The h Bar

frogeyedpeas

12:19 AM

anyone played with quantum frames of reference here?

danielunderwood

12:47 AM

Happen to have any good references? I've never even thought of reference frames in qm, but am intrigued

Jack Clerk

1:00 AM

Good morning/evening/night for everyone!

I would like to know when the very first concept of "warpdrive" was born (the first metric in GR literature that shows this concept).

Thank you.

I would like to know when the concept of "warpdrive" was born (the very first metric in GR literature that shows this concept).

**

Avantgarde

1:19 AM

Try Googling.

danielunderwood

1:57 AM

Is an operator just a map $A: \mathcal{H} \to \mathcal{H}$?

Cows

3:13 AM

@bolbteppa yes indeed it was quite hardcore. I had to do it on the board too. In retrospect it was a bit funny. I don't exactly recall the details of the question but he said something along the lines of he could do it while drunk on vodka. hehehehe . Yes, I will be getting the degree really fast. I also yes, I think a good internship will take me very far.

Jack Clerk

3:56 AM

@JohnRennie Please, how old are you?

Secret

Warp drive

A warp drive is a fictitious faster-than-light (FTL) spacecraft propulsion system in many science fiction works, most notably Star Trek. A spacecraft equipped with a warp drive may travel at speeds greater than that of light by many orders of magnitude. In contrast to some other fictitious FTL technologies such as a jump drive or hyperdrive, the warp drive does not permit instantaneous travel between two points, but rather involves a measurable passage of time which is pertinent to the concept. Spacecraft at warp velocity would continue to interact with objects in "normal space". The general concept...

arxiv.org/abs/1203.6902

Jack Clerk

Cool

John Rennie

@JackClerk Old

Jack Clerk

@JohnRennie Sorry, for the question. I've read your profile and I didn't expected the phrase "my PhD is listed on the web even though the World Wide Web didn't exist at the time!"

Anyway...

John Rennie

Well I finished my PhD in 1986

Jack Clerk

4:09 AM

hmmm

I'm a undergradute (2 year) yet...

As you said that you're from cambridge.... have you ever crossroads with Hawking?

John Rennie

I used to see him occasionally (he was already in a wheelchair by then) but I wasn't in his department so I never had any interaction with him.

Jack Clerk

I see...

Novalium Company

6:22 AM

Why is the shape of the crankshafts (the black rotating things under the pistons) so weird? Can't they just be circular? Maybe the shape is like that to produce rotating motion?

Maybe there is some weight spreading or something to do with the weights and center of mass?

Also what pushes the piston upwards when the exhaust gasses have to be taken out? The pistons go up, but what makes them? Maybe the crankshafts push them up?

Secret

7:53 AM

To be checked: machine learning that crawl through literature and identify for gaps. Requires being able to predict unknown unknown questions

Crankshaft

A crankshaft—related to crank—is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion; whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion. In order to do the conversion between two motions, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach. It is typically connected...

Crankshafts concert up down motion into rotational motion, thus those extra bearings provide the geometry to do so

Novalium Company

8:24 AM

@Secret Ok thanks, I'll give it a look.

Anonymous

8:44 AM

@lılostafa Apparently it's easy make those cylinder 3D bar plots using MATLAB. See bar3, Color 3-D Bars by Height and cylinder

Anonymous

The colour variation also depends on the individual heights (in the graph you posted). For that, see the second link.

Anonymous

9:28 AM

@lılostafa A bit of add-on. The bar3 doesn't have any direct option to change the style to cylinder. So probably you'd have to create your own cylindrical_bar3_fucntion(). Maybe start with creating individual cylinders of your required height and X,Y location and then overlap them using hold on. For colour variation, see: in.mathworks.com/matlabcentral/answers/…

Anonymous

And ffs change the first symbol of your name. It seems impossible to ping you directly. XD

Captain Bohemian

10:55 AM

it's like tonight the moon will be in its full phase, but as I saw the moon last night, it had been quite round.

lılostafa

11:43 AM

@Blue Thanks for looking into it; Last night I did this using cylinder but two problems remained: first, since my array of cylinders isn't necessarily square-shaped, the cylinders are shown as elliptical cylinders and I have to scale them manually in every case to compensate for that, which becomes very annoying. 2nd, apparently Matlab cannot export its 3D plots in true vector format. Even when you export as svg, it's actually rasterized. (since version 2014b)

watchme

12:40 PM

some newspaper say that a shift of the earth's magnetic poles will be somewhen near in the future.

But they don't shift instantly - many even say it takes up to 3000 year, so how does it makes sense to say "a shift is about to happen in near future"?

Captain Bohemian

1:39 PM

@watchme I read on web it may shift within 100 years.

watchme

Thats a bit more realistic yeah, since the poles are now shifting with a velocity of around 55-60km/year

(which I have not measured myself of course)

John Rennie

2:34 PM

@watchme I suspect you mean a reversal rather than a shift i.e. the north and south magnetic fields will swap places.

Captain Bohemian

@watchme I don't know that speed; even if I have read about it, I have forgotten because I can't remember a number for long.

@watchme I just summoned what I have read on Apr. 12, 2015, which said a full reversal of earth's magnetic field usually takes millennia to finish, but there is evidence showing some reversals in history took less than a millennium to occur and such a case may occurr due to the reversal is incomplete; such a reversal is called the Laschamp event, after the area in France where evidence of it was first discoveredin the 1960s.

@watchme livescience.com/31795-earth-magnetic-field-reversal.html

DarkRunner

Hi all, I'm a high schooler, and I just wanted to clarify what I know about the double-slit experiment. First, I'm thinking of particles & waves. If we have a single slit experiment w/ waves, they do not interfere, so they create one bright spot on the screen. But if we have a double slit w/ waves, the waves diffract and interfere constructively and destructively to create a band of dark and bright spots on the screen.

On the other hand, if we have one slit w/particles (i.e. electrons), there's just a single bright spot on the screen. And if we have a double slit w/particle, there's two bright spots on the screen. And light has a wave-particle duality, so if we have a one slit w/light, it interferes with itself (? I don't understand this), and if we have a double-slit it also interferes with itself? This is one of the parts I don't understand. First, how can a wave OR a particle interfere with itself?

If anyone could provide some resources, or clarity for these questions, that would be great. Thanks.

Anonymous

-1

Q: Is quantum computing just pie in the sky?

I have a computer science degree. I work in IT, and have done so for many years. In that period "classical" computers have advanced by leaps and bounds. I now have a terabyte disk drive in my bedroom drawer amongst my socks, my phone has phenomenal processing power, and computers have revolutioni...

Anonymous

Aaaaaaaaaaaaaaaaaaah

Anonymous

The JD is here :P

DarkRunner

2:49 PM

If anyone can help me with my questoin, that would be great

Anonymous

@DarkRunner It's not possible to explain that without quantum mechanics

Captain Bohemian

@DarkRunner Maybe you can resort to Huygens' principle.

Anonymous

@CaptainBohemian Not sufficient to explain the double slit experiment

DarkRunner

@CaptainBohemian OK

@JohnRennie Sorry to ping, but can you please help me with my question?

Captain Bohemian

@Blue but in quantum mechanics what I learnt is about matter, not photons. If you want to resort to quantum theory for that experiment, you may need quantum electrodynamics, which is not what I have learnt. In graduate school, we only had classical electrodynamics.

Anonymous

3:00 PM

> in quantum mechanics what I learnt is about matter, not photons

Anonymous

That's pretty weird....

Captain Bohemian

not weird because our course is about nonrelativistic quantum mechanics.

though I was taught about the idea of photons since high school, I have never learnt how an electromagnetic field is quantized in a textbook.

John Rennie

@DarkRunner Hi

The problem with your original question is that it's based on some fairly comprehensive misunderstandings.

I was reluctant to try and answer it because we'd have to go back to basics.

And that's quite a long discussion.

Anonymous

@CaptainBohemian I don't really see why you'd need relativistic quantum mechanics for photons

John Rennie

@Blue because the properties of photons and special relativity are intimately connected

Anonymous

3:16 PM

@JohnRennie While, of course, that is true (I don't know RQM though), I'm a bit surprised why Captain mentions that their course didn't cover photons. Polarization of photons , double slit experiment, etc are pretty much covered at the undergraduate level without any notion of relativity being introduced

Anonymous

Oh, wait

John Rennie

Polarisation and Young's slits are not particle properties

Anonymous

YDSE using photons would involve RQM

Anonymous

probably

DarkRunner

@JohnRennie I see, thanks. Are there any links I can read up to investigate the idea further?

Alex K Chen

3:17 PM

I have a question.

Although current flow is not dependent on the surface through which you're measuring it (i.e it's same irrespective of you take the surface to be slanted w.r.t to the direction the electrons are flowing or directly perpendicular to the direction the electrons are flowing), does't current density changes on which surface you're measuring it ?

(i.e high when measured w.r.t a surface perpendicular to the flow of electrons but low w.r.t a surface slanted to the flow of electrons ?) Then how is the current density in a hollow sphere or conical frustum or objects like that defined ?

Anonymous

@JohnRennie Yes, I agree. But neither is YDSE explainable using waves (afaik)

Anonymous

So perhaps my question is this:

Anonymous

1) How is RQM relevant to YDSE?

John Rennie

@DarkRunner Light and electrons are neither particles nor waves. They are states of quantum fields. The behaviour of those states gives rise to wave like behaviour in some circumstances and particle like behaviour in other circumstances.

Alex K Chen

(Related with my above question) How do you define resistance for a object which is not like a rod (say antipodal points in a hollow sphere or two ends of a conical frustum) ? The formula $R = \frac{V}{I}$ (or $R = \frac{\rho L}{A}$) don't work for the things I mentioned, because although $V$ is constant, $I$ is not constant through various slices of the objects right ?

John Rennie

3:20 PM

Light diffracting through double slits and electrons diffracting through double slits is an example of wavelike behaviour, and both light and electrons behave the same way and are described by the same equations.

Secret

4

Q: Calculating the resistance of a 3D shape between two points

If I have an arbitrarily shaped object made of a uniform material of some specified resistivity, how would I go about calculating the resistance between two measurement points with known contact geometry? Is there a general formula for this? (other than just Maxwell's equations) If so, where wou...

electromagnetism electric-circuits electrical-resistance

Physics Meta

0

Q: Can moderators take down comments that are not breaking any rules just because they don't like the comment?

Do mods have to follow a hard set of rules, or can they take down anything that they consider inappropriate? Can they take down questions or comments that are relevant to the community just because they don't like them?

discussion comments moderators

Captain Bohemian

I think photons are derived from the quantization of the U(1) group, but I have never learnt how to quantize a Lie group.

Anonymous

@JohnRennie But that (Huygens' wave theory) doesn't explain phenomenon like delayed choice (?)

John Rennie

@Blue I wouldn't describe that as the YSDE. That's a very different experiment.

By the YDSE I mean the development of an interference pattern after propagation through the slits.

Anonymous

3:33 PM

@JohnRennie Ah, okay, if you define it like that :P

Balarka Sen

@Blue Great title though

Anonymous

Anyhow, I wonder how RQM deals with photons in generality

Anonymous

@BalarkaSen lol

Anonymous

37

Q: What equation describes the wavefunction of a single photon?

The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a massless, relativistic spin-1 particle. What is the equivalent equation giving the quantum mechanics o...

quantum-mechanics quantum-field-theory electromagnetism photons

Anonymous

You can also say that the wavefunction of a photon is defined as long as the photon is not emitted or absorbed. The wavefunction of a single photons is used in single-photon interferometry, for example. In a sense, it is not much different from the electron, where the wave-function start to be problematic when electrons start to be created or annihilated... — Frédéric Grosshans Nov 17 '10 at 10:19

Anonymous

3:35 PM

Ah, this makes sense now ^

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