Q: Is reactionless propulsion possible?

Physicist: In a word: no.

A reactionless drive is basically a closed box with the ability to just start moving, on its own, without touching or exuding anything.  The classic sci-fi tropes of silent flying cars or hovering UFOs are examples of reactionless drives.

The problem at the heart of all reactionless drives is that they come into conflict with Newton’s famous law “for every action there is an equal and opposite reaction” (hence the name).  To walk in one direction, you push against the ground in the opposite direction.  To paddle your canoe forward, you push water backward.  The stuff you push backward so you can move forward is called the “reaction mass”.

In order to move stuff forward, you need to move other stuff backward.

This is a universal law, so unfortunately it applies in space.  If you want to move in space (where there’s nothing else around) you need to bring your reaction mass with you.  This is why we use rockets instead of propellers or paddles in space; a rocket is a mass-throwing machine.

But mass is at a premium in space.  It presently costs in the neighborhood of \$2000/kg to send stuff to low Earth orbit (a huge improvement over just a few years ago).  So, the lighter your rocket, the better.  Typically, a huge fraction of a rocket’s mass is fuel/reaction mass, so the best way to make spaceflight cheaper and more feasible is to cut down on the amount of reaction mass.  The only way to do that at present is to use that mass more efficiently.  If you can throw mass twice as fast, you’ll push your rocket twice as hard.  Traditionally, that’s done by burning fuel hotter and under higher pressure so it comes shooting out faster.

In modern rockets the exhaust is moving on the order of 2-3 km per second.  However, your reaction mass doesn’t need to be fuel, it can be anything.  Ion drives fire ionized gas out of their business end at up to 50 km per second, meaning they can afford to carry far less reaction mass.  Space craft with ion drives are doubly advantaged: not only are they throwing their reaction mass much faster, but since they carry less of it, they can be smaller and easier to push.

The drawback is that ion drives dole out that reaction mass a tiny bit at a time.  The most powerful ion drives produce about 0.9 ounces of force.  A typical budgie (a small, excitable bird) weighs about 1.2 ounces and, since they can lift themselves, budgies can generate more force than any ion drive presently in production.

Compared to rockets, ion drives pack a greater punch for a given amount of reaction mass.  However, they deliver that punch over a very long time and with less force than a budgie.

Given the limitations and inefficiencies, wouldn’t it be nice to have a new kind of drive that didn’t involve reaction mass at all?  You’d never have to worry about running out of reaction mass; all you’d need is a power supply, and you could fly around for as long as you want.

That’s not to say that propellantless propulsion isn’t possible.  There are ways to move without carrying reaction mass with you.  You can use light as your exhaust (a “photon drive”), but you’ll notice that a flashlight or laser pointer doesn’t have much of a kick.  And you can slingshot around planets, but then the planet is your reaction mass.

The problem with reactionless drives, fundamentally, is that Newton’s third law has no (known) exceptions.  It is one of the most bedrock, absolute rules in any science and a keystone in our understanding of the universe.  On those rare occasions when someone thought they had found an exception, it always turned out to be an issue with failing to take something into account.  For example, when a neutron decays into a proton and electron, the new pair of particles don’t fly apart in exactly opposite directions.  Instead, the pair have a net momentum that the original neutron did not.

When a stationary neutron (gray) decays into a proton (red) and electron (blue), the new pair flies apart, but always favor one direction.  Newton’s laws imply that there must be a third particle moving in the other direction to balance the other two.

The implication (according to Newton’s law) is that there must be another particle to balance things out.  And that’s exactly the case.  Although the “extra particle that’s really hard to detect” theory was first proposed in 1930, it wasn’t until 1956 that neutrinos were finally detected and verified to exist.  The imbalanced momentum, a violation of Newton’s laws, came down to a missing particle.  Today neutrinos are one of the four ways we can learn about space, along with light, gravity waves, and go-there-yourself.

There are plenty of ideas floating around about how to create reactionless drives, such as the Woodward Effect or the Albecurrie warp drive.  But in no case do these ideas use established science.  The Woodward effect depends on Mach’s principle (that somehow inertia is caused by all the other matter in the universe), and reads like a pamphlet a stranger on the street might give you, while the Albecurrie drive needs lots of negative energy, which flat-out isn’t a thing.

Science is all about learning things we don’t know and trying to prove our own theories wrong.  While scientific discovery is certainly awe inspiring, it is also the exact opposite of wishful thinking.  That said, good science means keeping an open mind much longer than any reasonable person would be willing to.  In the ultimate battle between theoretical and experimental physics, the experimentalists always win.  If someone ever manages to create a self-moving, reactionless drive, then all the theories about why that’s impossible go out the window.  But as of now, those theories (standard physics) are holding firm.  We can expect that for the rest of forever, all space craft will have a tail of exhaust behind them.

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15 Responses to Q: Is reactionless propulsion possible?

1. Dan says:

One of my personal heroes is Kelly Johnson. Born in an obscure town in the Upper Peninsula of Michigan. As a student at the University of Michigan, he performed wind tunnel testing on the P-38 Lightning and contributed to the design. Importantly, he was the founder of the Skunk Works at Lockheed . A selection of his accomplishments include leading the team for: first US Jet, the U-2 Spy plane (he advised the government to fly if for only two years, but they flew it for four), the record holding SR-71, etc. His successor was a fellow by the name of Ben Rich, who wrote a book worth reading about the Skunk Works. On his death bed, Mr. Rich reportedly stated ‘Anything you can imagine, we can do’. I can imagine quite a lot, but have seen a few things outside the bounds of commonly known physics… which is ‘division’ of math.

In my college years (circa early ’90s) I saw a news brief showing a video recording from an Astronaut aboard the Space Shuttle. It showed a bright object approaching the Earth, and suddenly two very fast objects from Earth’s surface were ‘sent?’ at it (Earth to orbit in about a second, not laser beams), and the approaching object made a V turn, yes V turn, and moved out of camera. I looked for the video on YouTube since then, but the video was highly degraded. I am a trained theoretical and empirical scientist and know what I saw; I wouldn’t bother typing this if it weren’t factual. The ‘official’ explanation was that it was space dust bouncing off the window. This is highly, highly improbable.

I’ve taught my kids F=MA and about the equation’s discoverer. If M drops to zero, somehow, then F and A could be whatever you want them to be. All I am saying is all. I’ll race you all to figure it out and put the folks at JPL to shame… or prove me wrong.

To paraphrase Shakespeare: There are more things in Heaven and on Earth that can be dreamt of by our philosophers, and you my friends are great philosophers if you know it or not.

2. Pavel Aymaliev says:

Isn’t the warp drive concept also sort of reaction propulsion which reacts with space itself ? Like the canoe example the drive pulls space infront of it and than pushes it around and behind it (similar to diver in deep sea that swims).
And also why is it imminently considered that the Alcubierre drive always needs negative energy densities ? For example, according to my humble understanding of cosmology and relativity negative pressure should also do the bending space-time trick. And there are actually known phenomenons which rely on the effects of negative pressure: the cosmological constant/vacuum energy (if we could manipulate it) and the inflaton field that drove the universe’s expansion (if we could reproduce it somehow).

3. Edwin Friskey says:

Since space is an almost perfect vacuum, would it be possible to make the space in front of you less dense than the space behind you causing you to move forward? Obviously that wouldn’t be easy, would it work?

4. Leo says:

When hot gases leave rocker nozzle, the particles move chaotically and not only in the direction opposite to the rocket flight. So great part of energy is wasted. Is there a way to make all particles (say atoms) to move just in one direction? Kind of laser but not of photons but of atoms, so all of them are coherent and move in exactly same direction?

5. The Physicist says:

@Leo
“Coherent matter waves” (mass lasers) are a thing, but they’re far more trouble than they’re worth in this case. Even coherent beams have some spread; there’s no way to get it to zero.
If you look at the exhaust from a rocket it does form a cone, but it’s a pretty sharp cone. It’s mostly going in the right direction.

6. The Physicist says:

@Edwin Friskey
There are a couple of ideas that are close to that.
A “ramscoop” is a hypothetical (but fairly reasonable) device that would use big magnetic fields to direct the stray ions in front of a ship into a scoop to be used as fuel.
A “solar sail” is literally just a really big, really light-weight sail that catches light (photon pressure) and solar wind flying out of a star. Solar sails are an extremely efficient way to move around in a solar system, if you’re not in any kind of hurry.

7. The Physicist says:

@Pavel Aymaliev
Unfortunately, space isn’t a substance that can be grabbed. If it could be, then we’d have a way to determine whether or not something is moving (by dropping a “space anchor”), which is a big violation of the base principles of relativity.
Dark Energy, assuming it is an actual “stuff” that exists rather than a mathematical artifact, seems to have an absolutely fixed density. Alcubierre’s warp drive requires us to be able to manipulate how that energy is distributed. But as of now, we don’t know how to do that, whether it’s possible, or if the stuff we want to manipulate even exists.
That could certainly change in the future, but I wouldn’t hold out too much hope.

8. David Martin says:

The question is, can you push against the quantum vacuum? That’s the question being asked by the NASA team who test new ideas for drives, some of which seem to work at this stage, according to papers they’re published, though it’s too early to tell.

We know there’s energy in the vacuum, it was predicted by quantum field theory, and then confirmed by experiment, with the Casimir effect. Experiment gave the same number as theory. But when we try to calculate the vacuum energy another way, we get a number 120 orders of magnitude different. This enormous discrepancy, the vacuum catastrophe, means that if you ask, do we know enough about the vacuum energy to answer the question about whether a drive could push against it, the answer would unavoidably be ‘no way’.

9. Leo says:

@ The Physicist: yes, the gas jet from the nozzle has nearly straight shape, but the atoms in the jet move chaotically, i.e. in all directions, because the gas is very hot. If it were nearly absolute zero temperature and moving just in one direction, then energy would not be wasted, right?

10. The Physicist says:

@Leo
Being hot means there is variation in the velocities of the exhaust molecules. But ultimately, they’re going in the right direction, so that variation takes the form of some of them traveling a little faster or slower than the mean. If you look at a rocket when it’s active, there isn’t a lot of gas spraying out sideways. Heat does make it spread out faster, but it’s not a major hurdle.

11. Tom Kirby says:

Just one small point. If you have a limited supply of ejection mass available for propulsion then you must eject it at as as high a velocity as possible…but that raises a question. In the text, a statement to the effect, says if you eject at twice the speed you get twice the reaction…of course…but if you are talking kinetic energy then doubling the speed of ejected mass quadruples the kinetic energy…i.e. E=1/2 mv²….so there.

12. craig says:

Seems the ultimate solution is to minimise the exhaust while maximising the applied energy. Couldn’t a COIL in a solenoid in a rocket be “moulded” by the thinnest of conductive plasma or gas – and then be supercharged by an onboard generator/capacitor. Thus the coil becomes the propellant

13. Tom Kirby says:

I am out-thunk by Craig in that I have no idea what he is talking about….

14. craig says:

I get that a lot. I remember Einstein’s statement that only half a dozen people really understood relativity.
I thought for a moment you could be right … but then thunk that was inthunkable.

15. Tom kirby says:

The trouble with not understanding other people’s thinking is that you are never sure where the deficiency lies…a sublimating solenoid moulded from thinnest of plasma leaves me defenceless. I just CANNOT envisage such a thing…and I have been making plasma, winding solenoids and sublimating for 50 years. Help me. You might be on to something…or a complete numpty. I know the feeling.:)