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greasyhorror t1_j9n9xx2 wrote

Reply to comment by Baron_Ultimax in Physicists Use Quantum Mechanics to Pull Energy out of Nothing by Vailhem

how is a quantum particle entangled? I see this term but have not the best understanding of what it is or how it works

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Baron_Ultimax t1_j9nhfzw wrote

So to describe Entanglement we have to understand that fundamental particles. Like electrons are not like a tiny pellets with a negative charge, but represent a point where an electron feild is exited or has a bit more energy. Visualize a droplet hitting a pond. These feilds can have different states. It can have a twist that gives particles a form of angular momentum like a gyroscope. But in multiple directions.

Because of this knowing where a particle is how fast its moving or its spin direction isnt possible. And measuring one property effects the particle so the others cant be measured.

Now Entanglement happens when two particles interact or 1 particle decays into two. The universe insists that everything is conserved so if particle a is spinning one way. Then particle b must have the opposite.

This means when we measure a property of particle A we know what that same property is on particle B Now because you can only know one aspect of a particle if you are measuring say up or down spin on A you can't find out the left and right spin. What's weird is the particles seem to be connected and measuring particles a also effects particle B. Its like the can communicate with each other. And the understanding of why or how the do this represents the bleeding edge of physics.

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GhostRid3r16 t1_j9pj6hp wrote

> Because of this knowing where a particle is how fast its moving or its spin direction isnt possible.

No, that’s due to wave/particle duality leading to the uncertainty principle. There’s uncertainty in where a quantum particle exists or goes thanks to the fact that all of our measurement tools are too big and waves don’t exist at a point. Consider using a meter stick to measure the width of a piece of paper. You will have an uncertainty wave packet width of 1mm at best. That is a better analogy for uncertainty. The paper is the electron and the meter stick is the observation/measurement, where instead of 1mm it’s h/4π meter.

> And measuring one property effects the particle so the others cant be measured.

Also not true, and a common lay misinterpretation (by those who haven’t been instructed on quantum I mean). You can measure Px with certainty but not simultaneously X. You can meaaure Py with certainty but not simultaneously y. Likewise Pz and not z. You can however simultaneously measure Px without affecting the state or certainty of y,z or Py, Pz. And so on for [Py,x]=0, [Pz,y]=0 etc.

It’s only σPxσx = σPyσy = σPzσz = σEσt >= h/4π; where σPxσy = σPyσX etc = 0 believe it or not. The uncertainties are coupled to the vectors. If you want you can find absolute certainty in X momentum and y position without collapsing the wave function for Pz,z. Absolute certainty for the complete vector components or the particle as a whole is not possible.

Mathematically that looks like this: https://i.imgur.com/XbCrYh3.jpg : expectation of Δx^2 • expectation of Δp(x)^2 = h^(2)/16π^2 where <Δx^(2)> = <x^(2)> - <x>^2 , same for <Δp(x)^(2)>. For quantum systems; Scale up and these uncertainties become so small as to be negligible to the system, and we’re back in Newtonian kinematics (correspondence principle).

Let’s also take note that the form is of ΔxΔPx>=h/4π: so this means, you can measure x with certainty much higher than h/4π; for instance let’s say you measure x to a certainty of h/10^(5)π. ΔPx must therefore be required to have an uncertainty that satisfies ΔxΔPx>=h/4π where ΔPx >= h/4πΔx, or ΔPx >= h/4πh/10^(5)π >= 10^(5)/4, and that’s in kgm/s. So our uncertainty in momentum rises to 25,000 kgm/s for a certainty in position of h/10^(5)π.

It’s plain to see here that as Δx goes to zero as we would approach absolute certainty, ΔPx must go to infinity, and to absolute uncertainty. You can know exactly where a particle was and know nothing about where it will be, or you can know exactly where a particle is going and know nothing about where it was. And keep in mind without both of those you can’t model the motion. Enter the wave interpretation of quantum systems, aka Quantum Mechanics, a(less)ka Wave Mechanics, and statistical analysis of the wave function provides us a model of behavior before and after measurement within the parameters of ΔrΔPr>=h/4π.. where you actually don’t need both parameters of initial position and momentum to model the wave function through time as it’s only a first derivative with respect to time!! YAY! And rejoice because if it was δ^(2)/δt^2 we’d all be fucked and stuck only with experimental data and no closed form solutions.

As a side note about entanglement: consider what I’ve said about measurements collapsing the wave function: let’s say you have 2 electrons and they interact. That is, they bounce into each other and are deflected. We know from Newtonian mechanics that if we solve the current position and momentum of one particle, we can wind back time and reconstruct the collision. The consequence of this in quantum means that when you measure the physical properties of one of the entangled particles, you necessarily have measured the properties of the other particle. This collapses both waveforms, since you have gained knowledge of the system of particles through measurement. You can reconstruct b from a. Therefore you have collapsed b’s waveform as well when you measure and collapse a’s. And thus, the particles are said to be entangled at the quantum level, the same way a cue ball is paired to an 8 ball at the Newtonian level. Measuring the properties of the 8 ball necessarily tells you the properties of the cue ball.

Also interesting aside: black holes produce a pair of electrons at the event horizon boundary, where at a certain probability, one of the electrons has been pulled into the black hole, and the other released into the universe so to speak. For one instant in time, this is the ONLY truly unpaired particle in the universe.

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Farklurth t1_j9ox052 wrote

Does the distance matter? Can we still measure the properties of the entangled particles that are 1 light year away?

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subjectwonder8 t1_j9oynth wrote

Yes. In current understanding distance doesn't matter. It could be few atom widths apart or light years. The fact that distance doesn't matter is one of the very interesting things about it and why there was some resistance to it (notably from Einstein) when the idea was first introduced.

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Farklurth t1_j9p1n1n wrote

That's absolutely amazing. So in theory when we have FTL spacecrafts we can communicate over vast distances without any problems.

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videopro10 t1_j9pblra wrote

Actually no, you would have to know the state of the particle at your departure point, which you can't know without that info being transmitted at the speed of light.

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zabuu t1_j9piwn2 wrote

Not quite... once you observe (read: measure) an entangled particle, it is no longer entangled.

Imagine 2 people face to face on a perfectly slippery frozen lake. If they push apart from each other, they would slide away. If you know the mass of each person, and you catch person A (this is like measuring the speed), then you'll know how fast person B is going. But measuring A changed the system and you'll get no additional info about B.

I'm no pro though, this is just how I understand it.

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tinwhistler t1_j9nb34l wrote

> how is a quantum particle entangled? I see this term but have not the best understanding of what it is or how it works

https://www.livescience.com/what-is-quantum-entanglement.html

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ToothlessGrandma t1_j9noce1 wrote

Asking for someone to explain this stuff on reddit is almost impossible. This stuff is very hard to understand, even for those with advance degrees and years of schooling. This is the cutting edge of science that can't be summed up in a comment.

There's a famous saying that goes that if anyone says they know how this stuff works, they're lying.

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tinwhistler t1_j9nuavp wrote

Obviously. But that article seemed good enough for a layman's grasp to me

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ToothlessGrandma t1_j9o897j wrote

Sometimes you can simplify a complex topic so much that what you're trying to say is really meaningless.

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subjectwonder8 t1_j9oxrj9 wrote

Entanglement is a fancy way of saying the properties of two or more things rely on each other.

Imagine I have 10 balls which I'm going to put into two bags. I might put 3 in one and 7 in the other or 1 in one bag and 9 in the other. You don't know what I'm going to do, only that I will put all 10 balls in the bags.

Now imagine I gave a bag to you and you looked inside to find 6 balls. Since you know there are 10 balls in total you know the second bag must have 4.

These two bags are entangled as they have a property that relies on the other. This is mundane and isn't interesting at macroscopic level.

If we go smaller though we run into some more interesting things as things act less like solid things and more like waves.

So imagine I showed you a video of a wave in the sea. You could from that video see how fast the wave is moving but if I asked you to point to where the wave is that becomes slightly harder. The wave was in many places in that video because it was moving. (ok this simplified but just go with it)

If I showed you a picture of a wave. It would be easy to point to where it is. But if I asked you for the speed of that wave, that becomes hard.

As you can see the more we know about movement of something the less we know about its position. And the more we know about position the less we know about movement. This is uncertainty principle.

(Obviously that is simplified for the metaphor, but it is close enough in principle to how it drops out of the math. Just know that in the math, knowing more about one thing and less about the other is much more like a hard rule that must be obeyed than the metaphor implies. So following that principle is super important)

Now particle act a lot likes waves. The more I know about a position the less I know about its velocity. The more I know about its movement the less I know about where it is.

Think of our bag metaphor, imagine if the one bag was red balls only and the yellow balls only. You can either feel the bag to count the balls or open the bag to check the color of one ball. (presume there is always at least 1 in the bag). You will only ever know 1 property, but once tested you'll know it for both bags.

But what if you tested one bag for one and the other bag for another. So the one had 6 balls and other bag is yellow. Which means the bag has 6 red balls. Now I know two properties of one thing. But this isn't allowed by uncertainty principle.

To think of it in waves or particles. I check where something is (and know nothing about its movement) and then I check its counterpart's movement. Since I know velocity like the balls is shared between the two, I know the particles movement and position.

This isn't allowed so what happens?

First know that at the small scale things become probabilistic. You look in your bag and you have 6 balls. Look again now you have 5. Look again now you 7. Again 6, again 6 again 6 again 8. It's probabilistic, it is most likely going to be 6 but it could also be 5-7 and maybe even 4-8, even more unlikely but it could even be 1 or 2.

This is where the wave properties comes from, if you draw probabilities on a graph, you would see high point at 6 and it slows away like a wave. This is superposition (because it could be considered multiple things at the time) and where you check its wave-function collapse (because wave goes away and it becomes a thing) and also where all the talk about multiple things at the same time comes from. (bit more complex in practice but simplified it is reasonably accurate)

Now here is the "spooky action at a distance" or the part where everybody freaks out. The other bag was entangled. Every time you check the other bag somehow knows what value the bag you check has. If it 6 the other bag knows it must have 4 balls. But if you check again it has 5 and so the second has 5. If it is red it must be yellow. But you check again now you are yellow and the second knows to be red.

How does it know that? And how can it transfer that faster than light. You can't transfer information this way because you have no control of what answer your test will give.

But that is what entanglement is. One way this forms is if a particle decays into 2 or more particles. Those particles would be entangled because the velocity is shared between them.

In practice it is even more interesting because there are other quantum phenomena which interact with this to produce even more interesting phenomena.

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greasyhorror t1_j9ploxc wrote

Thank for the explanation. This kind of helps explain the slit experiment too, why you see the wave of probability.so is the act of observation what entangles the particles?

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Spactaculous t1_j9nlqyq wrote

You are not alone. No one understands how it works.

We can observe it, calculate it, create experiments, but no idea how it works.

Other parts of physics that we do not understand, like dark matter, have many competing hypothesis. Entanglement is pretty lean on that front. Even though I am sure string theory had something about it, because it has something about everything.

It's just the way it is. Like the rest of quantum physics.

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