# Tag Info

10

The first thing that should jump out to anyone is the following excerpt from the abstract of the recent NASA paper: Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed ...

9

One of my mentors likes to say, "Nothing resembles a new effect quite so much as a mistake." Conservation of momentum is a fundamental principle of mechanics supported by hundreds of years of experimental evidence since the language needed to discuss it was codified by Newton. Certainly it's the case that electromagnetic radiation carries momentum and can ...

7

It is possible to generate thrust using EM radiation such as a laser or microwaves. Discussed in this XKCD blog. However this relies on momentum being transferred from the photons of the EM radiation to the object being propelled. This is not possible if the microwaves are completely sealed within a container as indicated in the article. This paper will ...

4

Consider the following diagram: This shows a mass $m$ moving past a point $P$ in a straight line. Note that the mass isn't connected to $P$ in any way - it's just moving past in a straight line. The angular momentum of $m$ about $P$ is given by: $$\vec{L} = \vec{r} \times m\vec{v}$$ So the direction of $\vec{L}$ is normal to the screen and the ...

3

it seems curious to me that a system can have a mass, and then accelerate to the speed of of a massless particle There is no paradox here. Special relativity doesn't preserve velocity - in fact the opposite - it allows one to move between frames of reference in which the velocity of objects is relative to the observer. [do the particles] accelerate ...

3

In Feynman diagrams all four vectors conform with special relativity algebra, BUT the internal lines, even though they have the name "photon" are virtual, which means the mass can be different than zero, only the quantum numbers identify them as a photon, quark, electron, etc.. Feynman diagrams are an iconic representation one to one with the integrals and ...

3

Yes. There are only internal forces of your body. Without external forces, the center of mass of your body cannot change position. As your center of mass did not move, the main body should move in the opposite direction.

2

(1) Since $u(\textbf{r}) = u(\textbf{r}+\textbf{R})$, we can expand this part in terms of reciprocal lattice vectors, $u_k(\textbf{r}) = \sum_\textbf{G}{e^{i\textbf{G}\cdot \textbf{r}}u_\textbf{k-G}}$. We can therefore write: \psi_{\textbf k+\textbf K} = e^{i(\textbf k + \textbf K)\cdot \textbf r}\sum_\textbf{G'}{e^{i\textbf{G'}\cdot ...

2

From the OP's comments I have a feeling that the real issue is not the virtual particle, but an actual annihilation of a positron and electron into 2 real photons. Like here: And the question is, how can the system of 2 massive electrons accelerate into the speed of light, when accelerating to c requires infinite amount of energy. The answer has two parts: ...

2

The engine isn't designed to use solar power. The thrust$^1$ is due to the motion of microwaves within a resonant cavity, and the microwaves are generated by a magnetron on the space ship. The power source for the magnetron would presumably be a nuclear generator or some other compact way of storing energy. The original 2006 paper describing the drive is ...

2

In inelastic collisions, kinetic energy is not conserved, so I'm going to assume you mean a totally elastic collision since you say energy is conserved. O.K, so when the ball hits the wall, the speed of the wall before and after is 0, so that means the kinetic energy of the ball is conserved and thus the magnitude of the velocity is the same before and after ...

1

of course it is possible.In my opinion the weight of cyclist is acting on his seat and as he bends the seat become an inclined surface.The body of thew cyclist is an object in the inclined plane and the friction and normal reaction offered by the seat can provide the necessary centripetal force. Another method is to throw a stone in the opposite direction ...

1

It seems like the previous answers were based on the abstract or third-party articles about the abstract, but I gather from this Wired article that the full paper provides more details (behind a paywall, so I can only speak for what the article says). The described tests were on the Cannae drive, the inventor of which believed required slots in the drive to ...

1

Tong is alluding to the standard trick in the derivation of Noether's theorem by promoting the (infinitesimal) $x$-independent parameter $\epsilon$ to become $x$-dependent, see e.g. this Phys.SE post.

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