Submitted by Vailhem t3_119jgi5 in technology
Baron_Ultimax t1_j9nhfzw wrote
Reply to comment by greasyhorror in Physicists Use Quantum Mechanics to Pull Energy out of Nothing by Vailhem
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.
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.
Farklurth t1_j9ox052 wrote
Does the distance matter? Can we still measure the properties of the entangled particles that are 1 light year away?
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.
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.
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.
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.
Viewing a single comment thread. View all comments