[HN Gopher] An experiment caught a quantum system in the middle ...
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An experiment caught a quantum system in the middle of a jump
(2019)
Author : airstrike
Score : 73 points
Date : 2021-05-11 16:47 UTC (6 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| davidhyde wrote:
| > "Yes, it is shot through with randomness -- but no, it is not
| punctuated by instantaneous jerks. Schrodinger, aptly enough, was
| both right and wrong at the same time."
|
| What an awesome way to conclude an excellent article!
| bosswipe wrote:
| Discussion 2 years ago:
| https://news.ycombinator.com/item?id=20105091
| ulam2 wrote:
| It wouldn't surprise me if they find that time too is discrete.
| SideburnsOfDoom wrote:
| Correct https://simple.wikipedia.org/wiki/Planck_time
|
| https://en.wikipedia.org/wiki/Planck_units#Planck_time_and_l...
| krastanov wrote:
| Very much not correct. The Plank time is a rough scale at
| which we know that our theories do not work. That might mean
| that a new discrete theory appears there (which is frankly
| quite probable), but we most certainly do not know whether
| that is the case and it is just as possible that the
| underlying theory is continuous.
| zepto wrote:
| That doesn't say it's discrete. Only that there is a limit to
| precise measurement.
| advanced-DnD wrote:
| I'm working in a field where m=1 and hbar as a semiclassical
| scale is given as order of N (number of bosons or fermions),
| i.e. I don't do actual real physics...
|
| But.... a thought experiment:
|
| If a particle experiences time differently dependent to its
| speed in relative to a frame of reference, i.e. that time
| experienced is scaled according to its speed. Since speed is
| definitely not discrete, scaling it with time, even though time
| in itself is discrete as you claimed, still makes the time
| experienced for said particle to be continuous, no? Is this not
| a contradiction?
| BoiledCabbage wrote:
| > Since speed is definitely not discrete,
|
| Why is it definite that speed isn't discrete? Distance is
| discrete in plank length, and time is discrete in plank time.
|
| Speed being distance over time, is one discrete unit over
| another. Isn't it by definition discrete?
|
| At a macro level we may abstract over it's discreteness, but
| isn't it necessarily discrete if it's made up of discrete
| units?
| RhodoGSA wrote:
| Isn't that one of the things that fall out of relativity?
| that i could never be able to say if i'm the one at rest or
| if you are at rest?
| brainfish wrote:
| I would be surprised if it is not, given the success of
| spacetime treating it as just another dimension.
| SideburnsOfDoom wrote:
| "A Planck time unit is the time required for light to travel
| a distance of 1 Planck length in a vacuum"
|
| https://en.wikipedia.org/wiki/Planck_units#Planck_time_and_l.
| ..
| wetmore wrote:
| What does that have to do with it being discrete?
| nine_k wrote:
| I don't know about any experimental proofs that either time
| or space is definitely discrete, or definitely continuous.
|
| Both Planck length and Planck time are _way_ too short for
| current experimental techniques.
| baja_blast wrote:
| I am unconvinced that time is a real dimension, sure we can
| model movement/interactions of particles as a 4th dimension,
| but nothing in our understanding of physics requires it being
| a physical reality. IMO it's less space-time and more just
| space, it's just the rate of change slows with more mass.
| jhayward wrote:
| There's at least one prominent physicist [1] who is working
| on showing that time is an emergent property of quantum
| physics rather than a fundamental one.
|
| [1]
| https://www.preposterousuniverse.com/blog/2013/10/18/is-
| time...
| AnimalMuppet wrote:
| If time isn't real, then when you say "rate of change", I
| ask "rate of change _with respect to what? "_
| advanced-DnD wrote:
| He's not claiming time isn't real, which the definition
| of "real" is difficult and troubling to define in itself.
|
| I think he doesn't consider time to be 4th (or part of
| 3+1 or \R^3 \times \R ) dimension... or in heat equation
| language: that the domain is a not parabolic cylinder.
| thricegr8 wrote:
| I'm not sure "quantum leaps" were ever seriously described as
| "instantaneous". Headlines like OP haven't done much service to
| the more concrete (albeit layman) definition; when we are talking
| about quantum speed limits and propagation, what we are really
| getting at is _the speed at which information is propagated
| through spacetime_.
|
| When we talk about the cosmic speed limit and why nothing can
| travel faster than light, that's what we are really trying to
| describe. Not that nothing can travel faster than light, but
| rather information propagates at a certain speed, which also just
| so happens to be the speed of light. The consequence is of course
| that nothing can travel faster than the speed of information
| propagation, not even "quantum leaps".
| TchoBeer wrote:
| What is "information"? Is it a physical thing?
| vladTheInhaler wrote:
| That appears to be an increasingly common perspective, summed
| up in the phrase "it from bit". It seems to owe its
| popularity to this essay by John Wheeler from 1989 [1]. I'm
| not really familiar with the topic so I can't say more than
| that.
|
| [1] https://philpapers.org/archive/WHEIPQ.pdf
| [deleted]
| walleeee wrote:
| Landauer would say so: https://www.sciencedirect.com/science/
| article/abs/pii/037596...
| jakeva wrote:
| It is in the same sense that entropy is a physical thing.
| https://en.wikipedia.org/wiki/Information_theory
| TchoBeer wrote:
| I don't know, something about this feels more like an
| abstraction than a real thing. If you say "the speed of
| light is the speed limit for information" that doesn't feel
| like you're saying something about the way the universe
| works directly.
| krastanov wrote:
| Pretty much the entirety of modern physics is this type
| of abstraction (https://en.wikipedia.org/wiki/The_Unreaso
| nable_Effectiveness...). You are just facing the
| discomfort felt when your intuition clashes with the
| tools usually required to make reliable predictions and
| explanations about the universe.
|
| Consider, how is your current discomfort with this
| statement any different from the discomfort felt through
| the centuries when ideas like fields instead of forces,
| entropy, phase spaces, quantum amplitudes / complex
| numbers, matrix mechanics, spacetime, curvature, etc were
| being introduced. They are all abstractions that end up
| being more reliable than our intuition.
|
| Whether this "abstraction" is a "real thing" is a
| question for the philosophers. For me there is no
| difference between the two.
| jakeva wrote:
| Sure it's a bit different (especially if you're concerned
| with how it makes you feel) than, say, describing
| tectonic plates or the acceleration of a falling body due
| to earth's gravity but that doesn't make it not a real
| thing.
|
| Quantum spin is one that gets me, or the uncertainty
| principle. It makes me very uneasy, but whether or not
| I'm comfortable is irrelevant. Those are to the best of
| our knowledge actual features of the universe.
|
| Abstractions aren't really in the language of the pure
| sciences. Analogies, metaphors, etc can all serve to help
| explain but the speed of the propagation of information
| in this universe is very much defined as the velocity of
| light in a vacuum in a completely literal sense. There's
| no abstraction here. Maybe some confusion about what we
| mean by 'information' but I'm sure there are better
| resources if you want an afternoon rabbit hole.
| galimaufry wrote:
| I'm not sure this answers the question. The wikipedia page
| is about the entropy of a probability distribution. But the
| information speed limit is supposed to apply even if
| everything is totally deterministic.
|
| If I write a single, 100% certain message and put it in a
| spaceship it still cannot go faster than light, even though
| there is no information transfer (entropy of my message (a
| constant random variable) is 0).
|
| (I'm not saying you are wrong, I am asking to be corrected)
| kgwgk wrote:
| Which is not, really. At least not as clearly as other
| things.
| avmich wrote:
| It's a bit like saying Maxwell equations are not as clear
| as Newton's laws.
| kgwgk wrote:
| "entropy is an anthropomorphic concept, not only in the
| well-known statistical sense that it measures the extent
| of human ignorance as to the microstate. Even at the
| purely phenomenological level, entropy is an
| anthropomorphic concept. For it is a property, not of the
| physical system, but of the particular experiments you or
| I choose to perform on it."
|
| http://www.lptms.u-psud.fr/membres/trizac/Ens/M2MQPL/Jayn
| es_...
| zkmon wrote:
| Very important question. I believe information is a
| percievable or measurable difference in states of a thing at
| two different time instants or difference in states of two
| things. A state of a thing exists only because it can be
| distinguished from another state; that is, the state exists
| only because it can encode information relative to another
| state.
|
| How fast information can travel? Just as fast as one can
| distinguish one state from another. This assumes that there
| exists an agent capable of distinguishing between the states.
| How fast can this be done? It doesn't start at one state and
| end at another state. It is instantaneous recognition of the
| difference. The information is created just when the
| difference in states is recognized. There is no start or end.
| It is an event. Events do not have speed.
| criddell wrote:
| I struggle with understanding just what a physical thing is?
| If a particle is an excitation of a quantum field, then what
| is a quantum field? Is it anything more than just
| information?
| dang wrote:
| Ok, we've replaced the title with the subtitle. It was objected
| to in the previous discussion too:
|
| _Quantum Leaps, Long Assumed to Be Instantaneous, Take Time_ -
| https://news.ycombinator.com/item?id=20105091 - June 2019 (95
| comments)
| krastanov wrote:
| "Quantum speed limit" as is usually used in the field has
| little to nothing to do with the speed of light. "Finite speed
| at which information propagates through space or time" appears
| also in theories that do not know about relativity or about
| space and time being unified. E.g. the "quantum speed limit"
| that has more to do with differential equations and control
| theory (the "boring" engineering field). Otherwise you are
| right, but the term you used will regretfully cause much
| confusion if people start googling its usual meaning
| https://en.wikipedia.org/wiki/Quantum_speed_limit
| liuhenry wrote:
| Right, quantum jumps as used here is actually a specific
| technical term. It seems like the parent comment takes it to
| be a lay analogy, but it's from a formulation called quantum
| trajectory theory [1]. Rather, the result is about the
| electrons in an atom jumping between energy levels and
| experimentally observing a continuous evolution.
|
| [1] https://en.m.wikipedia.org/wiki/Quantum_Trajectory_Theory
| RcouF1uZ4gsC wrote:
| > rather information propagates at a certain speed, which also
| just so happens to be the speed of light.
|
| Are these fundamentally related, or could we imagine a universe
| where these are two different numbers?
| at_a_remove wrote:
| First, attempt to imagine information in a complete void. No
| space, no energy. On what is that information coded?
|
| You have to pick _something_. Now, it doesn 't have to be
| self-propagating packets of electromagnetic waves, but
| whatever you pick will be that universe's equivalent of
| light.
| avmich wrote:
| We can be imaginative and declare that the whole universe
| has "states", and those states are different. The whole
| universe encodes states - without anything in the universe
| required to exist. For example, "laws" of universe depend
| on states.
|
| Frankly, we are not required to have time there.
| morelisp wrote:
| If something was faster than photons and could carry similar
| information, we'd probably evolve to see those instead of
| photons. So it seems more like a semantic question.
|
| On the other hand things get really weird if we say the
| fastest way to transmit information has speed in some way a
| function of that information (e.g. frequency).
| https://www.gregegan.net/ORTHOGONAL/00/PM.html
| throwaway894345 wrote:
| The time it takes light to propagate is not a determining
| factor in whether or not a particular animal reproduces
| (namely whether or not it survives to reproduce), so what
| evolutionary advantage could "faster light" confer?
|
| In a more general sense, just because something exists
| doesn't mean we would evolve the ability to capitalize on
| it. Think of all of the abilities that would obviously be
| pretty advantageous which have evolved in species other
| than ours--that we didn't evolve a hyper-sensitive sense of
| smell is clearly not because of some deficiency in the
| universe because dogs and bears and others managed to
| evolve it..
| morelisp wrote:
| edit: lol ok HN hates thought experiments, got it.
| kadoban wrote:
| The latency caused by the speed of light around us is so
| tiny as to be irrelevant.
|
| Our reaction times are far greater than the time it takes
| the light to get to us from anything that could affect us
| quickly (anything we can eat or that could eat us).
|
| If there were another similar-but-faster thing out there,
| even if it were thousands/millions/infinity of times
| faster it would give no real advantage. This feels like
| basically Amdahl's Law applied to fight-or-flight timing.
|
| Human reaction time is ~0.25 seconds. Light from even 100
| meters away takes 300 nanoseconds to get to you. Those
| are ~6 orders of magnitude off.
|
| If light were millions of times slower, this would stop
| being true (but then physics wouldn't work anything like
| the same either, so the point is a bit moot).
| JackFr wrote:
| Which gives me another opportunity to hype "Dragon's Egg"
| a novel by Robert Forward.
|
| From the synopsis in Wikipedia: "Dragon's Egg is a 1980
| hard science fiction novel by Robert L. Forward. In the
| story, Dragon's Egg is a neutron star with a surface
| gravity 67 billion times that of Earth, and inhabited by
| cheela, intelligent creatures the size of a sesame seed
| who live, think and develop a million times faster than
| humans."
| throwaway894345 wrote:
| It takes light 33ns to travel from a cheetah to a gazelle
| 10 meters away. The time it takes the gazelle to process
| and react to that information takes many orders of
| magnitude longer, so optimizing or even eliminating that
| 33ns isn't going to affect the gazelle's chances of
| survival.
|
| The bottleneck is never the speed of the photons.
| kbelder wrote:
| Evidence: The usefulness of ears.
| morelisp wrote:
| fuckin nerds
| throwaway894345 wrote:
| > edit: lol ok HN hates thought experiments, got it.
|
| > fuckin nerds
|
| I don't know man, you proposed a thought experiment, we
| engaged with it. Doesn't sound like HN hates thought
| experiments, nor that we're particularly nerdier than you
| (and I've engaged with you enough in the past to know
| that you're _delightfully_ every bit as nerdy as I am :)
| ).
| tsimionescu wrote:
| > I mean yes in our world, but also no, the bottleneck if
| the universe is literally the speed of photons.
|
| Not literally, no. The bottleneck is the speed of things
| with 0 mass, of which the photon is just one example (as
| far as we know).
|
| I also think it's very likely that, had there been
| another force that was ubiquitous but carried by a
| particle that traveled at 200 km/h, that would be easier
| to detect than photons or happened to have other
| favorable properties, we could have easily evolved to
| detect that instead.
| [deleted]
| Enginerrrd wrote:
| You can envision any universe you want, but it would be
| pretty tough to do this honestly... You'd have to break
| almost everything familiar.
|
| You can rule out universes with light having a propagation
| speed greater than information for obvious reasons but, for
| less obvious reasons, you can also mostly rule out a universe
| with the same physics as ours but a lower light propagation
| speed. The only way to make that second part not true that I
| can think of is to give photons a very small but non-zero
| rest mass. Otherwise, it's hard to reason about it at all.
|
| To proceed, I'd then assume there's still lorentz invariance
| but with the information speed rather than light speed.
| Additionally, they'd have to be pretty close in value to
| produce what we see.
|
| The big sticking point I have though, is what on earth the
| electromagnetic force would look like in that scenario. I've
| no idea.
| saalweachter wrote:
| Depending on what you mean by "the speed of light", it's
| quite easy.
|
| The speed of light in water is 0.75c, but other things
| sometimes travel faster than that in water -- Cherenkov
| radiation (the blue glue in reactors) happens when you have
| eg electrons moving through the water at speeds faster than
| 0.75c.
| tsimionescu wrote:
| The universal limit that results from relativity (c) is the
| speed of massless particles. The photon is believed to be
| massless; if it is, its speed must be exactly c. All
| experiments do far have shown the photon to have 0 mass.
| However, if it turned out it had some very small but non-0
| mass, that wouldn't directly contradict any fundamental
| theory (though it may have more complex indirect implications
| on the Standard Model etc).
| bostonsre wrote:
| Disclaimer: I don't know what I'm talking about, just
| interested in trying to slightly grasp stuff like this.
|
| Have they proven in this research (or other research) or is it
| generally understood that quantum teleportation propagates at
| the speed of light?
| nine_k wrote:
| Yes, it has been; the question whether QT allows for
| superluminal transmission of information has a firm negative
| answer, AFAICT. See e.g.
| https://en.wikipedia.org/wiki/Quantum_teleportation#Non-
| tech...
|
| QT does not allow for mass transfer, it's purely about
| information. So, no ansible for us yet.
| ShamelessC wrote:
| I believe they're asking about the speed at which the
| actual teleportation happens? Not a hundred percent on
| this, but my understanding was that theory suggests it is
| instantaneous but we can't prove it (and the ultimate limit
| is still just the speed of light) for the reasons you
| mentioned.
|
| Is that correct?
| krastanov wrote:
| It is not as much incorrect as it is ill defined. The
| "quantum teleportation protocol" inherently requires the
| transmission of a classical bit (the weirdness is that a
| single bit + entanglement is sufficient to "teleport" a
| quantum state represented by a complex number). So, which
| part are you asking about when you ask whether it is
| "instantaneous". I feel like the need for a classical bit
| makes the question rather moot.
|
| Or maybe the question is about the speed of the collapse
| of the entangled pair upon measurement. That question
| makes a bit more sense, but its answer is simply
| "unobservable". The observable effects are all the same
| independently of the answer of the question, so we do not
| care. The theory does not say it is instantaneous, the
| theory says the answer does not matter.
|
| You can decide for yourself what this means
| "philosophically". For most working scientists
| "unobservable" means that science does not care about it,
| because it does not matter when considering the workings
| of the universe. Some of us do try to see whether some
| extension of the current theory would make the answer
| observable and also help explain other difficult ideas
| (but Bell inequality experiments do confirm that it
| simply does not matter).
| thewakalix wrote:
| But a scientific theory can have well-defined
| implications about unobservable things. One example is
| "if you send a photon past the cosmological horizon, does
| it suddenly cease to exist?", which (assuming
| conservation of energy) would be false.
| krastanov wrote:
| That distinction might be more subtle than you think.
| Just picking on your example first: The math describing
| that cosmological horizon is actually the same as the one
| describing the black hole horizon (including things like
| Hawking radiation). The question of what that photon does
| is pretty much equivalent to the question about a photon
| falling in a black hole. That question probably matters
| to a grand unified theory (and has an explicit answer in
| it), but at the level of sophistication of current
| physics, the question is mostly moot.
|
| But notice that this particular question is something we
| can at least contemplate mattering. While on the other
| hand, we hardly can even make up an imaginary setting in
| which the speed of the "teleportation" matters. It is
| more of a sign that the mathematical treatment we are
| using is unnecessarily obtuse, because in a "good and
| proper" mathematical theory describing the effect, such
| "nonsensical" questions would not be able to appear.
|
| Of course, it might turn out that the question matters in
| some extension of quantum mechanics that leads to a grand
| unified theory, but that would be a surprisingly boring
| outcome. It is more probable that it does not matter and
| we need to find a language which explicitly shows it does
| not matter. Kinda reminds me of this comics
| https://calamitiesofnature.com/post/19171164647/fairies
| Strilanc wrote:
| In this case the theory doesn't have well-defined
| implications. The different interpretations of quantum
| mechanics, which agree on the observable details,
| disagree on the unobservable details.
|
| It's like an entity within a game of life simulation
| trying to figure out if the simulation is being run by
| Xlife or Golly. There's just no way to tell, and no slam
| dunk prior to eliminate one of them.
| bostonsre wrote:
| Is it possible to use entanglement for communication? I
| could be incorrect, but I vaguely recall reading that
| researchers were pursuing it a while ago. Even if it's
| just information, being able to communicate faster than
| light would be neat and have some big implications. But
| in the article, it sounds like it's not instantaneous..
| so if it's faster than light, but not instantaneous, that
| sounds like some kind of new speed limit?
| czzr wrote:
| It's not possible to use entanglement to send information
| faster than the speed of light.
| krastanov wrote:
| You can use entanglement for communication, as long as
| you also use some classical communication channel in
| addition to it. If you just want to transfer bits,
| entanglement does not provide much more than classical
| communication channels (maybe a bit better signal to
| noise ratio when used together).
|
| Quantum teleportation requires both entanglement _and_
| classical communication channels to be present.
|
| And all of this has nothing to do with FTL communication.
| No matter what piece of known physics you use, FTL
| communication is simply not possible. It is not just "not
| known how to do it", rather it contradicts and causes
| unresolvable paradoxes in an enormous body of extremely
| well verified laws of nature.
| inopinatus wrote:
| If all energy is information, then this would lead to a
| cosmology such that perception is the lazy evaluation of a
| functional universe. Unfortunately this doesn't necessarily
| bridge classical and quantum.
| junon wrote:
| Sorry, but what?
| odyssey7 wrote:
| I read this as: the universe doesn't bother with bringing
| things where they would go, until their spot needs to be
| computed due to some other spot depending on that spot's
| state.
| sandebert wrote:
| _When I 'd studied physics, they thought the duration of a
| collapsar jump was exactly zero. But a couple of centuries later,
| they did a complicated wave-guide experiment that proved the jump
| actually lasted some small fraction of a nanosecond. Doesn't seem
| like much, but they'd had to rebuild physics from the foundation
| up when the collapsar jump was first discovered; they had to rear
| the whole damned thing down again when they found out it took
| time to get from A to B. Physicists were still arguing about it._
|
| The Forever War (1974) by Joe Haldeman
| [deleted]
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