I just saw a comment that linked to an article explaining how Nobel prize winners recently discovered the universe is not locally real. My brain isn’t functioning properly today, so can someone please help me understand what this means?
In: 2860
Locally real means that things have a property even when not being observed and locally means can only be affected by whatever is immediately around them.
The discovery is that particle don’t have definite spin-up or spin-down properties until they are observed. Which in turn means an objects property can be changed simply by being observed.
It means that our universe is either local or real and cannot be both at the same time. “Local” here means that all actions happen through direct transmission of the fundamental forces, for example you kicking on earth can’t move a ball that’s located in space, the force can only be transmitted through collision and not just jump from one object to the next. “Real” in this sense is referring to a highly theoretical concept of property of quantum objects and whether they are inherent or created with observation, the following analogy is oversimplified to the point of being a bit incorrect, but that’s the best way of simply explaining it that I can think of: it’s like having an orange fruit, is it really the orange colour? If the universe was “real” in this sense the orange colour would be inherent to the fruit and always present, but if it’s not “real” then the fruit doesn’t have an actual colour and only becomes orange when you look at it and need the information about its colour. The Nobel prize was awarded for proving that within laws of quantum mechanics and when operating on quantum objects these two properties are exclusive, either our universe was local but not real, or was real but not local, but we don’t know which of the two it is yet, thus it was named “not locally real”.
Classic physics assumes that things exist in the way that we perceive them. That is to say, if I have an apple sitting on a table, classic physics assumes that the apple has a fixed position (on the table) and a fixed speed (sitting still) and that these are absolute, 100% true values.
In reality, things only look that way because humans are gigantic compared to subatomic particles, and at human scales things do behave in a predictable manner.
On subatomic scales this isn’t true. All fundamental particles have a sort of dual existence. If you could somehow freeze time for a particle, then while it is frozen in time it will have the absolute properties that classic physics ascribes to it. IE, that particle would have a fixed position and speed.
So if you imagined the universe as a strip of movie film, where each frame was a distinct moment in time, you could theoretically capture a particle’s position and speed in each of those frames. But what about in between the frames? The answer is that in between frames, the particle ceases to exist in the way that we understand existence.
In between frames, particles exist as a probability. So imagine that frame 1 had a particle in position 0, traveling forward at a speed of 1. In frame 2, the overwhelming majority of the time, the particle will appear to have advanced by 1 and moved to position 1. However, that’s not always true.
The particle will rarely advance by 2 and move to position 2 or advance by 0 and stay in position 0. It will even sometimes move backwards by 1 and end up in position -1. This also means that particles can teleport through one another.
So for example, if our particle was at position 0 and another particle was at position 1, sometimes our position 0 particle will move to position 2 despite the fact that it *should* have been blocked by the particle at position 1.
Even if you know everything about a particle at a given point in time, that isn’t enough information to *know* where that particle came from in the past, or where it will be in the future. Particles have an element of randomness to their movement that makes them unpredictable, which is what physicists are talking about when they say that the universe isn’t locally real.
The universe appears to be locally real to us, as humans, because this randomness is affecting particles that are very, very, very small. An electron is about the same relative size to you as you are to the entire universe. Because all of this randomness is happening on such a tiny scale, it ends up cancelling itself out to give the appearance of a fixed reality.
So what does this all mean? You can know what the position and speed of an apple sitting on your desk is. But in the real world where you can’t just freeze time, it is impossible to know what the exact position and speed of any of the subatomic particles that make up the apple are because those particles don’t really have a fixed position and speed as humans typically understand those concepts.
Local means cause and effect apply. For A to affect B, a signal has to have time to travel from A to B.
Real means things like particles have set properties. A particle has spin up or down.
When particles are entangled, they if one is measured with spin up, the other must be measured with spin down, for example.
So you might say when we perform an experiment where we entangle two particles and then separate them, one has U and one and D assigned at the moment of entanglement. This makes sense to us. This would be local realism.
This prize was won for determining that the particles don’t have U and D assigned. It had been done before, but the recent experiment rules out all remaining loopholes.
The particle exist in a “superposition “, and both particles assume a value when ONE is measured. There is no time for the communication to occur to somehow signal that one of the particles has been measured, so the other should assume to correct value. So in other words, local realism does not apply. (Note that locality still applies, just not realism).
It is absolutely AMAZING that we can know this. Look up YT videos on Bells Inequality for some relatively easy to understand videos on how we know particles don’t have values before we measure them.
I wrote up an explanation back when this was making headlines. Linked and quoted below: https://www.reddit.com/r/QuantumPhysics/comments/y1dqgy/comment/irx9x44/
> “Locality” is the principle that things can only affect and be affected by other things in their immediate vicinity.
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>You can push someone right next to you, but you can’t push someone a mile away from you. In order to do that, you have to physically travel to them. Even things which seem to affect distant other things require something else to travel that distance.
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>You can see far away objects because a photon bounced off that object where it was, traveled towards you and hit a sensitive cell in your eyeball. The interactions happened between the object and the photon at the object’s location and between the photon and your eye at the eye’s location.
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>So a “local” universe is one where all interactions happen like this and any interaction between distant object requires that something (another object or signal of some kind) travels between those objects, and that thing is limited in how fast it can travel by the speed of light.
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>“Realism” is the principle that objects have definite properties even when they aren’t interacting with anything.
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>Let’s say you have two particles that are going to collide. If you want to know how the collision will affect each particle, you need to know their speeds and masses, so their momentum.
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>In a universe where realism holds, each particle has a definite momentum and when they collide, they interact with each other based on those values and then fly off each with a new momentum.
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>If realism does not hold, then before they collide, each particle has a range of possible values it could have for its momentum, and interacting with each other forces the momentum of each particle to become a single definite value. The particles then interact using those definite values for their momenta before flying off with a new range of possible momenta until they interact with something else.
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>For a long time, scientists thought that the universe was locally real. That means that particles only interact with particles that are near them with all interactions over distance being restricted by the speed of light, and particles have definite values for all of their properties even when not interacting with other things. We may not know what the value is when they aren’t interacting, but the interaction reveals the pre-existing value to us, it does not cause the object that didn’t have a defined value at all to take one on for the purposes of the interaction.
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>Quantum mechanics, and entanglement in particular, threw a wrinkle into this view.
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>If you prepared a set of particles so that they are entangled, it means that measuring a property of one particle will tell you something about the other particle, because they are correlated.
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>If I take a pair of shoes and stick each shoe in a separate box, opening one box to find a left shoe will tell you that you would find the right shoe in the other box if you were to open it.
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>Similarly, you could prepare a set of particles so that they have opposite spins. If you measure one and find it is spin up, it means that a measurement of the other will have a value of spin down.
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>Curiously, however, the math of quantum mechanics says that these properties are indeterminate until they are measured, and that both particles are in a superposition of spin up and spin down until a measurement or other interaction forces them to take on one or the other state.
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>Furthermore, even if you separate the entangled particles over a great distance and measure them at the same time, the results will still be correlated. This presents a bit of a problem, because if the properties of each particle aren’t determined until they are measured and the measurements happened so far apart that no signal traveling at the speed of light or slower could have been exchanged by the particles, how does particle A “know” that it should be spin up to particle B’s spin down and vice versa?
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>This is what Einstein referred to as “spooky action at a distance” and he and others at the time proposed that our understanding of quantum mechanics must be incomplete and there is some value we have not yet discovered that pre-determines the result of the measurement ahead of time. The result isn’t random, it just looks that way because we have not discovered the thing that causes the result to be what it is, a so-called “hidden variable.” This would neatly solve the problem and take us back to a world with both locality and realism, since the properties of each particle are set from the time they are entangled and no communication would need to take place for the results to be correlated.
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>Much later, in comes John Stewart Bell who is able to demonstrate mathematically that there are certain predictions that quantum mechanics makes that can never be replicated by any theory that incorporates a hidden variable in this way. This means that either quantum mechanics is not just incomplete but wrong or else locality and realism cannot both be true. You could have one or the other (or neither) but not both.
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>The Nobel prize was awarded for devising and conducting experiments for which these two competing theories give different results for the expected outcome, and determining that the actual results in the real world match the predictions of quantum mechanics, which precludes both realism and locality from being true together.
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>Thus one or both of the following must be true:
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>Particles only have defined properties when interacting with other things and not between interactions
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>It is possible for a particle to directly interact with a distant particle without having to send a signal at or below the speed of light.
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>Thus “local realism”, the concept that objects always have defined properties and all interactions are limited by distance and the speed of light, cannot be true of the universe that we live in.
Look at all the random objects around you. Are they “real”? Meaning do they have properties like position, mass, velocity etc. that are fixed regardless of who, if anyone, is observing them? Or did they just only come into existence when you asked that question and looked at them? If you rewind time and check again, could these properties now be completely different than the first time?
From the perspective of classical physics the answer is that they are all real and have fixed properties. Moreover these properties are only determined by the environment around them and not some other magical force. This is called local realism. This also implies that given a list of all the particles in the universe and all of their properties at any given point in time, you can perfectly simulate the universe indefinitely into the future or the past.
All of this holds for large objects, but when you go down to the quantum scale the rules go out the window and things get weird.
A quantum particle, say an electron, does *not* have a fixed position, momentum, spin etc. Each of these properties only exists as a probability distribution. So the answer to “where is this electron” is really “3% chance at position A, 5% chance at position B, 1% chance at position C…” The electron is nowhere and everywhere at once. And this is the case for every property of every particle in the universe. The property only becomes “real” the moment you observe it, and if you rewind time and observe again you may get a different answer.
Locality is the idea that cause precedes effect limited by the speed of light/causality.
Realism is the idea that the rules of the universe apply the same to everything equally.
The most common demonstration that the universe cannot be locally real is that if you take two entangled particles in an indeterminate state (superposition). Because they are entangled the definite state one particle ends up in will match the other particle.
This holds true even if the particles zoom in opposite directions and resolve into their deterministic states despite being separated by a distance that rules out any kind of synchronizing intersection between the two particles
Other experiments have proven that there aren’t hidden properties where the particles have somehow determined what state they have ended up in from the start.
You may have heard it referred to as spooky action at a distance. Particles despite having no cause/effect relationship simply end up in consistent states.
The overall conclusion is that despite our instinct to the contrary, that’s just how physics works. Time and distance are simply irrelevant to how entangled indeterminate quantum systems evolve into definite systems. No cause, only effect.
Thus the rules of the universe are non-local.
The alternative is that the rules of the universe are not real.
That is instead of having nice consistent physics, the rules just change to make things work out. Maybe we really are living in a simulation. Maybe every single possible interaction occurs and the multiverses that aren’t consistent wink out of existence. Or other equally extreme and or unsettling propositions. Most people would rather believe in non locality than non reality though.
Lastly physics may be both non local and non real, which is again not a super common take on the matter.
None of these people explained it like you were five. Here’s my take:
You looking at that thing? It’s there.
You stop looking at that thing? It might not be there anymore.
Entangled particles change spin without any possible idea which way they will spin, because they don’t spin like a ball and because nothing near touches it to make the spin change.
Get a ball.
Local means if you push the ball, the ball moves. Your hand, your foot, a stick, the wind, something nearby that touches the ball moves the ball. Not something far away that can’t see or touch the ball.
The ball is real because it reacts to pushes and pulls and kicks and things that move it. We can say the ball is right there, it is moving or not moving, spinning or not. If you want, you could say the ball knows this too.
The ball is made of small particles. Sometime the particles can move like the ball. But when we look at particle spin, it is different. The ball can spin one way or another, but it is only spinning one way at a time until something local touches the ball and changes its spin. The ball changes based on how it was spinning and how it was last touched. In math and physics, we can say the ball has a local spin property.
When we look at entangled particles, we notice the spin is either one direction or the other. After we look.
Before we look at entangled particles, they spin every possible way, until we look to see which way they spin. Entangled particles are common, not weird. Spin is everywhere too.
Now the weird not locally real part.
With two entangled particles, before you measure spin on one particle, you don’t know the direction and you don’t know the direction of the particle either. After you measure one particle, you know its spin, and immediately know the other entangled particle has the opposite spin. But you never know which direction the particle you measure will spin.
So the locally not real part is that the particle does not know which way it is spinning before measure, and does not know which way it will spin after measure, and no other particle or known force locally touched the particle we measured.
So the particle is not locally real. Sort of.
Okay there are some GREAT answers but they are too long for an ELI5, so I’ll try my best at 5am and no sleep, feel free to correct me:
The term atom is for something that can’t be divided. Solid. The end of the scale.
Turns out atoms are made of of even smaller things that we can’t pin-point where they are, they aren’t still, they move all the time like they’re flickering.
Because that is what EVERYTHING is made of those things, while to us a table in front of you is always real and “solid” and you can always pin-point where it is about as well as your hand, in reality, nothing is.
It is very abstract, but if you try to scale up to how we see things… It is like this table is made of smoke, one that doesn’t fly away but keeps making the shape of the table. Yes, it exists, yes, it is there, but good luck trying to pin-point every single part of it.
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