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Comment by thricegr8 | Hacker News Reader

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".

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"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

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

What is "information"? Is it a physical thing?

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.

It is in the same sense that entropy is a physical thing. https://en.wikipedia.org/wiki/Information_theory

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)

The spaceship itself represents a huge chunk of information (such as the atomic arrangement of the metal atoms making up the bolts).

I agree that there is "information" in the colloquial sense there, or even in the Kolmogorov sense. I don't understand how there is information in the entropy sense, because I do not see a random variable anywhere in this story.

I believe we are talking about Shannon's direct analogy between information and entropy here. The low probabliity of the atoms of the spaceship being arranged as they are - a specific design of a spaceship - out of all their possible arrangements, is a state of low entropy and high information content.

https://en.wikipedia.org/wiki/Entropy_(information_theory)

That was what I was talking about, yes.

Entropy isn't necessarily random. I think a better description of entropy might be unimportant states. For the rocket, we might care about the total mass and we might even care about the temperature of those bolts. Those parameters represent information.

But there's even more information in the rocket, specifically the momentum*position of each of the atoms within the bolt. That quantity for each atom is measurable/knowable and represents information.

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.

Pretty much the entirety of modern physics is this type of abstraction (https://en.wikipedia.org/wiki/The_Unreasonable_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.

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.

Which is not, really. At least not as clearly as other things.

It's a bit like saying Maxwell equations are not as clear as Newton's laws.

“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/Jaynes_...

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

Landauer would say so: https://www.sciencedirect.com/science/article/abs/pii/037596...

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?

I'd say a quantum field is a combination of both information and a rule.

The rule determines the quantum field's next state given it's current state (the schrodinger equation).

The state of the quantum field could probably be considered nothing more than information.

Isn't it simple? Information is the content of a message from one intelligent being to another such that the recipient can understand the intended meaning of the message.

Random bits a can randomly arrange into a a configuration that looks like information. But that is not information because there is no intended message behind it. Whereas if an intelligent being organizes a set of bits into a configuration say like in a zip-file which looks like random bits, it is information because it can be decoded into meaning.

Information can not exist without "meaning".

That's just not true. Whether the message is from an "intelligent" source or not, it can still contain information. When you make an observation of a random process, you decrease your uncertainty about its state, or in other words, you lower its entropy. In the process, information has been communicated to you.

Are you saying that the act of observation decreases entropy of the observed system?

Or are you saying that entropy is fully in the "eye of the beholder"? If I observe something and you observe the same thing but longer, you reduce the entropy further (for yourself) than I do?

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)

> 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?

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.

The assumption that the photon has 0 mass is still being tested, so it's not absurd to imagine it, and the universe may not look really differently if it was non-0 but small enough.

According to [0], the most accepted upper limit on a possible mass of a photon is m < 10^-14 eV/(c^2).

[0] https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_...

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.

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.

If you can talk about it, and you can say that they are different, they exist.

I'm trying to imagine information without a material carrier. I'm sure it's a philosophical question by now.

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).

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.

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

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..

edit: lol ok HN hates thought experiments, got it.

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).

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.”

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.

Evidence: The usefulness of ears.

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?

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.

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?

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).

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?

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.

Good to know, thanks.

It’s not possible to use entanglement to send information faster than the speed of light.

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.

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

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.

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.

Sorry, but what?

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.

Ah, yes, sorry, that’s a fragment of something I’m mulling, but it’s basically nonsense out of context (and may be nonsense in context too).

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