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Aseyhe t1_jb17c9w wrote

Perhaps I should say there's no unique choice for a universal "now". "Now" would be a 3D surface in 4D spacetime that intersects the point "earth in the year 2023". But there are lots of possible choices you could make in defining this surface, and none of these choices are more correct than the others.


Teo_Filin t1_jb1dzzk wrote

What about simultaneity of entangled particles? Same "now"?


Aseyhe t1_jb1pypu wrote

Entanglement doesn't have any connection to time. It just means that the outcome of a measurement of one particle is correlated with the outcome of a measurement of the other. Those measurements could be performed at any times.


Teo_Filin t1_jb1quc6 wrote

And entangled particles are "connected" via their past moment of interaction?


[deleted] t1_jb1tzq5 wrote



Aseyhe t1_jb1vjul wrote

This is a good way to think of entanglement, as long as you also keep in mind that not every configuration of entangled particles corresponds to a set of local hidden variables in this way (those would be the spooky outcomes you're referring to). But most entanglement scenarios people think of are indeed equivalent to local hidden variables.


OldWolf2 t1_jb3un0o wrote

Your scenario is not entangled, it's just a lack of knowledge on the part of the other person .

Lack of knowledge is NOT the explanation of entangled particles.


PercussiveRussel t1_jb1kli4 wrote

I don't know a lot about cosmology, but I do know about quantum and relativity.

This is a very interesting question! The best way I 'mangle' entangled particles into relativity is to avoid it all together. This is based on the fact that, in order to know about the simultaneity of entangled particles, you'd need a classical channel to transfer information.

Take this experiment: you and I have an entangled particle pair (that's either in AA or BB) and we're lightyears apart. I measure my particle and measure it to be in state A. This means I know yours is in state A too. However, I don't know whether mine is in state A because you collapsed it to be in A, or whether I was the first and have therefore collapsed yours to A. There is no "now your particle is in A" because it could've been there for years (depending on how long ago we separated the particles). The only thing that my experiment proves is that now I know that your particle is in state A. In this way, it's safe to say there is no real simultaneity occurring. This is one of the more prevalent explanations at the moment and one that I personally believe is the most robust since it doesn't require us to completely rip up either relativity or quantum (which are both among the most verified theories we have)


In other words: to me relativity describes the speed of information. Entangled particles don't break this speed as no information is changing hands. This means there is no shared now between the particles.

(The math also shows this, the order of observations is symmetric (they commute), so they have no impact with regards to causality. No causality = no "now")

Edit: To clear something up, for those who don't live and breathe this stuff: entangled particled are two or more particles that share a common set of states. A "quantum object" (which is not a scientific term, but I'll use it anyway) can occupy a quantized (meaning discrete) set of states. When the object is said to be in a superposition, this object is in an undefined state, where it has an x% chance to be in state A, a y% chance to be in state B etc. When we measure the state of this object, it "chooses" one of these states at random and will be that state.

An entangled state means that two seperate objects share a combined superposition. This means that there are less possible states than just all possible permutations. Eg: two objects who can each be in state A or B and are not entangled if they're in the following (equally likely) superposition: AA, AB, BA and BB. Knowing that the first one is A leaves us with the following possible equally likely states: AA and AB. We know precisely nothing about the second object from knowing the first.

Now an entangled object removes some of these possible states. For example the particles are in a superposition of AA, AB or BB. Now knowing that the first one is B tells us that the second one has to be B too. Knowing the first one is A leaves us with no knowledge on the second one. (they're still entangled though, even if the AA and AB states are really useless)

This is "all" entanglement is. It's not something whereby moving the first from state A to B automatically moves the second one from A to B too.

Since you can't control the outcome of a random measurement, there is no way to encode any information in the entanglement itself that is transported on observation. It's pure probabilistics. I mean, you could make the states in a "superposition" of AA and BA so you know the second person always gets a state A after measuring, but you need to send them the particle which goes with locality, so that's just sending an email with extra steps.

The reason that so called "quantum communication" is so interesting though, is that no one can listen in on your conversation. Take the following scheme:

We agree on the following protocol: We have an entangled pair that's in a superposition of AA and BB. I measure mine, then send you a phone call with my measurement. Then you measure your particle (and therefore verify my phone call, beceause our measurements have to be the same).

If I lie, I mean to transmit a 0, if I speak the truth I mean to transmit a 1

No one knows what we are talking about, because they don't know what our entangled pairs are doing. For them, when I tell you "A", it's either a 0 or a 1, with 50% chance each. However notice I do need to say something to you, because I can't affect the outcome of the observation.

I think this explanation clears up a lot about what entanglement actually is. Some people imagine it's like two switches where if you flip one, you instantaneously flip the other one. It's not.


BedrockFarmer t1_jb22lez wrote

This made sense to me, a non mathematician and non physicist. So basically the entangled particles will behave identically when observed. So there is no information linking the particles across spacetime.

So like if I had two cans and two six sided die and I “entangled” the die and closed the lid. I could then send one can to the moon and keep the other and when opened, both die will show the same result because of entanglement when normally there would be a 1/6 probability of what is observed for a single die.


PercussiveRussel t1_jb25fro wrote

Bingo! This is effectively the same thing.

However, I have to be a bit pedantic here, in your example the dice might always have been "destined" to be the same, becauase a simple explanation could be that I glued the dice to the bottom of the can, both facing the number 3 up, and that you and a friend measure the same thing because the dice were always going to show 3. This is what we'd call a 'hidden variable theory' and is almost surely not how quantum probability works.

But yeah, entanglement simply means that knowing the outcome of 1 of the experiments gives you some sort of knowledge about the other experiment (like I said, this could be knowing the exact outcome of the other, or just give you better odds than pure luck for guessing the other experiment). The key concept is that you can't control the outcome of the experiments, you just improve your chances of guessing the other experiment correctly, which is exactly what happens in your dice example.


Teo_Filin t1_jb1o0zq wrote

Cool! So no way for instant info exchange? Such a pity.

Though I can't accept "information" as physical measure at macro-level (it's our description of some properties), just at quantum-level (numbers defining a particle).

I even doubt that time is a coordinate meaning some "future" and "past" do exist as places we can visit (present is real with some intensity of processes influenced by gravity and speed; future is our anticipation, past is our memory).

I'm not deep in advanced physics and higher mathematics, just evolving my perception and checking cosmology with common sense sometimes.


PercussiveRussel t1_jb240js wrote

>So no way for instant info exchange?

Reddit was being reddit, so while I could see your reply in my replies, I couldn't find it to reply to. Reply.

I've added a bit at the bottom of my post to explain why. This isn't just aimed at you, I don't know your understanding of quantum mechanics, but I notice that entanglement is a pretty misunderstood topic. And to be honest, actual quantum researchers talking calling their incredibly cutting edge cool research "quantum teleportation" isn't exactly helping this.

Not accepting or even understanding information at a macro level is perfectly understandable. I'm a physicist and I can't marry my knowledge of quantum with the macro world, I don't understand "information" as a quantum concept in macro scale either. (to be honest, I don't think I fully accept "energy" on the various scales)

I think the most simple way to accept "the speed of information" is to just imagine someone transmitting "Hello world", with laser beams through the vacuum of outer space. Since we know that the speed of light is finite, and furthermore light is the fastest thing in the world, no message can get to us faster than a message sent with light. There are lots of caveats to this of course (there always are), but this conceptually at least made sense to me when I first started to learn about this. Once you start to trust this concept, just flip it on its head: the speed of information is 3.00e8 m/s, so there is no way light is faster than information.

(this is a way to get to grips with the concept, this is in no way a proof nor meant as such)