Yes, you can determine your speed relative to the CMB. Or at least, you can determine it relative to your CMB. Two observers, each of whom is at rest relative to their observations of the CMB, will not be at rest relative to each other (assuming they’re separated by a cosmological distance).
“Favoured frame of reference” is to vague a term I think For example take a rocket, the rest frame of that rocket might be regarded as “favoured”, because it is the only frame at which that rocket is at rest, but that doesn’t prevent the rocket from having a rest frame.
This kind of discussion can get involved and nuanced very quickly, but I would draw the distinction between the symmetries of a given situation and the physical theory describing that situation. For example many physical theories are invariant under rotation: i.e. rotating an arbitrary object around an arbitrary angle around an arbitrary axis doesn’t change the physical nature of that object. However it doesn’t mean the object itself is invariant under a rotation: for example rotating a cube by 2 degrees about any axis, doesn’t change the physical nature of the cube, but it changes the description of that cube.
Small hijack:
I thought negative mass existed. Isn’t that how Hawking Radiation works? A negative mass and positive mass particle are formed at the edge of an event horizon and the negative mass particle falls in the black hole. This causes the black hole to essentially evaporate over time (a very, very long time).
Of course getting our hands on the stuff may be a practical impossibility but still…doesn’t that mean negative mass exists?
The negative energy needed for the Albecurrie drive is different from the negative energy seen in the Hawking effect. To understand what is going on with energy in the semiclassical Hawking effect it is best to start with the entirely classical Penrose process:
In the Penrose process a particle starts from infinity (we could just consider infinity to be a region very far from the black hole for practical purposes) and enters the ergosphere of a rotating black hole where it splits into two particles. One of the daughter particles then falls into the black hole and whereas the other escapes back to infinity. If this happens in the right way the daughter particle that escapes back out to infinity ends up with more energy than the parent particle starts with!
Energy conservation can be a sticky subject in GR, but this is one of those situations where it still applies and the gain in energy at infinity can be seen as the loss of energy from the black hole. Or in other words from infinity it ‘looks’ like the daughter particle that falls into the black hole has negative energy.
It’s important to point out though that to an observer watching the daughter particle that falls into the black hole from near that particle rather than from infinity would still observe it as a positive energy particle and there is no violation of the condition that prohibits negative energy densities. The apparent negative energy of one particle is to do with how an observer at infinity extends their local definition of energy over the whole of the spacetime.
The reason this relevant is that the Penrose process is the classical limit of the Hawking effect. The differences are that Hawking radiation can occur in a non-rotating black hole too and it is how an observer at infinity sees the quantum vacuum near the event horizon of the black hole rather than how the see particles that have travelled in from infinity.
In the Hawking process, the observer at infinity “sees” the quantum vacuum near the event horizon not as a vacuum but full of thermal radiation. Some of this radiation arrives at the observer at infinity as Hawking radiation, but some of it falls into the black hole. When the observer at infinity extends their definition of energy over the whole spacetime the particles that fall into the black hole have negative energy from their point of view. However from the point of view of an observer near the black hole the vacuum can still look like a vacuum (I say “can”, because Hawking radiation is intimately linked with the Unruh effect whereby an accelerating observer sees a quantum vacuum as being filled with thermal radiation).
The Albecurrie drive is different because it requires that energy conditions are violated, i.e. it requires negative energy that still looks like negative energy even to local observers. There is a thin crack of light though for would-be interstellar travelers in that the energy conditions prohibiting negative energy densities which sensibly apply to classical fields can’t sensibly be applied to quantum fields. Even then though it is still thought averaged versions of those conditions apply to quantum fields which is probably enough to put the kibosh on the Albecurrie drive.
TL;DR: The Albecurrie drive requires negative energy that looks like negative energy when you’re close enough to hold in you hands. The negative energy particles of the Hawking effect are like mirages in that if you get close enough to them to hold them they turn into a vacuum.
NB I am sure this a gross oversimplification, but the overall point is good.
Doesn’t the Casimir effect demonstrate that you can have a locally negative energy density?
Yes the Casimir effect is a specific violation of the weak energy condition.
The vacuum state in a QFT carries zero momentum, so if at the start of whatever supposed process occurs, we have vacuum, and at the end, as well, then nothing can have acquired momentum without violating conservation of momentum. And if you, in some way, want to end up with some non-vacuum final state, then you’d have to use some particle creation process, which however creates particle-antiparticle pairs; if you’d try to get some purchase on those via some EM-fields, then you’d produce effects that cancel each other out—say, you produce an electron and a positron, then if you subject them to an electric field in order to accelerate them and use them as some sort of effective ‘reaction mass’, due to their opposite charges, they get accelerated into opposite directions, and the total effect on your spacecraft is nil.
So I don’t think one can really might sense of any claims that ‘it pushes against virtual particles’…
I believe that’s how the NASA team is going about simulating negative energy, IINM.
Not necessarily. You could produce an electron-positron pair, separate them inside your engine using one field, and then switch the field configuration so it points in different directions at different points of your engine, and thus accelerate both particles out the back. But this isn’t really anything new or impressive: You’re not meaningfully “pushing against the virtual particles”; you’re just chucking real particles out the back of your rocket engine. Which is not, at that level, any different from what a conventional rocket does.
If I tried to build a drive that functioned this way, it seems unlikely that I could achieve pair creation with radiofrequency energy, I suppose.
There should be some measurable radiation in the exhaust as well, I’d think.
The press around this reminds of an old SF story, where FTL travel was a matter of piling up rocks in the right way. Most civilizations discovered this randomly, some by contact with other peoples. No one had a stark clue why it worked, just that it did. At any rate, the “something might be happening for some reason, maybe” reminded me of it.
(The amusing aspect was that, at the end, it describes the galaxy’s greatest conquering power mustered its elite guard thirty-thousand bent on planetary conquest, the soldiers readied their mukets and wondered… why did that planet have those scintillating light arrayed in webs and splotches all over the place?)
Yes we are the child who never figured out how to make a paper airplane.
To be clear, I’m not suggesting that the EmDrive works that way. I’m just saying that something along those lines would be possible (and uninteresting).
Your definition of “interesting” makes no sense to me. If the EmDrive turns out to be a more efficient way of propelling ourselves around the solar system then it has enormous implications on a scientific and economic scale. Many many missions of exploration that are not possible now could become feasible. If the EmDrive “works” but turns out to not rely on new physics that’s still very interesting to most people.
Chronos was referring to the possibility of it using pair production for thrust. It’s uninteresting because it would be vastly less efficient than even a photon drive, which is already vastly less efficient than any other rocket. Generating an electron+positron pair would require at least its own mass in energy input, so you’d be better off just grinding your reactor into little bits and shooting it out the back.
The EmDrive claims to be more efficient than that… but it’s still not clear how it could work, even if it did.
Indeed :“Eagleworks researcher Harold White went so far as to predict that a manned mission could get to Mars in just 70 days by producing just 0.4 Newtons/kW, or about 10 times the power efficiency of a modern ion thruster.”
According to Eagleworks, it does work in a vacuum, so there seems to be something interesting here.
Yes, you’re right, but besides neither being new nor effective, this also doesn’t seem to be something you could do by just pumping microwaves into a resonator.
Anyway, if the idea is that you start from vacuum and end with vacuum, then this is something that violates conservation of momentum, because you’d start with a zero-momentum state, and end with nonzero momentum, no matter what magic you appeal to in between. And since conservation of momentum follows from the statement ‘physics here works the same way as physics over there’, its nonconservation would break quite a lot of things.
I’m confused on this.
How can these two observers disagree? Won’t they both measure the BH as shrinking? If so then how does the local observer who has seen nothing but positive energy flow into the BH explain it getting smaller?