Reply
 
Thread Tools Display Modes
  #1  
Old 10-11-2019, 11:54 AM
naita is offline
Guest
 
Join Date: Jun 2002
Location: Norway
Posts: 6,642

New propellant-less drive proposal


It's entirely theoretical, someone's pet project, I noticed it through a NewScientist article with the dumbest headline and bad choice of illustration, but still!

Quote:
A new concept for in-space propulsion is proposed in which propellant is not ejected from the engine, but instead is captured to create a nearly infinite specific impulse. The engine accelerates ions confined in a loop to moderate relativistic speeds, and then varies their velocity to make slight changes to their mass. The engine then moves ions back and forth along the direction of travel to produce thrust. This in-space engine could be used for long-term satellite station-keeping without refueling. It could also propel spacecraft across interstellar distances, reaching close to the speed of light. The engine has no moving parts other than ions traveling in a vacuum line, trapped inside electric and magnetic fields.
https://ntrs.nasa.gov/search.jsp?R=20190029657

I suspect this guy is not entirely up to speed on his relativistic calculations and that it's never even going to go on a test bench (being somewhat more difficult to construct than the EM-drive that NewScientist inexplicably chose to illustrate the article with), but the guy admits he's out on a limb. And we all would love some breakthrough space tech, wouldn't we.

So anyone know enough to shoot this guy down authoritatively? Or know where to find some expert opinions on this paper?

Personally I think he's just missing something obvious and that proper calculations would show perfect symmetry, but I'm not a particle physcicist.
  #2  
Old 10-11-2019, 12:04 PM
Jasmine's Avatar
Jasmine is offline
Member
 
Join Date: Jul 1999
Location: Chicagoland
Posts: 2,257
It's sounds suspiciously like a "perpetual motion machine" kind of idea, and we all know how valid those are.
__________________
"The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge."
--Daniel J Boorstin
  #3  
Old 10-11-2019, 12:07 PM
XT's Avatar
XT is offline
Agnatheist
Charter Member
 
Join Date: Apr 2003
Location: The Great South West
Posts: 35,515
Quote:
Originally Posted by naita View Post
It's entirely theoretical, someone's pet project, I noticed it through a NewScientist article with the dumbest headline and bad choice of illustration, but still!



https://ntrs.nasa.gov/search.jsp?R=20190029657

I suspect this guy is not entirely up to speed on his relativistic calculations and that it's never even going to go on a test bench (being somewhat more difficult to construct than the EM-drive that NewScientist inexplicably chose to illustrate the article with), but the guy admits he's out on a limb. And we all would love some breakthrough space tech, wouldn't we.

So anyone know enough to shoot this guy down authoritatively? Or know where to find some expert opinions on this paper?

Personally I think he's just missing something obvious and that proper calculations would show perfect symmetry, but I'm not a particle physcicist.
Not sure the problem is with the 'slight relativistic speeds' thingy, but how do you change the mass of particles by just changing velocity? I mean, I suppose if you COULD do that you could make a closed loop sci-fi reactionless engine, but that seems kind of a show stopper right there, especially since changing mass would release energy...but then how do you get that energy back later? Maybe one of the physics oriented dopers will wander in, but seems like a version of perpetual motion and violates conservation of energy, at least just based on my quick skim.
__________________
-XT

That's what happens when you let rednecks play with anti-matter!

Last edited by XT; 10-11-2019 at 12:08 PM.
  #4  
Old 10-11-2019, 12:15 PM
Andy L is offline
Member
 
Join Date: Oct 2000
Posts: 6,680
Violates conservation of momentum, too.
  #5  
Old 10-11-2019, 12:24 PM
Shodan is offline
Charter Member
 
Join Date: Jul 2000
Location: Milky Way Galaxy
Posts: 39,990
How do you move ions back and forth in the direction of travel?
Quote:
how do you change the mass of particles by just changing velocity?
I thought the mass of something increased as you approach the speed of light, and actually attaining light-speed meant it would have infinite mass. So you accelerate the particles to near-light speed, they increase in mass, and then somehow that pushes the ship forward. But I don't understand how they push the ship - are they bouncing off the accelerator unit?

And where does the energy come from to accelerate the particles?

It does sound like perpetual motion.

IANAMathematician.

Regards,
Shodan
  #6  
Old 10-11-2019, 12:25 PM
ASL v2.0's Avatar
ASL v2.0 is offline
Guest
 
Join Date: Jul 2019
Location: Various
Posts: 307
Quote:
Originally Posted by XT View Post
Not sure the problem is with the 'slight relativistic speeds' thingy, but how do you change the mass of particles by just changing velocity?
Mass as distinguished from rest mass is a function of the total energy. You can absolutely (relatively?) change the mass by making something go faster. Whether or not the rest of it works, IDK.
  #7  
Old 10-11-2019, 12:34 PM
ASL v2.0's Avatar
ASL v2.0 is offline
Guest
 
Join Date: Jul 2019
Location: Various
Posts: 307
Quote:
Originally Posted by Shodan View Post
And where does the energy come from to accelerate the particles?

It does sound like perpetual motion.
Missed the edit window. Replying to the above, it may be that the question of where the energy comes from to accelerate the particles is something he has left to others, in much the same way that someone designing a steam turbine engine doesn't necessarily need to decide for the engineer whether the thermal energy to transition water to steam is being generated through nuclear power, fossil fuel, or waste heat from a fusion reactor.

ETA: But, again, I don’t know if the physics of it actually work, just addressing this one possible objection as perhaps beyond the scope of the problem he is setting out to solve.

Last edited by ASL v2.0; 10-11-2019 at 12:35 PM.
  #8  
Old 10-11-2019, 01:01 PM
Shodan is offline
Charter Member
 
Join Date: Jul 2000
Location: Milky Way Galaxy
Posts: 39,990
Quote:
Originally Posted by ASL v2.0 View Post
Missed the edit window. Replying to the above, it may be that the question of where the energy comes from to accelerate the particles is something he has left to others, in much the same way that someone designing a steam turbine engine doesn't necessarily need to decide for the engineer whether the thermal energy to transition water to steam is being generated through nuclear power, fossil fuel, or waste heat from a fusion reactor.
That's fair enough. How does the energy drive the ship? That is, how do you transfer the energy of motion of the ions into forward motion? It's apparently not like a rocket, where the propellant pushes the rocket forward. How does it work?

Somehow or other, you pump energy in. Fair enough - how do you get the energy to do work?

Regards,
Shodan
  #9  
Old 10-11-2019, 01:31 PM
TriPolar is offline
Guest
 
Join Date: Oct 2007
Location: rhode island
Posts: 40,712
This kind of sounds like the juggler on the bridge or the truck full of birds. How could you change the velocity of ions in a way that doesn't counteract whatever effect you get from the ions changing their mass?
  #10  
Old 10-11-2019, 01:38 PM
Shodan is offline
Charter Member
 
Join Date: Jul 2000
Location: Milky Way Galaxy
Posts: 39,990
Quote:
Originally Posted by TriPolar View Post
This kind of sounds like the juggler on the bridge or the truck full of birds. How could you change the velocity of ions in a way that doesn't counteract whatever effect you get from the ions changing their mass?
The bellhop keeps it. But only if he doesn't know what day he is going to be executed.

Don't particle accelerators add mass to the particles they affect? Does that push the accelerator in any particular direction? I believe they are circular - I wouldn't think that counts as the particles going back and forth.

Regards,
Shodan
  #11  
Old 10-11-2019, 01:45 PM
XT's Avatar
XT is offline
Agnatheist
Charter Member
 
Join Date: Apr 2003
Location: The Great South West
Posts: 35,515
Quote:
Originally Posted by ASL v2.0 View Post
Mass as distinguished from rest mass is a function of the total energy. You can absolutely (relatively?) change the mass by making something go faster. Whether or not the rest of it works, IDK.
It sounded to me as if they were saying that they were getting their acceleration by changing the mass, not just saying that mass changes as a particle accelerates to relativistic speeds. I mean, if they were just tossing the ions out the back, it wouldn't be a reactionless drive...it would just be a regular old ion propulsion drive that, admittedly, was accelerating the particles a bit faster than most of the current drives and thus getting more thrust, albeit using more energy to do so...or something.

ETA: Admittedly, I only skimmed it on my phone, so maybe I was missing something. That was the objection that passed through what pretends to be my mind at the time, but I might be totally misuderstanding the limited info saying how this is supposed to operate.
__________________
-XT

That's what happens when you let rednecks play with anti-matter!

Last edited by XT; 10-11-2019 at 01:47 PM.
  #12  
Old 10-11-2019, 01:52 PM
Chronos's Avatar
Chronos is offline
Charter Member
Moderator
 
Join Date: Jan 2000
Location: The Land of Cleves
Posts: 85,111
Quote:
Quoth Shodan:

I thought the mass of something increased as you approach the speed of light, and actually attaining light-speed meant it would have infinite mass.
Yup, a lot of people think that. But it's only true if, for some bizarre reason, you define the word "mass" to mean "energy". In which case this idea still doesn't work.

If you want a more detailed explanation: In Newtonian mechanics, the momentum of a particle is given by p = m*v. In special relativity, however, it turns out that that's only an approximation for low speeds, and for higher speeds, you need to instead use the formula p = m*gamma*v, where gamma is a unitless value that's approximately 1 at low speeds, but approaches infinity as v approaches c. Well, early on in the history of relativity, some folks decided that they really wanted to treat the Newtonian formula as correct, and so they re-defined "mass" (sometimes called "relativistic mass") as m*gamma, so the formula could still be p = m*v. By this definition, mass increases, but it's really not a very useful definition: It's usually not a useful quantity to deal with, and anyway it's just equal to the energy, so if you do need to talk about "relativistic mass", you might as well just say "energy". And even if you absolutely insist on forcing the Newtonian momentum formula to be "correct", it works a lot better to attach the gamma factor to v, instead of m, creating a quantity called "proper velocity", or u (which has a number of other applications): u = gamma*v, and therefore p = m*u.
  #13  
Old 10-11-2019, 02:10 PM
Buck Godot's Avatar
Buck Godot is offline
Guest
 
Join Date: Mar 2010
Location: MD outside DC
Posts: 6,017
As Andy L states, this would appear to be in clear violation of conservation of momentum. If you want your ship to go forward something has to go backwards. I'm not good enough with relativity to follow the math, of the original paper, but I notice it does discuss radiation being released. So it may be high energy photons shooting out the back are what is generating the thrust. I also noticed this little tidbit.
Quote:
Regrettably, the accelerator section of the design defined in Table 2 requires 165 MW of power for continuous operation. This power can also be approximated using the change in ion relativistic kinetic energy and the amount of time required to make this change. The actual “wall plug” power will depend on many other factors. Fortunately, power produced in the inner core’s decelerator is nearly equal to this amount. If all the power collected in the decelerator could be applied to the accelerator, the design described in Table 2 would require less than 10 watts to offset momentum and radiation losses for all ions during operation.
So this thing is running off of a 165MW reaction all of which needs to be recaptured except for the 10 watts that is providing momentum. I think they would be better off just shooting a laser out the back.

Last edited by Buck Godot; 10-11-2019 at 02:11 PM.
  #14  
Old 10-11-2019, 02:13 PM
ASL v2.0's Avatar
ASL v2.0 is offline
Guest
 
Join Date: Jul 2019
Location: Various
Posts: 307
For those with the time, this is an interesting take on the concept of mass (by someone seeking to make the case that there is no such thing as relativistic mass, only mass, and that the only way to properly define mass in a way that almost makes sense is in terms that include all that relativistic stuff). The lecture is about 50 minutes, and in case you’re worried he’s some quack, the lecture is before The Royal Institution:

The Concept of Mass - with Jim Baggott
(Video is through theRI's YouTube channel)

Anyway, I’m convinced, but then I’m no physicist myself.

Last edited by ASL v2.0; 10-11-2019 at 02:15 PM.
  #15  
Old 10-11-2019, 02:35 PM
Snarky_Kong is offline
Guest
 
Join Date: Oct 2004
Posts: 8,355
Is this different than the other propellant-less drive that was making the rounds a couple years ago? That one at least had prototypes built, IIRC.
  #16  
Old 10-11-2019, 02:44 PM
XT's Avatar
XT is offline
Agnatheist
Charter Member
 
Join Date: Apr 2003
Location: The Great South West
Posts: 35,515
Quote:
Originally Posted by Snarky_Kong View Post
Is this different than the other propellant-less drive that was making the rounds a couple years ago? That one at least had prototypes built, IIRC.
Yeah, this one is different than the EM drive, which is what I figure you are talking about. There have been a series of tests on the EM drive, which, I think, the latest ones pretty much found nothing. Though it's been a while since I heard/looked at anything about it. I think the last video I saw on it was from Scott Manley who has a YouTube video channel that's mostly about the Kerbal Space program.
__________________
-XT

That's what happens when you let rednecks play with anti-matter!

Last edited by XT; 10-11-2019 at 02:45 PM.
  #17  
Old 10-11-2019, 05:47 PM
iamthewalrus(:3= is offline
Guest
 
Join Date: Jul 2000
Location: Santa Barbara, CA
Posts: 12,029
Quote:
Originally Posted by Jasmine View Post
It's sounds suspiciously like a "perpetual motion machine" kind of idea, and we all know how valid those are.
Assuming it requires energy input to accelerate the particles, it's not a claim of perpetual motion.

I am not nearly a good enough physicist to evaluate this idea on its merits, but it doesn't seem totally impossible to me? A device that takes in energy and results in thrust might be possible?

Sure, conservation of momentum is required, but if you can change the masses of the things you're pushing with, you could maybe still have conservation of relative momentum, just not Newtonian momentum.
  #18  
Old 10-11-2019, 06:42 PM
Dr. Strangelove's Avatar
Dr. Strangelove is offline
Guest
 
Join Date: Dec 2010
Posts: 7,965
Quote:
Originally Posted by iamthewalrus(:3= View Post
Assuming it requires energy input to accelerate the particles, it's not a claim of perpetual motion.
It's equivalent to a claim either of perpetual motion or that the universe has a preferred reference frame.

If the force of the drive for a given power input varies with velocity, then relativity is wrong and there must be some preferred reference frame, since there must be some velocity at which the drive is most efficient.

But if that's not true--if the force of the drive is constant with velocity (for a given power), then you can turn the drive into a perpetual motion machine. Because kinetic energy goes up with the square of velocity, whereas the energy input is linear with time, if you wait long enough the KE will eventually exceed the input. You can then use some external device to capture the energy excess.
  #19  
Old 10-11-2019, 07:03 PM
Andy L is offline
Member
 
Join Date: Oct 2000
Posts: 6,680
Quote:
Originally Posted by iamthewalrus(:3= View Post

Sure, conservation of momentum is required, but if you can change the masses of the things you're pushing with, you could maybe still have conservation of relative momentum, just not Newtonian momentum.
The problem with the conservation of momentum in this situation is that if you want to device to have momentum in some particular direction, there needs to be something with momentum in the opposite direction.
  #20  
Old 10-12-2019, 06:04 PM
Hari Seldon is offline
Member
 
Join Date: Mar 2002
Location: Trantor
Posts: 13,067
Energy is still conserved. But I read a few years (unfortunately I cannot recall where) that reactionless motion was possible near the event horizon of a black hole. I think that the dependence on the black hole had to be relative (no pun) but the effect would be near unobservable in a small gravitational filed. But at near light speed?
  #21  
Old 10-13-2019, 02:32 PM
k9bfriender is offline
Guest
 
Join Date: Jul 2013
Posts: 11,500
Quote:
Originally Posted by Dr. Strangelove View Post
It's equivalent to a claim either of perpetual motion or that the universe has a preferred reference frame.
regardless of the other mechanics, its claim to be able to create thrust while only consuming 10 watts is dubious, as that means that there is only 10 watts of power available to create forward motion.

That's not very much. Your car, for instance, provides tens of thousands of watts of locomotive power.
Quote:
If the force of the drive for a given power input varies with velocity, then relativity is wrong and there must be some preferred reference frame, since there must be some velocity at which the drive is most efficient.

But if that's not true--if the force of the drive is constant with velocity (for a given power), then you can turn the drive into a perpetual motion machine. Because kinetic energy goes up with the square of velocity, whereas the energy input is linear with time, if you wait long enough the KE will eventually exceed the input. You can then use some external device to capture the energy excess.
I think that the idea is that they don't understand "relativistic mass", or at least, are pretending that they don't. The idea that something that actually gains mass as you accelerate it and decreases mass when you decelerate it could be used for something like that. Unfortunately, that's not actually how it works. It doesn't actually gain or lose any mass, it only gains or loses momentum that acts like mass only to a naive observer. If something hits you with enough relative velocity that it has a Lorentz factor of 2, then it will seem as though delivered twice the punch that its rest mass times its velocity would indicate, making your naive assumption being that the object had doubled in mass.

This isn't the case at all, as even the naive observer would note, should they choose to jog alongside the test particle. They reach out and weigh the particle, and find that it is equal to its rest mass, even though its moving. Of course, the naive observer could note that while they are doing their little jog, somehow or other the entire universe has doubled in mass. You can say that the energy to "double the mass" of the test particle comes from whatever you used to accelerate it. But where did the energy to double the mass of the universe come from? That's where it really starts to break down that "relativistic mass" does not and can not have an actual meaning that can be used to describe or predict physical phenomena.

Anyway, that's what I see them doing here, misusing relativistic mass to create a reaction-less drive and perpetual motion machine on paper. If it works, great, we've not only solved every energy crisis and space travel, but also revolutionized every idea we've ever had about the way the universe works. My money is on it not panning out.
  #22  
Old 10-13-2019, 11:11 PM
Dr. Strangelove's Avatar
Dr. Strangelove is offline
Guest
 
Join Date: Dec 2010
Posts: 7,965
Quote:
Originally Posted by k9bfriender View Post
I think that the idea is that they don't understand "relativistic mass", or at least, are pretending that they don't. The idea that something that actually gains mass as you accelerate it and decreases mass when you decelerate it could be used for something like that.
I'm not sure that's at the root of their confusion. Relativistic mass may be a dubious concept, but relativistic momentum is not, and that's really all that matters here: a momentum transfer between some internally moving masses and the external device.

But relativistic momentum doesn't change anything, because the only way to exchange momentum is via a force, and that force is exactly symmetrical forwards and back (yet another one of those pesky physical symmetries). So sure, accelerating those ions to relativistic speeds produces a greater momentum transfer than if Newton were correct. But to get those particles back to the front of the drive, you first have to cancel that backward velocity, and to do that you have to apply the same greater-than-Newtonian force to decelerate them.

In fact the nature of the function is irrelevant. The integral of a function f(x) from a to b is the negative of the integral from b to a. Whatever forces you had to apply to accelerate some particle from 0 to whatever are the exact negative of those needed to decelerate it from whatever to 0.

It's the same problem with basically all perpetual motion machines. It's like walking around a hilly landscape, eventually coming back to the origin, and hoping that somehow your altitude has changed. It's obviously impossible, and the path you take and the shape of the landscape is utterly irrelevant.
  #23  
Old 10-13-2019, 11:34 PM
DPRK is offline
Guest
 
Join Date: May 2016
Posts: 3,852
Quote:
Originally Posted by Hari Seldon View Post
Energy is still conserved. But I read a few years (unfortunately I cannot recall where) that reactionless motion was possible near the event horizon of a black hole. I think that the dependence on the black hole had to be relative (no pun) but the effect would be near unobservable in a small gravitational filed. But at near light speed?
You can theoretically extract energy from a rotating black hole by throwing stuff in it or by surrounding it with a magnetic field.
  #24  
Old 10-13-2019, 11:39 PM
Dr. Strangelove's Avatar
Dr. Strangelove is offline
Guest
 
Join Date: Dec 2010
Posts: 7,965
Quote:
Originally Posted by Hari Seldon View Post
But I read a few years (unfortunately I cannot recall where) that reactionless motion was possible near the event horizon of a black hole.
I have a hard time believing it would be truly reactionless. Using some frame-dragging effect to push yourself off the black hole without propellant? Sure, I can believe that. But the black hole would then be pushed in the opposite direction. No different from jumping off a small asteroid.
  #25  
Old 10-13-2019, 11:42 PM
k9bfriender is offline
Guest
 
Join Date: Jul 2013
Posts: 11,500
Quote:
Originally Posted by Dr. Strangelove View Post
I'm not sure that's at the root of their confusion. Relativistic mass may be a dubious concept, but relativistic momentum is not, and that's really all that matters here: a momentum transfer between some internally moving masses and the external device.

But relativistic momentum doesn't change anything, because the only way to exchange momentum is via a force, and that force is exactly symmetrical forwards and back (yet another one of those pesky physical symmetries). So sure, accelerating those ions to relativistic speeds produces a greater momentum transfer than if Newton were correct. But to get those particles back to the front of the drive, you first have to cancel that backward velocity, and to do that you have to apply the same greater-than-Newtonian force to decelerate them.

In fact the nature of the function is irrelevant. The integral of a function f(x) from a to b is the negative of the integral from b to a. Whatever forces you had to apply to accelerate some particle from 0 to whatever are the exact negative of those needed to decelerate it from whatever to 0.

It's the same problem with basically all perpetual motion machines. It's like walking around a hilly landscape, eventually coming back to the origin, and hoping that somehow your altitude has changed. It's obviously impossible, and the path you take and the shape of the landscape is utterly irrelevant.
This is obviously crank woo bullshit, and only deserves as much analysis as one finds entertaining, certainly not worth going into great detail about, but if you turn off your brain for a minute, then their misuse of "relativistic mass" makes sense if you don't think about it too much.

You send the particles really fast in one direction. Because of how fast they are going, they have more mass. You send them in the other direction at a slower speed, so they have less mass. Therefore, you have more mass moving in one direction than the other, and bob's your uncle, forward thrust!

That's how I read the claims, anyway. There may one day be some great breakthrough that changes our view of physics, but this is not it.
  #26  
Old 10-14-2019, 10:33 AM
Lumpy's Avatar
Lumpy is offline
Charter Member
 
Join Date: Aug 1999
Location: Minneapolis, Minnesota US
Posts: 16,666
Quote:
Originally Posted by Jasmine View Post
It's sounds suspiciously like a "perpetual motion machine" kind of idea, and we all know how valid those are.
Quote:
Originally Posted by k9bfriender View Post
This is obviously crank woo bullshit, and only deserves as much analysis as one finds entertaining, certainly not worth going into great detail about, but if you turn off your brain for a minute, then their misuse of "relativistic mass" makes sense if you don't think about it too much.

You send the particles really fast in one direction. Because of how fast they are going, they have more mass. You send them in the other direction at a slower speed, so they have less mass. Therefore, you have more mass moving in one direction than the other, and bob's your uncle, forward thrust!

That's how I read the claims, anyway. There may one day be some great breakthrough that changes our view of physics, but this is not it.
This. They're claiming that relativistic momentum introduces an asymmetry between the fast moving and slow moving parts of the cycle. I haven't the math skills to prove it but almost certainly not true.
  #27  
Old 10-14-2019, 11:21 AM
eburacum45 is online now
Guest
 
Join Date: Feb 2003
Location: Old York
Posts: 2,883
Quote:
Originally Posted by DPRK View Post
You can theoretically extract energy from a rotating black hole by throwing stuff in it or by surrounding it with a magnetic field.
This only works for a rotating black hole. If you managed to remove all the charge and angular momentum from a black hole, you couldn't get any more energy out by these methods.

Luckily most, or all, black holes are probably rotating quite fast, so this trick will normally be possible.
  #28  
Old 10-14-2019, 11:50 AM
iamthewalrus(:3= is offline
Guest
 
Join Date: Jul 2000
Location: Santa Barbara, CA
Posts: 12,029
Quote:
Originally Posted by Dr. Strangelove View Post
It's equivalent to a claim either of perpetual motion or that the universe has a preferred reference frame.

If the force of the drive for a given power input varies with velocity, then relativity is wrong and there must be some preferred reference frame, since there must be some velocity at which the drive is most efficient.

But if that's not true--if the force of the drive is constant with velocity (for a given power), then you can turn the drive into a perpetual motion machine. Because kinetic energy goes up with the square of velocity, whereas the energy input is linear with time, if you wait long enough the KE will eventually exceed the input. You can then use some external device to capture the energy excess.
Thanks for explaining so clearly.

So does that mean that no propellant-less propulsion system is consistent with relativity? I guess you could have a system that somehow deflects exterior mass to push against but didn't have to eject its own mass?
  #29  
Old 10-14-2019, 02:38 PM
k9bfriender is offline
Guest
 
Join Date: Jul 2013
Posts: 11,500
Quote:
Originally Posted by iamthewalrus(:3= View Post
Thanks for explaining so clearly.

So does that mean that no propellant-less propulsion system is consistent with relativity? I guess you could have a system that somehow deflects exterior mass to push against but didn't have to eject its own mass?
Depends on what you mean by propellant.

You could have a laser pointing out the back, and that would give you thrust. The "propellant" in this case is photons. Downside is that the power to thrust ratio is terrible, upside is the the specific impulse is effectively infinite.

It is possible for some of these proposed "reaction-less" drives to give thrust, but it would not be because they are revealing some new physics, but because they leak radiation preferentially in one direction. Of course, if that is what is giving the thrust, there are less complicated ways of doing so.

You could also use the magnetic field of the Earth or of the Sun (or other body with a magnetic field. Great for Jupiter too, but not so much for Venus or Mars.) to produce thrust, requiring only electricity to run, but in this case, the reaction mass is whatever is coupled to the other end of the magnetic field.

There is the possibility that someone will figure out how to interact with dark matter or neutrinos or some other fundamental aspect of space-time, and create a drive that does not use any conventional particle as reaction mass, but even in this case, it would be the dark matter or neutrinos or fundamental aspect of space-time that it is acting upon. A naive physicist with only contemporary tools at their disposal may well consider such a device to be reaction-less, but only because it is not fully understood.
  #30  
Old 10-14-2019, 04:32 PM
Lumpy's Avatar
Lumpy is offline
Charter Member
 
Join Date: Aug 1999
Location: Minneapolis, Minnesota US
Posts: 16,666
Some proposed "Mach" drives supposedly use the entire background mass of the universe as the reaction mass; and/or equivalently decouple inertia from the rest of the universe.

Sometimes it's hard to distinguish speculative physics from pure woo.
  #31  
Old 10-16-2019, 08:44 AM
Andy L is offline
Member
 
Join Date: Oct 2000
Posts: 6,680
Quote from the paper explaining this drive:

Basic concept is unproven
Has not been reviewed by subject matter experts
Math errors may exist!
  #32  
Old 10-16-2019, 07:05 PM
Darren Garrison's Avatar
Darren Garrison is offline
Guest
 
Join Date: Oct 2016
Posts: 11,693
Here is an anal-lysis of the proposal. (It breaks apart something pulled from the author's butt.)
  #33  
Old 10-17-2019, 08:40 AM
naita is offline
Guest
 
Join Date: Jun 2002
Location: Norway
Posts: 6,642
Great! I expected it was flawed in the manner, but wanted someone who actually understands the details to spell it out!
  #34  
Old 10-17-2019, 09:57 AM
Just Asking Questions is offline
Guest
 
Join Date: Jan 2014
Posts: 7,381
Quote:
Originally Posted by Jasmine View Post
It's sounds suspiciously like a "perpetual motion machine" kind of idea, and we all know how valid those are.
"That's impossible. But I don't suppose there'd be any harm in looking over diagrams on it. "
  #35  
Old 10-17-2019, 10:29 AM
Half Man Half Wit's Avatar
Half Man Half Wit is online now
Guest
 
Join Date: Jun 2007
Posts: 6,856
Quote:
Originally Posted by Hari Seldon View Post
Energy is still conserved. But I read a few years (unfortunately I cannot recall where) that reactionless motion was possible near the event horizon of a black hole. I think that the dependence on the black hole had to be relative (no pun) but the effect would be near unobservable in a small gravitational filed. But at near light speed?
Quote:
Originally Posted by Dr. Strangelove View Post
I have a hard time believing it would be truly reactionless. Using some frame-dragging effect to push yourself off the black hole without propellant? Sure, I can believe that. But the black hole would then be pushed in the opposite direction. No different from jumping off a small asteroid.
No, this is a real effect in general relativity, known as 'spacetime swimming'. In a gravitational field, a body can perform a series of local deformations involving only internal forces, and undergo a positional translation as a result---see the original paper here and a discussion here.

Conservation of momentum, however, isn't violated, as the 'swimmer' never has a nonzero net momentum.

Furthermore, the Alcubierre metric---better known as the 'warp drive'---would also be an example of a 'reactionless' drive---although of course no thrust is really generated, as the 'ship' isn't actually accelerated. But even for sublight speeds, such a configuration would need exotic forms of matter currently not known to exist.

The drive discussed here won't work, though. If you correctly account for the four-momenta of the ions and the fields used to accelerate and decelerate them, you will not obtain a net momentum in any direction.
  #36  
Old 10-18-2019, 01:51 AM
Dr. Strangelove's Avatar
Dr. Strangelove is offline
Guest
 
Join Date: Dec 2010
Posts: 7,965
Quote:
Originally Posted by Half Man Half Wit View Post
No, this is a real effect in general relativity, known as 'spacetime swimming'. In a gravitational field, a body can perform a series of local deformations involving only internal forces, and undergo a positional translation as a result
That's super interesting--thanks!

From your initial description, I thought "hmmm--sounds a bit like how a cat can change its orientation in space while maintaining zero angular momentum", and I see from your second link that it's a close analogy.

Of course the effect can't be used for a drive that continuously accelerates, as is being posited here. You can only accelerate a very small amount before having to pull in your limbs and decelerate again.
  #37  
Old Yesterday, 01:19 AM
GreenWyvern's Avatar
GreenWyvern is online now
Guest
 
Join Date: Jan 2008
Location: Cape Town
Posts: 1,746
The latest xkcd is relevant to this discussion.
  #38  
Old Yesterday, 04:08 AM
Half Man Half Wit's Avatar
Half Man Half Wit is online now
Guest
 
Join Date: Jun 2007
Posts: 6,856
Quote:
Originally Posted by Dr. Strangelove View Post
That's super interesting--thanks!

From your initial description, I thought "hmmm--sounds a bit like how a cat can change its orientation in space while maintaining zero angular momentum", and I see from your second link that it's a close analogy.
Yes, that's a great intuition! But one has to be (as always) careful with analogies---the other one Koelman suggests, of the ball thrown once around a closed universe, doesn't actually quite work, as you could also do that on a torus with zero curvature. So this is actually a topological effect, not one of curvature!

Quote:
Of course the effect can't be used for a drive that continuously accelerates, as is being posited here. You can only accelerate a very small amount before having to pull in your limbs and decelerate again.
Still, it illustrates a dilemma one sometimes faces in these debates: before knowing about spacetime swimming, I would have categorically denied that such a thing is possible. So, if I claim that this sort of drive discussed here is impossible (which it is), what's to prohibit anybody from claiming, 'well, you were wrong about spacetime swimming, so maybe you're wrong about this, too!'.

It's hard to conclusively argue that there's no 'wiggle room' for the kind of thing proposed here---it's two different levels of being wrong: being wrong about spacetime swimming involves being wrong about the consequences of established physical theory, while being wrong about the 'helical drive' would entail being wrong about its fundamental principles. The former is routine---even established theory may hold surprising and intriguing consequences (that's what makes the whole thing fun). The latter, however, would essentially overturn vast swaths of our current scientific understanding---so being wrong about that would mean having to clear a much larger hurdle of empirical evidence and theoretical reasoning than being wrong about spacetime swimming does. To most laypeople, however, both just look like 'being wrong' in the same sense.

Last edited by Half Man Half Wit; Yesterday at 04:12 AM.
  #39  
Old Yesterday, 04:29 AM
eburacum45 is online now
Guest
 
Join Date: Feb 2003
Location: Old York
Posts: 2,883
The whole universe is permeated by gravity fields that curve space - so it should (in theory) be possible to perform 'space-time swimming' anywhere. But the effect is so small that in most of the universe this effect would not be measurable.

One place where the effect would be noticeable would be close to the event horizon of the supermassive black hole at the centre of the galaxy. Space is so sharply curved there, that the circumference of a circle can be significantly greater than 2πr.
  #40  
Old Yesterday, 08:02 PM
Lumpy's Avatar
Lumpy is offline
Charter Member
 
Join Date: Aug 1999
Location: Minneapolis, Minnesota US
Posts: 16,666
Quote:
Originally Posted by GreenWyvern View Post
The latest xkcd is relevant to this discussion.
That's not so hard; just divide the 52 cards into an infinite number of infinitely convoluted pieces, then put them back together slightly differently. Set Theory, it's magic!
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 06:42 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright 2019 STM Reader, LLC.

 
Copyright © 2017