Double Slit Experiment Explained Step-By-Step

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Hey guys I'm Jon and welcome to Respect Your Intellect. In this video, we're gonna be talking about what is in my opinion the single most impressive experiment ever conceived: the Double Slit Experiment. It's not just impressive for how was first derived but also for all the variations that were done in later years that all give us a glimpse of how wonderfully weird the quantum world is. But as weird as it is, the quantum world forms the building blocks of our reality so it's important to understand how things work. The Double Slit Experiment is an experiment that demonstrates how particles can display characteristics of both waves and particles at the same time. This is called the wave-particle duality. This experiment also illustrates the fundamental nature of quantum mechanics which is all about uncertainty and probability. The first person to conduct this experiment was Thomas Young in 1801. He performed it with light and was able to show that the theory at the time of light being a wave was correct. The Double Slit Experiment is sometimes referred to as Young's experiment or Young's slits, in his honor. This experiment is so easy to perform that anyone can make it at home with a laser and a sheet of paper or aluminum. I'll leave a link in the description of step-by-step instructions, if you're interested. Here's how the experiment is set up. First we need a light source that outputs coherent light for best results so it's best to use a laser. You can also do it with sunlight but the results are not as well-defined due to the different wavelengths that are present in sunlight. Next we have a plate in the middle with two very narrow slits in it which we'll be shining the laser on. And lastly, we have a screen at the end that will capture or detect the light so that we can see the resulting pattern. Now let's start with a quick question. What's the pattern that you would expect to see on the screen if we shine the laser through the two slits? Feel free to pause the video here and think about it. If you guessed that you would see two lines of light on the screen that corresponds to the slits themselves, then you would have the same expectation as most people do. This is mostly due to our experiencing the world in classical physics so we have a tendency to think along those lines. But the reason why you're watching this video is because weirdly enough it is "not" two lines of light corresponding to the slits that we see on the end screen. What we see is a pattern like this which initially seems to go against logic "completely". How can we get this pattern if there are only two slits where light can pass through. And what about if we block one of the slits and only let light go through a single one; what do we get then? Feel free to pause here again and think about what you would expect to see in this case. Well if you guessed that you would see a single line corresponding to the slit then you would be "mostly" correct. We actually see more intensity in the center that corresponds to the slit but it does fade away a little bit towards the sides. This, at least, seems to be a lot more in line with what we would expect. So how is it possible to see this pattern when there are two slits available when there's no such pattern with only one slit? Well this is due to what we call diffraction and it's what happens when a wave encounters an obstacle; in our case, the walls around the slits. Here's an example with water where the large waves have to go through a narrow entrance and diffract into nice circular waves as they enter. It's worth noting that if the slits are not narrow enough you won't get nice circular waves so we'll just assume for the rest of this video that the slits are narrow and that we get circular waves coming out of the slits. In our experiment, the slits will create two circular waves almost as if they were a completely new source. These two circular waves will start to interfere with each other and cause different peaks and troughs in the pattern. When two peaks meet up they're amplified and that's called "constructive" interference. When a peak meets a trough though the peak is weakened, and can even get canceled out completely, and that's called "destructive" interference. This results in a series of alternating strong and weak lines that form this interference pattern on the screen at the end. I'll link to a video from Veritasium here that has one of the best demonstrations of this interference shown in a pond. Before we get to how this pattern emerges when using light, let's talk about what happens when we send out single photons at a time instead of a stream of them. If we send only a single photon at a time through the slits, what do you think will happen now? We already know we were getting an interference pattern with a constant stream so maybe the particles are simply interfering with each other to create the interference pattern. But will you still see the interference pattern on the screen if you don't have multiple photons to interact with each other or will it go back to only seeing two lines on the screen? Feel free to pause the video here again and think about it. Well the answer here is that even if you send only one photon at a time and accumulate the impacts on the screen at the end over time you'll still see the interference pattern emerge. Now this is crazy isn't it? How can a single particle create an interference pattern if it has nothing else to interact with? Well the answer to that is that while a single particle is behaving like a wave, it's actually able to interfere with itself. So how are individual photons able to create the interference pattern in this Double Slit Experiment? Well let's start by talking about what exactly happens on the journey from the beginning to the end. The first thing that happens is that the photons are shot out towards the slits. Since we haven't asked the photon to be in a specific position yet by performing an observation, it's essentially everywhere at the same time as a wave of probable locations. This wave is not a physical thing but rather a set of pure probabilities of where the particle could be found "if" an observation was made. Until we ask the photon a question about where it is or what path it took, it will continue to be a wave and propagate like a wave in a pond. As soon as we ask it any of those two questions, it will immediately decide, based on a probability of locations "on" the wave, where it is and what path it took to get there. This is what we call the "collapse" of the wave function and it happens for every observation regardless of what the observation is. You can think of an observation as any interaction at all with the real world. Once the wave is collapsed into a particle in a precise location and we get its position, it will return to a wave behavior from this new source position after the observation. Ok so that was a lot of information. So let's try to get a better feel for how the photons can interfere with themselves with a nice little video. Here we see single particles being shot out and the wave is coherent before hitting the slits. Since there hasn't been any interaction yet with the real world, the photon is still a wave when it reaches the slits. This means that the wave is actually going through both slits at the same time. Once it's through the slits it then diffracts and interferes with itself, forming peaks and troughs. The final step in this example is the end screen which is the interaction with the real world that counts as an observation and collapses the wave function. If you repeat this many times, you eventually see the interference pattern re-emerge, showing that the photon was in fact a wave that went through both slits at the same time. So what happens when we add a detector to perform an observation right before the slits so that we know which slit the particle is going through? This variation of the experiment is called a "Which-way" experiment. What we're essentially doing here is that we're asking the particle to choose its actual position so that we can measure it. This interaction results in the wave collapsing into a particle and it can no longer go through both slits at the same time. This means that with simply having added an observation, the interference pattern is destroyed which shows that the photon did not go through both slits at the same time. Now if you leave the detector there and quietly go unplug it to try and trick the photons into thinking that they're still being observed, what do you think will happen? Feel free to pause the video here again and think about it. Well the answer is that since the interaction between the particle and the real world is removed, the particle can continue to be a wave, go through both slits simultaneously, and interfere with itself. This restores the interference pattern on the end screen. So if you guessed that, you were correct. So essentially just plugging and unplugging the detector will change the pattern on the end screen between the interference pattern and the two lines corresponding to the two slits. Is your mind blown yet? But wait, there's even more. Physicists are pretty clever so they've been trying to figure out ways of measuring the particles in different ways without collapsing the wave function. One of these variations of the experiment is by changing the detectors position to be after the slits and before the end screen. This variation of the experiment is called the "Delayed Choice" experiment. By putting the detector after the slits, the particle should be behaving like a wave, going through both slits simultaneously as we've seen before, and have time to interfere with themselves before the detector collapses the wave function. Feel free to pause the video here again and think about whether or not you would see an interference pattern on the end screen in this version of the experiment. Well the result here is that the interference pattern is once again destroyed so if you guessed that, you were right. What this Delayed Choice experiment shows is that the particles are somehow able to retroactively choose which slit they went through; even if there were still a wave when they actually went through. This means that an observation that collapses the wave function doesn't just turn the wave into a particle at that particular point, it also determines the entire path that was taken for the particle to end up where it is; even if it wasn't yet a particle at the time. I personally find this simply "amazing". There's also another variation of this experiment called the "Quantum Eraser" that is even more impressive but that will have to wait to be done in a future video dedicated to it. Now as if all this wasn't weird enough, we've only been talking about photons up to now but it doesn't stop there. These experiments were also later performed with electrons and showed that even matter acted in the exact same way. This was repeated with particles of matter with higher and higher masses until 2013 where we did it with molecules comprised of 810 atoms of over 10,000 atomic mass units. The results showed that even if the matter was a particle at the beginning where it gets emitted, it still acts like a wave until it hits the screen at the end or an observation is made. This means that all the matter we're made of is also behaving as waves until it's necessary for their locations to be determined. Otherwise, they're occupying all the possible points in their wave simultaneously and interfering with themselves and each other. Oddly enough, this is also a technique used in programming to avoid having to compute everything if it's not necessary to do so. So it's as if the universe is trying to save resources by reducing computations until it's absolutely necessary to do so. Before we end, I want to also address one of the most common misconceptions about the collapse of the wave function, which is that consciousness is required to collapse it. This was proven to be false when they stored the information in atoms about which slit the particles went through instead of larger macroscopic devices. The result was that the interference pattern was still destroyed even though there was no consciousness involved in the measurement. You can read more about this in a scientific paper called "Quantum Mechanics Needs No Consciousness". I'll leave a link in the description. The Double Slit Experiment is one of the best examples of why we need to use both classical physics and quantum mechanics at the same time. Classical physics is all about smooth, orderly, and predictable patterns while quantum mechanics is completely unpredictable and random. The collapse of the wave function, where a wave of probabilities collapses into a specific position, is only one example of the fundamental unpredictability and randomness of quantum mechanics. So what do you think? Do you agree that this experiment and its variations is one of the most impressive you've ever heard about? Let me know in the comments which aspect of this experiment you liked the most and how many of the questions you answered correctly. If you liked this video, consider subscribing and I'll bring you more content just like this. Also don't hesitate to hit that like button, share, tag someone; all that good stuff. You can also find links to my social media in the description. Until next time, thanks for watching and remember: Respect Your Intellect!
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Channel: Respect Your Intellect
Views: 247,470
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Keywords: respect your intellect, double slit experiment, double slit, physics, science, quantum, quantum physics, particles, double-slit experiment, particle, photon, electron, wave function, light waves, superposition, constructive interference, destructive interference, interference, diffraction, universe, quantum mechanics, which way, delayed choice, education, consciousness, atoms, peak, phase, probability, proton, slit, modern physics, measurement problem, uncertainty principle, explainer, explained
Id: 2VZ6dMXpxeU
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Length: 12min 19sec (739 seconds)
Published: Mon Mar 18 2019
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