6 ms·

I'm completely out of my element here. Hoping maybe for some clarification. I've never heard of a laser, at any scale, having a force pushing backwards against

by jfoutz 8y ago

I'm completely out of my element here. Hoping maybe for some clarification. I've never heard of a laser, at any scale, having a force pushing backwards against the direction of light. It seems like one of two things is possible. 1) you could move a spaceship by shooting a laser. The force would be tiny, but it would keep accelerating as long as the laser is powered, with no reaction mass. i've never heard of such a thing being possible (but, again, way out of my element). 2) you could play the classic impossible machine game, with the fan on the back of a boat blowing into a sail. except with a laser and a sail. If 1) doesn't work, then 2) has to work, right?

majewsky 8y ago
1) is absolutely correct. This is also how solar sails work in proposed interplanetary spacecraft designs. For more details, I can recommend this video from Because Science: https://m.youtube.com/watch?v=K6-q2edmiGk https://m.youtube.com/watch?v=K6-q2edmiGk (hope I got the right one, didn't watch it because I'm on mobile)
- jfoutz 8y ago
  That was the right one. I never got far along enough to learn about e^2=p^2c^2+m^2c^4 Thank you for the amusing explanation.
gus_massa 8y ago
1) Yes, but it's easier to use the light from the Sun. More in https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail Also, it is possible to measure this in the lab. For example https://www.youtube.com/watch?v=Ng1X8mPJziA https://www.youtube.com/watch?v=Ng1X8mPJziA Beware that some popular experiments measure tiny difference of gas pressure instead of actual radiation presure https://www.youtube.com/watch?v=r7NEI_C9Yh0 https://www.youtube.com/watch?v=r7NEI_C9Yh0 2) If you put the laser an the sail in the same spaceship, and the sail absorbs 100% of the radiation, then the when the laser emit the photons it is pushed in one direction and when the sail absorbs the photons it is pushed in the other direction. So if the sail absorbs 100% of the radiation, the effect in null. You can use a laser as a strange kind of thruster (if you remove the sail, and point the laser in the other direction). It's very inefficient, but it produces a force. It is not necessary to use a laser, you can use any kind of lamp (and it would be even more inefficient). The heat that escaper from the Pioneer is an involuntary example of this method https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly
- tzs 8y ago
  That made me think of a totally ridiculous idea for an interstellar ship: The Inchworm Drive. Put a laser complex in space, which has a laser, a power source for the laser, and enough extra mass (if the power source isn't massive enough) to make the laser complex way more massive than your ship. Use the laser to push our ship via a light sail. Have our ship connected to the laser by cables, which we spool out from the ship as it travels, spooling them out fast enough that the cables are not exerting a retarding force on the ship, nor a pulling force on the laser complex (you can probably actually get some thrust from spooling out the cables). Yes, this will require several light-years worth of cables. Go until the ship reaches the next solar system, and enters orbit around one of the system's outer planets. You need to pick a planet that has resources you can use to refuel the laser power source. Reel the cables in, using the planet's gravity to keep the ship from getting pulled back to the laser. Doing this, the ship can now pull the laser after it, to the outskirts of the planet's solar system. Then use the cables to pull the ship back to the newly positioned laser, and refuel it with resources you got from the planet. Repeat for the next hope of the trip.
  - comboy 8y ago
    https://www.goodreads.com/book/show/34466963-why-we-sleep https://www.goodreads.com/book/show/34466963-why-we-sleep
  - gus_massa 8y ago
    The problem is that it's not easy to spool out the wires. Just disconnect the laser for a moment to simplify the discussion. If the ship is traveling at speed V and the wires are at speed 0 in the usual reference frame [1]. If you imagine that you are over the ship, you will see that the wires are initially at rest and you have to accelerate them so they go away from the ship at speed V. So you will have to use a lot of energy to accelerate the wires [2]. And also the wires going away will act like the exhaust of a rocket. If you push the wires backwards, the wires will push you forwards. Now you have a very strange rocket that uses wires instead of some propellent. [1] whatever it means, perhaps the initial reference frame of the space station that launched the ship, or the average of the local stars. [2] Assuming that the wires are not so thin, because they have to pull a space station. Also, we are comparing them to a laser beam, so they are in comparison much heavier.
tgb 8y ago
For (2) you can cut out the middle man (the mirror) and have the laser just point in the opposite direction of your desired acceleration. It amounts to the same thing, assuming the mirror reflects perfectly (by conservation of momentum - the momentum of the released photon is the same whether it bounces off a mirror first or just starts off in that direction).
mikeash 8y ago
Laser propulsion does use reaction mass. Mass-energy equivalence means that the emitted light has mass. Conservation means that the spaceship loses mass as it fires the laser. It’s basically a standard rocket with a ridiculously high specific impulse.
- gus_massa 8y ago
  In an old comment throwaway_yy2Di made me notice that in spite it has a ridiculously high specific impulse, the momentum to power ratio is very small, it's only 1/c = 3.3E-6 N/kW.
  - mikeash 8y ago
    I believe that’s an inherent tradeoff. The faster you make your reaction mass go, the more efficient your use of mass, but the less efficient your use of energy.
    - gus_massa 8y ago
      IIRC if you don't need to create the particles like in a ion thruster, it is better to accelerate them as much as possible. The problem with photons is that you need energy to create them.
      - mikeash 8y ago
        If you double your exhaust velocity, you double your thrust but quadruple your power requirements. If you have a fixed amount of power, then you’ll have to cut thrust by one half. Spacecraft are usually severely mass limited so less thrust and more energy consumption in exchange for better mass efficiency is usually the right tradeoff.
mdturnerphys 8y ago
(2) isn't actually impossible with a fan and sail. The gas molecules recoil off the sail, imparting nearly 2x their original momentum (given by the fan) to the sail. Not as efficient as just using the fan as a propeller, but it does work. One of my college professors demonstrated it in class and published a paper about it in a physics education journal decades ago [0]. [0] https://aapt.scitation.org/doi/10.1119/1.2341931 https://aapt.scitation.org/doi/10.1119/1.2341931

by jfoutz 8y ago

majewsky 8y ago

1) is absolutely correct. This is also how solar sails work in proposed interplanetary spacecraft designs. For more details, I can recommend this video from Because Science: https://m.youtube.com/watch?v=K6-q2edmiGk https://m.youtube.com/watch?v=K6-q2edmiGk (hope I got the right one, didn't watch it because I'm on mobile)

jfoutz 8y ago
That was the right one. I never got far along enough to learn about e^2=p^2c^2+m^2c^4 Thank you for the amusing explanation.

gus_massa 8y ago

1) Yes, but it's easier to use the light from the Sun. More in https://en.wikipedia.org/wiki/Solar_sail https://en.wikipedia.org/wiki/Solar_sail Also, it is possible to measure this in the lab. For example https://www.youtube.com/watch?v=Ng1X8mPJziA https://www.youtube.com/watch?v=Ng1X8mPJziA Beware that some popular experiments measure tiny difference of gas pressure instead of actual radiation presure https://www.youtube.com/watch?v=r7NEI_C9Yh0 https://www.youtube.com/watch?v=r7NEI_C9Yh0 2) If you put the laser an the sail in the same spaceship, and the sail absorbs 100% of the radiation, then the when the laser emit the photons it is pushed in one direction and when the sail absorbs the photons it is pushed in the other direction. So if the sail absorbs 100% of the radiation, the effect in null. You can use a laser as a strange kind of thruster (if you remove the sail, and point the laser in the other direction). It's very inefficient, but it produces a force. It is not necessary to use a laser, you can use any kind of lamp (and it would be even more inefficient). The heat that escaper from the Pioneer is an involuntary example of this method https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly

tzs 8y ago
That made me think of a totally ridiculous idea for an interstellar ship: The Inchworm Drive. Put a laser complex in space, which has a laser, a power source for the laser, and enough extra mass (if the power source isn't massive enough) to make the laser complex way more massive than your ship. Use the laser to push our ship via a light sail. Have our ship connected to the laser by cables, which we spool out from the ship as it travels, spooling them out fast enough that the cables are not exerting a retarding force on the ship, nor a pulling force on the laser complex (you can probably actually get some thrust from spooling out the cables). Yes, this will require several light-years worth of cables. Go until the ship reaches the next solar system, and enters orbit around one of the system's outer planets. You need to pick a planet that has resources you can use to refuel the laser power source. Reel the cables in, using the planet's gravity to keep the ship from getting pulled back to the laser. Doing this, the ship can now pull the laser after it, to the outskirts of the planet's solar system. Then use the cables to pull the ship back to the newly positioned laser, and refuel it with resources you got from the planet. Repeat for the next hope of the trip.
- comboy 8y ago
  https://www.goodreads.com/book/show/34466963-why-we-sleep https://www.goodreads.com/book/show/34466963-why-we-sleep
- gus_massa 8y ago
  The problem is that it's not easy to spool out the wires. Just disconnect the laser for a moment to simplify the discussion. If the ship is traveling at speed V and the wires are at speed 0 in the usual reference frame [1]. If you imagine that you are over the ship, you will see that the wires are initially at rest and you have to accelerate them so they go away from the ship at speed V. So you will have to use a lot of energy to accelerate the wires [2]. And also the wires going away will act like the exhaust of a rocket. If you push the wires backwards, the wires will push you forwards. Now you have a very strange rocket that uses wires instead of some propellent. [1] whatever it means, perhaps the initial reference frame of the space station that launched the ship, or the average of the local stars. [2] Assuming that the wires are not so thin, because they have to pull a space station. Also, we are comparing them to a laser beam, so they are in comparison much heavier.

tgb 8y ago

For (2) you can cut out the middle man (the mirror) and have the laser just point in the opposite direction of your desired acceleration. It amounts to the same thing, assuming the mirror reflects perfectly (by conservation of momentum - the momentum of the released photon is the same whether it bounces off a mirror first or just starts off in that direction).

mikeash 8y ago

Laser propulsion does use reaction mass. Mass-energy equivalence means that the emitted light has mass. Conservation means that the spaceship loses mass as it fires the laser. It’s basically a standard rocket with a ridiculously high specific impulse.

gus_massa 8y ago
In an old comment throwaway_yy2Di made me notice that in spite it has a ridiculously high specific impulse, the momentum to power ratio is very small, it's only 1/c = 3.3E-6 N/kW.
- mikeash 8y ago
  I believe that’s an inherent tradeoff. The faster you make your reaction mass go, the more efficient your use of mass, but the less efficient your use of energy.
  - gus_massa 8y ago
    IIRC if you don't need to create the particles like in a ion thruster, it is better to accelerate them as much as possible. The problem with photons is that you need energy to create them.
    - mikeash 8y ago
      If you double your exhaust velocity, you double your thrust but quadruple your power requirements. If you have a fixed amount of power, then you’ll have to cut thrust by one half. Spacecraft are usually severely mass limited so less thrust and more energy consumption in exchange for better mass efficiency is usually the right tradeoff.

mdturnerphys 8y ago

(2) isn't actually impossible with a fan and sail. The gas molecules recoil off the sail, imparting nearly 2x their original momentum (given by the fan) to the sail. Not as efficient as just using the fan as a propeller, but it does work. One of my college professors demonstrated it in class and published a paper about it in a physics education journal decades ago [0]. [0] https://aapt.scitation.org/doi/10.1119/1.2341931 https://aapt.scitation.org/doi/10.1119/1.2341931