flashlights and space ships

newton said that for every action there is an equal but oposite reaction. so why is it that when i turn on my flashlight, and photons go streaming out one end at an alarming rate, that i don’t get thrown against (through?) the wall? is it the friction between me and the floor? if i powered a space ship with a realy big flashlight, why wouldn’t it go realy, realy fast?


I realy, realy (sic) think maybe a good physics book will help. (sorry, had to do it…)

Photons have no mass. Therefore, no inertia. Therefore no Newton’s Third Law.

Men will cease to commit atrocities only when they cease to believe absurdities.

The reason you don’t get hurled against the opposite wall when you switch on the flashlight is because although the photons the flashlight emits DO exert a force in the opposite direction from where it’s aimed, that force is vanishingly tiny. Certainly it’s not anywhere near enough to move any object big enough to see with your naked eye.
Now, if you were out in space with a really big reflective surface (as in tens of miles across), then you could get useful thrust out of the photons emitted by the sun. Nobody’s built a lightsail yet, but there’s no reason it wouldn’t work once we do build it. You could also build a gigantic kick-ass laser and propel something with the beam. If you do an internet search on ‘lightsail’ you’ll undoubtedly turn up enough reading material to keep you busy for a couple weeks…

Someday we’ll look back on this, laugh nervously, and change the subject…

Hey, Cowboy, did I screw up? I thought photons were massless creatures (still think they are).

Isn’t the “lightsail” you mentioned more like a “solar wind sail”?

According to NASA,

I think you might be confusing terms here.

best book (fiction) I know of about lightsails is “The Mote in God’s Eye” by Larry Niven and the other guy.

I can see where the confusion would come in, E1… the terminology is similar. Now, the solar wind, as we all know, is hydrogen nuclei, free electrons, and various and sundry other particles, all of which have mass. The photons, on the other hand, DO have zero REST mass, which is why they travel at lightspeed. Photons, however, also have non-zero RELATIVISTIC mass, which is how they come to have momentum, which can be transferred to other objects and how you can get useful work out of them.
Now, you can get useful thrust out of them both… but, as Niven and Pournelle pointed out in ‘The Mote in God’s Eye’, you can’t tack against the solar wind. It STICKS to your lightsail, and you can’t reflect it as you can with light. You can tack against the light coming from a star, and that in part is how you’d maneuver a lightsail craft.
This excerpt, taken from this site might clear things up. To wit:
—In consideration of the physics behind the sail a common misconception is that the propulsive force is provided by the solar wind.
The solar wind is a stream of charged particles, mostly high velocity electrons and protons, emanating from the solar corona. It is a
highly dynamic phenomenon, influenced by factors such as solar flares and X-ray bursts. The energy attatched to these particles
however is several orders of magnitude smaller than the energy attatched to the flux of solar photons itself (9x10-6 newtons for each
m2 of sail).

Hence the lightsail makes use of the solar photon flux for propulsion. While photons have no rest mass, they do of course have mass
while in motion, and thus momentum. Upon collision with the reflective sail material the photon will be reflected and, in the process,
apply a force to the sail. The size of the total force on the body of the craft will thus be proportional to the sail’s area.

The applied force can be directed by tilting the sail with respect to the incoming photon flux, this will change the direction of the
acceleration applied to the sail and thus allow changes in the sail’s orbit.

There’s much more at that site, which I commend to your attention… :slight_smile:

Someday we’ll look back on this, laugh nervously, and change the subject…

They do have a good example of light moving an object. There are vacuum globes that have paddles in a cross formation. The axis is a tiny almost frictionless point at the base. Looking at the globe from the side you see white on the back side of the paddles, and black on the front side. The paddles spin like a top. The stronger the light, the faster the paddles move. All my science class rooms have had one of these. I have also seen them in specialty shops in the mall. Same shops that have the floating globe thermometers

Phobia, don’t those move not so much because of the light itself but because of the difference in temperature between the light and dark sides of the paddles? Like some kind of convection current or something?

My apologies to all! I guess I opened my mouth way too soon…

I’ve been doing a little reading and it seems that I’m the one confusing terms.

Guess the one who needs to get a NEWER physics book is me. :o

De nada, E1. It was many years before I grasped that massless particles might have relativistic mass, and thus attained Buddha-nature. :wink:

Phobia, pldennison… the device you’re referring to is called a Crookes Radiometer. More detail than you ever wanted to know about how the damn thing works (it’s not convection) can be found here.

Someday we’ll look back on this, laugh nervously, and change the subject…

Well I’ll be darned. I always thought I knew how those things worked, but I was wrong.
Thanks, CG for that link.

PBS had a mini-series on space flight a few months ago. One of the topics was the problem of getting heavy equipment into orbit without using heavy rockets.

They showed these guys out in the desert lifting metal cones a few hundred feet with lasers. They would take these metallic cones, spin them thousands of r.p.m.s (for stability), and then zap the under side of 'em with super concentrated laser beams. The lasers were so strong that they had to place a screen over the launch site to shield overhead satellites from getting fried by the laser.

I don’t remember what the physics was behind it. I think it had something to do with pulverizing the air molecules along the serface of the underside of the cone causing it to lift (WAG). Get Cecil in here and maybe he can explain that part.


The laser-propelled rockets don’t use light pressure. They use the energy from the laser to heat a reaction mass inside the rocket chamber, which gets blown out the back at high velocity just like any other rocket. In the case of the small metal scale rocket you saw, I believe the reaction mass is just air, which gets superheated and expelled. In larger versions the interior surfaces of the reaction chamber can be designed to continually vaporize and add reaction mass, or the rocket can actually carry reaction mass in tanks and add it to the mix. Obviously, once the air gets thin enough you need some other form of reaction mass if you want to keep on accelerating.

Dang, I must be using the wrong brand of flashlight, or perhaps the wrong brand of batteries. Every time I turn on my flashlight, the photon stream knocks me backwards a few steps. Makes it hell to go out to take a leak at night when we’re camping.

Actualy, Dex, you’d be surprised. The people at MagLite have devised the UPD ( Urinary Photon Deflector). It allows the holder to …um…hold…you know…whilst holding a large MagLite,with no “backlash”.
They come in a variety of attractive anodized aluminum colors. :slight_smile:

Well, even if the vacuum globe thing isn’t an example of the momentum of light, there is another example-- the photocell.

Photocells work because photons knock electrons off of one electric plate on to another, creating a voltage. Einstein invented the concept of the photocell, and used it to prove his idea that light could act like a particle, with momentum. Although he invented the idea of the photon, he didn’t coin the name. He called them “light quanta”. Einstein won the Nobel Prize for the concept of the photocell.

I still find the description of light having “relativistic mass” to be a shaky idea. Certainly it has momentum, and perhaps saying that it has relativistic mass simplifies explanation. But it isn’t quite the same as mass in the normal sense, because light doesn’t demonstrate inertia (which is one of the two major definitions of mass). There isn’t a valid way to define the mass of a photon as E=mc^2 specifically refers to rest mass, and E=gamma*mc^2 would give you infinity times zero.

So personally, I prefer to simply refer to the photon as being massless yet having momentum.

Cowboy Greg:
Thanks for the post. Now I know they’re called a Crookes Radiometer. Good link.

For definitions of relativistic mass and invariant mass I find the following link helps me. It has taken me several readings just to grasp the concept though.


I remember an essay by Isaac Asimov about solar wind sails. His calculations showed that any appreciable energy would require a sail kilometers across, at the very least.

Aside from the material properties of such a sail, what would significant motion through space do to that sail, considering dust and meteorites?

Congratulations, you’ve realized one of the big engineering difficulties involved in designing a lightsail craft.

You are correct that a lightsail would need to be extremely large in order to catch enough light to be useful. The craft would also need to have a low total mass, so that it would accelerate more readily. This means that we’d want the sail to be very thin, to save weight. (Think of aluminum foil, but thinner.)

The problem is that a thin sail would be fragile. Since a lightsail craft accelerates for such a long period of time, it can reach a significant fraction of the speed of light. At that kind of velocity, even a speck of dust packs a lot of momentum, and would likely put a tiny hole through the sail. Over the many-years-long journey, this “swiss cheesing” effect would add up, very slowly reducing the efficiency of the sail.

Encountering a bigger meteoroid could be trouble. A large hole in one part of the sail would make the whole thing assymetrical, meaning that the thrust would not be applied uniformly. As acceleration continued, the ship would tend to start rotating because of the unequal thrust. The sail would then not be perpendicular to the propelling force; like a ship tacking across the wind, the craft would begin to travel at an angle, and could miss its destination by a huge margin.

If the meteoroid did not simply tear through the sail, but instead hit some more solid part of the craft, there would be even more serious problems. A fist-sized rock, hitting a spacecraft moving at 5% of the speed of light, would pack the same punch as a Mack truck driving at 2000 mph. Unless you’re Larry Niven, material that can withstand this magnitude of impact is hard to come by.

Of course I don’t fit in; I’m part of a better puzzle.