้ เตรียมพร้อมบริการทุกท่านแล้ววันนี้

    ReplyDelete
  40. confident that technology is paving the way for a safe comeback to live events. virtual edge and thank you for your accompany

    ReplyDelete
  41. thanks for sharing this post, Get a superior quality Swimming pool for kids to give a source of playing in your home comfort. Connect with me to receive more information on portable swimming pools. I am a certified seller of portable pools in India, especially for children, and keep their safety in mind. If you need to buy the right portable pool according to your requirements, feel free to contact me at any time.

    ReplyDelete
  42. I found your article one of the best. Please keep going. If you anytime need any paid sofware for your
    work you can visit shanpc

    ReplyDelete
  43. First period they have remained related to the unknown of the experiential extra infrared laser lights & lighting. The attractive airs are similar to what takes home on the superficial of our Sun, anywhere moving attractive arena lines spur marvelous solar fames to flare up in big rings.

    ReplyDelete
  44. Thankful to you for posting such a mind blowing blog post! I discovered your website perfect for my needs and I am really so happy with this. It contains amazing and strong releases about many niche. Continue doing great! Thankful to you for this useful posts! help with assignment - Assignment Provider - auditing assignment - online homework help australia

    ReplyDelete
  45. Get best and professional assignment help services with experts. Our assignments writing services are probably the fastest on the web. GoAssignmentHelp.com offer superior quality and, best assignment help, with the team of highly qualified and skilled professionals. Each assignment writer with us needs to go through a specialized process of training such that they can complete the given online assignment help task within time by complying all the assignment need perfectly. Our team is completely dedicated to handling all kinds of assignments such as essay writing, dissertation writing assignment and much more. GoAssignmentHelp.com writers are completely capable of accepting the challenge and meet it and Our experts constantly improve their methods of work so, they improve the speed of execution. They know how to choose the most suitable assignment help singapore strategy to complete this or that piece of writing. As a result, almost 100% of all the orders made by our clients were written before the time limit was over. You will always be on time with us, For more information about uk assignment help please visit our website.

    ReplyDelete
  46. A Marble pavers will improve the curb appeal and value of your house. However, as with any other home improvement project, the type and colour of your driveway fittings, as well as the layout and overall design, are all important considerations. The ultimate goal, of course, is to create a gorgeous entryway to your home that can withstand the weight of traffic without shifting or cracking. StoneHardscapes is a well-known company that specializes in pavers made of natural stone. When you deal with us, finding driveway pavers that fit your needs and expectations is simple.

    ReplyDelete
  47. Shop Support is the best flat-rate solution for entrepreneurs and shop owners looking to scale their businesses without the worry of hiring a team of designers and developers.
    Our Happy Client: Pure Life Bag
    About Purelifebag
    Canada All Locations
    Product

    ReplyDelete
  48. This comment has been removed by the author.

    ReplyDelete
  49. Our team has over 30 years of marketing experience.
    Learn More
    We have helped businesses create their brand presence and achieve their goals. Our process is designed to empower your brand and outfit your business with the marketing tools needed to succeed. Talk to us today about how we can support your growth, limit your turnover, and put you on a solid track to success and profit.

    ReplyDelete
  50. United airlines baggage policy - Packing is always a challenge, especially when travelling by air. To avoid unneeded turmoil, United Airlines has a rather clear luggage policy for customers. However, before boarding the aircraft, you should carefully review United Airlines luggage rules.

    ReplyDelete
  51. Dr. Tahmasub Faraz Tayyab | Dentist in Sargodha | Implantologist , Oral & Maxillofacial Surgeon

    Assoc. Prof. Dr. Tahmasub Faraz Tayyab is a Dentist, Implantologist and Oral & Maxillofacial Surgeon. His qualification is BDS & FCPS. He is practicing at MAXFAX Clinic which is a Multi-specialty Boutique Dental Practice situated in the Niazi Medical Complex (VIP Block) with Latest Facilities and Modern Diagnostic Equipment for Maxillofacial and Dental trauma which is first of its kind in entire Sargodha Division, where high standard services are offered following International Standard Sterilization and Cross Infection Protocols. He is committed to give a wide range of services which includes but is not limited to; Oral/Head & Neck Cancer, Salivary Gland Diseases, Wisdom Tooth Surgery, Facial Pain and Implants
    Contact : 03324962596
    Address: 103 Club Road, Sargodha, Punjab 40100
    Dentist in Sargodha

    ReplyDelete
  52. Dr. Anila Gillani | Skin Specialist in Sargodha

    "Dr. Anila Gillani is one of the best Skin Specialist in Sargodha. Not only does She have a number of certifications including M.B.B.S. and F.C.P.S. in dermatology but She also has 15 years of experience in her field. She is specialized in treating and managing Skin Problems, Hair Problems, Acne, Acne Scars, Benign and Malignant Skin Tumors, Eczema, Psoriasis, Scabies, Skin Aallergy, Skin Diseases, Skin Fungal Infections, Vitiligo, Warts, and Hair Fall.
    Phone: 0345 8401282
    Address: 103 Club Road, Satellite Town, Sargodha, Punjab 40100

    Skin Specialist in Sargodha

    ReplyDelete
  53. The Cricut Setup Explore Air 2 Setup machine and are wondering how to install Cricut Explore Air 2 model. Well, we have got you covered with our installation guide. You might be confused about purchasing a Cricut model or wondering how this model differs from its other versions. To resolve all these confusions for you, here we have brought up the common and useful features that can be of use to you. Let’s start the step-by-step procedure to install Cricut Explore Air 2 to create desired projects.
    Cricut.com/setup
    install Cricut Explore Air 2
    Cricut Setup Explore Air 2

    ReplyDelete
  54. Very helpful information thanks for sharing with us. Hope to see more related to this! Visit Us: India Trekking Blog

    ReplyDelete

  55. There has been these news about the Places to Visit in the town lately. Though I have not seen much about it anywhere here. Anyways, you have done quite a good job with this one, thanks!

    ReplyDelete
  56. Rehana Ruhy Clinical Psychologist In Lahore

    Rehana Ruhy is a Clinical Psychologist and has been working as a mental health professional since 1995. She is trained in Hypnosis, NLP, Reiki, Meditation, Cognitive therapy, behavioural and humanistic approaches of therapy. She is one of best Psychologist in Lahore and uses approaches that are research driven, contemporary and scientifically based to help individuals and couples to reconnect with themselves, finding solutions to their problems and in developing a higher sense of self esteem and self awareness. Rehana has a passion for seeing individuals and couples establish and restore healthy relationships and in helping people live their lives to the fullest. She has a holistic approach towards therapy, healing all aspects of one’

    Phone: 03334976126

    Address: Plot 24, Chenab Block Allama Iqbal Town, Lahore, Punjab 54570

    ReplyDelete
  57. Finding the right Best Acne Treatment in Lahore can be challenging. Has anyone here tried a dermatologist in Lahore they'd recommend?

    ReplyDelete
  58. Soda ash prices have historically exhibited volatility, influenced by factors like supply-demand dynamics, production costs, and global economic trends. Prices for soda ash, a key raw material in various industries including glass, detergents, and chemicals, tend to fluctuate based on shifts in production capacities, energy costs, and environmental regulations impacting the manufacturing process. In recent years, market variations have been observed due to factors such as changes in raw material costs, energy prices, and trade dynamics, affecting the overall pricing of soda ash on both regional and international scales.

    ReplyDelete
  59. Discover the perfect solution for your home at Alfen, your go-to destination for premium. Explore our extensive selection of top-notch products, including various aluminium sliding windows price per square feet, meticulously crafted to enhance your living space. Whether you seek durability, style, or energy efficiency, Alfen has you covered. Upgrade your home with our exquisite offerings today.

    ReplyDelete
  60. Struggling with your programming assignments? Don't let complex coding problems hold you back! Our dedicated service offers experthelp in programming assignments to ensure your success. With a team of experienced programmers, we provide personalized assistance tailored to your needs. Whether you're stuck on algorithms, debugging, or syntax errors, our reliable help in programming assignments ensures that you meet deadlines and achieve top grades. Say goodbye to coding dilemmas and hello to confidence in your programming skills with our comprehensive assistance.

    ReplyDelete
  61. Name: Aap Indian Restaurant | Indian Restaurant | Cincinnati OH
    Address: 6430 Glenway Ave, Cincinnati, OH 45211
    Phone: +15138327448
    Step into our Indian Food Restaurant In Cincinnati OH, and you'll be transported to the heart of India. We take pride in preserving traditional recipes and cooking techniques that have been passed down through generations. Our commitment to authenticity means that every meal you enjoy with us is a true reflection of India's culinary heritage. Join us at our Aap India restaurant for a dining experience that honors tradition while embracing the spirit of innovation.

    ReplyDelete
  62. Name : Rajaji Curry House Indian Restaurant
    Address: 2603 Connecticut Ave NW #2634, Washington, DC 20008
    Phone: +12022657344
    When searching for an Indian restaurant in Washington, DC , Raja Ji is the place to be. Our restaurant specializes in authentic Indian dishes, featuring a menu filled with favorites like spicy vindaloo, creamy butter chicken, and aromatic biryani. Each meal is prepared with fresh ingredients and traditional spices to deliver an unforgettable dining experience. Located in the heart of Washington, DC, Raja Ji offers a warm and inviting ambiance that enhances your visit. Discover why we are considered one of the best Indian restaurants in Washington, DC.

    ReplyDelete
  63. Delhi Indian Cuisine, a top-rated Indian restaurant in Las Vegas, Nevada, offers an authentic dining experience with a rich array of traditional Indian dishes. Whether you're craving flavorful curries, aromatic biryanis, or mouthwatering tandoori specials, our menu is crafted to delight your senses. Located conveniently in the heart of the city, we provide a welcoming ambiance perfect for both casual dining and special occasions. Our chefs use only the freshest ingredients, ensuring that every meal is a taste of India. If you're in Las Vegas and searching for a genuine Indian culinary experience, Delhi Indian Cuisine is your go-to destination. For reservations or inquiries, call us at +1 702-383-4900 and savor the finest Indian flavors today.

    ReplyDelete