Conservation of momentum

[PingOfPong]

A Brittish engineer has developed an engine which claims to violate the law of momentum. Forumites are discussing it here. The engine has the supposed ability to achieve acceleration without any propellant.

Like everyone else who has taken a couse or two in physics, I was taught that momentum will always be constant. I always took this law for granted because it makes perfect sense to my experience. I wonder though, does the law of momentum share the same logical problem as stating that all swans are white? Is there a compelling reason to assume that it's true at all times?

 

[Dancing David]

Because as of yet no violation of the law of conservation has been demonstrated. It is not a true scotsman arguement. No one has demonstrated a violation of the law you can transform energy but not destroy it that we know of.

Here is the home site:
http://www.emdrive.com/

From the Theory section:

Thus if the EM wave travelling in a tapered waveguide is bounced between two reflectors, with a large velocity difference at the reflector surfaces, the force difference will give a resultant thrust to the waveguide linking the two reflectors. If the reflectors are separated by a multiple of half the effective wavelength of the EM wave, this thrust will be multiplied by the Q of the resulting resonant cavity, as illustrated in fig 1.

...
The inevitable objection raised, is that the apparently closed system produced by this arrangement cannot result in an output force, but will merely produce strain within the waveguide walls. However, this ignores Einstein’s Special Law of Relativity in which separate frames of reference have to be applied at velocities approaching the speed of light. Thus the system of EM wave and waveguide can be regarded as an open system, with the EM wave and the waveguide having separate frames of reference.

So this person is claiming to boost the EM wave and create momentum from that. They are not creating momentum from nothing, they are generating it by 'boosting' the EM wave. Now why you would not just use a directed microvae without the funnel, i don't know.

If it works , it should be easy to test.

In fact the person who developed it say it doesn't violate the law of momentum. From the Dynamic tests area

The engine was mounted on a dynamic test rig enabling it to be “flown” on a rotary bearing, as shown in fig 3. The tests simulated the engine moving a 100Kg spacecraft in weightless conditions. The programme included acceleration and deceleration runs in both directions, and confirmed the thrust levels measured in the static tests. The dynamic operation also conclusively proved that the engine obeys all Newton’s laws, and that although no reaction mass is required, the engine is not a reactionless machine. Reaction occurs between the EM wave and the reflector surfaces of the resonator, and the law of conservation of momentum is maintained with the transfer of the momentum of the EM wave to the engine.

When it is used or replicated i will beleive it.

 

[PingOfPong]

Whatever the specifics of the claim, Sol's discussion centered on the momentum problem. Anyway, I'm really interested in the broader question.

ETA: No matter what the inventor says, if the thing speeds off into space without emitting something behind it that carries opposite momentum, it is still creating momentum from nothing.

 

[The Man]

A Brittish engineer has developed an engine which claims to violate the law of momentum. Forumites are discussing it here. The engine has the supposed ability to achieve acceleration without any propellant.

Like everyone else who has taken a couse or two in physics, I was taught that momentum will always be constant. I always took this law for granted because it makes perfect sense to my experience. I wonder though, does the law of momentum share the same logical problem as stating that all swans are white? Is there a compelling reason to assume that it's true at all times?

A charged particle moving through a magnetic or electrical field experiences acceleration without “any propellant”, as does a mass moving through a gravitational field. Acceleration is just a change in momentum (or specifically change in velocity over change in time). Velocity (like momentum) is a vector having both magnitude and direction. In an orbit (or free fall) no propellant is required, but the craft is constantly decelerating in one direction while accelerating in another, although the magnitude of velocity and momentum vector tangent to the radius do not change in a circular orbit, the direction that is tangent to the radius (and thus the direction of that tanget vector) does change as the body orbits. In an elliptical orbit both the magnitude and direction of the momentum and velocity vectors change as the body orbits, again without propellant (something shooting out the back).

 

[Theophage]

I wonder though, does the law of momentum share the same logical problem as stating that all swans are white? Is there a compelling reason to assume that it's true at all times?

Yes, it is called Noether's Theorem.

 

[Verde]

Here is the home site:
http://www.emdrive.com/

Thanks for the link.

Didn't we do this a while back?

The drawing in the theory section implies that the forces are only acting on the flat portions of the cavity, but in fact they will be impinging on all surfaces.
Integrating over the complete cavity surface will result in a zero net force.
Invoking relativistic frames of reference to try and change the outcome is simply silly. EM operation of cavity resonators has been been well known for many decades.

I see no mention of how they propose to power the magnetron.

V.

 

[Zeuzzz]

A Brittish engineer has developed an engine which claims to violate the law of momentum. Forumites are discussing it here. The engine has the supposed ability to achieve acceleration without any propellant.

This reminds me of roy thornston who built a device that had three independant spinning wheels that when spun in a certain 'specfic' way, apparently, when the the wheels alligned the momentum of them would create a small net force in one direction. Which the law of conservation of momentum would certainly not allow. When put on water the device would move in a direction, as if creating a reactionless drive.

Of course though, when tested in a vacuum it did not show any effects. And this sems to be the problem with all reactionless drives. So far, to date, I have not seen any that can violate conservation of momentum in a vacuum. Though I certainly dont want to rule it out, the chinese seem to be pretty confident they've made something.

But the thing is that if this chinese device does work, it wont violate the conservation of momentum, there will be an extra law added to explain why this effect can occur within the conservation of momentum. It doesn't mean that the conservation of momentum is wrong in any way, its one of the most well tested laws in history, just that (like with GR in certain domains) its applicability can be questioned in very specific and well defined conditions.

You may want to check out the reactionless drive page at wiki, which is basically what this is about: http://en.wikipedia.org/wiki/Reactionless_drive

 

[sol invictus]

Like everyone else who has taken a couse or two in physics, I was taught that momentum will always be constant. I always took this law for granted because it makes perfect sense to my experience. I wonder though, does the law of momentum share the same logical problem as stating that all swans are white? Is there a compelling reason to assume that it's true at all times?

Conservation of momentum follows if the fundamental laws of physics are invariant under translations in space; that is, if they are the same over there as they are here. A good example of a case where that does not hold is phonons (vibrations) propagating through a solid. Momentum is not conserved for them, because they can transfer it to the lattice they are moving through (since the lattice isn't the same everywhere it breaks translation invariance). If one ignores the momentum transferred to the lattice, which is usually a good approximation, momentum isn't conserved. Of course if you keep the momentum of the lattice, it is.

So that's the assumption - translation invariance of the laws of physics. It's a very compelling one, I would say, and it has been tested to extreme accuracy in many ways, but one can never prove anything in science.

 

[MattusMaximus]

A Brittish engineer has developed an engine which claims to violate the law of momentum. Forumites are discussing it here. The engine has the supposed ability to achieve acceleration without any propellant.

Like everyone else who has taken a couse or two in physics, I was taught that momentum will always be constant. I always took this law for granted because it makes perfect sense to my experience. I wonder though, does the law of momentum share the same logical problem as stating that all swans are white? Is there a compelling reason to assume that it's true at all times?

Yes, it's called Newton's Third Law.

The law of conservation of linear momentum is a direct consequence of Newton's Third Law.

 

[Theophage]

Noether's Theorem explains why Newton's third law applies.

 

[69dodge]

A charged particle moving through a magnetic or electrical field experiences acceleration without “any propellant”, as does a mass moving through a gravitational field. Acceleration is just a change in momentum (or specifically change in velocity over change in time). Velocity (like momentum) is a vector having both magnitude and direction. In an orbit (or free fall) no propellant is required, but the craft is constantly decelerating in one direction while accelerating in another, although the magnitude of velocity and momentum vector tangent to the radius do not change in a circular orbit, the direction that is tangent to the radius (and thus the direction of that tanget vector) does change as the body orbits. In an elliptical orbit both the magnitude and direction of the momentum and velocity vectors change as the body orbits, again without propellant (something shooting out the back).

You're considering only the momentum of the orbiting body. What about the momentum of the body about which it orbits?

Gravity doesn't pull one object towards another. It pulls two objects towards each other.

 

[PingOfPong]

A charged particle moving through a magnetic or electrical field experiences acceleration without “any propellant”, as does a mass moving through a gravitational field.

In this case, the propellant is whatever system that created the field. Think of a rail gun for example. It uses magnetic fields and current to shoot a metallic slug. A rail gun has recoil just like a regular powder gun. No matter how you apply a force to a charged particle, there is still an equal and opposit force somewhere as well as conservation of momentum.

Noether's therem

So if I'm reading Theophage's and Sol's responses right, the assumption that physical laws are the same in all places leads to a theorem of conservation. That's a rational assumption and it is pretty compelling.

 

[The Man]

In this case, the propellant is whatever system that created the field. Think of a rail gun for example. It uses magnetic fields and current to shoot a metallic slug. A rail gun has recoil just like a regular powder gun. No matter how you apply a force to a charged particle, there is still an equal and opposit force somewhere as well as conservation of momentum.

Well, that was part of my point, Electromagnetic or Gravitational fields causing acceleration (angular or linier) while conservation of energy and momentum still hold. The other part of my point being that this is happening “without emitting something behind it that carries opposite momentum”. In the case of a gun, powder or rail, it is the gun itself emitting something that carries opposite momentum, namely the slug.

Ever action has an equal and opposite reaction, while one might be obvious the other may not be, especially when neither is “emitting something behind it that carries opposite momentum”, as in the case of gravity.

Sorry, if I was not clear on both of those points.

 

[The Man]

You're considering only the momentum of the orbiting body. What about the momentum of the body about which it orbits?

Gravity doesn't pull one object towards another. It pulls two objects towards each other.

It may have seemed that I was considering only one momentum but that was part of the point that the opposing change in momentum is not always as obvious as that of the obviously accelerating body.

 

[Dancing David]

So this theory says that flashlights would propel themselves in zero G?

 

[Dave Rogers]

ETA: No matter what the inventor says, if the thing speeds off into space without emitting something behind it that carries opposite momentum, it is still creating momentum from nothing.

Having given this some thought, it's clear that it violates conservation of energy as well. The inventor claims that it uses microwaves in a resonant cavity to produce reactionless thrust, because some backwater of general relativity makes the momentum of the wave packets greater at one end of the cavity than at the other. It's trivial to hypothesise an idealised form of the device in which all reflecting surfaces are perfect and no loss occurs in the medium within the cavity. If the physics of the device were correct, microwaves could be injected into this idealised cavity, allowed to persist indefinitely, and would still produce thrust, as there is no mechanism for energy loss within the cavity. Therefore the device would be generating kinetic energy without losing an equal or greater amount of electromagnetic field energy.

Dave

 

[Dave Rogers]

So this theory says that flashlights would propel themselves in zero G?

Why wouldn't they? Photons have momentum.

Dave

 

[sol invictus]

So this theory says that flashlights would propel themselves in zero G?

Standard physics says that.

What this theory says is that a flashlight attached to and shining into an opaque, sealed container would propel itself.

It's a bit like trying to propel a ship by filling the hull with water and putting the propellers on the inside.

 

[Dancing David]

Why wouldn't they? Photons have momentum.

Dave

Yes , I know that but it is the same priciple. Why is the EM drive different?

Standard physics says that.

What this theory says is that a flashlight attached to and shining into an opaque, sealed container would propel itself.

It's a bit like trying to propel a ship by filling the hull with water and putting the propellers on the inside.

Now that I can understand.

 

[Dave Rogers]

Yes , I know that but it is the same priciple. Why is the EM drive different?

Because nothing leaves the cavity. Firing photons out of one end is a perfectly viable way of creating thrust. The concept of a lightsail is based on this, although it gets double the momentum per photon because they're reflected back on themselves. The EM drive doesn't fire photons out of one end; it's based on the principle that the photons bounce harder off one end of the cavity than they do off the other. I strongly suspect that there's a force on the tapered walls of the cavity that balances out the difference exactly, but I'd have to revise a lot of stuff I haven't looked at for a quarter of a century before I could work it out.

Dave