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Where can I read more about how/why this is impossible? Isn't it just the reverse of a solar sail to send out the photons yourself rather than catching them? It

Where can I read more about how/why this is impossible? Isn't it just the reverse of a solar sail to send out the photons yourself rather than catching them? It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here?

SideburnsOfDoom 12y ago
> It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here? Yes, there does seem to be something else. Classical mechanics cannot explain thrust from shining a flashlight inside your spaceship. But "the EmDrive converts electric power into thrust ... by bouncing microwaves around in a closed container"
nsxwolf 12y ago
I guess a related question is "why does a flashlight in space not propel itself". And I don't know the answer to that. Edit: Ok. That makes sense. I guess it's just not a practical source of propulsion.
- oneandoneis2 12y ago
  It does. But the propulsion is really, really tiny.
- leishulang 12y ago
  https://answers.yahoo.com/question/index?qid=20111214233110AAicrQm https://answers.yahoo.com/question/index?qid=20111214233110A...
- IanCal 12y ago
  A flashlight in space would propel itself, since photons have momentum.
- dragonwriter 12y ago
  A flashlight in space does propel itself, but that's not really a related question. A more related question is why does a flashlight in a fixed mount inside of a windowless spacecraft not propel the spacecraft (or, similarly, why does a fan inside of a sealed, ventless housing attached to an airplane wing not function as an engine.) Of course, if there's some way this does work despite what we think we know about physics, its a pretty gigantic breakthrough (well, at least, understanding and practically applying it would be -- otherwise its just an anomaly that provides a pretty gigantic question mark...)
IanCal 12y ago
> Isn't it just the reverse of a solar sail to send out the photons yourself rather than catching them? No, that'd be the case if it were open but it isn't. As I understand it, it's an entirely closed system. That's what's controversial, there's nothing coming out of this "thruster", not even photons. There's an image on page 3: http://www.emdrive.com/yang-juan-paper-2012.pdf http://www.emdrive.com/yang-juan-paper-2012.pdf (disclaimer, I only have a hazy understanding of what this is supposed to be)
- davorak 12y ago
  > As I understand it, it's an entirely closed system. You got the gist correct. The microwaves are supposed to be completely contained.
- Gravityloss 12y ago
  Wouldn't it be relatively straightforward to repeat the Chinese test in a garage? We're talking a whopping 70 millinewtons here, so equivalent to the weight of 7 grams. Some shrews are lighter than that. Microwave science has this strange aura, since the stuff that happens in cavities etc is not exactly obvious at first sight. http://en.wikipedia.org/wiki/Cavity_magnetron http://en.wikipedia.org/wiki/Cavity_magnetron
  - leishulang 12y ago
    sorry to disappoint you, but it's actually 30~50 micronewtons, which is only 0.04g.
  - leishulang 12y ago
    I checked again. Some insider said the Chinese machine does produce 80 millinewtons, but it's less efficient than the NASA one since they take way more power input.
phkahler 12y ago
>> It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here? That is absolutely true. Electromagnetic radiation effectively has momentum. Take the energy in joules and convert it to mass via E=mc^2 and then plug it in the momentum equation p = mv but using c as the velocity and you get p = E/c. Change in momentum over change in time is force, so we can then get to F = P/c. I believe the units are Newtons, Watts, and meters/sec. So with 300 Megawatts you can produce a whole Newton of force! I used this on a physics final once, but hid the work because the prof expected us to use formulas for radiation pressure derived from electromagnetics and I had missed that lecture. Later I derived the general case of my approach and it matched the EM derived formulas. 50/50 if he would have appreciated it or marked it down. The question was "a 50mW laser is reflected off a mirror, what force is exerted on the mirror due to the beam?". Note that the change in momentum is doubled for reflection vs emission or absorption.
- evincarofautumn 12y ago
  > the prof expected us to use […] and I had missed that lecture You just gave me a wave of nostalgia. That’s how I survived all of my math and physics classes. If you develop an intuitive understanding of the material, you can derive things as needed from a few core principles you do remember. A bit sad when learning is the way to subvert an educational system, though.
  - pavel_lishin 12y ago
    That's how I passed quite a few math exams that I couldn't be bothered to study for.
- deleted 12y ago
  [deleted]
  - schiffern 12y ago
    Yes, charged batteries have more mass per e=mc². That means that a good AA battery weighs ~100 picograms more than a dead one.
daveloyall 12y ago
I think that when you send out photons, there is energy in them. Like, energy you would have rather have converted into momentum. ...IANAP.
- fixermark 12y ago
  The momentum those photons are carrying implies a change in momentum to the emitter that is equal and opposite. Conservation of momentum applies even though the emitted substance is electromagnetic radiation, not matter particles. What's weird about this device is that the radiation chamber is completely closed---the microwaves emitted are then re-absorbed by the chamber, but the claim is that the net momentum of the chamber has changed... Somehow. If it is true, it'd be a violation of conservation of momentum (unless the microwaves are somehow passing momentum to something else, which is why people are talking about quantum mechanics possibly coming into play).
  - daveloyall 12y ago
    We're well beyond my physics knowledge here. (See IANAP above!) So, I have questions. Is momentum the only type of energy a photon has? By allegory: Suppose a cannonball can be red hot, or cool. Separately, it can be flying or at rest. So, there are four cannonball types: hf, hr, cf, and cr. The hot, flying cannonball has the most energy and the cool, resting cannonball has the least. Yet, the momentum of both flying cannonballs is equal--temperature is orthogonal. Is all that right? If it is, then... I thought photons contain some energy that is similarly orthogonal to momentum. ...I guess I don't recall learning that, so maybe I just made it up. But, I have seen videos of laser pointers that can burn paper--they don't seem to have recoil. If the energy that is burns that paper is ... momentum, then I've clearly misunderstood the SCALE of mechanical energy vs. thermal energy. But if the thermal (or something?!) energy is ... separate, then... Well, then we're back to the original point: most of the energy you use to create a photon isn't translated into momentum, so light isn't the propellant you're looking for. NB I don't have a comment about the microwave chamber other than "NFW.".
    - MereInterest 12y ago

SideburnsOfDoom 12y ago

> It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here? Yes, there does seem to be something else. Classical mechanics cannot explain thrust from shining a flashlight inside your spaceship. But "the EmDrive converts electric power into thrust ... by bouncing microwaves around in a closed container"

nsxwolf 12y ago

I guess a related question is "why does a flashlight in space not propel itself". And I don't know the answer to that. Edit: Ok. That makes sense. I guess it's just not a practical source of propulsion.

oneandoneis2 12y ago
It does. But the propulsion is really, really tiny.
leishulang 12y ago
https://answers.yahoo.com/question/index?qid=20111214233110AAicrQm https://answers.yahoo.com/question/index?qid=20111214233110A...
IanCal 12y ago
A flashlight in space would propel itself, since photons have momentum.
dragonwriter 12y ago
A flashlight in space does propel itself, but that's not really a related question. A more related question is why does a flashlight in a fixed mount inside of a windowless spacecraft not propel the spacecraft (or, similarly, why does a fan inside of a sealed, ventless housing attached to an airplane wing not function as an engine.) Of course, if there's some way this does work despite what we think we know about physics, its a pretty gigantic breakthrough (well, at least, understanding and practically applying it would be -- otherwise its just an anomaly that provides a pretty gigantic question mark...)

IanCal 12y ago

> Isn't it just the reverse of a solar sail to send out the photons yourself rather than catching them? No, that'd be the case if it were open but it isn't. As I understand it, it's an entirely closed system. That's what's controversial, there's nothing coming out of this "thruster", not even photons. There's an image on page 3: http://www.emdrive.com/yang-juan-paper-2012.pdf http://www.emdrive.com/yang-juan-paper-2012.pdf (disclaimer, I only have a hazy understanding of what this is supposed to be)

davorak 12y ago
> As I understand it, it's an entirely closed system. You got the gist correct. The microwaves are supposed to be completely contained.
Gravityloss 12y ago
Wouldn't it be relatively straightforward to repeat the Chinese test in a garage? We're talking a whopping 70 millinewtons here, so equivalent to the weight of 7 grams. Some shrews are lighter than that. Microwave science has this strange aura, since the stuff that happens in cavities etc is not exactly obvious at first sight. http://en.wikipedia.org/wiki/Cavity_magnetron http://en.wikipedia.org/wiki/Cavity_magnetron
- leishulang 12y ago
  sorry to disappoint you, but it's actually 30~50 micronewtons, which is only 0.04g.
- leishulang 12y ago
  I checked again. Some insider said the Chinese machine does produce 80 millinewtons, but it's less efficient than the NASA one since they take way more power input.

phkahler 12y ago

>> It seems like shining a flashlight in one direction should produce thrust in the other. Is that not true or is it something different at work here? That is absolutely true. Electromagnetic radiation effectively has momentum. Take the energy in joules and convert it to mass via E=mc^2 and then plug it in the momentum equation p = mv but using c as the velocity and you get p = E/c. Change in momentum over change in time is force, so we can then get to F = P/c. I believe the units are Newtons, Watts, and meters/sec. So with 300 Megawatts you can produce a whole Newton of force! I used this on a physics final once, but hid the work because the prof expected us to use formulas for radiation pressure derived from electromagnetics and I had missed that lecture. Later I derived the general case of my approach and it matched the EM derived formulas. 50/50 if he would have appreciated it or marked it down. The question was "a 50mW laser is reflected off a mirror, what force is exerted on the mirror due to the beam?". Note that the change in momentum is doubled for reflection vs emission or absorption.

evincarofautumn 12y ago
> the prof expected us to use […] and I had missed that lecture You just gave me a wave of nostalgia. That’s how I survived all of my math and physics classes. If you develop an intuitive understanding of the material, you can derive things as needed from a few core principles you do remember. A bit sad when learning is the way to subvert an educational system, though.
- pavel_lishin 12y ago
  That's how I passed quite a few math exams that I couldn't be bothered to study for.
deleted 12y ago
[deleted]
- schiffern 12y ago
  Yes, charged batteries have more mass per e=mc². That means that a good AA battery weighs ~100 picograms more than a dead one.

daveloyall 12y ago

I think that when you send out photons, there is energy in them. Like, energy you would have rather have converted into momentum. ...IANAP.

fixermark 12y ago
The momentum those photons are carrying implies a change in momentum to the emitter that is equal and opposite. Conservation of momentum applies even though the emitted substance is electromagnetic radiation, not matter particles. What's weird about this device is that the radiation chamber is completely closed---the microwaves emitted are then re-absorbed by the chamber, but the claim is that the net momentum of the chamber has changed... Somehow. If it is true, it'd be a violation of conservation of momentum (unless the microwaves are somehow passing momentum to something else, which is why people are talking about quantum mechanics possibly coming into play).
- daveloyall 12y ago
  We're well beyond my physics knowledge here. (See IANAP above!) So, I have questions. Is momentum the only type of energy a photon has? By allegory: Suppose a cannonball can be red hot, or cool. Separately, it can be flying or at rest. So, there are four cannonball types: hf, hr, cf, and cr. The hot, flying cannonball has the most energy and the cool, resting cannonball has the least. Yet, the momentum of both flying cannonballs is equal--temperature is orthogonal. Is all that right? If it is, then... I thought photons contain some energy that is similarly orthogonal to momentum. ...I guess I don't recall learning that, so maybe I just made it up. But, I have seen videos of laser pointers that can burn paper--they don't seem to have recoil. If the energy that is burns that paper is ... momentum, then I've clearly misunderstood the SCALE of mechanical energy vs. thermal energy. But if the thermal (or something?!) energy is ... separate, then... Well, then we're back to the original point: most of the energy you use to create a photon isn't translated into momentum, so light isn't the propellant you're looking for. NB I don't have a comment about the microwave chamber other than "NFW.".
  - MereInterest 12y ago