10 ms·
I have a bag with two balls, one red, one blue. I randomly send one of the balls to Australia, one of the balls to the USA. The moment you look at your ball in
by 317070 6y ago
I have a bag with two balls, one red, one blue. I randomly send one of the balls to Australia, one of the balls to the USA. The moment you look at your ball in Australia, you also know the color of the ball in the USA. How would you use that fact to send information?
The crux with quantum mechanics is that you can show (very easily, understandable by the layman, look up the Bell inequality) that in the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well.
However, the basic setup still applies. You cannot send information by merely observing something.
- smusamashah 6y agoHow does a ball know that it has been observed and in effect changing the color of the other paired ball?
- EliRivers 6y agoThat's the "spooky action at a distance" (in the case of the entangled particles, obviously; with the two balls it's not so spooky). Spooky.
- nemothekid 6y agoMaybe this is a basic question - but what I don’t understand is why this is called “spooky” action. My intuition is you have two particles, and you don’t know what concrete states they are in, but you know all possible states (that may be represented as some sort of system of equations). By observing a single particle you unlock a variable in that system of equations and can therefore solve the whole thing. To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed. The concept of superposition seems like an overly complex description for this phenomenon. I understand my view is wrong, but I don’t understand how I’m wrong
- rrss 6y agoWhat you are describing is a hidden variable theory - i.e. there is some concrete state of the particles, but it is hidden. John Bell demonstrated that in order for a hidden variable theory to make predictions in agreement with quantum mechanics, it must have nonlocal interactions, which means any workable hidden variable theory must also be pretty spooky. https://en.wikipedia.org/wiki/Bell's_theorem https://en.wikipedia.org/wiki/Bell's_theorem
- tabtab 6y agoHere's a layman's explanation of why "hidden local variables" theory doesn't match experiments: https://www.wired.com/2014/01/bells-theorem/ https://www.wired.com/2014/01/bells-theorem/ In other words, modeling particle pairs as having matching static hidden "meta data" in them doesn't work. They do act as if there is instantaneous communication between the particles, but in a limited way that prevents us from using them for instant communication. Quantum mechanics is a weird tease, having magical properties that always serve up loopholes when we try to leverage the magic for real-world benefits. The quantum universe seems built by insurance lawyers who are masters at screwing consumers with fine-print when they go to make a claim.
- EliRivers 6y agoI observe my particle here, and in doing so its state is decided. The state of the entangled particle over there, a light year away (for example) is also decided. Instantly. Faster than the speed of light. Nothing travelled from here to there. No particle, no photon, nothing. How does over there "know" that I did something over here? Sure feels kind of spooky. To me it would be more straightforward to say the concrete state of the particle is simply unknown until it is observed. It's not just unknown. It's undecided. It has no concrete state. It's not that it IS a one or a zero and you just don't know it. It's not yet been decided whether it's a one or a zero, but as soon as the decision is made for one of the entangled particles, the decision is also made for the other one, a light year away. Instantly. Spooky.
- 317070 6y agoWhen you observe it, you collapse the wave-function of which color the ball has into a particular value (red or blue). Before you observed it, the ball was in a superposition of the two colors. And this collapse instantaneously also collapses the wave-function to the American ball. Now, that is obviously not true for macroscopic objects like balls. Those are not in a superposition of colors until they are observed, but it is true for quantum objects like electrons.
- AnthonyMouse 6y ago> Now, that is obviously not true for macroscopic objects like balls. Those are not in a superposition of colors until they are observed, but it is true for quantum objects like electrons. But then what is it that can I do with two entangled electrons that I can't do with two literal billiard balls known to be different colors than one another?
- Koshkin 6y agoYou cannot paint a billiard ball with a superposition of colors.
- AnthonyMouse 6y agoBut what is the implication of that? What do those words cause to be different in practice?
- Koshkin 6y agoThe randomness in the observed color of the ball. (The observable represented by the "color operator" could have two or more distinct eigenvalues.)
- kgwgk 6y agoYou can prepare a pair of photons in a state such that when you measure the polarization of both of them along the same axis, for whatever direction you want to choose, you get the same result. But they are entangled, each photon considered separately is not in a well-defined state. You can also prepare two photons in the same state, so the have the same polarization for some direction chosen at that time. But the measurements along other axis won't be perfectly correlated (if they are correlated at all). The red/blue color example is too simple to be interesting.
- brianush1 6y agoObligatory "not a quantum physicist," but the only way to observe something is to throw something at it (e.g. a photon) and see what bounces back. The problem is that when you throw something at it, you're interacting with and affecting the ball.
- monktastic1 6y agoThis is not quite correct. There are ways to gain information about a quantum state without interacting with it: https://en.m.wikipedia.org/wiki/Interaction-free_measurement https://en.m.wikipedia.org/wiki/Interaction-free_measurement
- mrkstu 6y agoDo (non)measurements taken this way 'collapse the wave function' anyway? Or can you only get information that is still open to change during the actual measurement?
- monktastic1 6y ago"Collapse" is just one way to explain how measurements happen. These are "actual" measurements, because they give you actual information.
- Koshkin 6y agoYeah, this is another popular non-explanation. (It does not explain why the ensuing randomness is subject to strict and perfectly deterministic laws).
- tim333 6y agoParallel realities. In one red goes to the US in another blue. In another vice versa. They decohere to one with someone who's seen red one who's seen blue.
- aeternum 6y agoThe ball example is insufficient and misleading. It is unfortunately too simple as you need 3 inputs and 2 output to demonstrate the effect (aka Mermin device). The way to think about it is a box with 3 buttons. There is no such thing as 'observation', the only way you can interact is to push one of the 3 buttons and as a result the box will output either a red or green light. You must push a button to get the light, but the button may mutate the internal state of the box. Using this model, there's nothing special about human or conscious observation. Every interaction via a particle or otherwise is simply pushing a button. The crazy thing is.. no matter how clever an algorithm you write to drive the lights from the buttons, you cannot match the observed probabilities. (100% if the same button is pushed, 25% if different buttons are pushed).
- kgwgk 6y ago> Using this model, there's nothing special about human or conscious observation. Every interaction via a particle or otherwise is simply pushing a button. But there is something kind-of-special about the box with the buttons and the lights. Not every interaction is simply pushing a button that lights one lamp or another. Keeping the analogy, the result of an interaction between two particles may be a combination of the "red on", "green on" states. You need to keep adding particles to have a box with buttons and lamps that works as expected.
- onlyrealcuzzo 6y agoI've always wondered - what constitutes an "observation"? How does the particle know it has been observed?
- burkaman 6y agoParticles don't know anything, but you have to interact with it in order to observe it. You have to bounce a photon off of it or something like that in order to get any information out of it.
- thaumasiotes 6y agohttps://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence You can't observe something without sending information. In order to make an observation, you must interact with whatever is being observed, so that information about the interaction can come back to you. In the bag example above, we can observe the Australian ball and know the color of the American ball, and we cannot use this interaction in Australia to send information to America. But we cannot avoid sending information to the Australian ball when we observe it. >> You cannot send information by merely observing something. This is, at the least, very poorly phrased. As explained above, not only can you send information by observing something, it's impossible not to do so. The question here is where the information goes.
- warent 6y agoMy understanding is that any kind of measurement will do, it doesn't matter how you get it. You could use photons to do the measurement but there are other ways which all have the same result. It's a mystery how the particle "knows" (In other words, nobody knows when the wave function collapses) but one popular interpretation is that the particle exists in all states, i.e. in a pure description of reality. When any quantum system interacts with it, then it becomes entangled with the result of that measurement, branching it into a new universe (edit for clarification: a new world where it was as if it was never a wave, and it was always a particle). That's my understanding of the many-worlds theory. That entanglement propagates across nearby particles, so it doesn't have anything to do with eyes or consciousness. If the air molecules around your body interact with the particle then that entanglement propagates through your body and places you in the new world.
- centimeter 6y ago> the quantum case the ball only takes on a color the moment you look at it, and so instantaneously is setting the color in the USA as well We don't actually know that this is an accurate description of what is happening, although it is consistent with what is happening. Very likely, the underlying physical process still operates below the speed of light. "Instantaneous" isn't something that makes physical sense in this context.
- gmadsen 6y agoThere have been experiments confirming entanglement. It does not "tell" the particle at the speed of light. The mutual wave function collapses as a whole unit. it is very easy to measure something entangled at the same time (or at least within a margin that is faster than light travel) and confirm you always get the correct results. If the wave function didn't collapse together, you would get results that break quantum laws, such as measuring two entangled particles with both up-spin.
- centimeter 6y agoYou misunderstand the objection. The fact that you didn't expand on your definition of simultaneity suggests that you're missing a lot of background here. There are many ways that physics could appear to violate causality with collapse without actually sending information faster than light (e.g. how most of the multiverse theories work).
- TrinaryWorksToo 6y agoIf the ball only takes a color when it's observed (loosely), couldn't that timing be exploited to convey information?
- mwaitjmp 6y agoBy viewing the color of the ball you’re not notifying the other ball to change its state(in a way that can be measured). You will only know that once the ball is observed it will have a certain color. The only way I can foresee this being used to transmit data faster than light is that if you both agree to perform an action depending on what color ball they see. If you both view the ball at some agreed point in the distant future, you will instantly know what action the other person will do.
- Groxx 6y agoBy looking, you're forcing it to take a color if it didn't have one. You can't tell if it was randomly chosen by your action, or if it was chosen by the other side's action before yours, without communicating by some other means.
- GuB-42 6y agoNo, in fact it doesn't matter who opened the box first. Imagine we both open our boxes at around the same time, and communicate each other results at the speed of light. - From my point of view, I opened the box first, got red and caused you to get blue. Your message confirms my hypothesis. - From your point of view, you opened the box first, got blue and caused me to get red. My message confirms your hypothesis. We can repeat this experiment thousands of times, every time, we both get a 50/50 chance of red vs blue, and every time, the hypothesis is confirmed. In order to know who really was first, the only way is to wait for the other person result. For example: when you open the box, start a timer, stop it when you receive my message. It the time is less than the times it takes for light to travel between us, I was first. But in order to have this information, you have to wait, so it is not faster-than-light anymore.
- steve76 6y agoThat's not right. The colors need to be able to change in transit. Otherwise, how does a double slit adjust after a particle passes through, and the opening then closes? Here's my approach: 1. Each ball starts with red or blue as the easiest way to transfer heat through electromagnetic radiation. 2. You are good at quantum chemistry. You convert the ball to a microstate. You turn it into a voltage without chemical reactions or heat loss through phase change. 3. That voltage grounds out to Earth. Earth gets brighter. Earth's internal clock ticks faster. Earth's orbit increases in radius. Good telescopes far away all agree that bigger Earth in orbit around bright sun transfers heat the fastest. 4. That propagates at the speed of light to everything. As time passes, all that is leveraged. You will need to give all that a different explanation if you want to start reversing things now. 5. You alter the macrostate of AUS so if a ball materializes, it's easier to transfer heat as blue. 6. Any ball to materialize in the USA would be red. It's a big difference. Cosmic objects are spinning coins growing in locality at the speed of light. Stopping them with chemicals in orbit or matter wave satellites or systems that start with our sun or a black hole at one end have very profound effects. We can increase our precision here based on our greater ability out there. A slow acting photosensitive drug or genetic treatment that works because what a solar probe did in the past. Sounds desperate but I am tenacious in the pursuit.