The 'impossible' EmDrive could reach Pluto in 18 months? • Hacker News Board

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

#21

Post by nerd_stuff » Mon, Jul 27, 2015, 4:30 AM UTC

yincrash wrote: post #18

DenisM wrote: post #8

Let's cut to the chase: 5. Q. Why does the EmDrive not contravene the conservation of momentum when it operates in free space? A. The EmDrive cannot violate the conservation of momentum. The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection. The momentum gained by the EmDrive plus the momentum lost by the electromagnetic wave equals zero. The direc…

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

One derivation starts with E = mc^2, which is a special case of E^2 = m^2c^4 + p^2c^2 but with velocity set to zero. In the second equation if you instead set the mass to zero you get p = E/c (momentum equals energy divided by the speed of light) for massless particles.

The deeper reasons are above my pay-grade.

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

#22

Post by eropple » Mon, Jul 27, 2015, 4:37 AM UTC

jdmichal wrote: post #17

n0us wrote: post #14

The worlds biggest unsolved mystery about microwaves has still not yet been answered; how exactly does one microwave on HIGH?

While your comment doesn't add much to this discussion, I won't turn down a chance to educate. Conventional microwave ovens only have two states: on and off. (I won't rule out the possibility of some exotic build having multiple powers, but the ones you interact with on a normal basis don't.) So, power levels cannot change the output level of the magnetron. Instead, the power levels control what percentage of time th…

> I won't rule out the possibility of some exotic build having multiple powers, but the ones you interact with on a normal basis don't.

Mine does, and it cost $120.

http://www.amazon.com/Panasonic-Countertop-Microwave-Technol...

(I don't find that it actually makes a difference when using it, though.)

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

#23

Post by gizmo686 » Mon, Jul 27, 2015, 4:44 AM UTC

yincrash wrote: post #18

DenisM wrote: post #8

Let's cut to the chase: 5. Q. Why does the EmDrive not contravene the conservation of momentum when it operates in free space? A. The EmDrive cannot violate the conservation of momentum. The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection. The momentum gained by the EmDrive plus the momentum lost by the electromagnetic wave equals zero. The direc…

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 the speed of light. This is why we say that the mass of an object increases with its velocity, and that any object with mass traveling at the speed of light will have infinite mass.

Now consider the momentum, p, of a photon. We have:

     p=m*y*v
     m=0
     y=infinity
     v=c

this gives us p=0*infinity, which is indeterminate, so we cannot use this equation to determine the momentum of a photon.

Instead, we can use the engery-momentum relationship, which states:

    E^2 = (mc^2)^2 + (pc)^2

(This is a generalization of the famous E=mc^2 equation to also consider the momentum). In this equation m refers to the rest mass, not inertial mass. Because we are dealing with a photon, we have m=0, which gives us:

    E^2=(pc)^2
    p=E/c

Indicating that the momentum of a massless object is proportional to its energy.

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

#24

Post by [deleted] » Mon, Jul 27, 2015, 4:46 AM UTC

Natsu wrote: post #15

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…

What would it take to get one of these into space?

[deleted]

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

#25

Post by api » Mon, Jul 27, 2015, 4:48 AM UTC

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…

Why the assumption that if it works it must violate a conservation law? That's the last thing I would suggest after exhausting every other possibility, even weird ones like space time distortion thst would still operate within known conservation laws.

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

#26

Post by api » Mon, Jul 27, 2015, 4:49 AM UTC

saalweachter wrote: post #11

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…

Smaller effect size as studies improve is the classic sign of a non-event. ESP and the like went through similar cycles - impressive early studies followed by a series of more rigorous studies with diminishing effects until people got bored and moved on to the next big thing.

In this case different tests have occurred at different power levels. Has anyone normalized the results? Of course with only a couple replications its early for a meta study.

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

#27

Post by ChuckMcM » Mon, Jul 27, 2015, 4:49 AM UTC

Seriously, it isn't that expensive to get a shot on a possibly blowing up test rocket that is going into orbit, launch something and fly it out to mars and back. We'll believe you then.

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

#28

Post by simcop2387 » Mon, Jul 27, 2015, 4:58 AM UTC

eropple wrote: post #22

jdmichal wrote: post #17

Earlier quoted context omitted.

While your comment doesn't add much to this discussion, I won't turn down a chance to educate. Conventional microwave ovens only have two states: on and off. (I won't rule out the possibility of some exotic build having multiple powers, but the ones you interact with on a normal basis don't.) So, power levels cannot change the output level of the magnetron. Instead, the power levels control what percentage of time th…

> I won't rule out the possibility of some exotic build having multiple powers, but the ones you interact with on a normal basis don't. Mine does, and it cost $120. http://www.amazon.com/Panasonic-Countertop-Microwave-Technol... (I don't find that it actually makes a difference when using it, though.)

That's got to be the first one I've ever heard of that can do that. I don't imagine it'll make much difference in most cooking but for things like softening butter that you only do for 10-15 seconds I could see it making a big difference.

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

#29

Post by duskwuff » Mon, Jul 27, 2015, 5:17 AM UTC

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.

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

#30

Post by teraflop » Mon, Jul 27, 2015, 5:28 AM UTC

api wrote: post #25

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…

Why the assumption that if it works it must violate a conservation law? That's the last thing I would suggest after exhausting every other possibility, even weird ones like space time distortion thst would still operate within known conservation laws.

The whole reason the EmDrive is supposedly so revolutionary is that it's claimed to produce thrust without requiring any propellant. If conservation of momentum isn't going to be violated, something with mass-energy has to be carrying that momentum away from the spacecraft, and if it's not matter, it has to be radiation. But the claimed efficiency is about 5 orders of magnitude higher than radiation pressure could possibly produce.