for today's video I want to do a little experiment follow the instructions in this video what it's going to do is it's going to show you how to shine a laser through two tiny little slits and then project what that does against the wall I really want you to try this because you should see with your own eyes the phenomena that has led to a series of discoveries that has challenged everything we know about light matter and reality itself super answer file MLP Luigi said can you do a video on the double slit experiment yes I can oh right here we go let's step in the Wayback Machine to the late sixteen hundreds where it was pretty much settled science that light travelled as a particle there have been a lot of experiments to back this up and it even had the support of Sir Isaac Newton and he was not known to be wrong about things but then a physicist named Thomas Young created the double slit experiment and it consisted of a box through which light traveled through two very tiny slits and then a little I hole where he could look in and see what that did when I projected to the other side of the box the thinking was that if it travelled like a particle it would create two parallel lines on the other side of the box because it was projecting through those lines just like bowling balls flying through pens at the end of the bowling lane that's not what happened what he saw instead was the same strange pattern of lines that you're seeing in this experiment young recognize what he was saying right away it's an interference pattern you see when waves combine that creates what's called interference like if you take waves in water when they hit each other the peaks will combine to create taller peaks and the peaks and troughs will cancel each other out which creates this pattern of higher peaks and then stillness and when I get projected against the surface you get the weird double slit experiment pattern this only happens if light is traveling as a wave that's the only explanation for this the test was replicated hundreds of different ways in the years following this discovery and even Isaac Newton himself had to concede that yes light can travel his way but other experiments showed it traveling as a particle so what is it light are you waver particle this particle wave duality became known over time as well the particle wave duality and it was a mystery that confounded for the next 200 years by the 1920s the physics community had much better instruments at their disposal and they decided to add another layer to the double slit experiment you'll see this over and over again experiments building on top of other experiments increasing in complexity and weirdness so then you twist in the experiment was basically asking the question that if the photons were behaving as waves and colliding with each other and the other side of the slit what would happen if he fired the photons through one at a time the guess is that you would get the particle pattern at two complimentary parallel lines after all how can you get a wave interference pattern when there's nothing to interfere with so they gave it a try into everyone's shock and amazement they still got the interference pattern but how how there is nothing for that photon to be bumping into what could it possibly be colliding what to create this pattern the answer itself you see somehow this photon was going through both slits at the same time and then colliding into itself on the other side of the barrier and this revealed a fundamental truth about quantum mechanics in a waveform state quantum objects exist in a probability state meaning that at any given time it could go through the left slit or the right slit or hit the barrier while in a waveform state that photon is all three at once it is 33% in the left slit 33% in the right slit and 33% hitting the barrier actually that's incredibly simple math there are highly complex equations out there that calculate probabilities States with a high degree of accuracy but eventually the waveform has to go back down to a particle and that's called the waveform collapsing in the case with the double slit experiment that happens when it hits the back wall and this experiments been replicated over and over and over again even with different particles like photons electrons protons even whole atoms and molecules so now we know that fundamental elements of matter can pop in and out of existence through probability States but that's where things just start getting weird because for the next evolution of the double slit experiment they wanted to see if they could measure the probability state ed Li went through the slits so they set up a little detector on one of the slits to see if they could measure it this was called the whichway experiment the results shook scientific understanding to its core and remains unexplained to this very day because this time the pattern on the back of the wall all displayed what you would expect with particles going through the slits two lines no interference no waves this means that the wave form collapsed before it went through the slit why because it was being observed this proved that the act of observing the particle caused the wave form to collapse how is this possible how can it know that it's being observed and what is the connection between the observation and the wavefunction that's causing it to collapse this experiment has been repeated countless times to the exact same results and we have no explanation for this but wait it gets weirder still dark about wielder created a version of this experiment that he called delayed choice which I had a cool name like Archibald it gets pretty complicated but the basic gist is that he puts a second barrier behind the first one and through beam splitters and interferometers he makes it so that the second barrier could be one slit or two and he changes that while the photon is mid-flight and when he found out that by changing the structure of the second slit it altered the behavior of the particle at the first slit in other words the particle was being altered by things that hadn't happened yet future events were changing the present out the hell Oh son anyway when Thomas Young created the double slit experiment he had no idea that endless series of mysteries he was about to unleash on the world and he had no idea that these mysteries would shape and confuse our very ideas of the nature of the universe and reality itself how does conscious observation change the nature of matter truth is we don't know but it brings up some fascinating ideas so now that I've melted your brain I want to hear what you think go ahead and talk it all up in the comments there are no wrong answers here thanks so much for watching and to MLP Luigi for asking an awesome question if you have a question you can hit me up in the comments or tweet me a question at joe scott writer and we can get smarter together if you learn something this 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