As other commenters noted, the title is very misleading. The force exist but it is tiny, very very very tiny.
For radiation, force = power / speed of light
You can add x2 if the radiation bounce against the surface like in a mirror. For the calculations, let's pick a random value for power: 100W
* 100 W it's all the power that used an old incandescent lamp, most of it was wasted as heat instead of visible light, and in many directions.
* A modern LED lamp for a room has about 10W, so 100W are like 10 LED lamps.
* And 100W is HUGE for a laser. And 5W is huge for a laser. (Class 3R is up to .005W and Class 3B is up to .5W.)
So if we divide the force by the acceleration of the gravity https://www.wolframalpha.com/input/?i=2+*+100W+%2F+(speed+of... we get 7E-8Kg = 70ug that according to wolfram alpha is approximately 1/40 of a typical snowflake and 1/20 of a typical mosquito.
Note that if you suddenly put 100W into a snowflake or a mosquito, they will be destroyed instantly, and you need x20 or x40 more power to make them float, and they will absorb most of the energy so you need x2 the power.
(The article is about manipulating the light/material to stabilize the object, not to magically make the radiation force strong enough to levitate an elephant.)
Not to detract from your comment, but the most interesting thing about what you said (at least to me) is that a mosquito weighs half as much as a snowflake.
> And 100W is HUGE for a laser. And 5W is huge for a laser. (Class 3R is up to .005W and Class 3B is up to .5W.)
It's kind of irrelevant, but I think it's worth pointing out that while those are legal/safety/sanity limits [0], there's a lot of companies that completely ignore them. Here's one that has quite a few 5W lasers [1], and some that are supposedly up to 50W which would be insane if true. As for 100W being huge... the US Navy is testing ones in the tens of kW [2]. Might eventually make for one hell of a point defence system.
That being said, photon drives still SUCK. Not having to carry the drive helps, but it's still in the Newton-per-100MW-power range.
A few years ago, I had for a year a green laser of 5W pointing in my direction over an optical table. It was redirected and used before it reach me, and there were a few security measures like a "wall" to stop the beam. Anyway, I was always slightly nervous ...
5w isnt all that much for a laser these days. There are consumer laser "pointers" in the 5/10w range. 2w lasers are a norm for desktop engraving machines. So putting a 100/200w bank together wouldnt be a big deal.
(Ive personally verified some "legal" laser pointers in the 1.7w class. Weapons imho.)
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?
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
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.
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.
(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].
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 (hope I got the right one, didn't watch it because I'm on mobile)
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).
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.
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.
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.
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.
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.
For radiation, force = power / speed of light
You can add x2 if the radiation bounce against the surface like in a mirror. For the calculations, let's pick a random value for power: 100W
* 100 W it's all the power that used an old incandescent lamp, most of it was wasted as heat instead of visible light, and in many directions.
* A modern LED lamp for a room has about 10W, so 100W are like 10 LED lamps.
* And 100W is HUGE for a laser. And 5W is huge for a laser. (Class 3R is up to .005W and Class 3B is up to .5W.)
So if we divide the force by the acceleration of the gravity https://www.wolframalpha.com/input/?i=2+*+100W+%2F+(speed+of... we get 7E-8Kg = 70ug that according to wolfram alpha is approximately 1/40 of a typical snowflake and 1/20 of a typical mosquito.
Note that if you suddenly put 100W into a snowflake or a mosquito, they will be destroyed instantly, and you need x20 or x40 more power to make them float, and they will absorb most of the energy so you need x2 the power.
(The article is about manipulating the light/material to stabilize the object, not to magically make the radiation force strong enough to levitate an elephant.)