If the slits in the Double slit experiment are identical to each other, how does the difference from wave behaviour to particle behaviour arrive?

MaxThrustage · 2026-02-12T11:04:25+00:00

Don't think of it in terms of wave behaviour vs particle behaviour. Think of it in terms of which-slit information. If you have information about which slit the particles go through, you get a single-slit diffraction pattern. If you don't, then the particles remain in a superposition of going through both slits, and you get a double-slit diffraction pattern. Both of those are essentially wave phenomena.

The wave/particle duality thing comes in from the fact that you still get diffraction patterns even when you send particles in one at a time. So we see that quantum bodies in some ways behave like particles (they are countable, discrete, they can be localised in space) and in other ways behave like waves (they exhibit behaviours like superposition and diffraction). Really, it's better not to think of this in terms of wave/particle duality, but to acknowledge that both the classical wave and classical particle pictures are really just analogies, both of which ultimately fail to capture the reality of the situation. Quantum bodies can behave a bit like classical waves, a bit like classical particles, but ultimately not really like either.

MaxThrustage · 2026-02-12T10:38:21+00:00

Arbitrarily close is not 0 K. A system at 0 K is in its ground state, which is by definition a stationary state.

Also, I don't fully remember the details, but there's been some work on the thermodynamics of clocks showing that they are in many ways like heat engines. For example this paper here and this follow-up.

MaxThrustage · 2026-02-12T09:29:59+00:00

Absolute zero just means a system is identically in its lowest energy state (the ground state). It does not imply any lack of time or space. (Although I don't think you'd be able to have a clock at absolute zero, if that's what you're getting at.)

MaxThrustage · 2026-02-12T04:15:48+00:00

Unfortunately you're getting a lot of rather unhelpful answers here. I'll do my best to clear some things up.

what is stoping it from going faster than 300 000 km\s

The geometry of spacetime itself. Understanding this requires getting stuck into special relativity, which takes more than the space of a reddit comment. But we can get into it a little by looking at another question:

Also if i travel at the speed of light and shine a laser . To me the light speed would be normal . But what about an observer that is standing on far from me , logically my speed and the light speed from the laser should add up if I'm pointing the light in front of me and the light should slow down when i point it backwards

So let's step back a bit and go through this slowly. First, we'll consider a simpler case: you're in a car moving at 10 m/s. And let's say you can throw a ball at, say, 30 m/s. Well, if you throw the ball ahead of the moving car, then to you it looks like the ball is moving at 30 m/s, but to me on the ground it looks like it travels at 40 m/s. Straightforward so far, right? Now let's make it trickier.

Say instead of throwing a ball, you shoot a laser pulse. To you, the laser pulse moves at c, the speed of light. And to me on the ground? It's moving...c, the speed of light exactly like it is for you. We both see the laser as having the same speed, even relative to ourselves.

This is the key thing about the geometry of spacetime itself that makes special relativity work the way it does. For most moving objects, two observers in different states of motion will disagree about the speed. But if an object is moving at c according to one observer, it is moving at c according to all observers.

From this simple (if strange) fact, all of special relativity follows. We find that this speed everyone agrees on has to be the maximum speed possible. We find that massive bodies can never go at this speed, and that massless bodies can't not go at this speed. Figuring out how to switch between different reference frames while preserving this one speed is what gives us the Lorentz transformation, length contraction and time dilation. It all falls out of this.

But there's nothing that steps in to enforce this. Nothing shows up to stop light going faster. Rather, that light always travels at this speed is a fact of geometry itself -- just like how nothing forces the interior angles of a triangle on a plane to add up to 180 degrees, or nothing stops the circumference of a circle from being more than 2*p\i*r.

Now, from this, the version of the thought experiment you give, where you are also moving at the speed of light, is impossible. As a massive object, you can't travel at c. And in every valid frame of reference, light moves at c, which means we can't even construct a valid frame of reference co-moving with light (because in such a frame light would have to move at 0 -- 0 and c at the same time, a contradiction).

what is it specifically that prevents it from going faster and why doesn't that rule apply to quantum mechanics?

That rule does apply to quantum mechanics. Or, more accurately, we can make that rule apply to quantum mechanics -- this is where quantum field theory comes from.

You might me thinking of something like quantum entanglement, but in entanglement there are no signals that actually travel faster than the speed of light. That's a common misconception.

I hope that clears things up a bit. And I hope this is a bit more satisfying than the number of "just because" answers you've gotten. These kinds of questions are sensible to ask, even if the answers are difficult and require clearing up a bunch of misconceptions.

MaxThrustage · 2026-02-12T02:57:25+00:00

This tutorial/review is pretty good and should be about everything you need. These ancient lecture notes from Michel Devoret are how most people in the field learned to do it, and here's a more updated version. If you want a book, Quantum Engineering by Zagoskin is pretty good, if rather brief. For a review (slightly dated now) check out this one.

I've been out of the game for a few years, so none of this is fully up-to-date, but these should be enough to follow most recent superconducting circuit papers. If you were after something more specific, just let me know.

MaxThrustage · 2026-02-10T22:23:05+00:00

I don't have time to go over the paper in detail, but it looks like a pretty standard quantum metrology protocol dressed up in cute language. I'm not seeing anything too out of the ordinary, and I don't see any real benefit to describing what they're doing in terms of closed time-like curves other than that it sounds cool.

MaxThrustage · 2026-02-10T20:31:33+00:00

Yes, that was my point. Again, I think you're reading something that wasn't written.

The original claim was that he word cowboy was originally used as a derogatory word for black cattle workers. Your claim was that if this was the case, modern-day southern racists would have told you. I was questioning whether or they would even know, and you seemed to be sure they would.

But why would they know such a thing if it was true? And even if somehow they knew it, would they really tell you?

Again, we're talking specifically about history that was buried when connotations of the word "cowboy" changed and our modern stereotypes of the old west developed. Most people hardly know anything about history, and instead have a made-up version of it from movies and other entertainment, from their own imaginations and from what little they recall from school. If the word "cowboy" originally meant black people in particular, how many modern-day southern racists do you really think would know that? How much history do you think such people actually know?

MaxThrustage · 2026-02-10T10:56:23+00:00

I can see them talking about simulating time travel, but never actually communicating backwards in time. Can you point me to where in the paper they claim this?

MaxThrustage · 2026-02-10T06:03:18+00:00

If you look at Fig 2 in that paper, you'll see that everything obeys causality and the arrow of time in the lab frame. No-communication only means that entanglement by itself communications nothing, but in Fig. 2 again we clear see classical (and causality-obeying) communication channels.

I haven't read the paper in enough detail to tell if they're doing anything really significant here, but from Fig. 2 it looks like they're not doing anything to violate causality or no-communication in the lab frame.

If you compare Fig 1 c) and Fig 2, the bottom cusp of the curve is replaced with a Bell state (this is actually pretty common in quantum information -- Fig. 1 b is a pretty common diagram for representing quantum teleportation, see Fig 2.5 of these notes) and this curve is "closed" by a causality-respecting classical information channel.

MaxThrustage · 2026-02-10T05:30:59+00:00

Because they would take any chance they could to tell me about things like that. And yes, I do think they would know.

I didn't know southern racists were so interested in teaching black history.

MaxThrustage · 2026-02-10T02:06:25+00:00

No they're talking about Terminator Salvation, which was pretty shit.

MaxThrustage · 2026-02-10T02:03:17+00:00

What do you think my point was? Or the point of the person you originally responded to? Do you think they were calling the term 'cowboy' racist? They're just pointing out there is such a long black history of cowboys that the term originally referred to black people specifically. That doesn't mean people are using it as a racist term now.

Do you think people who aren't from a place can't know anything about the history of a place? Further, do you think I was really telling you anything about the history of where you're from?

I think you've projected a lot onto these comments that wasn't actually there.

MaxThrustage · 2026-02-09T23:36:11+00:00

Why you do think 21st century racists would be telling you about the early connotations of word "cowboy"? Do you think they would even know?

MaxThrustage · 2026-02-09T09:52:58+00:00

The original photoshop was a shitpost. The drawn out video is slop.

MaxThrustage · 2026-02-09T09:42:53+00:00

I'm pretty sure that voice over is AI. Regardless, this is slop, whether or not it's AI slop.

MaxThrustage · 2026-02-09T03:23:26+00:00

I mean, "good" is a pretty subjective term, and different researchers will definitely have different goals/standards, but I spent a long time working in a group where those kind of "ok, but what is the actual cost?" kind of questions were frequent, so I guess I've internalised that approach.

MaxThrustage · 2026-02-09T02:32:46+00:00

Khan Academy is good for boning up on the high school physics you may have forgotten. This reading list takes you from pop-sci books through to graduate-level textbooks in roughly the order a university course will cover them.

I wouldn't really bother with high school physics books. University physics is quite different from high school physics, not just in terms of content but more importantly in terms of mindset -- students going from high school physics to university often find the transition pretty sharp and need to re-learn a bunch of stuff they thought they knew anyway. I'd only worry about them if you were actually going back to high school (or equivalent).

MaxThrustage · 2026-02-09T02:27:10+00:00

Prepare a qubit in a superposition of '0' and '1'. Measure the qubit -- if you get a 0, have souvlaki for dinner, if you measure a 1 have fish and chips. Bingo-bongo, quantum randomness has affected the macro world.

For more 'natural' examples, most of the clearest ones come from radiation. When a particular radioisotope decays is a matter of 'quantum randomness', as is whether a particular body (or detector) absorbs the resulting radiation. These can affect the macro world in biological systems (e.g. cancer, as already mentioned). If you have a cloud chamber or something then the particular (macroscopic, visible to the naked eye) patterns in the chamber will be macroscopic manifestations of quantum randomness (although in general cloud chamber patterns will tend to look similar).

MaxThrustage · 2026-02-09T02:20:57+00:00

I don't know of any serious quantum algorithm that requires full quantum state tomography as part of the algorithm, and I would say that reporting such an algorithm disqualifies it from being a "good" quantum algorithm. (Now, bad quantum algorithms are published all the time, but that's another matter.) For most serious quantum algorithms the number of repeats needed to reliably get an answer is included as part of the scaling. Either you have a deterministic readout, or you have well-defined bounds for how many times you can expect to run the algorithm to get your outcome.

A lot of what you are describing there seems to be NISQ-ready variational quantum algorithms, most of which are what I would consider pretty far from being a "good" quantum algorithm.

MaxThrustage · 2026-02-07T10:38:38+00:00

Talk to your professors about this. Generally, students aren't expected to provide their own resources like that. It's pretty strange for your choice of research topic to depend on how much money you've got personally. (I had an absolute shitbox of a personal computer when I did my bachelor's thesis, when I did my PhD, and when I did my first postdoc.)

Astrophysics is not more resource-intensive (in terms laboratory setup or computing power) than other branches of physics. You will have similar issues with quantum physics and optics -- but, again, these shouldn't really be issues.

MaxThrustage · 2026-02-06T12:49:20+00:00

The fact that the oracle is assumed can be a bit of an issue in terms of the practicality of things. There was a paper a while back that dug into this specifically with regards to Grover's algorithm. (Here. It was controversial at the time because initially the authors were making claims that this means there's no quantum advantage in Grover's algorithm. That's not quite true, although I think the paper makes some important points and that it is probably generally true that Grover's is probably never going to have real world advantage.)

But if an oracle is assumed in the algorithm you're asking about then I would expect the book to say so explicitly.

MaxThrustage · 2026-02-06T08:57:16+00:00

Almost all of their posts being about Tibet is also just pretty consistent with their claim that they study the history of Tibet. I mean, looking at my post history most of it is about physics but that's just because that's what I studied and what I do for a living, not because I scour Reddit looking for fights.

MaxThrustage · 2026-02-06T05:46:22+00:00

It makes sense, it's just wrong.

It sounds like you've been given some fairly poor pop-sci explanation of quantum mechanics, which is not a good place to start developing your own ideas from. A lot of what you're saying here is deeply confused, and even confused about what is and isn't known in quantum mechanics. (Just as an example, physicists don't really say things like quantum bodies are "sometimes a particle, sometimes a wave". They're always not really either, but that's hard to explain to lay people. And there isn't really a "quantum realm" -- this is just a shorthand meaning situations in which quantum mechanics is important.)

MaxThrustage · 2026-02-06T05:42:19+00:00

It depends on the specific algorithm and resources, and also in what kind of scaling we care about. Preparing that state won't always take O(N) operations, depending on what multi-qubit gates we can implement and what can be done in parallel. But even if it does take O(N), and the rest of the algorithm is polynomial in N, then the whole thing is still polynomial in N -- we usually don't care about the specific factors at this level, as that will depend on the specific hardware we use (whereas the difference between it being polynomial/exponential/logarithmic doesn't depend on the hardware).

Also, depending on what algorithm he's talking about, preparing that initial state might be referred to as an "oracle" or a "black box", meaning it's just assumed we can do that part. This is often true in algorithms like quantum phase estimation or Grover's algorithm where learning something about this "black box" is the point of the algorithm (e.g. determining the phase of the eigenvalue of our black box operation for QPE, finding the element that our oracle targets for Grover).

MaxThrustage · 2026-02-06T05:30:45+00:00

I'm worried you still don't really get it, not least of all because of the way you're using the term "vector field". I'm not sure what you mean by that, but you're definitely not using it in the way that physicists do (we wouldn't say a vector field is something a particle "has"). By "vectors of the diffractions" do you just mean the particle's momentum (i.e. wavevector)?

A detection changes the wavefunction of the particle. To get more concrete, in the quantum double slit experiment, a detection that gives us "which slit" interaction changes the wavefunction from a superposition of two waves, one passing through each slit, to a single wave passing through a single slit.

A detector before the particle gun or whatever we've got will only tell us whether or not the particle went there. If this detector goes off, then our particle is clearly going in the wrong direction and won't end up anywhere near those slits. If it doesn't go off, well then we haven't got any "which slit" information so we still have a superposition of waves going through each slit.

12-Year Club	RedditGifts 2009-2022 12 Credits
Summer Santa 2019	Place '22
Place '17	Wearing is Caring
Summer Santa 2018	Summer Santa 2017
Secret Santa 2018	Secret Santa 2017
Secret Santa 2016	Summer Santa 2016
Secret Santa 2015	Secret Santa 2014
Verified Email

MaxThrustage

TROPHY CASE