What is Quantum Mechanics Really Trying to Tell us about Reality? Featuring @SabineHossenfelder

quantum mechanics is one of the most successful theories in all of science using its mathematical equations scientists can accurately predict how tiny particles like photons of light and electrons and electrical currents behave it can be used to explain everything from the atomic structure of the elements to the shapes of molecules to the cosmic structure of the universe but amid all its useful equations and accurate predictions a troubling question remains although quantum mechanics clearly works what does it mean what is it telling us about the true nature of reality the way the universe actually is the rules at the level of atoms and subatomic particles seem to be different from the ones we're used to in our everyday experience they seem weird for example particles appear to be fuzzy and spread out they seem to be in two places at once or two different states at once or affect each other instantly over any distance but is this really what quantum mechanics says all these claims are no more than clumsy attempts to interpret the theory using everyday language but everyday language wasn't designed for the quantum world it was designed for the macro world of classical physics which is what we observe at human scales here objects can be seen and touched and watched as they follow predictable paths in space at precise speeds that we can measure surprisingly for such a well-established and accurate theory as quantum mechanics there is still no consensus on what it really means physicists differ in what they think quantum theory tells us about the world some think it's every bit as concrete and definite as our classical world some see a multiverse where all possibilities exist for others quantum mechanics sets the ultimate limit of how much we can really know about reality if you hear an expert insist that they know what it means be cautious in this video i'll tell you why it's so hard to be sure of the answer but also what we can say for sure about it that's coming up right now many things inspired me to make this video but one documentary which really influenced me is what i watched on magellan tv today's sponsor it's called einstein's nightmare hosted by one of my favorite educators jim alcaley jim takes us on a journey starting with einstein's dislike for the idea that nature could be fundamentally probabilistic and shows how we came to the conclusion that it must be this way this is only one of thousands of fascinating documentaries on magellan tv it's a new kind of documentary service created by the filmmakers themselves who bring you the highest quality content feature topics include science history travel and technology magellan has a special offer right now for arvind ash viewers if you click the link in the description you'll get a free one month trial it's totally worth giving it a try i highly recommend it so be sure to click the link in the description if we want to understand quantum mechanics we have to start with the math because that's where the heart of the theory lies but don't worry i'm not going to derive any equations just describe them arguably the most important quantum equation was figured out in the mid 1920s by austrian physicist erwin schrodinger he was picking up on the suggestion of french scientist louis de bruy that maybe quantum particles can behave like waves schrodinger came up with an equation to describe this waviness and it looked a lot like the equation scientists had used to describe ordinary waves like those in water this equation may look complicated but at its heart is just an equation describing energy kinetic energy plus potential energy equals total energy it includes a mathematical function called the wave function which encapsulates all that can be known about a quantum object like an electron or atom or even a molecule the object's wave function usually written as the greek symbol psi has a particular value at all points in space and this value depends on the state that the quantum object is in for a single quantum particle for example in a square box the wave function of the lowest energy state has a single peak in the middle if you know the wave function schrodinger's equation lets you figure out for example where you'll find the particle in space or how fast it is traveling or how much energy it has but what is the wave function that's the key question we need to answer to understand what quantum mechanics is telling us about the world in some ways pretty much all the arguments that still rage about this question boils down to disagreements about a wave function really is if we plot out a typical wave function for a particle it looks a bit like a wave but a wave of what for example a water wave is made of water molecules but a wave function for a quantum object isn't really made of any substance other than the information about the object the amplitude of a water wave is simply the height of the wave the amplitude of a wave function is simply its value at any point in addition the amplitude of a quantum wave function is generally a complex number which means that it contains some term that is an imaginary number which is the square root of a negative number it's not a whole number like 1 2 3 and so on what does an imaginary number mean well it's not a physically meaningful quantity but they are useful in simplifying the mathematics of anything that oscillates but when we square an imaginary number we get a real number so the square of a wave function the value of psi squared becomes a real number this has no imaginary component so it can correspond to the value of some real measurable quantity at first schrodinger himself thought that the value of psi squared at any point in space tells us what the density of the quantum object like an electron is at that point in other words maybe the particle really is smeared out almost like a cloud of gas with different densities in different places but he was wrong german physicist max born said that the square of the wave function isn't the density of the particle instead it's the probability it tells us the probability that we'll find the particle at that point in space if we look if we add up psi squared for every point in space it will equal 1 meaning that if we look everywhere we're bound to find the particle somewhere bourne's rule is at the heart of why quantum mechanics is such a peculiar theory and why it's so hard to say what it means compared to what we find in classical physics the equivalent of schrodinger's equation is an equation of motion like the one that describes the path of a basketball we can use newton's laws of motion to write down a mathematical expression for its trajectory and use it to calculate exactly where the ball will be at each moment in time there are no probabilities no ambiguity we can say for sure that if we look at one particular point in space at a particular time we'll find the ball there if we look elsewhere in space at that moment we know that we won't find the ball there but a wave function says that for a quantum object we might find the object at many different places in space at any given instant with various probabilities we can never be sure which of these it will be in and in fact if we can measure several different particles with the same wave function in successive experiments we're likely to find it at different places each time the wave function itself can change over time rising and falling like an ordinary wave but this simply changes the probabilities of finding the particle at different places for each instant as the physicist rolandonis has put it the wave function is the fuel of a machine that manufactures probabilities now here's what's particularly perplexing you might think that the wave function tells us where the particle probably is at any point in time but it doesn't tell us that it tells us as i said earlier only the chance of finding it there if we look what if we don't look well then all we have to go by is the wave function itself which says nothing about where the particle is it only lets us figure out the probabilities of where we'd find it if we looked there or made a measurement of some sort there it's often said that the wave function implies that until we look the particle is everywhere or at least it's smeared out but that's not really right because born's rule doesn't mean that borne's rule is only about the probabilities of the outcomes of measurements this mathematical machinery tells us not what the quantum world is like but only what we'll see if we measure it so the idea that before we measure it the particle is everywhere is not quite correct it doesn't tell us what is it only tells us what we might expect if we look but this also doesn't mean that nothing exists until we look a quantum world exists whether we look at it or not it's just very hard to know exactly what we can say about it before we look beyond what its wave function is and that's what's so peculiar about quantum mechanics because unlike most other scientific theories it doesn't give us a clue about what's really happening but what we might experience when we measure things it says nothing obvious about what quantum reality itself is like all we can do is try to find the best story to tell about what's really happening now one story from a famous experiment really does look as though quantum particles can be smeared out this is the famous double slit experiment richard feynman called this the central mystery of quantum mechanics the setup of this experiment is something like this we fire photons or electrons or any quantum object one at a time towards two closely spaced slits and then place a fluorescent screen on the other side what we see is a point-like single flash when the electron hits it after going through the slits this appears normal and nothing special however if we keep firing multiple thousands of electrons one at a time we begin to see an interference pattern this may at first appear to be mysterious how could single electrons fired one at a time interfere when no two electrons had the opportunity to interact with each other but this really shouldn't be such a surprise because it is precisely what quantum mechanics predicts it predicts that we should see an interference pattern because individual electrons are like waves it's very much like a water wave if we sent a water wave through the slits we would see an interference pattern on the other side similarly individual electrons would behave like a wave that passes through both slits too and this wave would split and emerge from each slit to interfere with each other you might say okay then why don't we see a wave interference pattern on the fluorescent screen when we send just one electron through the slits the reason is because the particle behaves like a wave as long as we don't measure it but the fluorescent screen acts like a measuring device when we measure it at the screen it takes a particle-like position so when electrons hit the screen they're no longer in superposition they lose their quantumness they take on a point-like appearance because they are measured the wave becomes localized to a fuzzy point the interference pattern is not visible when an individual electron is measured by the screen but if we measure thousands of individual electrons one at a time the interference pattern emerges this happens because more electrons show up on parts of the screen where the probability amplitude is higher due to constructive interference of the wave and they don't show up where the probability amplitude has destructive interference in other words because the schrodinger equation only gives us probabilities we have to make many repeated measurements of these individual electrons to see if what we measure fits those probabilities and it always does so the double slit experiment should not be a mystery it shows us exactly what quantum mechanics predicts that individual particles have wave-like behavior which can be seen in the collective pattern created by shooting lots of particles through the double slit and measured on the screen had we not gotten this result we would have to rethink quantum mechanics so again what really is the wave function is it just math that lets us predict what we'll see in a quantum experiment or is it a real physical object like an ocean wave that tells us about the underlying reality that causes those observations some theories attempt to define the collapse as a real physical process to give you a fuller perspective i invited fellow youtuber and friend sabina hasenfelder to tell you about these as well as her favorite theory hi alvin i have one thing to add quantum mechanics may seem weird to us because it's not actually the right theory this is why some physicists have tried to improve quantum mechanics by turning the collapse of the wave function into a physical process this removes the ambiguity about how to interpret the wave function it's a real thing and it really collapses there are three major approaches to this at the moment the probably best known one is penrose's gravitationally induced collapse the idea is that a superposition of quantum particles should also come with a superposition of quantum space times and those parallel cells don't exist long so the wave function collapses another idea is called spontaneous collapse and the best known example is the grw model in this model the wave function stochastically collapses a little bit every now and then until it's entirely localized and then there is super determinism super deterministic models are also collapse models but they are not stochastic instead the apparent randomness of quantum measurements is explained by hidden variables it's not much of a secret that personally i think super determinism is correct thanks for letting me join thank you sabina and be sure to check out her channel at the link in the description and while you're there subscribe for some great physics content the problem is we can't really see the wave function of a single particle because when we observe the particle like when the electron hits the screen in the double slit experiment its spread out wave function seems to collapse to a single sharp spike where we find the particle because standard quantum theory with its schrodinger equation at smooth wave functions doesn't actually provide any description of the measurement process we don't really know why or how the collapse of the wave function happens we have to add in the collapse to the theory in a makeshift sort of way it's even been suggested that our own conscious awareness of the measurement result somehow alters the wave function there's not really any good reason to think that's true though the collapse happens whether we are looking at it or not how it happens and what causes it to happen is called the measurement problem and it's one of the biggest causes of arguments about what quantum mechanics means some researchers say that we can't deduce much about that underlying reality if all quantum mechanics gives us is probabilities about measurements we just have to accept that that's all we can know and who really cares anyway about a question that seems more philosophical than physical as long as the theory correctly predicts the outcomes of experiments the extreme version of this was summed up by physicist david marman in the expression shut up and calculate and it's a view that some quantum physicists take today they don't want to bother about questions that lie beyond what we can measure and observe because that doesn't seem like science they just use the theory to calculate what their molecules or devices will do others go to the opposite extreme they say that at the most fundamental level reality really is a wave function this view is commonly shared by scientists philosophers who believe in the many worlds interpretation of quantum mechanics often called the evoredian view because it was first proposed by the physicist hugh everett in the 1950s in this view the entire universe is a gigantic wave function and we can probe little bits of this wave function with our experiments because a quantum wave function encodes many different possible outcomes all those places in space for example that we might observe a particle to be in have ready and say that all of these are equally real we only observe one outcome but the measurement also elicits all the other possible outcomes too in different worlds or universes so while you might measure an electron to be in one location in this world there are other worlds where you measure it in a different spot we experience just one because the measurement itself causes the universal wave function to split into separate universes where a copy of you measures something else these worlds are isolated from each other many worlds is seen by many who are growing in number as the conservative option the one that adds the least to the bare math of quantum mechanics the fact that there are different interpretations of what might really be happening is unusual for a scientific theory it's a frustration that there doesn't appear to be a good way to test which interpretation is correct quantum mechanics seems to say that reality works in certain mathematically predictable ways but we can never see it working that way because when we try to see it by taking a measurement it seems to work a different way and that's what many physicists find so disturbing that quantum mechanics doesn't appear to show that there's a set reality out there independent of how we choose to look at it that seems to fly in the face of all that we've come to believe about science that it can tell us about how reality really is regardless of what we believe but this doesn't mean that an objective reality doesn't exist quantum mechanics gives us a theory that makes our measurements predictable and it works the same for everyone so it's repeatable predictability and repeatability are the hallmarks of an objective reality but it appears we are not completely independent of this reality since we are also part of the universe that quantum theory describes perhaps quantum mechanics is simply the kind of theory we need to understand a universe of that kind and if you enjoyed this video give us a like and subscribe for a future follow-up video on the same topic if you have a question for me or our viewers put in the comment section i'll see you in the next video my friend [Music] you

Channel: Arvin Ash

Views: 349,733

Keywords: quantum mechanics, quantum physics, quantum reality, wave function collapse, quantum mechanics explained, quantum mechanics for dummies, quantum reality explained, what is the meaning of quantum mechanics, what does quantum mechanics tells us about reality, double slit experiment explained, what is the physical significance of wave function, what is the wave function, what is the wave function made of, the born rule, quantum mechanics interpretations

Id: -hxljpxTaiA

Length: 19min 48sec (1188 seconds)

Published: Sat Apr 09 2022