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[personal profile] jack
To expand on the point in the previous post, is it right that electrons bend round corners, like sound etc? Aka diffraction? This is how electron microscopes work, right?

That means that a probability wave is an actual thing, right, not a description of a particle? Does it?

But if so, how can anyone cling to the idea that they're a particle with a particular position. Particles don't do that. Do they?

And yet, there's massive amounts of effort to come up with interpretations of quantum mechanics that retain the "in a particular position" idea. Or the idea of hidden variable theory seems to be that the electron is in multiple places at once, but when you finally measure it, it was predetermined what value you were going to find[1]. If you've *already accepted* the multiple-places-at-once thing, AND the wave-physically-exists thing, what do you gain by assuming it then suddenly stops doing that at some unspecified point?

[1] "Predetermined" to avoid the "spooky action at a distance" problem, of, if you have a probability wave describing *two* particles (say, emitted in opposite directions with opposite spin), and measure them waaaaaaay far apart, how do they "know" what value to take to ensure they end up opposite, when there's no way for a signal to travel between them. Leaving aside the absurdity of a "hey, collapse this way" message even if it were slower than light.
jack: (Default)
[personal profile] jack
Hypothesis 1: Electrons are tiny objects that have a specific position

Evidence: If you bounce something off an electron it hits the electron in one place. For all the talk of "in multiple places at once", you never shoot something at an electron, it bounces off the electron, and it bounces off the electron *somewhere else*.

Evidence: There's always a particular number of electrons. You never have two and a half electrons.

Hypothesis 2: Electrons are waves

Evidence: If you have an electron "orbiting" an atom, it's not at a particular place, it's smeared out over a whole sphere (or sphere-ish shape?) round the electron aka "an electron shell". Indeed, if you have two electrons in an electron shell, I don't know if you can even tell them apart, just that there's two. In metal, ALL the electrons are ALL OVER. They really don't have a particular position.

Evidence: If you fire one at a corner of an object, they diffract round it (is that right??)

Evidence: If you fire one through one or two narrow slits, you get interference bands, where "electron from here" and "electron from *here*" combine to give a dark band of "no electrons detected". This happens to waves. It does not happen to objects.

Hypothesis 3

This takes longer to explain. Imagine you have an object, but its position isn't certain, you're doing a calculation like, "if there's an x% chance it's here, and a y% chance it's there, and it bounces off this, then it might be anywhere along this line with a chance proportional to the distance..." etc. We do that all the time instinctively. But we mostly expect that the object actually *is* in one particular place, we just don't know what it is.

Suppose that instead of a mathematical convenience, what an electron *actually is* is a probability distribution like that, except for:

(a) When something interacts with it, it interacts with one point in the distribution chosen with the relative likelihood of that point, and from then on only that matters. Except if the other particle is of uncertain distribution too, then you get a probability distribution over both of them, until you actually check at least one of them.

(b) The probability distribution changes obeying equations which mostly describe a particle moving in a straight line (or a curve according to a force acting on it), except that it's all continuous, and if you have a sharp corner, the probability spreads out round it (as if the particle's path was bending).

(c) The probability not only has a magnitude, it has a direction (usually represented as a complex number, where the actual probability is the magnitude). If two probabilities have opposite signs, they cancel out. And it changes as it moves, analogous to wave oscillating, eg. light consisting of electric field waxing while a magnetic field wanes, etc.

The third point (c) is par for the course for waves: waves almost always involve something oscillating in both directions away from a rest point. But it's very spooky to see with things that look like particles: if there's a 5% chance of an electron hitting this particular point on a screen having gone through slit A, and a 5% chance of an electron hitting this particular point having gone through slit B, what's the chance of it hitting that point at all? Well, it might be 10% or it might be 0% or it might be somewhere between, depending

Evidence: Everything above in both previous hypotheses

Evidence: Everything behaves like a particle even if you didn't expect it (eg. light has photons)

Evidence: Everything behaves like a wave even if you didn't expect it (eg. you can fire small molecules through slits and see them do wave-like things like interference).

Evidence: The cancelling-out thing. You can construct this out of specific particles with clearly defined values (qubits) in building a quantum computer, and this is exactly how you find probabilities behaving. (Right?)

Correct me?

Is (b) really true? That's what it looks like from what I've read. But is that basically accurate?

If not, where have I gone wrong?

If so, it seems such an obvious "this is how we know these probability thingies actually exist" why isn't it front and centre in more explanations?

Is the description of probabilities right?

Conclusions

Hopefully I will think myself through some more examples. But this is the major point to get your head around first with quantum mechanics.

I think everyone would say the first two hypotheses are more natural. But they don't fit the evidence. The third hypothesis fits ALL the evidence, even though the hypothesis itself looks screwy.

And as far as I can tell, physicists still argue about which parts of this are actually there, and which are mathematical descriptions of something else, but agree that if you take Hypothesis 3 and just assume everything works like that, then you get all the right answers.

HOGWARTS HOUSES

Apr. 24th, 2017 04:27 pm
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[personal profile] aroceu posting in [site community profile] dw_community_promo

GO SORT YOURSELF!

LFG: Hordie Needs Help

Apr. 24th, 2017 10:15 am
elynne: (Default)
[personal profile] elynne
I want to play WoW again. I'd love to get into a guild. Finding a guild that matches my criteria has been vanishingly difficult. If you know of a guild that is:

1. Horde
2. on any US server (I'm willing to transfer my main! and start new alts! anywhere! I'd prefer not PVP but I'm even willing to try that!)
3. not Raid-Focused (raiding is fine, I'm just not into it, and I'm not interested in a guild that demands a 910+ gear score with Heroic Steam Vaults cleared or whatever the current bar is)
4. active (preferably more than like three people on at a time, and people who are willing to talk about stuff)
5. not a seething bag of dicks (you know what I mean here)

please, please, PLEASE let me know? I'm currently investigating a guild on Emerald Dream, which is an RP-PVP server, and PVP servers scare me, but I'm that desperate. If you know someone who you think once mentioned a guild like this, pass on a mention? I promise I'm a good guildie...

Rocinante

Apr. 24th, 2017 06:09 pm
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[personal profile] kayim posting in [site community profile] dw_community_promo
I swore I didn't want to mod another community, but I couldn't hold out any longer. I needed a place to squee, so please come and join me!


[community profile] rocinante 
For all of your Expanse squee, whether it's about the books, the TV series,
the amazing Belter language, or about the actors themselves.
 

RPG Citadel

Apr. 24th, 2017 09:37 am
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[personal profile] jennandanica posting in [site community profile] dw_community_promo


Genre: Celebrity BDSM RPS
Name: Citadel
Contact: the mods - [personal profile] jennandanica, [personal profile] fiercy and [personal profile] alley_oops
Main Comm: Citadel
Minimum Age Requirement: 18+
Deadline: None

More Information on the Game )

Current Pups (Already Taken) )

QotD

Apr. 24th, 2017 05:24 am
dglenn: Me in kilt and poofy shirt, facing away, playing acoustic guitar behind head (Default)
[personal profile] dglenn

"One lesson America needs to understand is that the Holocaust did not begin with killing or gassing, but rather with words.

"In my work as a scholar of American Jewish history, I examine the Jewish experience in the United States and consider whether the near-obliteration of European Jewry could have been avoided through the adoption of different policies by countries throughout the world in the 1930s. For example, the experience of the infamous SS St. Louis teaches us the important power of executive orders, the words they use, and the messages they send -- not only to those immigrants who clamor to come to the United States but also to the larger world.

"The SS St. Louis left Hamburg in May 1939, carrying 937 German Jews -- many of whom had been imprisoned in concentration camps -- seeking to flee Nazi Germany. They all had valid visas for entry in the coming years but had to leave Germany immediately for their safety. Denied entry to the United States out of the preposterous popular misconception of their being German spies, the 937 Jews were sent back to Europe to await the calling up of their visa numbers. These passengers resettled in Europe, but many fell back into Nazi hands. As a result, over a quarter of these US visa-holding Jews perished in the Holocaust."

-- Rebecca Kobrin, 2017-04-23

"'[Spencer] said that America belongs to white people. His statement that white people face a choice of 'conquer or die' closely echoes Adolf Hitler's view of Jews and that history is a racial struggle for survival,' the museum said. Then it offered a history lesson to anyone who has forgotten: 'The Holocaust did not begin with killing; it began with words.'

"Those words eventually led the Nazi party, which came into power legitimately in Germany, to kill 6 million Jews and millions more Communists, Gypsies, Poles, gay people and people with disabilities."

-- Julie Zauzmer (quoting from a statement from the U.S. Holocaust Memorial Museum), 2016-11-22

[You can find plenty of hits for reminders that the Holocaust began with words. That remains a really important reminder, but also look at the other reminders different authors attach to that one.]

Today is:
Gregorian: 2017 April 24
Julian: 2017 April 11
Hebrew: 5777 Nisan 28 -- Yom HaShoah, Holocaust and Heroism Remembrance Day (started last night)
Islamic: 1438 Rajab 27
Persian: 1396 Ordibehesht 04
Mayan: 0.0.0.13.0.4.7.5
Indian: 1939 Vaisakha 04
Coptic: 1733 Paremoude 16

two comms

Apr. 24th, 2017 07:33 pm
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[personal profile] timetobegin posting in [site community profile] dw_community_promo

[community profile] capshare
share those caps collecting dust on your harddrive!


[community profile] iconthat - Come join icon that a challenge community for icons!

calling all Yu Yu Hakusho nerds

Apr. 24th, 2017 01:58 am
effingunicorns: four hellebore flowers in a dusty dark blue, two shades of pink, and white (Default)
[personal profile] effingunicorns posting in [site community profile] dw_community_promo
In an attempt to consolidate the fandom where I can, I've created [community profile] yu_yu_hakusho for lovers of the classic manga and anime! All fannish content is welcome, and there may be group activities in the near future depending on what sort of traffic we get, so don't hesitate to drop in.

And it just gets better and better

Apr. 23rd, 2017 10:55 pm
baratron: (cn tower)
[personal profile] baratron
Saturday, April 22, 2017, 10:00 p.m. — Monday, April 24, 2017, 5:00 p.m.

Due to system maintenance during the dates and times listed, there will be a partial shutdown of AirTrain JFK service. There will be no service to/from all airport terminals and no service at the Howard Beach and Lefferts Blvd stations. AirTrain JFK service will continue to operate a normal schedule to/from Jamaica Station and Federal Circle. Free shuttle buses replace service and will be available at the affected AirTrain stations and airport terminals. Please allow extra time when traveling to and from John F. Kennedy International Airport and see a Customer Care Representative for assistance if needed.


Argh! I just LOVE the idea of having to travel around the airport in a shuttle bus instead of a nice, accessible train! *facepalm*

*turns around several times*

Apr. 23rd, 2017 03:58 pm
elynne: (Default)
[personal profile] elynne
I am unaccustomed to 1. writing like this 2. being on this site 3. things 4. stuff 5. hi 6. I'm working through it 7. but it's probably going to be slow and awkward while I figure things out 8. how are you 9. *shoots terminal* 10. boring conversation anyway
baratron: (boots)
[personal profile] baratron
I have many photos of my trip to the US to post, when I get A Round Tuit. They are crappy mobile phone pictures, but I cannot be bothered to carry a real camera with me any more. I am a terrible photographer with the best equipment in the world, and I'm more interested in recording memories than attempting to take good shots.

Currently I am worrying about being stranded in New York tomorrow once Delta drop me off there.

It turns out that Aer Lingus are morons. I've had several "incidents" with them this trip and am never flying with them again, but in the meantime they are supposed to be getting me home to London tomorrow.Read more... )
brainwane: My smiling face in front of a brick wall, May 2015. (Default)
[personal profile] brainwane
I saw Antitrust on an airplane in the summer of 2001. I didn't leave with a high opinion of it; it seemed campy fun.

I found a used DVD at a local thrift shop last weekend, so last night I watched it with my spouse.

It actually holds up better than I predicted on a technobabble level! We freeze-framed a lot and marvelled at how reasonable (mostly) all the command-line stuff was. And as mainstream fiction movies go, I think there still hasn't been a movie that takes the conflict between proprietary and open source software more seriously than Antitrust (I'd welcome corrections on this point).

details, including spoilers )

At some point in the future I will watch the special features and listen to the commentary. (One of the special features is a music video for the Everclear song that plays at the end of the movie. The music video includes clips from the movie. It's like Everclear made a vid!) I imagine I'll have more thoughts then.

Forgive us our Trespassers

Apr. 23rd, 2017 06:35 pm
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[personal profile] hairyears
Tomorrow we celebrate the anniversary of The Kinder Scout Mass Trespass, in which hundreds of Ramblers and other members of the public forced their way past gamekeepers to hike on the moors and peaks of Derbyshire in 1932.

I have many reasons to be thankful that they did, having walked upon those very hills myself; and on others, which were long fenced off against the public as the playgrounds of a privileged few.

Some of the original Trespassers were imprisoned, and I do not doubt that they would suffer harsher treatment if they tried such acts of civil disobedience today.

So, a question for the legally-minded Journallers who read this: what would the Trespassers be charged with today? And what would be their fate if any of them could not prove their cizenship, or looked ever such a little bit foreign?

Would my long-lapsed membership of the Ramblers Association, and my evident sympathy for their aims - I have led a ramble myself! - turn out to be a liability? Worse, perhaps, than the social consequences that arise when I reveal my ability to hold a lengthy conversation about 1980's hiking socks?

QM: Quantum eraser experiment

Apr. 23rd, 2017 05:56 pm
jack: (Default)
[personal profile] jack
There's an experiment. "Quantum eraser". This is "me asking advice", I don't understand it to explain it.

It involves, producing two entangled photons, and doing the double-slit experiment on one of them with a different polarisation-changing filter over each slit. Repeat lots of times and see if you get an interference pattern, or actually not, because the polarisation-changing filters make the photon not destructively-interfere with itself (because the two states "at this point coming from slot A" and "at this point coming from slot B" are no longer exactly the same).

The mysterious bit is, if you put a linear polarisation filter in front of the *other* photon, this ruins the polarisation and the interference pattern goes away. Which looks like a specific physical effect of waveform collapse. People go to lots of effort to make sure that the same effect applies if you make the path between the other entangled photon and the "linear polarising filter or not" really long, so you make that choice *after* the other photon hits the screen, and yet, still seems to affect it.

This seems really mysterious. In fact, it sounds so mysterious it's actually impossible.

But what I was missing was, every diagram has a "coincidence counter" which only counts photons if one from each path both arrive (at the same time, if the paths are the same length, or at corresponding times otherwise). This seems like a standard precaution, to ensure you're only counting the actual photos, and not stray cosmic rays or whatever.

And yet, normal two-slit experiments don't (seem to?) need to use one.

And specifically, the linear polarising filter *throws away* half the photons, which means that at the screen you DON'T get an interference pattern. Whereas if you only look at the half of the photons which correspond to ones which passed the linear polarising filter, then you DO. (If you look at the OTHER half of the photons, you'll see an opposite interference pattern, which adds up to a smooth non-banded pattern of photons if you overlay the two halves).

What actually happens does (as always) correspond to "things only interfere if they're smeared out over multiple potential possible values (in this case two different paths through the slits), if you've already interacted with them, then not". And I don't quite follow what *does* happen because I've not tried to follow the equations. But the whole "mysterious effect travels back in time causing waveform collapse" seems to just not exist, except in how people choose to interpret the experiment.

So, I'm confused, many physicists seem to agree this is important, but I don't quite see how.

And "you get exactly the same experimental results but only look at half of them according to the result of the other entangled particle" seems a really important concept but all explanations seem to leave it out and say "you get a different result" instead. Do I understand that right??
jack: (Default)
[personal profile] jack
This is a bit earlier in the sequence than I'd intended but I wanted to rant about it.

What is so-called quantum teleportation?

Imagine you have a small particle. If this were a classical world, you could measure everything about it (it's speed, it's spin, etc), and then use a bunch of fiddly experiments to recreate one (or more) new copies of it that had all those same properties. Of course, it's *practically* impossible, to scan the state of millions of particles so this actually only happens to single particles (or we mass-manufacture consumer goods, but we don't try and make sure they all have corresponding atoms in the same place in each).

As we live in a quantum world, you can't "measure everything about it". Electrons don't exist at a particular point, they exist as a wave of possibility in a sphere round an atom, and only when another particle interacts with them, does it interact with them at one particular place on that sphere. Each photon isn't "in a particular place", even if you have a single photon you have a very very very faint beam of light and if you repeat the experiment, you find "places a photon hits" and "places the beam of light would cover" are the same thing. If you have a qubit made up of a single atom, you can measure its value as 0 or 1, or send it through a quantum logic gate, and find out about the parts of its state you can't measure directly *instead* but you can't do both.

Hence, in a quantum world, even in theory, it's weirder to construct a new particle the same as an existing particle, because you can't "measure everything, and then move the new particle so it has all those values".

So you *can't* make multiple copies.

What can you do

However, it turns out, there *is* a way of making an exact copy of a particle's property. You create two other objects (photons?) with opposite values for polarisation or something, even though you can't measure what that value is. (aka "an entangled pair", although all "entangled" means is "they have the opposite polarisation even if you don't know what it is"). You interact the original with that one and measure some values. Those values don't tell you what the property is (because if it WAS one particular thing, you'd have destroyed the information you were trying to copy). But you can apply them to a new particle via the second entangled particle. And you don't know what the state *is*, the original particle no longer has it, but the new one does.

That is, "You might imagine that you could copy a quantum electron the same way you could copy a classical particle by measuring the values and applying them to a new electron. But you can't, that's actually a meaningless concept. Knowing that, you might give up. But there's a way to do sort-of do that."

Specifically, "quantum teleportation" means "there's a special and fiddly way you can construct a new particle exactly the same as an old particle, but only EXACTLY ONE, and it destroys the original state". As in, you can do SOME of what you'd expect to be able to do to a classical particle, but not all of it.

What doesn't it mean?

What doesn't it mean? Firstly, it means "teleportation of quantum", not "teleportation by means of quantum". It doesn't give you some magic way of scanning macroscopic objects or reconstructing them elsewhere. It just means that, if you happened to already have one, you might be able to copy quantum states too.

Secondly, nothing anyone cares about day-to-day is encoded in quantum states. It might matter for quantum computers. Maybe for quantum cryptography. Certain scientific experiments. That sort of thing.

If you actually cared about quantum states, this might be exciting. Suppose brains encoded what they did in something like a quantum computer. Then startrek teleportation would only be normally impossible because you can't scan a human like that, not logically impossible. However, brains don't do anything of the sort[1].

If you care about startrek-teleporting a human, you probably want to end up with the same DNA molecule. But you probably don't need each atom to have the same quantum state. So it doesn't really matter.

Takeaways

A: Startrek is awesome, right?
B: Yeah.
A: But teleporting people is impossible right?
B: Pretty much. I suppose there might be some way discovered, but it doesn't seem very practical.
A: But, doesn't quantum say something about this?
B: Oh right. Yes, it says if you care about replicating all the quantum states in the transportee, you can only have one source (which is destroyed) and one copy.
A: That seems fair. That's how it works in startrek.
B: Well, it rules out "lets keep a backup of our most valuable engineers and seconds in command". Which did happen in startrek but only by accident.
A: Oh yeah, I guess.
A: So, *do* I care about replicating the quantum states in the transportee?
B: No, not really.
A: So quantum doesn't really change the answer?
B: No.
A: What about "quantum teleportation". Doesn't that let you... teleport people?
B: No. It just means, you CAN do the up-to-one perfect-quantum-states copy (assuming you have a way of teleporting people at all).
A: So why do people keep writing news articles about it?
B: Because it sounds startrek-y.
B: And to be fair, is relevant for how QM works.

Footnote 1

How do I know that? Well, I might be wrong. But firstly, maintaining atoms in a particular quantum state which can encoded a qubit used for quantum computing needs a whole bunch of vacuums and stuff. MAYBE brains could do that, but it seems unlikely. Sorry Penrose, I know you're a genius and I'm not, but I don't believe you.

Secondly, quantum computers have certain distinguishing features. They're about the same as classical computers for most problems. Notably, most every-day stuff. Also, NP-complete problems they're not significantly better. However, they ARE better than normal calculations for some specific things, like factoring numbers with thousands of digits, and other maths problems which share some features in common with that. If you look at a human brain, do you think, "boy, that's optimised for simple but powerful heuristics used for catching balls, recognising objects, and social interaction, but is mediocre at factorising incredibly large numbers"? Or the reverse?

Thirdly, there's no reason to think brains DO have quantum information used in any particular way. If they did... it wouldn't change anything significant. It wouldn't make the free will argument any different. It wouldn't give them a magical insight into parallel universes (as awesome as Anathem makes it sound). So why would you think that?

Snippets

Apr. 23rd, 2017 04:25 pm
wildeabandon: photo of me with wavy hair and gold lipstick (Default)
[personal profile] wildeabandon
Yesterday I went to Walsingham for the wedding of Fr Daniel and his lovely bride Elise. It was a beautiful service, presided over by the Revd Canon Stephen Gallagher, Elise's father, and with a sermon from +Philip North which was both moving and highly entertaining. The music was exquisite - the ordinary was William Walton's Missa Brevis, which I'd not heard before, and sounded beautiful but also very hard! It was marvellous to spend some time in Walsingham, and I somehow managed to come away from the shrine shop without buying any tat, and only one book.

As Fr Daniel had other places to be this morning, we had a visiting celebrant today, and since she had enough on her plate getting to grips with an unfamiliar ceremonial, I was asked to step in as cantor for the Alleluia. I'd had enough warning about this that I was able to ask CN to turn half my piano lesson the previous week into a singing refresher, and that helped me feel a lot more confident. Despite being frightfully anxious, I think I did a pretty decent job, and I might ask about possibly doing it more regularly, so I can work on getting over my nerves.

We continue to settle in to the new house. We still lack a functioning dishwasher, and have slightly more furniture in the spare room than we actually need, but apart from that we've done all the urgent jobs, and are starting to move on to the longer term nice-to-haves. Today I put together a new standing desk, which I'm hoping will have the twofold effect of being good for my back and shoulder tension, and discouraging me from wasting too much time hitting refresh on social media.

TRON Community seeking Users!

Apr. 23rd, 2017 08:35 am
skye_writer: Cropped screencap of lightcyles (one blue, one red) from the film TRON: Legacy. (TRON)
[personal profile] skye_writer posting in [site community profile] dw_community_promo
 [community profile] tron_comm is a new TRON community on Dreamwidth! There's not much now (I'm very new at this), but plans for the future include community rewatches of the franchise, weekly fanfic recs and promos, graphic and icon contests, and prompts for both fanfic and fanart! Please join us if you love the TRON franchise in all its many-splendored forms!

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