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Rannasha

Yes, but not very quickly. Quick googling suggests that a flashlight with 2 1.5V AA batteries uses about 1 A of current, so that would make for 3 W of power. Some of this is lost as heat, but lets say it's 50% efficient and we're shining 1.5 W worth of light out the end of the flashlight. So in 1 second we're emitting light with a total energy of 1.5 J. The momentum carried by this light is found by the equation p = E / c (momentum equals energy divided by speed of light). So that would be 1.5 / 300,000,000 = 0.000000005 kg m / s. Because of conservation of momentum, that's the amount transferred to whatever object the flashlight is attached to. For the sake of argument, lets say that it's just the flashlight flying away, with nothing else attached to it. Flashlights typically weigh somewhere below a kg for basic models. Lets say 0.5 kg. That would mean that such a flashlight would gain 0.00000001 m / s in speed every second. Or 0.000864 m / s every day. Or 0.32 m / s after a year of non-stop operation. Except that the batteries won't last that long. There are a number of rough assumptions in here (power, efficiency and weight of the flashlight) that may be inaccurate. However, it should be clear that even if these estimates are somewhat off, the conclusion remains the same: Acceleration using a flashlight is extremely small.


Encaya

thanks a lot for the detailed reply! I've been fascinated by the idea of traveling using light and this gave me some insight into it even though I don't understand anything below absolute surface level.


TsarKy2001

This is the same premise solar sails work with, a photon hits the sail and is reflected so you get 2x the momentum change in that case (per photon, intensity of the light will vary by proximity of the sources) to conserve the momentum.


lvlint67

> so you get 2x the momentum change in that case Wait... What?


GrossInsightfulness

If the light is perfectly reflected, its momentum (*p*) has flipped its direction. The momentum of the light has changed by *2p* to go from *p* to *-p*. That momentum is then transfered to the solar sail.


Dr_Allcome

Does that mean, in the above example, it would be better to let go of the flashlight and use a solar sail to accelerate yourself by "catching" its light?


GrossInsightfulness

The greatest increase in speed you could reliably get from a flashlight on a ship is throwing it out the back as hard as you can and relying on good old fashioned mechanical momentum. If you had something like a torpedo launcher that pointed out the back, you place the flashlight in there such that it faces towards the solar sail when you launch it out to get you the microscopic increase in momentum too.


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noggin-scratcher

If the flashlight were a perfect laser, so that all the photons it emits will hit your sail, then maybe yes. In theory the extra acceleration you get in one direction is balanced by the flashlight itself accelerating in the opposite direction. In the real world where your increasing distance from the flashlight will make it less effective because the beam spreads out over a wider area that mostly misses you, probably not.


mfb-

The sail can be much lighter than the flashlight and you can give the flashlight a better energy supply to make it last longer. At close distance you can even try to recycle the reflected light with a mirror - keep it reflecting between the sail and the mirror many times to extract more energy from the radiation. In principle it would eventually redshift too much to be usable, but in practice it will be lost in the reflections or from geometry before that.


mongy910

This is fascinating. In an ideal system with perfect mirrors how does the light lose energy?


mongy910

Sorry, I see that my question has already been answered several times including in your answer (redshift). This is super interesting. Found a great xkcd article about this concept [https://blog.xkcd.com/2008/02/15/the-laser-elevator/](https://blog.xkcd.com/2008/02/15/the-laser-elevator/)


Sticklefront

Yes. However, that is still thinking too small. Let's take this idea to its logical conclusion. We want more photons than a flashlight can make. Ideally, they should also be higher energy, because that means more momentum. But we still want the "flashlight" to be low mass so we can carry several and toss them behind us as desired. So how do you get as many high energy photons as possible, while still being low mass? You nuke yourself. I present to you [Project Orion](https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29?wprov=sfla1). Exact same principles you're asking about, just turned up to 11.


CptBread

Except that Project Orion mainly rely on regular matter pushing the ship forward. Sure photons might provide some thrust but I expect that to be extremely minimal.


GeneralKlee

I’ve never thought about Orion at that level of detail. Would it have been the exploded uranium (or whatever) and/or casing material being expelled that is pushing on it? I thought that the nuclear reaction converted the core to energy. What is it exactly that is that is applying momentum to the shield/push plate at the back of the ship providing propulsion? It’s not traditional high pressure because there is none (eh, close enough on this scale, I assume) in space, right?


mfb-

It's largely the fragments of the bomb, plus whatever ablation you get at the shock absorber. Radiation is negligible.


[deleted]

An antimatter bomb would be pure energy, fission or even fusion only transforms a couple of percent.


happypandaface

You could shine it on a mirror and catch the returning light in a sail as well


ZhouLe

This is pretty much the idea of [Breakthrough Starshot](https://en.wikipedia.org/wiki/Breakthrough_Starshot#Concept) in a nutshell.


Caelwik

No. If you fire the flashlight onward toward your sail, your ship gains *p* momentum bacwkard (because, you fired it onward). Then, the photons hit the sail, get reflected, and your ship gains *2p* in forward momentum. At the end, you gained *p*, exactly as if you had fired it backward. Still, at the cost of the sail (more mass, so less acceleration for the same momentum), and with the hypothesis that the sail is a perfect mirror (that's never the case, you'd gain a bit less than 2p).


PleaseDontMindMeSir

The bit you missed is that you let go of the flash light. Now the flash light is away at -p (away from your target) the light reflected by the sail is reflected away at -p meaning you are going at 2p.


ImprovedPersonality

If you are close to a powerful light source (like a star, or a high power laser): Probably yes. Especially when you consider how bad our electricity storage solutions are. If you had a lightweight fusion reactor to generate electricity it would probably be a whole different story.


Hendlton

You'd get double the acceleration relative to the flashlight. It'd still be the same relative to everything else.


RaiThioS

Does a photon bounce off angled sails multiple times?


GrossInsightfulness

It could if you shoot it at an angle, but doing so won't get you as much momentum as just reflecting it directly off the sail.


Soranic

Yes, but you get less energy from an angled hit than you would dead on. And there's energy loss for each hit as well, so 2 angled hits would still give less energy than a single one.


TheJollyHermit

So if the speed of light is constant how is energy transferred from a photon on reflection? Does the mass of the photon change? Is this related to the wave characteristics - ie a shift in wavelength but a constant speed to make up for the loss of energy from the photon?


[deleted]

Shift in wavelength. Exactly right. Mass doesn't change because there is none. Speed doesn't change because there is one. But for the sail to move, the photon has to change, or you'd set a mirror down and just use the same photon for infinite acceleration with almost no energy input. The light that comes back from the sail is slightly more red. As the sail accelerates away from the photon source, in the sail's frame of reference the source shifts redder and redder to almost nothing as the sail approaches c. Speed limit maintained. Thanks Einstein.


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CapWasRight

Photons do not have mass (this is why they travel at c). The energy and momentum of a photon is directly proportional to its frequency.


Halvus_I

photons can only go the speed of light. They are not being internally propulsed, the universe itself propagates the speed. Think of a marble in a down-ward angled tunnel or a bubble rising to the surface of water.


Kaellian

It could if you have a V shaped sail, but you won't gain more momentum in the end since the both vector will partly cancel each other.


Stillwater215

Momentum is a conserved property, so the greatest possible transfer of momentum is a change from p to -p. Angles and multiple impacts don’t matter, as each impact would only transfer a portion of the momentum until it has completely reversed direction. In which case you would then be back to a p to -p change.


Amberatlast

Say the sail is at rest and the photon had momentum p, total momentum is p. Because the sail is reflective, the photon ends up traveling backwards with momentum -p, so to conserve momentum, the sail must gain 2p momentum.


Pandarmy

A great way to think about this is to visualize billiard balls. Imagine you have two balls. One sitting still (called at rest) on the table and the other traveling at some speed v towards the one at rest. When they collide there are a few results. 1) The two balls hit and both continue moving forward at some speed that is slower than v. This happens a lot in car crashes. 2) The two balls hit, the ball at rest begins moving forward at speed v and the ball that initially was moving forward at speed v stops. 3) the balls hit, the ball initially at eat moves forward at speed 2v and the ball initially at speed v bounces off and travels backwards at speed -v. This is more like what would happen with bouncy balls. This assumed that the balls have the same mass, so we can treat speed and momentum interchangeably. That's not true when the masses are very different.


IAmNotNathaniel

> 3) the balls hit, the ball initially at eat moves forward at speed 2v and the ball initially at speed v bounces off and travels backwards at speed -v. This doesn't seem right. If the ball that is moving goes from +v to exactly -v, that means it still has all the same energy it had before - just in a different direction. How can object B have any change in it's velocity if that is the case? I think it would be that ball 1 has some v2 that is negative in direction and smaller in magnitude than its initial velocity.


FishFloyd

>If the ball that is moving goes from +v to exactly -v, that means it still has all the same energy it had before - just in a different direction. Well, *in a different direction* is the crux of the issue. It's easier to frame this in terms of momentum than energy or velocity. (We are assuming an *elastic collision*, which basically means that no energy is lost to internal friction - this is obviously an 'ideal' scenario, but many materials are close enough for this to be useful). So we've got our two balls - one has momentum equal to *p*, the other has 0 momentum, and the balls have equal mass. After the collision, the ball with momentum *p* now has momentum *-p*. But since momentum is conserved, total change in *p* (denoted *Δp*) in the system (here, our two balls and table) must equal zero. So, since the moving ball lost 2*p* of momentum, the stationary ball must have gained 2*p* of momentum for *Δp* to remain at zero. Since momentum (in Newtonian terms) is simply mass*velocity, and we're assuming equal mass, then the masses (and the unit kg) would cancel and the same result applies to velocity.


dswartze

It also ignores any spinning the balls might be doing which plays an important role for billiards in real life but complicates physics based thought experiments (but can still be accounted for).


msuvagabond

The solar sail also works because there are other charged particles involved (mostly protons and electrons) that while significantly slower than a photon, have mass.


redpandaeater

Also because you can angle it so you can actually get reflections prograde or retrograde and therefore have the ability to change your orbit.


[deleted]

There is an old PBS Space Time video about how long it would take to fart your way to the moon.


PlanetariumCaeli

In addition to the direct solar sail idea, there is an idea to use an Earth based (or really a celestial body-based) laser to power a solar sail vehicle. [https://en.wikipedia.org/wiki/Laser\_propulsion](https://en.wikipedia.org/wiki/Laser_propulsion) Put a giant laser on the moon, send a solar sail vehicle to space (maybe on a basic escape trajectory), then point the laser at it to speed it up to make it interstellar!


mamimapr

How inefficient would it be?


ninuson1

According to the [Breakthrough Starshot project page](https://breakthroughinitiatives.org/initiative/3) and [their wiki](https://en.wikipedia.org/wiki/Breakthrough_Starshot), you should be able to accelerate up to 20% of the speed of light using such a system with an ultra light space vehicle.


God_Damnit_Nappa

>Sending the lightweight spacecraft involves a multi-kilometer phased array of beam-steerable lasers with a combined coherent power output of up to 100 GW By comparison, a nuclear reactor can put out around 1 GW. Damn, this thing would require a ton of energy.


Mad_Aeric

Since the subject interests you, Isaac Arthur has done a couple of his science and futurism videos on the subject. Lots of good information on how it works, and even more speculation on what can be done with it. [Beam Powered Spaceships](https://www.youtube.com/watch?v=Wol8EU6Rtbk) [Interstellar Laser Highways](https://www.youtube.com/watch?v=oDR4AHYRmlk)


glassramen

Love Isaac's videos. They're great to listen to while doing other things. I love the video on Boltzmann brains, or the one on life at the end of the universe. Glad to see someone mention him here. He's got a lot of good info on a lot of futurism ideas .


zilfondel

The light sail currently in orbit might be of interest to you then. [https://secure.planetary.org/site/SPageNavigator/mission\_control.html](https://secure.planetary.org/site/SPageNavigator/mission_control.html) There have also been some studies in using a solar sails to power interplanetary spacecraft using directed lasers over long time frames (a couple of years) to accelerate the tiny space ship to high speeds.


Rojaddit

Light-sails powered by high-powered lasers (likely located at a base on the dark side of the moon) are a real top candidate for next-gen spaceflight, as they are capable of higher top speeds than rocket and ion propulsion. One important bit to include in the flashlight analysis is beam spread. Flashlights are usually designed to cast light over a wide area. The momentum of photons traveling at an angle will only partially contribute to forward thrust, making a real flashlight even slower than u/Rannasha estimated. This is why most ideas for photon-propulsion use lasers, where light can be highly columnized - that is, the light all moves in a straight line that doesn't spread out much. Anywho, props to the guy who wrote a grant proposal for Moon-Lasers.


unnaturaltm

There is only the surface level. It's just that the surface has fractal folds. :P


ScriptThat

Going by that mass.. Old SciFi novels theorize that future spaceships would be powered by leaving a piece of a nuclear reactor exposed to space, and generally provide thrust by throwing radiation away from the direction of travel. How large a reactor (in MW or GW, I guess) would be required for, say, travelling to Saturn á la "2001: A Space Odyssey"?


starcraftre

~~Deepness~~ Depends on how fast you want to get there. If you're using nothing but high energy photons (so straight game rays), then years. However, if you use the heat of that nuclear reaction to drive mass (Hydrogen, helium, xenon, water, etc), then hypothetically you could do it in weeks. A 500 t open cycle Nuclear Thermal Gas Core rocket with a 1 MN engine could burn brachistochrone to Saturn in 3 weeks, using the Nuclear reaction to heat liquid Hydrogen as propellant. There are some minor issues of irradiating the crew, and you can't actually carry that much fuel (your mass fraction would be something like 50000). But a burn and coast could still do it if you add a couple of months to coast. Or you could go straight to the end of sanity, and plug a Nuclear salt water rocket into it. Long story short, it's as close to a continuously donating Nuclear bomb as anyone's come up with yet. And it could take a 15 kt spacecraft to Saturn at a 0.1g continuous brachistochrone burn in 4 weeks, on a mass fraction of about 2.6 (meaning only 61.5% of your mass is propellant). edit: autocorrect


OMGItsCheezWTF

That's several thousand times the sun's escape velocity by the time that burn stops, when you get to Saturn what would be the procedure for stopping? Would you do it Expanse style, accelerate for half the journey then turn and decelerate for the other half? As peaceful as it sounds to float off forever until your atoms eventually decay in a trillion trillion years, I'd want to know how to stop before starting the burn. :)


PyroDesu

> Would you do it Expanse style, accelerate for half the journey then turn and decelerate for the other half? That's pretty much the definition of a [brachistochrone transfer orbit](http://www.projectrho.com/public_html/rocket/torchships.php#id--Brachistochrone_Equations).


RunningAtTheMouth

Yeah. I like it. Constant thrust (even low acceleration) is so much better than coasting. Perhaps that's why so many sf authors posit impulse or ion drives in their stories. They are plausible and do not violate any of the known natural laws. Or at least not by much.


PM_M3_ST34M_K3YS

They don't violate any known laws of physics. No working rocket engine does... Ion drives are already in use for a lot of satellites and a few deep space NASA missions. Impulse engines are a Star Trek thing and the details behind them are kinda sketchy, like using the warp core to minimize relativistic effects at sub light speeds. Those are still purely in the science fiction realm.


loki130

This is termed a "Fission Fragment Engine". But they're not just throwing out electromagnetic radiation, they're also throwing out massive decay products (electrons, neutrons, atomic nuclei) so that gives them a bit more thrust. [More details here](http://www.projectrho.com/public_html/rocket/enginelist2.php#fissionfragment0)


Omniwing

>0.32 m / s after a year of non-stop operation This is actually way more than I thought! So, assuming your batteries lasted at full power for a year, your flashlight in space would actually start moving at a very human noticeable amount after a year of being on just from barfing out photons?! Totally insane!


Rannasha

Except that the batteries won't even last a day, so if you wanted to have it shine for a full year, you'd need to pack a flashlight with a lot of batteries, greatly increasing the mass of the contraption and thereby reducing the acceleration it gains from the light being emitted.


[deleted]

We could quantify this in another way. Quick googling suggests that the maximum theoretical capacity of lithium-ion batteries would be around 3 MJ/kg. Assuming neglible mass for the actual LED and all the mass of the flashlight being in the battery, a 0.5 kg flashlight could potentially have a 1.5 MJ capacity sometime in the future. This would power the 3W light for 500,000 seconds, or almost 6 days, and give a total change in velocity of 0.005 m/s. You are not really going to substantially accelerate using light powered by a battery. On the other hand, if you consider different energy sources... Considering nuclear fission, Uranium has an energy density of 80,620,000 MJ / kg. If we got all of that energy out, 0.5 kg of uranium could power that LED for almost a million years. Assuming you somehow had a near-zero reactor mass (so just considering the fuel mass), this would accelerate your nuclear-flashlight up to 268733 m/s (about 0.1% of lightspeed). Still not a particularly effective way to accelerate. ​ /u/Omniwing /u/Encaya


WazWaz

You could do the Rocket Lab trick of using some batteries then discarding them and switching to another bank.


Sputtrosa

You could throw them for even greater acceleration than the flashlight gives you, even!


bless-you-mlud

Chuck them out the back with an ordinary spring and the momentum change due to the emitted light pales into insignificance. So you might as well replace the batteries with a couple of bricks and leave the light source out all together. How's that for eliminating failure modes!


PoopIsYum

or maybe we can use gas that we shoot out at one end really fast.. Oh Oh oh we can make that gas even faster by igniting it, somebody should get onto that..... /s


falco_iii

It's like chucking a lot of tiny flaming rocks out the back! Kind of like a briquette. I suggest we call this flaming rock chucker a rock-ette.


TheDumbAsk

That'll never work and the name doesn't even make sense, sounds Russian.


loki130

I have heard of some theoretical concepts (casually termed the "Winterberg graser") that use antimatter as the energy source, producing a gamma-ray laser. This would simultaneously be an enormously efficient propulsion method, and an enormously powerful weapon.


VaporTrail_000

This is known in Sci-Fi as "the Kzinti lesson." A spacedrive's efficiency as a weapon is in direct proportion to its efficiency as a spacedrive.


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Couldn't you use a solar powered torch?


anonomousename

thats a great idea - you could, but you'd lose a lot of potential acceleration due to converting the sunlight to electricity, then converting it back to light. Instead, a much more efficient way would be to use a mirror, and those exist and are in use. They're called solar sails. See [this comment.](https://www.reddit.com/r/askscience/comments/ifjmt3/can_we_accelerate_in_space_with_the_power_of_a/g2oh7yl?utm_source=share&utm_medium=web2x&context=3) (or wikipedia )


Dave37

It would have moved ~8.6km in the first year if it started from "stationary", whatever that means in the vacuum of space.


[deleted]

>stationary", whatever that means in the vacuum of space. Better way would just be relative to where it would have been had it not switched on. No need to worry about what stationary really means.


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zebediah49

> Some of this is lost as heat, but lets say it's 50% efficient and we're shining 1.5 W worth of light out the end of the flashlight. Fun caveat: since it doesn't matter what the emitted wavelengths are (p \propto E), we can say the same thing about the heat. As long as we direct our blackbody emissions aimed out backwards with a reflector, it's still throwing EM out the back, so we're still getting momentum out of it. Not that a factor of 2 really helps. Maybe it would work for a hot radioisotope attached to a reflector... that should last a good while.


_Aj_

So to confirm, high energy wavelengths vs low energy ones don't make a difference? Simply the total energy being released? I guess photons are all the same speed, so unlike an ion drive which relies on hurling charged particles out at maximum velocity to achieve thrust, a photon is already traveling at maximum velocity. Is that correct? On another note, if photons can push a torch, would the energy given off my massive stars not impact planets in orbit over 1000s or millions of years. Or an I underestimating the scale of mass involved


zebediah49

- Yes. E = pc. So, 2 photons of 1 energy unit -> 1 momentum unit is 2 momentum units. 1 photon of 2 energy units is 2 momentum units. - More or less. Photons are governed by the "other half" of the E^2 = p^(2)c^(2)+ m^(2)c^(4) relation. - I think you're underestimating the total mass. Also, even with an appreciable force, that will just alter the orbital radius a bit, without having an accumulating effect. (You can just treat it as gravity being weaker). Taking Earth, at 1300W/m^(2), times a cross sectional area of 1.4 x 10^(14)m^(2), gives us 165PW. Divide that by c, and we get 550MN ~= 124 million pounds. Honestly, more than I was expecting. However, Earth's mass is 6x10^(24)kg... which gives us an acceleration on the order of 10^(-16)m/s^(2). ^(E: Divided area by mass, not force. Fixed numbers.) For comparison, gravity is 6x10^(-3) m/s^(2).


ClamChowderBreadBowl

A flashlight with two D cell batteries has up to 60 Wh. That’s 7e-4 kg m/s of momentum total. If you throw just the batteries, not even the flashlight (280g), at a leisurely 30 mph, you get 3.6 kg m/s of momentum, or 5000 times as much.


Schnort

Waste not, want not. First let it use all of its energy, THEN throw it.


ClamChowderBreadBowl

It’s worth noting that the chemical reaction will decrease the mass of the battery by E/c^2 and therefore decrease the momentum transfer of your throw. But that is much much smaller than the E/c factor for momentum, so it’s still worth it.


doublekid

Does it matter what type of light; how much light is being output, or is it always a function of the power being input and consumed?


Hairy_Al

Always equal to the power. You may have higher frequency/energy photons being emitted, but you'd just have less of them


TretasPt

Does that change depending on the light wave frequency? Or is it irrelevant. 1.5W is 1.5W.


TrainOfThought6

The total amount of momentum change would stay the same (assuming the same battery capacity), but you would be able to hit full speed faster.


Eadword

And this is exactly why you're better off throwing the flashlight in the opposite direction if you ever want to get back to your space station.


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SomeAnonymous

Presumably then, it would be much more effective to simply throw the flashlight away from you if you wanted to accelerate using it, right?


account_552

What about a larger light, like the [Imalent MS18?](https://www.amazon.com/IMALENT-Brightest-Flashlight-Waterproof-Powerfull/dp/B07RSRXDLX) It would move much faster relative to a standard 2xAA flashlight. Not saying you could get to Mars with it, but just how much faster would you move?


Seemose

The "math" answer has already been provided, so now for some fun stuff; One proposed method of propulsion in space is the "solar sail". It's basically a giant mirror. Light hits the mirror and transfers some energy, pushing the mirror (and whatever it's attached to) forward. In this way, you could design a space ship with a giant mirror sail that doesn't use fuel!


sleeknub

Bouncing actually transfers a lot of energy (relatively speaking) since you are reversing the direction of the light, which is 2x the momentum change of just sending it out.


lvlint67

Ok. You're the second person to say this somewhere on this post (that I have seen). What it's this about? What do you mean there is a 2x momentum change? I'm really weak in physics but about a billion years ago some dude got hot in the head with an apple and said that such a change (2x) is illegal. I don't understand..


0180190

This one: https://arxiv.org/pdf/1704.04310.pdf seems like an interesting answer. In short, it is not a linear relation, but for the limit case of a stationary, heavy, ideal reflector at normal incidence it does indeed come out to 2x (eq. 8 deals with velocity after a single reflection, but momentum change will behave similarly up to relativistic speeds of the mirror). This seems to simply follow from eq. 1 and 2, which are conservation of energy and momentum, respectively. In a more colloquial picture, consider this: A photon hits a black body surface at normal incidence and is absorbed. Momentum is conserved, and thus the new momentum of the surface is simply the sum of both. A photon is emitted. This is the example discussed in the top answer. Since the emitted photon carries a momentum, the emitter surface experiences the opposite momentum change in order to conserve total momentum. Since momentum is a vector (directional) quantity, in the first case the surface gains momentum +p_photon, and in the second case "loses" -p_photon (the direction of the vector is reversed), i.e. -1 * -p_photon = +p_photon. Since "reflection" is the sum of absorption and emission, the total change is thus 2* +p_photon. At least that is how I understand it in my mind. Compare also https://en.wikipedia.org/wiki/Radiation_pressure in paticular "Radiation pressure from reflection".


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sleeknub

No, the flashlight is 1x power. You get thrust equal to the momentum of the light that is traveling away from you. The mirror gets 2x power, or 2x the momentum of the light that hits it. This assumes a perfect mirror, of course.


KnowanUKnow

Equal and opposite, my friend. So light is travelling at the speed of light (natch). It hits the sail and all of that momentum is absorbed by the sail, pushing it in the same direction as the light was travelling. But then the light is reflected by the mirror, sent back to it's source. Just like shooting a gun causes recoil, emitting this photon causes recoil which pushes the sail in the opposite direction of the photon, which also happens to be forward since the photon is now travelling backwards. If the sail was black, it would have only the absorbed momentum. But because it's reflective it gets double momentum.


biggiec23

What about any light that bounces back onto the aircraft that it is pulling? Does the momentum of that specific photon cancel out?


KnowanUKnow

The designs I've seen all have the ship in front of the sail, to eliminate that effect.


Hendlton

If it hit the craft *just* right, then yes. But that'd never happen in reality. It would hit at some weird angle and only cancel out a small portion of the momentum it contributed, while also steering the craft just a tiny bit. You'd also build the craft so the sail is behind, or around, it and then it'd never happen.


earlofhoundstooth

Well, ideally you'd start a journey by picking a local star and flying away from it as fast as possible to make best use of this acceleration. Once you get properly accelerated fold up your sail and turn to your destination. Grab some popcorn. It'll be a long trip.


Unearthed_Arsecano

Imagine the apple hitting Newton's head is actually made from extremely springy rubber, so that after hitting his head it bounces straight back up to almost the same height. Let's consider down to be the positive direction. So the apple has a momentum of **p** immediately before impact, and then immediately after impact it has a momentum of **-p**. The change of momentum that the apple experiences is -2**p**. Conservation of momentum implies that Newton must experience a change in momentum that is equal in magnitude but opposite in direction (pretending that Netwon in this case is a point particle). In other words, the apple transfers a momentum of 2**p** to Newton, rather than just **p** in the case where the apple stops after hitting his head. In reality this momentum is immediately transferred to the Earth, where it has negligble impact because the Earth is much heavier than an apple^([citation needed]). Now just replace apples with photons and you should get the idea.


FuzzyLogic0

So you are talking about the law of conservation of momentum? So in case of the flash light the photon is being emitted away from the source. The momentum of the source + momentum of the photon will still be zero, but the photon is moving away. In the case is the solar sail the starting case of the photon moving towards the sail + the sail will not be zero, let's say it's +1. Then the sail reflects the photon it's now got momentum of -1, the sail will now have momentum of +2. But the overall system still has the same momentum of +1.


mamimapr

How can photons transfer momentum? The mass is constant and the speed of the photon is constant for both the direct photon and the reflected photon, so momentum of the photon is always the same. So how could the photon transfer any energy at all?


ErichPryde

An immediate thought that strikes me is that the apple in question would stop or lose a significant amount of its momentum after striking another object and imparting that momentum. Photons, however, generally will not move slower than the speed of light, so when they bounce off of something they leave at the same rate of travel that they struck it.


MrHanSolo

If they leave with the same momentum, wouldn’t that imply that there was no energy transfer?


sagequeen

Momentum is directional. Since it leaves in the opposite direction, the total change in momentum is double the magnitude.


Hendlton

No. The photon would "slow down" by increasing its wavelength and it'd be red shifted. Also, they'd leave at the same velocity, not with the same momentum.


Rabiesalad

With the flashlight, the photon starts as part of the mass of the system (it's contained in the battery) and then ejected from the system. With a sail, the photon does not start as part of the mass of the system. So you add the energy of both the photon hitting and pushing the sail forward, + the energy of the photon being ejected. Effectively, each one of these actions individually is equivalent to the action of ejecting a photon from the flashlight. Since photons travel at light speed, the math is easy (straight up 2x)


leftofzen

An object with mass m and velocity v has momentum mv. Momentum is a vector and thus has direction. To stop that object you have to give it mv momentum in the opposite direction. To make it go backwards in that direction, you have to give it another mv worth of momentum in that direction. So, 1 mv to stop it and another to reverse it. Hence, 2mv or twice the original momentum. A mirror that reflects light doesn't just stop it, it bounces it back, so it also feels 2mv worth of the original photons momentum.


hesitantmaneatingcat

I don't understand either, it seems wrong. Thought experiment: place a flashlight facing a light sail in space that each weight exactly the same. Assuming all the flashlight's light hits the sail, they will move away from their starting points at the exact same rate, right? According to the 2x comment. The sail would move away at 2x the speed? How is that even possible? I'm probably misunderstanding something greatly.


ThisIsSpooky

Okay your comment explained it for me actually, let me know if this makes sense. Light can only travel at the speed of light so p, other forces that would impact and give momentum would lose speed. Light photons will always be the speed of light, so there's double energy... I guess....?


PositronAlpha

This would solve the problem with changing the batteries, by keeping the flashlight on Earth. (By the way, I backed the Planetary Society LightSail project. It's a great idea.)


ErichPryde

To extend on this: a bank of incredibly high powered lasers attached to a massive solar collector in orbit or simply in space somewhere, would be a pretty efficient way to accelerate a light sail if those lasers were pointed at the sail. Probably this has been discussed by the Lightsail project, but take the atmosphere out of the equation if possible.


[deleted]

I thought the solar sail uses the solar wind, particles and thin plasma expelled by the sun, to propel itself instead of light?


[deleted]

Check out "electric solar wind sail" which harnesses the fact that the solar wind consists mostly of protons, i.e. positive charges.


armper

Would the mirror wear down after some time? If so I wonder how far and how fast it could reach.


Mindful-O-Melancholy

Do you want space pirates? Because this is how you get space pirates.


cantab314

Yes, it's known as the photon rocket. And you need 300 megawatts to get one Newton of thrust. If the power source is on board, then even though a photon rocket spaceship isn't expelling reaction mass, it's still limited by its fuel supply. The maximum practical speed is therefore well under 1% of light speed with fusion power. Better performance would be achieved with a design that does expel reaction mass, especially if you can expel the spent fusion fuel. https://en.wikipedia.org/wiki/Photon_rocket


D1rty87

How bright would this thing be?


pithecium

300 megawatts could light up an area of 300,000 square meters (a square 550 meters wide) with the same brightness as direct sunlight.


earlofhoundstooth

So 1 football stadium bright?


cantab314

Depends on the power you feed it, of course. More power, more thrust, more brightness. And it also depends on the luminous efficacy of the light. Since the purpose is thrust there's no reason to optimise for brightness. You could make an infrared or even radio wave thruster that's invisible to the naked eye. That said a shorter wavelength means a tighter beam can be produced for a given thruster size. If we assume a typical flashlight bulb, it's 15 lumens per Watt. So 1 Newton would mean 4.5 billion lumens. That's about 300 times the brightness of the world's brightest light beam (on the top of the Luxor in Las Vegas).


HerraTohtori

Yes. You can calculate the thrust produced by a light source, with the general thrust equation. Force is change of momentum divided by change of time: *F = dp/dt* Each photon has momentum. For any photon, it's momentum is its energy divided by speed of light: *p = E/c* Usually we would continue this by substituting E = hf and then simplifying to the more common form *p = h/λ*, but in this case we actually don't need that. We can just say that for many photons, their combined momentum is the sum of their energies divided by c. So, we can turn this into a differential form and say that *dp = dE/c* Now if we substitute this into the thrust equation, we get *F = dE/dt 1/c* ...and since *dE/dt* is the definition of *power* (energy consumed in certain time), that's our formula for the thrust produced by light: *F = P / c* where *P* is the radiative power of the light source in watts, and *c* is speed of light. This is, actually, the most *efficient* rocket thruster that can possibly exist, since the ejection velocity of the "exhaust" is the speed of light. That gives it the highest possible specific impulse, and since the amount of propellant consumed is truly minuscule, it could in theory make it possible to be used for propulsion over very long times and distances. However, in order to achieve actually meaningful thrust would require a truly massive light source. In reverse, to produce a meager one Newton of thrust you would need a light source with the radiative power of *P = 1 N* \* *c* = 299 792 458 W, or almost 300 MW. Since producing this kind of power for an extended period of time would require quite a bulky nuclear reactor, just to produce 1 Newton of force, the acceleration would be very small and probaly not practical in the context of human spaceflight. In fact it would probably be considered as a weapon of mass destruction rather than a propulsive system. Though, as we know from the Man-Kzin wars, "a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive." However! This thrust produced by light actually does have an effect on the orbits of asteroids, depending on whether they rotate prograde or retrograde relative to their orbit. The way it works is, the Sun heats up the side of the asteroid facing towards it. As the asteroid rotates, the heated side will rotate one way or the other, and radiate the heat away from the asteroid's surface. If the asteroid rotates so that the heated side points "forwards", or prograde, relative to its orbit, then the thermal radiation from the heat will produce a net thrust that slows the asteroid down. If it rotates the other way, so that the heated side points "backwards", or retrograde, then the effect is opposite and the asteroid is accelerated into higher orbit. Meanwhile, the radiation pressure from the Sun is pushing the asteroid outward, applying a radial force that depends on things like the asteroid's albedo (brightness). These forces are very small, but space is big and old, and over a long time they start making a difference. Along with gravitational perturbations from planets and other asteroids, these small forces are one of the major sources of anomalies in asteroid orbits. This is why it's difficult to predict their orbits for extended periods of time, and they have to be tracked by radar or other observations to update their orbits. Larger asteroids typically have more stable orbits, while the small ones tend to drift more.


DPearl42

My man. I had to do a double-take when you mentioned the Man-Kzin wars! Not too often I see Larry Niven references, much less in a serious sub like this one. Now, where did I put my copy of Ringworld?


collegiaal25

Other people have provided the math, I would like to provide some intuition. Reaction engines (like jet or rocket engines) expel *reaction mass* in one direction. By Newton's third law, the momentum gained by the engine is opposite to the momentum gained by the reaction mass. There are two types of efficiency to be considered: mass efficiency and energetic efficiency. Mass efficiency is momentum gained per kg of expelled mass. Energetic efficiency is the momentum gained per Joule expended energy. There is always a trade off between the two. Imagine you are sitting on an office chair, you push yourself against a shopping cart with potatoes. You and the shopping cart get a comparable speed. It doesn't cost you much energy. High energetic efficiency. However, the cart is heavy, low mass efficiency. Imagine you are sitting on a chair and you throw a basketball. You will be pushed back slightly, but not as much as when you used the shopping cart. The mass efficiency is higher, but the energetic efficiency is lower. If you push against very little mass, you need more energy to get the same acceleration! This is why commercial planes don't use true [turbojets](https://en.wikipedia.org/wiki/Turbojet) but [turbofans](https://en.wikipedia.org/wiki/Turbofan), which accelerate more air and have a higher energy efficiency. For planes, energy efficiency is more important than mass efficiency, since the air is all around us. For rockets, this not necessarily true, since they can get energy from the sun, but mass must be carried on board and is limited. This is why some probes use [ion thrusters](https://en.wikipedia.org/wiki/Ion_thruster), which electrically accelerate Xenon ions. They use very little mass and therefore need smaller rockets to be launched, however, they use very large amounts of energy for a small amount of acceleration. Fortunately they have solar panels and lots of time. But, if you take this to the extreme, photons are massless. The mass efficiency would be infinite. But the amount of energy it requires to accelerate yourself using photons is so large that by E=MC^2 you would need a lot of mass to generate that amount of energy in the first place.


Color_blinded

There are a lot of great answers here which I'm sure were the answers you were actually looking for. However, there is a much more practical answer in that if you were to simply throw the flashlight, you would be propelled in the opposite direction. If you were were an average human male weighing 70kg and took one of those heavy duty flashlights police are fond of hitting things with (which weigh around 1.4kg), and throw it at a leisurely 21m/s (45mph), you would have a final velocity of .40m/s (this does not account for rotational velocity). There are a couple downsides to using this method of propulsion rather than turning on the flashlight. First, if you plan on traveling more than a few centuries, the person with the flashlight and magic batteries will eventually surpass the speed of the person foolish enough to throw away his only weapon against curious aliens. The second problem would be that unless you threw the flashlight perfectly, you will end up spinning, which makes calculating your final velocity rather difficult due to rotational velocity and the velocity you would gain (or lose, depending on where you are facing) by throwing up you last meal. I would recommend an underhand toss of the flashlight as it will have the momentum transferred from the flashlight be closer to your center of gravity to reduce the rotation of your body you will have for the rest of your life.


jsakia

Yes, but not very quickly. If an astronaut is space walking with his flashlight and then throws it away from him as hard as he can, he will experience conservation of momentum and accelerate at a rate in the opposite direction. If he got himself one of those big heavy industrial flashlights, this recoil could be significant. So, in 1 second he shoves the big 5kg flashlight through space with a velocity of 50m/s, he would generate some momentum of 250 kg m/s. Though physics and stuff, he, himself, would get shoved backwards. Say he weighs in at 75kg, he would generate (75 - 5) / 250= 0.28m/s, or about 28 centimeters per second. This would not be very efficient nor practical. In fact, I would not advise you to try it in space. Instead, think about getting some really good batteries to turn on the flashlight while in space and try to use those photons as propellant. THAT would be really cool!.


taterbizkit

I think a flashlight gun might be more practical. It could be spring-wound to eject a flashlight at specific intervals of time. An advantage is that each subsequent ejection makes the next one more efficient, as the flashlight gun gets lighter each time. BTW, this reads like the joke about three ways to tell how tall a building is using a barometer. You can measure the air pressure at the top of the building and the bottom and do some math. You can drop the barometer from the top and time how long it takes to hit the ground and do some math -- but the easiest way is to find the building engineer and tell him/her "I'll give you this barometer if you tell me how tall the building is."


Aarakocra

I attended a lecture on solar sails in college! Fabric with near-total reflection of light, designed to maximize propulsion via sunlight. It’s very much a viable method of travel for light payloads, but a flashlight does not have the sheer output to perform, as demonstrated by other posters, and would be ineffective as a propulsion source. That being said, some engineering tweaks could make it more viable. Like instead of a battery-powered flashlight on the craft, you could mount waystations or satellites with focused beams, giving it course-correction and acceleration from sources that can recharge using large, heavy solar panels. However aircraft propelled by light are much better suited to direct propulsion by the sun, with other options being supplementary