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Old 3rd January 2017, 12:04 PM   #201
Beelzebuddy
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Originally Posted by 3point14 View Post
So an elliptic orbit? Or a circular one a long way out?
Circular. Just on the edge of leaving Earth's SOI entirely. It'll take years of very slight acceleration to get out that far, but that's acceleration you won't need to pack with you when you launch your astronauts.
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Old 3rd January 2017, 04:12 PM   #202
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So what's going on with this thing? If it was due to some kind of error, shouldn't it have been discovered by now?
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Old 3rd January 2017, 04:27 PM   #203
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'Success, I have thrust'

https://m.youtube.com/watch?v=Rbf7735o3hQ

That should settle the issue.
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Old 4th January 2017, 03:36 AM   #204
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Originally Posted by Fudbucker View Post
So what's going on with this thing? If it was due to some kind of error, shouldn't it have been discovered by now?
I'm really, really wanting it to work.

I'm pretty sure it won't - it breaks the laws of physics, Newtonian and relativistic physics - there's some bits upthread about how constant acceleration without a corresponding rise in energy levels required to accelerate is a recipe for an infinite energy machine. (I think, someone will be along to correct me if I'm wrong)

Oddly, this fiction writers resource has proved useful (I think someone posted it already, can't remember who, sorry)

http://www.projectrho.com/public_htm...nlessdrive.php


In conclusion, if it works it provides infinite energy and a fairly simple way of destroying planets if one desires.
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Old 4th January 2017, 08:32 AM   #205
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Originally Posted by Fudbucker View Post
So what's going on with this thing? If it was due to some kind of error, shouldn't it have been discovered by now?
Science doesn't work like that. There's no table full of men with long gray beards who dictate what is sufficiently Science.

Most of the obvious sources of error have been eliminated, enough for it to get serious looks, but plenty of others remain so there's still significant room for doubt.

Let's assume for the sake of interest that the EM drive does work as advertised. What you'll see is a series of increasingly obscure and unlikely errors being tested, as hypothesis after hypothesis of "well, it could be due to this" get ruled out. There's no actual end to it; some people will NEVER accept that it works, and gradually they will become the kooky ones. But on the whole, the scientific consensus (for that is what the term means) will accept that it does in fact work, and will move on.
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Old 4th January 2017, 12:27 PM   #206
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Originally Posted by Beelzebuddy View Post
Science doesn't work like that. There's no table full of men with long gray beards who dictate what is sufficiently Science.

Most of the obvious sources of error have been eliminated, enough for it to get serious looks, but plenty of others remain so there's still significant room for doubt.

Let's assume for the sake of interest that the EM drive does work as advertised. What you'll see is a series of increasingly obscure and unlikely errors being tested, as hypothesis after hypothesis of "well, it could be due to this" get ruled out. There's no actual end to it; some people will NEVER accept that it works, and gradually they will become the kooky ones. But on the whole, the scientific consensus (for that is what the term means) will accept that it does in fact work, and will move on.
I didn't make any claim about how science works. I'm wondering why this EM Drive is still chugging along.
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Old 4th January 2017, 01:25 PM   #207
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Originally Posted by Fudbucker View Post
I didn't make any claim about how science works. I'm wondering why this EM Drive is still chugging along.
You also asked

Originally Posted by Fudbucker View Post
If it was due to some kind of error, shouldn't it have been discovered by now?
The answer is "it doesn't work that way." A simple "no" would be incomplete.
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Old 4th January 2017, 02:19 PM   #208
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To the problem of free energy. Ie to increase speed we need constant force, but the kinetic energy increases with square of speed ..
That is not unique to EM drive. Fuel engines do that too, be it chemical energy like in normal rocket or electric energy as in ion engine. Yes, the vehicle is loosing mass, but not in a way it would perfectly outweigh linear vs. quadratic.
There must be some misunderstanding in play. Sadly I don't see it, I was never strong in energy, work and stuff like that ..
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Old 4th January 2017, 02:43 PM   #209
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Originally Posted by Dr.Sid View Post
To the problem of free energy. Ie to increase speed we need constant force, but the kinetic energy increases with square of speed ..
That is not unique to EM drive. Fuel engines do that too, be it chemical energy like in normal rocket or electric energy as in ion engine. Yes, the vehicle is loosing mass, but not in a way it would perfectly outweigh linear vs. quadratic.
The difference is that regular engines have an exhaust (or something that conservation of momentum acts upon).

For these engines, the increase in KE of the ship is balanced by a reduction in KE of the exhaust. And when you run out of exhaust or reaction mass, you have to stop accelerating. The total change in KE in all inertial frames is exactly balanced by the energy input from the combustion.

Because the described EM engine has no exhaust, it has no such constraints, and the change in KE depends on the frame you use to measure.
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Old 4th January 2017, 03:12 PM   #210
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Example: Rocket has a mass of 1kg. In an atomic fashion, it exhausts 1g of mass at a speed of 1000m/s. Conservation of momentum means the rocket accelerates by 1.001 m/s.

Assuming it was at rest initially, we can see the total change in KE is:
KE(fuel) = 0.5 (1g) (1000m/s)^2 = 500J
KE(rocket) = 0.5 (999g) (1.001m/s)^2 = 0.5005J

So the energy of combustion is giving us a bit over 500J of energy, but most of it is going into the exhaust.

Now we imagine the rocket is already moving at 10km/s and it does the same thrust. What's the change in KE of the rocket and the system?

dKE(rocket) = KE(10001m/s) - KE(10000m/s)
dKE(rocket) = 49,960,001J - 49,950,000J = 10001J

dKE(fuel) = KE(9000m/s) - KE(10000m/s)
dKE(fuel) = 40,500J - 50,000J = -9500J

dKE(total) = 10001J - 9500J = 501J

In this frame, the rocket gets a huge increase in KE from the same burn, but only with a corresponding decrease in KE of the exhaust mass. The total change in KE of the system is constant.
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Old 4th January 2017, 03:59 PM   #211
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Originally Posted by Beelzebuddy View Post
You also asked



The answer is "it doesn't work that way." A simple "no" would be incomplete.
It doesn't work what way? I assume scientists are trying to figure this thing out. We had a similar situation with supposed FTL-neutrinos. That was eventually resolved. Why hasn't this been? What specifically is making it so hard to tell whether this thing works or not?
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Old 4th January 2017, 05:30 PM   #212
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Originally Posted by Fudbucker View Post
What specifically is making it so hard to tell whether this thing works or not?
Because "works" in this case is really small and almost indistinguishable from "doesn't work".

It's easy with an eyeball and ruler to tell the difference between something 2 inches long and 3 inches long. It's difficult with an eyeball and ruler to tell the difference between something 2.0002 inches long and 2.0003 inches long.

If this thing has an effect of 0.000000 N, then it's useless and nothing new is happening. If it has an effect of 0.000001 N, then it's real. But seeing the difference while being in a laboratory on earth with vibrations, atmosphere, temperature differences, and gravity potentially creating forces many orders of magnitude larger is difficult.

Also, it's not like there are dozens of these machines being examined by labs around the world with replicated studies confirming each other. The number of experiments being done on it is very limited, so the information on it is similarly limited.
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Old 4th January 2017, 06:14 PM   #213
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Not only that, but because the actual mechanism of action is unknown, any real effect may be a consequence of some random aspect of construction that we are currently unaware of. So two different teams may think they are building the same test article may be building two completely different test articles.
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Old 4th January 2017, 08:00 PM   #214
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How is this different than a photon drive? A photon drive is perfectly consistent with GR, and I expect that if this works it will be very similar in the energy / thrust ratio.

(for those who wonder, a photon drive will shift its frequency as seen by inertial observers, conserving the energy and momentum relationships. The thrust comes from that photons have momentum in spite of having no rest mass)
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Old 4th January 2017, 08:07 PM   #215
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Originally Posted by RussDill View Post
Not only that, but because the actual mechanism of action is unknown, any real effect may be a consequence of some random aspect of construction that we are currently unaware of. So two different teams may think they are building the same test article may be building two completely different test articles.
I have wondered about that too. If a mechanism can be found, then then potentially a way to increase the effect by optimizing the required conditions might also be possible.

Lots of if's and might's though. They are a long way from all that so far.
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Old 4th January 2017, 10:04 PM   #216
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Originally Posted by ArmillarySphere View Post
How is this different than a photon drive? A photon drive is perfectly consistent with GR, and I expect that if this works it will be very similar in the energy / thrust ratio.

(for those who wonder, a photon drive will shift its frequency as seen by inertial observers, conserving the energy and momentum relationships. The thrust comes from that photons have momentum in spite of having no rest mass)
Specific to the claim of the EM drive is that the net amount of thrust is too large to be accounted for by simply flinging photons.
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Old 4th January 2017, 11:53 PM   #217
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Originally Posted by ArmillarySphere View Post
How is this different than a photon drive?
You mean like a flashlight? Photon drives are very (very) low thrust and are horribly inefficient from an energy standpoint.

The lighter your exhaust, the more mass efficient your rocket becomes, but the less energy efficient it is. You end up putting almost all of your energy into photons rather than into your ship's KE. The thrust is just too low.

Solar sails can work because all the energy to create the photons is done off-ship. If you need to generate the photons with on-board energy, then the penalties are huge.

As described, the EM drive is supposed to be much more powerful. It could use existing power sources (such as a nuclear reactor) and be useful.
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Old 5th January 2017, 01:19 AM   #218
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Originally Posted by BowlOfRed View Post
The difference is that regular engines have an exhaust (or something that conservation of momentum acts upon).

For these engines, the increase in KE of the ship is balanced by a reduction in KE of the exhaust. And when you run out of exhaust or reaction mass, you have to stop accelerating. The total change in KE in all inertial frames is exactly balanced by the energy input from the combustion.

Because the described EM engine has no exhaust, it has no such constraints, and the change in KE depends on the frame you use to measure.
Ah, of course. I didn't realize kinetic energy depends on reference frame. Makes sense now. Thanks.
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Old 5th January 2017, 11:16 AM   #219
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Originally Posted by Beelzebuddy View Post
Circular. Just on the edge of leaving Earth's SOI entirely. It'll take years of very slight acceleration to get out that far, but that's acceleration you won't need to pack with you when you launch your astronauts.
Define "very slight" for this? If you can get 25mm/s2 you can escape earth in a month or so.
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Old 5th January 2017, 12:36 PM   #220
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Originally Posted by BenBurch View Post
Define "very slight" for this? If you can get 25mm/s2 you can escape earth in a month or so.
That's a couple of magnitudes higher than existing ion spacecraft, though. The wiki figures for Deep Space 1 were the first I found; at 482 kg and 92 mN of thrust, assuming I can math today its acceleration was at most 0.19 mm/s^2.

The current EM drive (I maintain the best way to test if it works is to build one big enough to measure) is still a couple of orders of magnitude below *that.*
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Old 5th January 2017, 08:19 PM   #221
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Originally Posted by BowlOfRed View Post
Example: Rocket has a mass of 1kg. In an atomic fashion, it exhausts 1g of mass at a speed of 1000m/s. Conservation of momentum means the rocket accelerates by 1.001 m/s.

Assuming it was at rest initially, we can see the total change in KE is:
KE(fuel) = 0.5 (1g) (1000m/s)^2 = 500J
KE(rocket) = 0.5 (999g) (1.001m/s)^2 = 0.5005J

So the energy of combustion is giving us a bit over 500J of energy, but most of it is going into the exhaust.
As you clearly know from your result of 500J, 1J = 1kg m2/s2.

No problem with anything else in your post, but don't you think it would be better if you wrote 0.001kg?
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Old 6th January 2017, 01:46 AM   #222
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Originally Posted by Roboramma View Post
No problem with anything else in your post, but don't you think it would be better if you wrote 0.001kg?
Probably. But sometimes I make fewer typing-the-right-number-of-zeros mistakes by entering "1g" instead of ".001kg". And in this case, I wasn't actually doing the calculations; the computer was and it understood the units. I wasn't thinking about clarity when I typed it out.
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Old 8th January 2017, 06:59 AM   #223
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Originally Posted by Beelzebuddy View Post
That's a couple of magnitudes higher than existing ion spacecraft, though. The wiki figures for Deep Space 1 were the first I found; at 482 kg and 92 mN of thrust, assuming I can math today its acceleration was at most 0.19 mm/s^2.

The current EM drive (I maintain the best way to test if it works is to build one big enough to measure) is still a couple of orders of magnitude below *that.*
Well, of course, but there seem to be no challenges with scaling ion engines at least. Biggest issue is power, and we will have to revive the SNAP program to make a useful spacecraft.

And IF the EM-drive works, I am quite sure it can be scaled too as I see nothing to prevent that until the cavity power gets quite high.
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Old 8th January 2017, 09:37 AM   #224
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Originally Posted by BenBurch View Post
Well, of course, but there seem to be no challenges with scaling ion engines at least. Biggest issue is power, and we will have to revive the SNAP program to make a useful spacecraft.

And IF the EM-drive works, I am quite sure it can be scaled too as I see nothing to prevent that until the cavity power gets quite high.
Unfortunately, from the test data presented, increasing the power didn't increase the thrust. Look at http://arc.aiaa.org/doi/10.2514/1.B36120 (link thoughtfully provided by Jrrarglblarg), figure 19.
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Old 12th January 2017, 09:55 AM   #225
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Originally Posted by 3point14 View Post
I'm really, really wanting it to work.

I'm pretty sure it won't - it breaks the laws of physics, Newtonian and relativistic physics - there's some bits upthread about how constant acceleration without a corresponding rise in energy levels required to accelerate is a recipe for an infinite energy machine. (I think, someone will be along to correct me if I'm wrong)
Why does it it break the laws of physics?

Maybe the thrust can be explained by radiation pressure. Every form of electromagnetic radiation, including radio waves, produce radiation pressure. Radio waves carry both energy, linear momentum and angular momentum. So maybe the thrust comes from radio waves.

Doesn't the EM drive have a power source, however small?

The thrust that they measure is extremely small. So is the power draw. So I would anticipate large experimental error. Still, it is possible that the conservation laws are still satisfied.

Maybe the thrust is coming from radio wave photons carrying energy from the power source.

Then it couldn't be called a 'perpetual motion machine'. It would more accurately be called a 'photon drive'. The propellant would in this case would be radio wave photons.

Still, the good news may be significant. Maybe all the energy comes out the back of the rocket as a collimated beam of radio waves. This would be very useful for long range space probes. The proper acceleration would be small, but over time it would significant. The drive would not use up as much energy because it doesn't generate as much heat.

The significance would depend on how much heat the machine is giving off. if the drive is very efficient. For instance, maybe the drive doesn't generate any heat heat energy. Equivalently, the drive doesn't create entropy.

So what I would really like to see is figures related to the thermodynamic efficiency of the EM drive. They should measure the ratio of work being done by the drive divided by the total energy being used by the drive. If the efficiency is greater than that of a Carnot cycle, then the laws of thermodynamics are being violated.

Suppose the efficiency of the EM drive is very close to the Carnot limit. Then it would be very good! Most of our machines have a thermodynamic efficiency well below the Carnot limit. Certainly, a chemical thruster doesn't have large efficiency. It the efficiency of the EM drive is greater than the ion thrusters NASA is now using, then it would still be a breakthrough even without violating the laws of physics.

If the device did destroy entropy, then that would be revolutionary. I seriously doubt that the EM device destroys entropy. I don't think it violates the second law of thermodynamics. However, I do realize that calorimetric measurements under these conditions is very hard.

So I am hoping that they could at least give us some estimate of the thermodynamic efficiency. If the engine runs hot, then the device is useless. We already have thrusters that create lots of entropy. We need a drive that produces almost no entropy.

The researchers haven't said anything about the torque. They keep on talking about the thrust, which is a force. I wonder how much torque the EM drive produces, if any. There may be applications where the drive 'spins up' a sensor in outer space. Given sufficient time, a really efficient torque generator could produce a probe with a great deal of angular momentum. Perhaps we could make better tests of GR using a sensor with high angular momentum.

Last edited by Darwin123; 12th January 2017 at 09:57 AM.
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Old 12th January 2017, 10:08 AM   #226
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Originally Posted by dasmiller View Post
Unfortunately, from the test data presented, increasing the power didn't increase the thrust. Look at http://arc.aiaa.org/doi/10.2514/1.B36120 (link thoughtfully provided by Jrrarglblarg), figure 19.
Yes it did. Look again.

Originally Posted by Darwin123 View Post
Why does it it break the laws of physics?
It pushes against the luminiferous aether.

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Old 12th January 2017, 10:10 AM   #227
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The point of the EM drive is that the amount of thrust produced is many orders of magnitude stronger than just emitting photons. Photon drives are already a thing that needs no further experimental proof.

And as mentioned before, the simplest explanation for why it violates the laws of physics is that it increases velocity at a constant rate with a constant energy input, but kinetic energy increases as a square of velocity.

The only way I can possibly picture the results of this thing being actual results instead of experimental error is if it is somehow interacting with dark matter particles in a directionaly preferential manner and the momentum of the particles is being imparted on the emdrive.
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Old 12th January 2017, 10:15 AM   #228
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Originally Posted by Darwin123 View Post
Why does it it break the laws of physics?
Conservation of momentum is a law of physics, and it very obviously breaks that one; the cavity is closed, and no photons emerge from it. It therefore produces an acceleration in the object it's moving with no balancing acceleration of an object in the other direction. It's therefore always possible to transform its motion into a frame of reference in which the increase in kinetic energy is greater than the energy powering the device, because kinetic energy goes as V^2 so any increase in KE must have a V^2 term, and ultimately that will dominate the dependence. (That's the simplified Newtonian argument; the results are the same in relativistic motion but I'd have to revise for three months to do the maths.) So it violates conservation of energy as well.

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Old 12th January 2017, 10:41 AM   #229
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Btw. interesting and kinda counter intuitive result of Newton laws is that the center of gravity of a closed system does not change.

Imagine rocket with standard reaction engines off in the vacuum of space, far from anything. Mark it's center of gravity.
Rocket starts the engine and moves away. But the fuel is moving in the opposite direction. The center of gravity of whole rocket-fuel system is still the same, on the spot where the rocket was in the beginning, no matter what the rocket does.

Also as was stated before .. conservation of energy also depends on the Newton laws, otherwise you could create excessive amounts of kinetic energy. Those simple Newton laws tie everything down quite tightly.

Still even if the device works, we might be missing some kind of energy leaving the system .. imagine it accelerates neutrinos. It wouldn't be easy to detect. Or 'dark matter' whatever that might be. I don't think Newton is necessarily violated, even if the device works.
Or his laws at least ..
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Old 12th January 2017, 10:49 AM   #230
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You say the photons don't leave....what if there is a tunneling aspect.....didn't I see recently someone got a photon on the other side of a "barrier".

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If we describe a photons with a wave packet, moving towards a potential barrier and E smaller than V, there is a finite chance that it will tunnel to the other side. In this process it is likely that it will arrive before a photon that does not tunnel, not because it exceeds the the speed of light, but because the front of a wave packet will contribute most to tunneling. I would understand this if it was concerned with PARTICLES as follows: because each of the different waves that make a wave packet has a different momentum, the faster moving, higher energy, waves will move to the front of the wave packet when it disperses. According to the formula for tunneling probability indeed the higher energy parts will tunnel more often.

BUT, why would a photon wave packet also have the higher energy elements more in front of it's wave packet, since all light-frequencies move at the same speed in vacuum right? Or does the described system indeed not work in vacuum.
You have these rattling around in side the chamber and more tunnel in the wave guided direction....= thrust??
SInce it's very small thrust does that not imply only "some" are tunneling.
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Old 12th January 2017, 10:57 AM   #231
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Originally Posted by macdoc View Post
You say the photons don't leave....what if there is a tunneling aspect.....didn't I see recently someone got a photon on the other side of a "barrier".



You have these rattling around in side the chamber and more tunnel in the wave guided direction....= thrust??
SInce it's very small thrust does that not imply only "some" are tunneling.
The very small thrust is still way higher than a photon rocket produces.
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Old 12th January 2017, 10:59 AM   #232
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Not following you - the very small thrust being measured by the existing prototypes????
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Old 12th January 2017, 11:01 AM   #233
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Originally Posted by Darwin123 View Post
, it is possible that the conservation laws are still satisfied.
Unfortunately, the device is only interesting if it breaks these laws.

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Maybe the thrust is coming from radio wave photons carrying energy from the power source.

Then it couldn't be called a 'perpetual motion machine'. It would more accurately be called a 'photon drive'. The propellant would in this case would be radio wave photons.
If that's true, we can imagine the available thrust and energy requirements of a perfectly built engine that produces photons of any particular wavelength with 100% efficiency. Such an engine is almost worthless for space travel because the thrust per watt is so low.

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The proper acceleration would be small, but over time it would significant. The drive would not use up as much energy because it doesn't generate as much heat.
A big problem here is that in an event where you push one thing from another (in this case the ship and the photon), most of the energy goes into the smaller mass object. This means that for every bit of energy you produce, only a teeny, tiny fraction of it goes into increasing the KE of the ship and the reset goes into the photon beam. Heat loss is an minor point on the already horrible energy efficiency. It's orders of magnitude less thrust/W than an ion drive.
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Old 12th January 2017, 11:07 AM   #234
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Originally Posted by macdoc View Post
Not following you - the very small thrust being measured by the existing prototypes????
Yes. The thrust amounts measured are higher than what would be produced in a perfect photon drive, so photons escaping the engine doesn't work as an explanation.
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Old 12th January 2017, 11:16 AM   #235
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Originally Posted by macdoc View Post
Not following you - the very small thrust being measured by the existing prototypes????
Yes. Photon pressure is teeny tiny. Given the power, I doubt their setup could detect the force contribution from it.

Any measurement of thrust for their engine is coming from something else.

ETA: Hmm, maybe within range to detect after all. A megawatt of power can produce a few millinewtons of thrust at 100% efficiency. I'll have to go back and see what the power was in the experiment.

F = (E/c)
3mN = (1MW/c)

That means at their 80W run, if all the power were going into escaping photons, they should see a contribution of 0.3 micronewtons. I think that would be just visible on their graphs. So I'll retract "couldn't detect", but continue with "irrelevant to explain the measurements".

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Old 12th January 2017, 11:30 AM   #236
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Originally Posted by Beelzebuddy View Post
Yes it did. Look again.
At 41W, ~39 microNewtons

at 60W, ~86 μN

at 82W, ~87 μN

That 1 μN increase between 60 and 82W is waaay within their measurement uncertainty*. I don't think it's safe to assume, based on those 3 points, that thrust would be higher at 100W and higher still at 150, 200, etc.

*Notionally, their measurement uncertainty should be around 2.7 μN, based on 5 measurements each with 6 μN uncertainty, assuming that there's no systematic bias to their measurements. But since the scatter is so much larger than the quoted 6 μN measurement uncertainty, I don't think 2.7 μN is plausible.
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Old 12th January 2017, 12:08 PM   #237
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Originally Posted by dasmiller View Post
At 41W, ~39 microNewtons

at 60W, ~86 μN

at 82W, ~87 μN

That 1 μN increase between 60 and 82W is waaay within their measurement uncertainty*. I don't think it's safe to assume, based on those 3 points, that thrust would be higher at 100W and higher still at 150, 200, etc.

*Notionally, their measurement uncertainty should be around 2.7 μN, based on 5 measurements each with 6 μN uncertainty, assuming that there's no systematic bias to their measurements. But since the scatter is so much larger than the quoted 6 μN measurement uncertainty, I don't think 2.7 μN is plausible.
The relationship may not be as linear as the authors would like, but that's not the same thing as there not being a relationship.
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Old 12th January 2017, 01:35 PM   #238
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Originally Posted by Beelzebuddy View Post
The relationship may not be as linear as the authors would like, but that's not the same thing as there not being a relationship.
The actual problem is that the relationship suggests that the device will never produce more than a smidgen above ~87 μN since the data suggests an effect that is saturating.

That saturation is also an argument against real thrust which should increase with more power.
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Old 12th January 2017, 02:19 PM   #239
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Originally Posted by Reality Check View Post
The actual problem is that the relationship suggests that the device will never produce more than a smidgen above ~87 μN since the data suggests an effect that is saturating.

That saturation is also an argument against real thrust which should increase with more power.
With the measurement spread they're seeing, I wouldn't make any firm conclusions about the nature of the relationship beyond its simple existence.

It does make me want to reiterate the question I've been asking this whole time: why hasn't someone scaled it up? Take a welder and a 50-gallon drum and go to town. Either it works, or you have some yard art with an interesting story, right?
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Old 12th January 2017, 03:36 PM   #240
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Originally Posted by Beelzebuddy View Post
With the measurement spread they're seeing, I wouldn't make any firm conclusions about the nature of the relationship beyond its simple existence.
With "the measurement spread they're seeing" we could say that is no relationship at all and all we are getting are random results (noise, experimental or measurement error) !

The answer to your question is simple - no one would waste time and money spending lots of money and time doing a "50-gallon drum" experiment that has no hope of duplicating the results of a relatively sophisticated experiment. We are talking about measuring thrusts on the scales of microNewtons while trying to ruling out other possible causes of thrust in a vacuum chamber.

ETA: Another reason is that people do not usually waste their time doing experiments that violate the known laws of physics. The Advanced Propulsion Physics Laboratory is an exception because their purpose is to look at fringe technologies.
Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum
Quote:
A. Facilities
The thrust measurements were made using the low-thrust torsion pendulum at NASA Johnson Space Center. This torsion pendulum is capable of measuring thrust down to the single-digit micronewton level. Figure 2 shows a simple representation of the torsion pendulum’s major elements. The torsion pendulum is constructed primarily of an aluminum structure that is mounted on a slideout table within a 0.762-by-0.914 m vacuum chamber.
...

B. Test Article
The RF resonance test article is a copper frustum with an inner diameter of 27.9 cm on the big end, an inner diameter of 15.9 cm on the small end, and an axial length of 22.9 cm. The test article contains a 5.4-cm-thick disk of polyethylene with an outer diameter of 15.6 cm that is mounted to the inside face of the smaller diameter end of the frustum.

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