Rowing across the interstellar sea

December 14, 2020 7:44 AM

Physicist and Historian of Science Jim Woodward may be getting tantalisingly close to developing a propellantless thruster that can accelerate up to lightspeed. Just very slowly...
posted by Chairboy (42 comments total)

C...conservation of momentum??
posted by techbasset at 8:16 AM on December 14, 2020 [5 favorites]

I really think Wired does a disservice hyping yet another crank theory. Maybe crank isn't quite fair, they are trying to do real science and really do have a small NASA grant. But stilll.. The article is fairly responsible in reporting on all the skepticism, but it clearly is from an "I Want to Believe" perspective. The only new hook here is "a bold new result in June" where they say they stopped tamping some resonant vibration because it was essential to the thing working. Only now they say there's one specific frequency it works at, but they don't know what it is and it changes all the time anyway. Good luck with that.
posted by Nelson at 8:31 AM on December 14, 2020 [10 favorites]

Oh, Wired. Never change.

No, wait, I mean, do change. Please change.
posted by biogeo at 8:31 AM on December 14, 2020 [9 favorites]

If we want rockets that don't use propellant it's probably going to be photonic. At least photons can be imparted with momentum.
posted by Your Childhood Pet Rock at 8:36 AM on December 14, 2020 [1 favorite]

Too bad if your vehicle hits a tiny piece of matter while traveling at light speed, though.
posted by ZenMasterThis at 8:49 AM on December 14, 2020

There's an easy way to answer whether the physics is good or not.

During the next joint talks at the underground base on Mars, one of our representatives should casually drop some comment about Elon Musk buying up a bunch of piezoelectric materials.

If the other delegates start looking nervous, maybe these physicists are on to something...
posted by RonButNotStupid at 8:53 AM on December 14, 2020 [1 favorite]

If we're doing impossible fringe science, why limit ourselves to sub-lightspeed? Just build a warp bubble generator!
posted by kaibutsu at 10:00 AM on December 14, 2020

Maybe I'm just sentimental today but...

a 79 year old scientist, survivor of stage IV lung cancer living with COPD and being treated for relapsed Hodgkin's lymphoma, hunkering down at home during COVID to keep working on what he thinks is a way to expand humanity's reach among the stars...i dunno. Good for you, Jim. Godspeed.
posted by lazaruslong at 10:08 AM on December 14, 2020 [25 favorites]

Relevant XKCD

TL;DR; There is no such thing as a free lunch. Even if you hand wave about resonance chambers.
posted by nickggully at 10:36 AM on December 14, 2020 [3 favorites]

To expand on techbasset's question, something I wrote a few years ago:

Conservation of momentum comes directly from conservation of energy and Galilean relativity. (Yes, that Galileo. Described almost 500 years ago.)

Let's say I have a battery powered device. It weighs one kilogram. If I turn it on, it accelerates away from me at 1 m/s^2 for five seconds and continues away at 5 m/s forever. It does so without pushing anything back the other direction. How much energy does it use?

Well, because it weighs one kilogram and moves 5 m/s, the high-school-physics answer is that it has 12.5J of kinetic energy. Whatever else it does, it can't do what it does without converting at least 12.5J from its power source into kinetic energy.

Now, let's say I get on an airplane with this device. While moving 300 m/s, I activate the device. After five seconds, it's moving 305 m/s down the aisle, and it keeps going until it bumps into the cockpit door. How much energy did it require?

The same high-school-physics equation tells me that to go from 300 m/s to 305 m/s requires 1512.5J from its power source. Whatever else it does, to accelerate a 1kg object from 300 m/s to 305 m/s requires at least 1512.5J.

So, why can I walk forward on an airplane? After all, I'm going from 300 m/s to 305 m/s, and I'm certainly not feeling bogged down like I have to work a hundred times as hard as I would to walk on land.

The reason is that in order to walk forward, I have to push back on the airplane. Doing that causes it to move slightly slower, giving up an enormous amount of kinetic energy, which all gets concentrated into me.

This happens invisibly and most of us never really think about it. This is Galilean relativity, the fact that accelerating and decelerating an object by pushing against another object doesn't change based on how fast the system is moving. If two objects exert force on each other, all observers will agree on how much their relative speed will change. All observers will agree that a certain amount of energy needs to be expended to cause that change to happen, but they won't agree on the invisible answer of how exactly that energy got distributed into and from kinetic energy.

But any device that can accelerate without pushing on something else breaks Galilean relativity, because no two observers can agree on how much energy in needed to do what it did.
posted by Hatashran at 10:38 AM on December 14, 2020 [6 favorites]

It sure would be nice if all you needed to get a useful-on-human-scale acceleration was some electric power. You could go anywhere in a human lifetime, as long as you didn't get creamed by dust or fried by unruh radiation or the extremely blue shifted CMB. Everyone you ever knew back home would be dead before you could get back if you went farther than the closest stars, but you could visit the other side of the milky way out Andromeda or wherever. Indefinite constant acceleration combined with length contraction leads to some pretty crazy sounding things.

It would even be physically possible (ignoring the dust and radiation, anyway) if all known means of generating photons in a reasonably directional manner (at least those that don't require a stellar mass object) weren't so inefficient...
posted by wierdo at 10:59 AM on December 14, 2020 [1 favorite]

And for the record, there is actually stuff to push against everywhere in the universe, just not necessarily enough to be useful to a human with our short lives and significant mass density relative to things like the solar wind, interstellar medium, other charged particles that happen to be around, and the magnetic fields that all permeate the entire visible universe. Lasers and their ilk are a more realistic solution for a thruster that doesn't require reaction mass.
posted by wierdo at 11:13 AM on December 14, 2020 [1 favorite]

Aren't ion thrusters another working example of a low-but-constant acceleration drive with limited propellant requirements? Not no propellant, you need a source of ions, but Deep Space 1 has gotten 4300m/s delta-V with only 74kg of xenon propellant and 2.1kW of solar power.

The propellant was 15% of the launch weight. How much difference would getting that down to 0 make? Maybe it becomes interesting if you want to run for many years and get over 100,000 m/s delta-V and start approaching relativistic speeds?
posted by Nelson at 11:20 AM on December 14, 2020 [1 favorite]

It makes pretty much all of the difference. We can pack an enormous amount of power into a small package with radioisotopes or possible Future Fusion Thingies, but we can't carry near enough propellant to use up all of that power. A reactionless rocket would let us turn on the propulsion and leave it on for, e.g. 50 years. Imagine where voyager would be now if a thing like this were possible.

Articles like this make me sad. I don't begrudge the old man his hobby and I hope to be so lucky, but the reporter was gullible or complicit or both. Every single crackpot red flag, from operating outside his field of expertise up to and including crystals and energy waves.

We've been to this party before, Wired.
posted by Horkus at 11:33 AM on December 14, 2020 [8 favorites]

The advantage of a reactionless drive is you can just keep going. Ion drives are still limited by how much propellant they carry. And a reactionless drive doesn't have to impart velocity onto the propellant. Both of these things is an advantage normally (I mean it would totally revolutionize interplanetary exploration) but are especially advantageous if you want to continuously boost for years.

EG: Roughly, and keeping it modest, if you can get up to 20% of light speed with a year of constant boosting we can get and orbit an instrument package to Alpha Centari in roughly 23 years. Barnard's Star in about 32, Wolf 359 in 42, Lalande 21185, Sirius and Luyten 726-8 in ~45. Another dozen stars in less than 62.

I can't even conceive how much of a boon that would be to science. Cripes even if we can't build a transmitter that works at those distances a round trip to Alpha Centari would only be 40 years.

Seems unlikely I'll ever see such a drive developed in my lifetime.
posted by Mitheral at 11:51 AM on December 14, 2020 [1 favorite]

The spaceship, it vibrates?

This is going to need about 1000x the evidence before anyone should write a mass-market news article about it.
posted by GuyZero at 12:35 PM on December 14, 2020 [2 favorites]

Visualizing a dark matter paddlewheel starship.
posted by sammyo at 12:38 PM on December 14, 2020 [4 favorites]

The other thing I visualize is essentially a 200 mile long straight line linear accelerator powered by insanely huge solar panels. What I have not researched is what can be given a charge, can ground up regoth dust be charged and be effective as a charged stream rocket fuel? (This ship would never land, but be a ferry from planet to asteroid to planet.
posted by sammyo at 12:41 PM on December 14, 2020 [1 favorite]

Visualizing a dark matter paddlewheel starship.

Conservation of angular momentum is a harsh mistress.
posted by Your Childhood Pet Rock at 12:45 PM on December 14, 2020 [3 favorites]

Let's get back to the basics with stuff like FTL communication via an infinitely rigid rod between here and Uranus.
posted by GuyZero at 12:51 PM on December 14, 2020 [1 favorite]

The advantage of a reactionless drive is you can just keep going.

The disadvantage is it not existing in the universe we live in.
posted by GuyZero at 12:52 PM on December 14, 2020 [5 favorites]

I wouldn't put it past the universe to have a Secret Preferred Reference Frame.
posted by Pyry at 12:56 PM on December 14, 2020 [1 favorite]

I wouldn't put it past the universe to have a Secret Preferred Reference Frame.

Double secret probation reference frame.
posted by GuyZero at 1:20 PM on December 14, 2020 [1 favorite]

If we want rockets that don't use propellant it's probably going to be photonic.

Yeahbut a photon drive with as much thrust as a Falcon 9 would also be something like a petawatt laser that nobody in history could be trusted with.
posted by GCU Sweet and Full of Grace at 1:39 PM on December 14, 2020

Isn't there an adage about any reasonably powerful spaceship drive doubling as a WMD?
posted by rhamphorhynchus at 2:57 PM on December 14, 2020

That observation was certainly made in Niven's Known Space series. The humans have eliminated war and have no armed spacecraft, but when the Kzin attack expecting easy pickings, our starships are more than capable of fighting back with their reaction drives. And/or the fixed lasers used to launch starships from the Belt.
posted by mark k at 3:14 PM on December 14, 2020

I'm not sure if anyone ever ran the heat dissipation calculations on Niven's laser-ships and I'm not sure that they wouldn't end up as 2000K blackbodies.
posted by GuyZero at 3:17 PM on December 14, 2020

I wouldn't put it past the universe to have a Secret Preferred Reference Frame.

And with our luck, it'd be moving .9999c with respect to Earth, meaning that any acceleration against it would require ridiculous amounts of energy.
posted by Hatashran at 3:49 PM on December 14, 2020

Humanity once again turns to myth and fashions a new eschatology for the age. The evidence we have suggests the most likely scenario is eventually earth becomes uninhabitable and humanity is erased with the exception of a few space probes that will also eventually rot away from micrometeorite impacts and radiation. Our observations suggest that we are actively working to hasten our demise.
Rather than confront the blunt truths of our circumstances we instead create a myth of a future where humanity ascends to the heavens and becomes immortal star children.
posted by interogative mood at 3:53 PM on December 14, 2020 [3 favorites]

And with our luck, it'd be moving .9999c with respect to Earth, meaning that any acceleration against it would require ridiculous amounts of energy.

Oh we already know about that one. It's about 70km/sec/megaparsec.
posted by Your Childhood Pet Rock at 4:18 PM on December 14, 2020

"Conservation of momentum comes directly from conservation of energy and Galilean relativity."

Conservation of momentum comes directly from the invariance of a Lagrangian under translation in space. This comes from Emmy Noether... who is way cool and nowhere near as old as that dude.

The point of the Mach principle stuff is that this symmetry is kinda violated in general relativity, which is basically the marriage of Lorentz invariance and the equivalence principle.

(Note: this is not an endorsement of Woodward or his ideas. )
posted by dsword at 4:56 PM on December 14, 2020 [4 favorites]

The spaceship, it vibrates?

Yep. There's a class of pseudo-inertialless drives that work by jiggling, the most famous of which is the Dean Drive. That one, as best anyone can tell, worked by stiction: it would slowly draw a weight forward then shoot it back, or accelerate a weight forward into a stop, or do some other asymmetric pair of movements that relied on the object gripping the floor for slow impetuses but jerking forwards for large ones. The same asymmetry can exist even when an object is suspended as a pendulum: the claimed motion is too small to be visible, and the effect just has to fool the measuring device.
posted by Joe in Australia at 9:25 PM on December 14, 2020 [1 favorite]

If we want rockets that don't use propellant it's probably going to be photonic. At least photons can be imparted with momentum.
posted by Your Childhood Pet Rock

Shine me to the moon,
Let me play among the stars
posted by Pouteria at 4:58 AM on December 15, 2020

If this guy is a well-known crank whose reported experimental results can’t be replicated how did he get two NASA grants? Not defending his science, more curious about NASA’s process.
posted by MattD at 5:59 AM on December 15, 2020

Humanity once again turns to myth and fashions a new eschatology for the age. The evidence we have suggests the most likely scenario is eventually earth becomes uninhabitable and humanity is erased with the exception of a few space probes that will also eventually rot away from micrometeorite impacts and radiation. Our observations suggest that we are actively working to hasten our demise.
Rather than confront the blunt truths of our circumstances we instead create a myth of a future where humanity ascends to the heavens and becomes immortal star children.

sir this is a wendy's
posted by lazaruslong at 11:02 AM on December 15, 2020 [13 favorites]

If this guy is a well-known crank whose reported experimental results can’t be replicated how did he get two NASA grants? Not defending his science, more curious about NASA’s process.

why do you think this guy is a crank? He's a professor emeritus of physics at California State University, has a pretty decently long list of published and peer reviewed work. I'm not sure what the definition of a crank is as it tends to shift around a lot, but Dr. Woodward doesn't seem like one to me.
posted by lazaruslong at 11:06 AM on December 15, 2020

I'm not asserting an opinion on this guy or theory specifically, but I will say that this is in fact one of the classic crank phenotypes.

Professor emeritus at a mid level university who moves from a solid, mainstream career to novel opinions about a subject that wasn't really in their area when they were active. They've got the paper credentials to get people to listen them, a lot of self-confidence, and have lost the negative feedback channels they got earlier career.

In terms of going completely off the rails, the risk factors are up there with economists at right wing think tanks.
posted by mark k at 12:59 PM on December 15, 2020 [3 favorites]

Mmm, that's fair. I think my mental crank picture has "no credentials, or really shitty ones if that" as a strict requirement. I can see the phenotype you describe as approaching crankdom, though, for sure.
posted by lazaruslong at 1:20 PM on December 15, 2020

The NASA “NIAC” program has a history of funding studies in all kinds of of weird stuff. I suspect it exists to as a means of dealing with certain members of Congress and their crazy constituents.
posted by interogative mood at 8:47 PM on December 15, 2020 [1 favorite]

There have been lots of these over the years — the Dean Drive (1950s), Eric Laithwaite's gyroscopic drive (1980s), Shawyer's EmDrive (2000s), and more. The question you have to ask in each case is, "which do I trust more, the principle of conservation of momentum, or the ability of some guy to eliminate all sources of experimental error?"
posted by cyanistes at 2:41 AM on December 16, 2020 [1 favorite]

My position is that is entirely reasonable for scientists to support and pursue fringe theories as long as:

a) it's within your area of expertise
b) your efforts are spent trying to convince your colleagues, not the general public

That's the Scientific Method(tm) more or less. If some incidental press results, that's basically unavoidable and fine. Will anything come of it? probably not. Should I, a non-expert, care? also no.

As a non-expert I'll follow the expert consensus, but I won't begrudge those trying to change that. In physics, this happens a lot with cosmology - there's a lot of weird models that deviate from the general consensus that still have adherents. But the consensus is probably a lot weaker in this area to begin with than it is for stuff around reactionless drives, so maybe it's not so comparable.
posted by vibratory manner of working at 2:46 AM on December 16, 2020 [2 favorites]

I wouldn't put it past the universe to have a Secret Preferred Reference Frame

In a very real sense, the universe does have a preferred reference frame. It's called the zCosmic Microwave Background. It's not fundamental, of course, the laws of physics don't dictate it exist, but it is there and can be a quite useful measuring stick thanks to its freakish uniformity. Building instruments capable of detecting the very slight nonuniformity in its temperature was quite an impressive technological accomplishment.
posted by wierdo at 9:08 AM on December 19, 2020