The 9 Experiments That Will Change Your View of Light (And Blow Your Mind)

There is a lot of science that we understand. If I threw a ball into the air and was given the right data about the forces acting on it, I could tell you exactly where it would land. Science explains through chemistry the molecules that make up the ball. We can predict the energy levels of the sound it

will

emit when it lands. Like a candle held in the dark, science illuminates our

view

of the world around us... but there is a limit to how far the

light

currently reaches. Even today, when it seems that there is much of the world and the universe that we can explain, there is also darkness.

Answers that we do not yet have, and worse things; confusing results that erode our confidence in what we think we know. There are

experiments

that seem to suggest that

light

lies to us and call into question the very nature of reality. We are real? Is time linear? …Maybe not. But are you ready for the comforting veil of understanding to be torn away and for the strangeness that lies at the limits of our understanding to come to light? If so, do I have some

experiments

for you? I'm Alex McColgan and you're watching Astrum. And in today's supercut, I'll show you nine experiments that

will

challenge

your

understanding of the fundamental laws of physics in a way that will almost certainly leave you with something between a headache and existential dread.

You have been warned. And curiously, almost all of these experiments have something to do with light. The light is much stranger than you think. Sure, it may seem simple enough: traveling around the universe delivering energy from one place to another. It helps us see. It provides life to plants and, therefore, to our planet in general. It has a reputation for being very fast. And yet, for an energy source that has become synonymous with greater understanding, light is surprisingly difficult to understand. Light helps us see other things better, of course, but when scientists tried to observe light itself, it proved surprisingly difficult.

No, I don't mean that they started staring at any lamp (please don't do that at home), but experiments over the last 200 years have shown that what light appears to be and what light is actually are. two things. different things. The first experiment on our list highlights the following puzzling fact: light behaves differently when you're not looking at it than when you are. But to understand that, let's start with the basics. What is light? In the early 1700s, Isaac Newton theorized that light was made up of tiny particles he called "corpuscles," but in 1801, almost 100 years later, a man named Thomas Young discovered that light must actually have more of a waveform. than particle. as.

He demonstrated this using an important method known as the double slit experiment. He set up a light source and shone it through two narrow slits on a board. Young noticed that instead of two bands of light on the other side of the slits, a strange striped pattern was forming. This was known as an interference pattern and was indisputable proof that light had traveled as a wave. Because? Let's talk about waves for a moment. When waves travel, they oscillate up and down. But when two waves try to oscillate the same point in space at the same time, something known as interference occurs.

Imagine you have a bathtub with a rubber duck on the surface. Two waves hit the duck at the same time. One wave tries to pick up the duck at exactly the same time the other wave tries to drop it. What happens? As long as the waves are of the same magnitude and perfectly out of phase, they will cancel each other and the duck will not move at all. This is called destructive interference. Similarly, if both waves tried to lift the duck at the same time, the duck would rise twice as high. This is known as constructive interference.

Because waves tend to spread out in a circle, two waves next to each other will begin to interfere with each other constructively and destructively. There are two waves in the water here. Do you see these lines? In these quieter areas the waves cancel each other out: this is the effect we see when light travels through the two slits. As light from one slit propagates, it cancels out the other light wave at certain points, creating the interference pattern that Young noticed on the board. Thus, the mystery was solved. Light was a wave and not a particle. Except there's more to this experiment than meets the eye.

Fast forward another 100 years, to 1905. Scientists at the time were baffled by something known as the photoelectric effect. It turned out that when light was focused on a metal surface, electron-like particles came out of it. It was deduced that this was because electrons were being knocked out of the metal by the increase in energy imparted by the light. Imagine it as fruit on a tree. If you pluck the fruit from the tree, you need to use a certain amount of energy. Once the energy is greater than the strength of the fruit's connection to the branch, the fruit falls off.

This was happening with light and electrons. Once the light hit an electron and gave it enough energy to cross the threshold, it broke free of the metal. However, what surprised the scientists was that if the intensity of the light was increased, they expected the electrons to be removed faster. If you plucked the fruit from the tree with more force, it would fall off faster. More energy = more kinetic energy output. However, this did not seem to be the case. Instead, increasing the frequency of light increased the speed of the departing electrons. The intensity of the light did not affect the speed of the departing electrons at all, but it did affect the number of electrons emitted.

This was a bit disconcerting. Albert Einstein was the man who solved the puzzle. He deduced that light must travel in small packets of energy, so sending more of them (increasing the frequency) was the only way to increase the energy reaching the electrons. He called these packets photons and later won the Nobel Prize for his work. The light, it seemed, was becoming more like a particle again. Or a wave and a particle at the same time? Of course, even this is not the whole picture. To be honest, even now we are not completely sure of the full picture.

Instead, we have more results that are contradictory. Let's go back to the double slit experiment. Armed with the knowledge of photons, physicists revisited the double slit experiment. Experimental techniques had improved over the past 100 years and it was now possible to emit a single photon of light at a time. Then, the double slit experiment was repeated. This time, only one photon would be sent through the slit to a detector on the opposite side. Once this was done, the detector recorded the arrival of the photon at a single point. Then, the light again behaved like a particle. But then why had he interfered with himself in the previous version of the experiment?

The scientists had an idea. They sent several photons, one at a time, and plotted the results on the detector. And this is where the result got really strange. Once again, the detector began to see photons arriving at single points, one at a time. But surprisingly, the incoming photons began to create a pattern: it was the interference pattern. Proof that light behaved like a wave. But, strangely enough, this occurred only when a single photon passed through at a time. Somehow, the single photon, leaving the detector as a particle and arriving at its destination as a particle, apparently somehow traveled through both slits at once, enough to interfere with itself on the other side, like a wave .

If light were just a particle, when it passed through the slits this pattern would not be seen. You would only see two spots of light: one for the particles that passed through one slit and one for the particles that passed through the other. And yet here was the interference pattern with its multiple lines of light, disproving that. Scientists tried to pinpoint the light. They set up the experiment, but this time with two more detectors in the slit, so that the scientists could observe if it really passed through both at the same time. It was not so.

But at the same time it stopped creating an interference pattern in the farther detector. And from this, scientists began to realize something. The light cared about being observed. To be clear, it didn't matter whether it was observed by a human eye or a machine. The moment any particle interacted with light in some way (which is the only way we can detect light, there is no other way to observe it), it began to behave differently than if it had not been detected at all. It was as if the light came into focus each time the universe asked it exactly where it was, when without that scrutiny it seemed to relax into something a little more nebulous.

Oddly enough, to me this seems to imply that light is actually more like a probability wave than any discrete particle or wave. Every time he was asked where he was, he confidently provided a definitive answer: HE WAS at this point on the detector, HE WAS NOT at any other point. But without anyone controlling it, light appears to travel in all directions at once, according to certain probabilities. If you ran the experiment several times, you could quantify those probabilities and find that it was more likely to be in the interference pattern bands and less likely to be in the gaps.

But every time a single photon of light was asked, it gave a 100% concrete answer. This is highlighted by something known as the paradox of the three polarizers. Consider for a moment a pair of polarized sunglasses. Obviously, these reduce the amount of light that can pass through them; usually around 50%, depending on the type of lens and the wavelength of the light. They work by being formed by fine chains of molecules that run along the lens. Any light that oscillates in the same orientation as this lens is absorbed. Anyone who is perpendicular to the chains will be able to pass without problems.

The interesting case occurs when a single photon passes through a diagonal orientation to the lens. In this case, not even half a photon passes. Apparently you can't simply absorb the part of the oscillation that is parallel to the lines and let the part that is perpendicular pass. Instead, the photon "adjusts" to one orientation or the other. It is either absorbed completely or passes through completely, but now with a new perpendicular polarization, to match what it would have had to be to pass through it easily. How do we know that the photon did not have this orientation all along?

So what happens when you start adding more lenses. When you place a second lens behind the first, you can block the light completely, as long as the two polarizations are perpendicular to each other. Let's say we rotate the second lens 90 degrees compared to the first. Any light that passes through the first lens has a 0% chance of passing through the second, like trying to send a letter through a chain-link fence. As a result, we only see black. But add a third lens and place it at a 45 degree angle between the other two and strangely the light will start passing through all 3 lenses again.

This may seem counterintuitive: how does adding more blocking increase the amount of light that passes through? But this result actually rules out the possibility that light has a fixed orientation. You must focus with each new lens, rolling a quantum die each time to see if you had the correct orientation from the beginning or not. If it manages to pass through the first lens (a 50% chance), it only did so because it was oriented perfectly perpendicular to the polarization of the lens. Which means that once it gets to the second, it's coming from a diagonal polarization. So once again there is a 50:50 chance that you will make it.

You roll

your

quantum dice again and once again have a 50:50 chance of proceeding. If it also overcomes this obstacle, then it reverts to the new orientation, as if it were that new orientation from the beginning (which it obviously wasn't). Which means it is now diagonally polarized with respect to the third lens, which means it now has a final 50% chance of passing. Of course, some photons do not pass all 3 probabilistic challenges. Only about 12.5% ​​of them succeed. But that's more than 0%, which is what happened before when you only had two lenses. Light likes to behave in discrete quantities.

It's "quantum." It apparently reaches a discrete value when observed. And honestly, we don't really know why. If you think about a wave, there's no reason you can't just have half a wave. You could halve it over and over an infinite number of times and still have an answer that makes mathematical sense. And yet, it seems that at a low enough quantum scale, you can't halve light beyond a certain point. You can't have half a photon, not even one and a half photons. And if you try to do so, the photon adjusts to one or another nearest integer, based on probabilities: but only when asked.

Otherwise, it is content to exist probabilistically, interfering with itself like a wave throughas he travels, before jumping to an answer when later asked exactly where he is. What's going on here? This is still being theorized. The closest comparison we have is something known as harmonics, where on a tied string, only a certain number of waves can exist. On a guitar string, you can have one wave, two or more, but never a number that is not an integer. It seems that the light works the same way. Perhaps something squeezes the beginning and end of the path along which light travels, although what this could be and what mechanisms drive it are unknown so far.

Fundamentally, though, perhaps the craziest thing about all of this is that it's not just about light. Although we have focused on light behaving like a wave and behaving probabilistically, all particles of matter do the same thing. Light is simply another form of energy, and energy and matter are linked. Particles of matter (atoms and even complex molecules) have been shown to have wavelengths. Electrons are just as quantifiable and probability-driven as photons are. Apparently we are all driven by probability, if you narrow things down small enough. So what is everything really made of? What constitutes energy and matter that makes them behave the way they do?

What is happening under the hood of reality? Why does the universe behave differently when you look at it and when you don't? And what does it mean to think that even you are at some probabilistic level? What all this means is anyone's guess. The person who discovers it will be the Einstein of our time. But for now, all we can say is that when it comes to reality, it seems like the universe is playing dice. You and the world around you may be much less safe than you thought. Thus they complete our first experiments by highlighting the strangeness of light.

Take a breath for a moment. Give your brain a chance to untangle. From now on, everything will get stranger. If there's one thing I've learned about light, it's that for thoughtless energy, light seems to love to mess with us. As I just showed you, scientists debated whether it was a particle or a wave, because it continues to exhibit elements of both, apparently unable to settle. Interestingly, it behaves one way when you look at it, but another when you don't. But at least its speed is constant. Light travels at the speed of light. No matter your frame of reference, that thing remains the same...

I have bad news for you. It turns out that the constancy of the speed of light might not be correct either, and the next experiments I'm about to show you prove it. Under certain circumstances, light could travel more slowly than physics predicts. And no, I'm not just talking about light slowing down in denser media like glass, although I originally intended this video to be about. We have an explanation for that. What I'm saying is that, in some circumstances, light appears to travel a path through time and space that makes it go slower or faster than the speed of light, even if there are no dense media present.

But the really strange thing is that it ends up at the same destination in time and space anyway. Let me show you what I mean. Light travels at 299,792,458 m/s. According to relativity, this is the only speed at which light can travel, and interestingly, it seems to stay at that number regardless of your frame of reference. Two people could be traveling through space, one at 1% the speed of light and the other at 50% the speed of light, but if they both look at the same beam of propagating photons, they will see them traveling at the same speed. speed. .

Time and distance would apparently rather warp than allow you to see anything other than light traveling at the speed of light. Of course, when scientists say this, they are only referring to light traveling in a vacuum. We have known for a long time that as soon as matter is involved, light stagnates and travels slower. Light traveling in air only goes at 299,705,000 m/s, 87,458 m/s slower than light in a vacuum. Light in water is around 225,000,000 m/s. The light that passes through the glass covers leaves around 200,000,000. The reasons for this are intriguing, but fairly well understood, and certainly don't break physics.

When light travels through matter, its constantly churning electromagnetic fields cause the electrons within the matter to also begin to move, like boats bobbing in water. But, because electrons moving up and down also generate an electric field, which in turn creates a magnetic field, these moving particles create a second wave of light that crucially overlaps the waves of the original light. , although at a slightly different pace. to the original light (exactly what speed varies depending on the material). When two waves meet, they interfere with each other: they take an average, sometimes they interfere constructively to strengthen each other, and sometimes they work against each other.

So when you take the grand total of all the ups and downs of each wave, you actually end up with a new wave, one that travels at a different speed than the other two and that is slower than the speed of light. Over time, this propagating wave can reach the edge of the blocking material and, without those electrons interfering any further, you are left with just the original light again, which is then free to travel again along its original path to its original speed as if nothing. had ever happened. Scientists have had a lot of fun with this concept over the years.

Harvard researcher Lene Hau in 1999 was able to slow light to an astonishing 61 km/h by sending it through a cloud of sodium atoms that had cooled to a billionth. one degree above absolute zero. Two years later, Ella Hau managed to slow the speed of light to 0, before heating the cloud and sending it back into her path. You may find this result surprising. However, strange things have also happened in the opposite direction. In 2000, researchers at the NEC Research Institute in Princeton, New Jersey, sent a pulse of light through a cloud of cesium atoms. Alarmingly, when they timed it to see how fast the pulse came out of the cloud, it seemed like it came out before it had entered.

While this may seem to interfere with causality, after all, how can you exit a building before entering? – fortunately there was a simple explanation that saved us from creating too many paradoxes. Although the light pulse traveled faster than light, the light itself did not. This was more of an optical illusion than a refutation of Einstein's relativity. Let's take a closer look at a photon of light. As Einstein showed us, each photon represents a small packet of waves moving up and down. The speed at which waves propagate within the packet is known as the phase velocity, while the speed at which the packet as a whole travels is known as the group velocity.

It can also have a wavefront velocity, which is the speed at which the first photon of a photon wave can travel. This is a bit heavy on its terminology, so let's explain with an example. Think of a crowd of people doing a Mexican wave. The wave people are making is the phase velocity. You can see the wave traveling through the crowd, it may appear to be traveling quickly. But the crowd itself is not going anywhere, so the true speed of our wave is 0. These people are the speed of the group, or possibly the speed of the wave front.

Let's imagine that we wanted to send our crowd to march. They could do it and they could still do a Mexican wave while traveling. But although his waving hands could make the wave go very fast in the direction of its travel, it would disappear every time it reached the front of the crowd. The ex

change

of information could not go faster than the walking speed of the crowd, regardless of how fast the peaks of the waves seemed to travel. Einstein in relativity never stated that the phase velocity could not exceed the speed of light. He simply stated that information could not travel faster than light.

And if you're trying to get a message across to someone by sending a crowd of hesitant Mexicans in their direction, it doesn't really matter how fast they're waving. Until the first person in the crowd arrives, no information can be delivered. Still, this difference between light waves and the speed of light itself will be interesting in our next experiment. And this is where things start to get a little weird. Oh, did you already think that was weird? Oh no, this is the part that really defies physics. Let's think about the double slit experiment. There, researchers explored how light can sometimes behave like a wave and other times like a particle.

However, in 2023, researchers at Imperial College discovered a way to separate the slits of this experiment, not in space, but in time. The way they did it was simple. They took a transparent material called indium tin oxide, which under specific conditions can be made reflective. Indium tin oxide is what they use in most cell phone screens. They shot a laser at it and then quickly

change

d the material from transparent to reflective and then back again. This left only a small window (a few femtoseconds) where the laser was reflected. They called this “time gap.” They recorded what the laser looked like after being reflected and found that its frequency had spread a bit in the process, but other than that, nothing too crazy had happened.

The strange thing was what happened when they sent two laser pulses through these “time slots” in rapid succession. The position of the emitter, mirror and receiver remained the same; The only difference was the passage time of the lasers. Oddly enough, when two passed, an interference pattern occurred. However, this was not an interference pattern in the same sense as the normal double-slit experiment in 3D space. This was an interference pattern that affected the frequency of the laser. Certain frequencies of light within the laser faded, exactly in line with the way the intensity faded in the normal version of the double-slit experiment.

To visualize why this might be happening, let's do this experiment in terms of time. The time slit experiment can be drawn similarly to the double slit experiment, except that we will need to visualize the change in the experiment over time. To do that, let's create a 4D graph where space is along the x-axis and time is along the y-axis. This is quite easy to do: it just looks like this: the photon leaves the emitter on the left, arrives at the time slot, is reflected and arrives at the receiver. I drew this as a solid line just to simplify things later, but the idea works just as well either way.

Later, the emitter releases a second photon, it is reflected and reaches the receiver a little later, represented by how it takes place higher up (further in the future) on our time graph. If light behaved normally, traveling at the speed it was supposed to go, this would be the end. Instead, light interferes with itself. This means that you must travel a path that takes you through the other slit as well as your own. This is the only way the light will come in with the pattern we see. And just like the double slit experiment, it's probably happening on the other side of the slits too: As to why it's frequency and not intensity that's being altered here, think about the implications of what you could see if the light really came in. at a different angle like this.

Photons come in small packets of waves, as I mentioned above. Now, see what happens if you change the angle at which those waves arrive. Here's what it would normally look like: I added a black timeline here and highlighted each time the receiver sees a new peak in the wave. This is what happens when the direction of arrival of the wave is altered: suddenly, the peaks arrive much more frequently. The frequency of a wave over time is closely related to the color we perceive of light. Low frequency light is redder in color, while increasing the frequency changes the color of the light towards blue.

So this color variation makes sense. What makes less sense is what happens with the paths that this light takes through time. Remember, the straight lines we started with represent the 299,792,458 m/s that we see light travel. So what can we say about the photons that travel along these paths? During some parts of their journey, they travel slower than the speed of light, and take longer to reach a destination that is the same distance away. And yet, in other parts of their journey, they travel faster than causality should allow. From their perspective, they are traveling back in time.

As a re

mind

er, these two emitters on the left are actually the same, just at different points in time. Itsame for the receivers on the right. It is an amazing result. And yet, according to the results of this experiment carried out by a research team at Imperial College London, this is what is happening. The implications of this are surprising. Light always travels along the path of shortest time: the route that allows it to reach its destination by the closest path at 299,792,458 m/s, the fastest apparently anything in the universe can go. And yet, it seems to me that in its efforts to locate exactly what path that might entail, the light is testing the waters, laying out probes that test whether other paths, and apparently other paths through time itself, might present a more viable solution. . solution.

These sensors interfere with photons traveling next to them, but also with photons traveling slightly ahead or behind them in time. To be clear, we never actually detect photons that take any of these other paths. We don't see photons coming from the future. We never see photons traveling slower than the speed of light, as long as there are no supercooled gases to provide an explanation for why they slow down. And yet, for interference patterns to occur, at least to some extent the light must be trying alternative paths through time. Maybe it's like lightning, trying many different directions to find the optimal path to its destination, before finding the one that works and collapsing down that path in a giant boom, while all the other sensors fade and collapse: or maybe it's some other phenomenon at play.

Who can say? For now, all we know is that the light has proven once again that it doesn't play by anyone's rules. At least, not rules we can decipher. Again, now might be a good time to pause and reflect. This experiment that we have just seen suggests that not everything in the world of physics passes through time in the way we might expect. The light might be playing a little fast and loose with the linear nature of reality. We are comfortable with causality: with the idea that things happen one after another and that things in the past influence things in the future, rather than the idea that things in the past influence things in the future. another way around.

This last experiment could be interpreted as a complication in that sense. But, unfortunately for our aching

mind

s, it's not the only experiment that does this. Well, rest time is over. Can information travel back in time? It's the kind of thing that would be really useful, if it were true. You could tell your past self not to eat that burrito that didn't agree with you, or you could reveal the winning lottery numbers to yourself. But that just doesn't happen; the resulting paradoxes alone make the whole thing ridiculous. In our universe, time always seems to flow in one direction: forward.

The idea of ​​traveling back in time, or even just communicating with your past self, seems so far-fetched that it can't be true. So why at the quantum level does information seem to be doing precisely this? "Alex, stop!" you might be saying. “You have already shown us that the solid universe around us could be nothing more than probability waves, and that light has some strange element that causes it to interfere with other lights in its past and future. But this? Surely it is impossible for information to travel back in time.” I understand the feeling. It goes against all intuition and, according to all indications, does not seem possible.

In previous videos I mentioned that objects would require infinite energy to even go fast enough to reach the speed of light. So how could something go so fast as to reverse the usual direction of time and arrive at a destination not only instantly, but before leaving? Not even light can do that and it's the fastest we know. Well, this rule about the speed limit of causality seems to apply primarily to the macro-scale universe. And by macroscale I mean anything significantly larger than an atom. But at the quantum level, time might obey different rules... or at least, the speed of causality seems to come with some important caveats.

And to demonstrate this idea, we need to look at a man named John Stewart Bell and quantum entangled particles. I must apologize in advance for what I'm going to do with understanding causality from him. Okay, but what are quantum entangled particles? In quantum physics, it is possible to collide two particles in such a way that they stick together, so that by measuring one particle, you learn things about the other. For example, if you know that the particles originally had a total of 0 momentum and you learn the momentum of one of the new quantum entangled particles, you will know that the momentum of the other particle will be exactly the opposite, ensuring that the total momentum remained at 0 Indeed, by measuring one particle, you can learn things about the other.

This also works for other properties of the particles, such as position, polarization or spin. At first glance, there is nothing strange about this. It's no different than meeting up with a friend and discussing our plans for the evening. We agreed to go out and we agreed that I will pay for the evening and my friend will not. So no matter how far we go on our night out, or even if at some point we break up, I know I will pay and my friend will know he won't. This is how Einstein thought it worked. Only it turned out that Einstein was wrong.

Because it just so happens that my friend and I didn't discuss beforehand who would pay. And the strangest thing of all is that we both agree 100% of the time, no matter how far apart we are. This is the strange thing about quantum entanglement and quantum physics in general. We like to think that particles have fixed properties. However, our mind-

blow

ing penultimate experiment shows that particles only have properties when those properties are detected. Yes, it's like the double slit experiment again, only it focused on the position of one photon. It seems that particles are also a little vague about the whole "properties" thing, and instead rely solely on probabilities as defined by a quantum wave equation.

This doesn't make sense intuitively. Looking at a thing shouldn't be what gives it properties... right? Well how would you know? If a tree falls in the forest, does it make a sound? According to quantum physics, not necessarily. Let's talk about the Bell experiment. The math for this is quite complicated, but be patient, it's worth it. This experiment was fundamental to our modern understanding of quantum physics, and closing its gaps earned Alain Aspect, John F. Clauser, and Anton Zeilinger the Nobel Prize in physics in 2022. The experiment was first conceptualized by John Stewart Bell, who wanted to know if the particles really had secret properties that they carried with them, known as hidden variables, or if they were really making some up on the spot.

He observed an interesting mathematical fact about the spin of particles. Before continuing, he should probably mention that quantum spin is not the same as normal spin. Deceptively, quantum spin actually defines whether a particle is influenced (pushed or attracted) by a magnetic field. The name is not important, but it is important to note that these particles are not actually spinning and therefore can have different "spin" values ​​in almost any given direction. Now let's take two quantumly entangled particles and say that we have arranged them so that their spins total 0 between them. This means that if one particle is attracted by a field, the other will be pushed by it by the same amount in that direction (with the understanding that this says nothing about its spin in other directions).

One of the characteristics of quantum spin is that if we measure the spin of an entangled particle in a given direction, say up and down, it will have a 50% chance of spinning up and an equal 50% chance of spinning down. . But remember, once you measure the other entangled particle, it will have a 100% chance of spinning in the opposite direction to the first particle. Based on this fact alone, there is no way to know if the two particles already knew their spin, or if they are somehow deciding it on the spot and consulting with each other now that they have been asked.

But Bell noticed something clever about asking a clever question. If you were to measure two quantumly entangled particles from two randomly selected directions, what are the probabilities that their spins in different directions match? Let's define that every time a particle spins toward a detector, its spin is "up," and every time it moves away from a detector, its spin is "down." What are the probabilities that both particles will spin “up-up” or “down-down” when tested, and what are the probabilities that they contrast? Let's formalize this with a little experiment. Here we have two entangled particles, with three detectors reading their spin in different directions.

If both particle A and particle B are read with the upper detector, then one of their spins will be up and the other down. They are tangled and this is what we saw earlier. However, if particle A is read using the top detector, while particle B is read with one of the other two, these two spin directions are not opposite, so particle B has more flexibility in the direction it take. Quantum physics states that particles are making up their attributes on the spot, so once the spin of particle A was measured using the top detector, it was 50:50 if the spin of the other particle, using one of the other detectors, it would coincide or contrast.

But this is not what classical physics predicted. Let me show you what I mean. Classical physics states that each particle carries secret information that defines its spin in a certain direction. So for our 3 tested directions, each particle would already have a value. They're not making it up on the spot. Let's hypothetically say that the hidden information of our particles says "Up, Up, Down" for particle A and "Down, Down, Up" for particle B, since B must be opposite A for each of the directions 1, 2 and 3. Let's choose a random detector for A. We select detector 1. Detector 1 tells us that particle A is spinning.

Now let's select a random detector for particle B. We select 1 there too. This detector gives us a Down reading. In fact, we can plot all possible outcomes of this random selection process on this graph. There are 9 possible outcomes if you only measured from two detectors at any given time: 1-1, 1-2, 1-3, 2-1, 2-2 and so on. For each of these possible selections, we have set results for hidden variables that we already know, because we hypothetically defined them previously. Let's complete them now. Of course, if you test particles using the same detector on both particles, you will get a contrasting result because they are entangled, but we are not interested in these results.

Both classical physics and quantum physics agree on this. So, let's eliminate them. What are the probabilities that two different detectors for particles A and B will obtain the same result, and what are the probabilities that they will differ? Remember, quantum physics expected it to be 50:50. The particles take their values ​​on the spot, so it is perfectly random which one they will choose, since they are not confined by the rule of opposites. But in this table, classical physics says that contrasting results only occur one-third of the time. The other times, they are both up or both down.

If we do this many times, assigning different directions each time, and ignore the exceptions, for example when the particle spins are all Up-Up-Up or Down-Down-Down - once you do the numbers, the important thing to take It follows from all this that, according to this mathematics, classical physics predicts a coincidental result 55% of the time, while quantum physics continues to predict only 50%, pretty table be damned. This percentage difference was key. By quantum entanglement of particles and running this test over and over again, you could now see what percentage was correct. And it turned out that the winner was quantum physics.

Apparently, the particles were making up their spin results on the spot. Which is creepy. Because that not only calls into question our perceptions of reality itself, but it also means that the moment a particle decided the outcome of its spin, its quantum entangled partner instantly knew that decision had happened. You could try both particles at the same time, regardless of the distance, and you would get the same result. Somehow, the information had traveled from one particle to another in a very short time, much faster than light itself. So something strange was already happening here. This result refuted Einstein's predictions and showed that some information appears to travel faster than light.

But we can go one step further and have information that goes backto the past. There is another experiment, our last experiment, known as the “delayed choice” test. His main goal was to explore the fundamental nature of light, whether it was a wave or a particle, and discover when it decided to be one or the other. However, unlike the double slit experiment, this test focused more on the last part: trying to identify the moment when the waveform collapsed into something discrete. In the double slit experiment, light seemed to choose a different path through space depending on whether it was observed or not.

In 2006, several scientists asked an interesting question: what would happen if you tried to observe light after it had to choose a path? Consider the experiment: A single photon is sent to a beam splitter, with a 50/50 chance of being allowed to continue its path along path 1 or being reflected along path 2. Once in Either way, the photon bounces. outside the mirrors, with both paths reconverging here, where another beam splitter is inserted. Once again, the photon has a 50/50 chance of going in either direction, with an equal chance of reaching one of the two detectors. If light were just a particle, sending a single photon into this experiment would give it the same chance of reaching one detector or the other.

You wouldn't be able to tell which direction it went, since the two beam splitters make it impossible to tell, but you could see where it ended up. However, this does not happen. When the second beam splitter is present, the light produces an interference pattern, indicating that the single photon traveled both paths and eventually collided with itself, before passing to both detectors. This seems like compelling evidence that light is a wave; It certainly behaves like one here. But what happens if you remove the second beam splitter? Suddenly you know which path the light traveled: if the light reaches the top detector, it must have arrived via path 1.

If it reaches the side detector, it must have arrived via path two. And something in this knowledge scares the light. It stops following both paths and suddenly each photon only reaches one detector. Here's the question: what happens if you insert the beam splitter after the photon has already started traveling one or both paths? That is why the test is called “deferred election.” If you delay choosing exactly how you intend to detect the photon, either knowing which path it followed or making it ambiguous to you, what happens to the light? What happens is something very strange. When this experiment was performed, it was done several times, inserting or not inserting the beam splitter randomly, but always inserting it after the photon had entered one or both paths.

And yet the results were unequivocal. If the beam splitter was present, the photon suddenly, and apparently retroactively, stopped choosing a path. If the beam splitter was removed, the photon apparently knew it would later be detected and chose a specific path to adapt. In some ways, the beam splitter being added or removed in the future changed what the photon did in the past. So what is going on here? Is it really true that particles somehow saw the future? Did the experiment cause information to be sent back in time? Or is there some other principle at play here that explains this whole thing? which explains the instantaneous transmission of information between quantum particles and allows it to be perfectly rational that light can travel along one path or both at the same time.

Personally, I'm inclined to think this is more likely. We clearly don't understand what's going on here. But you have to admit it; If we don't understand what is happening, there is nothing to say that causality is not ignored. In some ways, maybe at the quantum level time really is more fluid than here in the larger universe. Maybe space and time just don't apply down there. And maybe one day someone will be able to come up with a theory that will allow all of these strange phenomena to finally make sense. Until then, we'll have to keep asking ourselves the same question: can information travel backwards in time?

Until then, we'll all have to agree on one thing: quantum physics is weird.