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The 'impossible' EmDrive could reach Pluto in 18 months?

wired.co.uk

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Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#31

Post by mizzao »

MattPearce wrote: post #19
Niten wrote: post #7

It's good to see that someone finally got around to testing this in a vacuum. Here's the paper mentioned in this story: http://arc.aiaa.org/doi/abs/10.2514/6.2015-4083 High levels of skepticism are still warranted, as much as I would love for this to pan out. Every time this gets tested—presumably more and more carefully—the measured force gets smaller. The original test at Chinese Northwestern Polytechnical Universi…

The Chinese test used kilowatts rather than watts of power: http://www.you-tech.it/index.php/content/download/12369/1124...

All the more possible for small extra forces to slip through the cracks from magnetic fields or whatever.

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#32

Post by nitrogen »

gizmo686 wrote: post #23
yincrash wrote: post #18

Earlier quoted context omitted.

Can anyone explain electromagnetic momentum? I thought that em waves (photons) are massless. How can they have momentum (mass * velocity)?

Because photons travel at the speed of light, we need to consider relativistic effects. Under special relativity, classical momentum (mass * velocity) is not actually conserved. To achieve conservation of mass, we consider the "inertial mass" of an object instead of the classical mass, where the intertial mass is given by: m'=ym y=1/sqrt(1-(v/c)^2) As you can see, y>1 and y approaches infinity as you speed approaches…

    E^2 = (mc^2)^2 + (pc)^2
Is it just coincidence that this looks like the Pythagorean theorem?

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#33

Post by Semiapies »

duskwuff wrote: post #29

Making projections like "this drive could get a probe to Pluto in 18 months" seems kind of premature, given that scientists still aren't certain that the drive works at all , let alone how efficient it actually is.

I'd say science reporting is a sad ghetto of journalism, but that's giving too much credit to the profession as a whole.

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#34

Post by ProcessBlue »

nitrogen wrote: post #32
gizmo686 wrote: post #23

Earlier quoted context omitted.

Because photons travel at the speed of light, we need to consider relativistic effects. Under special relativity, classical momentum (mass * velocity) is not actually conserved. To achieve conservation of mass, we consider the "inertial mass" of an object instead of the classical mass, where the intertial mass is given by: m'=ym y=1/sqrt(1-(v/c)^2) As you can see, y>1 and y approaches infinity as you speed approaches…

E^2 = (mc^2)^2 + (pc)^2 Is it just coincidence that this looks like the Pythagorean theorem?

Yes (but also no). The initial form of this equation is

    (mc^2)^2 = E^2 - (pc^2)^2
The Pythagorean theorem tells you the length of a 2D vector when you know x and y lengths. Here mc^2 is the length of the 4-momentum vector [E, pc^2] (where p is a standard 3D vector). However since our 4D spacetime is not Euclidian but Minkowskian the sign in the generalised Pythagoras theorem is a minus and not a plus.

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#35

Post by searine »

I've been following this story pretty closing for several months now. So here's the opinion of an actual research scientist.

Q. Does the Em drive produce thrust?

A. Undetermined. Several tests from multiple independent labs have reported thrust, including in a vacuum. However several of the labs that reported positive thrust come from a sketchy background.

Q. Does it violate conservation of momentum?

A. Undetermined. There are several competing theories trying to explain the phenomenon, but we lack raw data. I think the last thing anyone wants to see here is "new physics".

Q. Roger Shawyer seems like a scummy kook?

A. Yes. This is why more testing is really important.

Q. What about more tests?

A. A few dozen people have taken it upon themselves to hack together emDrives and test them. The first couple so far were plagued by poor construction, RF interference, and poor experimental design. However, the hacking community is slowly getting its shit together.

Q. Pluto in 18 months?

A. If it works. My intuition screams "its cold fusion all over again", but there is still a chance.

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#36

Post by searine »

I've been following this story pretty closly for several months now. So here's the opinion of an actual research scientist.

Q. Does the Em drive produce thrust?

A. Undetermined. Several tests from multiple independent labs have reported thrust, including in a vacuum. However several of the labs come from a sketchy background.

Q. Does it violate conservation of momentum?

A. Undetermined. There are several competing theories trying to explain the phenomenon, but we lack raw data. I think the last thing anyone wants to see here is "new physics".

Q. Roger Shawyer seems like scummy kook?

A. Yes. This is why more testing is really important.

Q. What about more tests?

A. A few dozen people have taken it upon themselves to hack together emDrives and test them. The first couple so far were plagued by poor construction, RF interference, and poor experimental design. However, the hacking community is slowly getting its shit together.

Q. Pluto in 18 months?

A. If it works. My intuition screams "its cold fusion all over again", but there is still a chance.

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#37

Post by jimmahoney »

nitrogen wrote: post #32
gizmo686 wrote: post #23

Earlier quoted context omitted.

Because photons travel at the speed of light, we need to consider relativistic effects. Under special relativity, classical momentum (mass * velocity) is not actually conserved. To achieve conservation of mass, we consider the "inertial mass" of an object instead of the classical mass, where the intertial mass is given by: m'=ym y=1/sqrt(1-(v/c)^2) As you can see, y>1 and y approaches infinity as you speed approaches…

E^2 = (mc^2)^2 + (pc)^2 Is it just coincidence that this looks like the Pythagorean theorem?

It's not entirely coincidence.

Four dimensional space-time has a metric analogous to but not quite the same as euclidean space: the time part has the opposite sign from the space parts.

  ds^2 = (c dt)^2 - (dx^2 + dy^2 + dz^2)
ds is an "invariant proper time" which has the same value in all frames of reference. Check any special relativity textbook for the details.

Just as you can start with distance and then build up to momentum and energy in classical physics, in relativistic mechanics you can start with this metric and build up vectors in 4-space for velocity and energy/momentum. (Turns out that the time part is an energy while the space parts are momentum.) The upshot is that

  (m_0 c^2)^2 = E^2 - (p c)^2
which is the frame-invariant length of the energy-momentum 4-vector. (I think that m is better written as m_0, the rest mass, since the "m" notation sometimes means relativistic mass, which is different.)

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#38

Post by evanb »

nitrogen wrote: post #32
gizmo686 wrote: post #23

Earlier quoted context omitted.

Because photons travel at the speed of light, we need to consider relativistic effects. Under special relativity, classical momentum (mass * velocity) is not actually conserved. To achieve conservation of mass, we consider the "inertial mass" of an object instead of the classical mass, where the intertial mass is given by: m'=ym y=1/sqrt(1-(v/c)^2) As you can see, y>1 and y approaches infinity as you speed approaches…

E^2 = (mc^2)^2 + (pc)^2 Is it just coincidence that this looks like the Pythagorean theorem?

No, it is not a coincidence [0]. It is a direct result of the fact that in relativity temporal and spatial information has to be unified into a single vector (a four-vector [1]). Since (in the Noether theorem sense [2]) energy is the conserved quantity related to symmetry of displacements in time and momentum is the conserved quantity related to symmetry under displacements in space, it is quite natural to unite them into one four vector, related to symmetry of displacements in spacetime.

What's peculiar is that the temporal entry of the four vector gets the "opposite sign" for the Pythagorean theorem. That is, if you have a vector (t, x, y, z), then the "hypotenuse" (called an invariant) is t^2 - x^2 - y^2 - z^2 (up to a conventional choice of overall sign). What's surprising is that the hypotenuse is the thing that's the same for different observers. So, if you do a Lorentz transformation [3], t and the spatial entries will change, but the invariant combination won't.

Everybody knows E = mc^2 is the relationship between a particle's mass and its energy. But what is its energy if it's moving? Certainly the particle gains energy the faster it moves, right? Yes. E = mc^2 is the 0-momentum version of a relativistic expression. Since E is temporal and momentum p is spatial, the four vector can be written (E, pc), which has an invariant E^2 - (pc)^2. We call this invariant the rest mass[4], up to some factors of c. Algebraically moving things around gives us the "Pythagorean" form.

[0] https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...

[1] https://en.wikipedia.org/wiki/Four-vector

[2] https://en.wikipedia.org/wiki/Noether%27s_theorem

[3] https://en.wikipedia.org/wiki/Lorentz_transformation

[4] https://en.wikipedia.org/wiki/Invariant_mass

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#39

Post by javert »

searine wrote: post #35

I've been following this story pretty closing for several months now. So here's the opinion of an actual research scientist. Q. Does the Em drive produce thrust? A. Undetermined. Several tests from multiple independent labs have reported thrust, including in a vacuum. However several of the labs that reported positive thrust come from a sketchy background. Q. Does it violate conservation of momentum? A. Undetermined.…

Thanks, nice summary.

> I think the last thing anyone wants to see here is "new physics".

I really want to see new physics, because I want to see us get off this rock and far away. I think most people want to see new physics. I think that's why this topic is so popular.

That said, I can understand physicists not wanting to see new physics.

Wait. In a way, that sentence is crazy. In a way, physicists should want to see new physics more than anybody.

Where does the "not wanting to see new physics" sentiment come from? Just not wanting to be wrong?

Re: The 'impossible' EmDrive could reach Pluto in 18 months?

#40

Post by eevilspock »

searine wrote: post #35

I've been following this story pretty closing for several months now. So here's the opinion of an actual research scientist. Q. Does the Em drive produce thrust? A. Undetermined. Several tests from multiple independent labs have reported thrust, including in a vacuum. However several of the labs that reported positive thrust come from a sketchy background. Q. Does it violate conservation of momentum? A. Undetermined.…

> I think the last thing anyone wants to see here is "new physics".

The article's responds with the nonsensical "Some damage to our theories of physics is an acceptable payoff if we get a working space drive."

EDIT: Let me clarify why it's a nonsensical statement:

First, I think it meant to say "cost", not "payoff". Second, there would be no cost. None of the things we were able to do with our existing theories would be lost. We'd only be pushed to refine the theories (or in rare cases come up with an entirely new theories) that would be at least as accurate as the prior theories and cover phenomenon the old didn't. If anything, we'd be able to go back and do better at all the things to which we'd applied the old theories.

It's like saying "Some damage to Newtonian physics is an acceptable cost if we get a working understanding of relativity and gravity." Einstein did no "damage" to Newtonian physics. He made it better.

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