The second experiment (the "Delayed Choice"), does not necessarily show communication backwards in time either. It shows the same as the first experiment: there is some form of coordination that happens instantaneously. In the Delayed Choice experiment, the photon always takes both paths, also when there is no beam splitter in place. It is just that all the other possible paths disappear instantaneously when the photon is detected. That is the "normal" QM quirk, called the measurement problem. Measuring a photons collapses all possible outcomes into a single one.

I remember when you questioned how we'd feel about you touching on topics other than planets outside of earth yet in our solar system. I'm happy to see you expanding your options for content

Just listened to Einsteins biography audiobook. The one sentence stuck with me:" even Einstein's mistakes are better than today's newest findings."

I love that you have branched into incorporating theoretical physics and quantum topics. Huge fan of everything you put out, thanks for teaching me so much🤙

it helps to look at the photon from einsteins perspective. for the photon itself there is no time passing. the entire path was taken in one moment. the photons moment is projected into the world we experience as a line. but from the perspective of the photon there is no before or after in relation to points on the photons way. the entire way is one single moment for the photon. so the 2nd beam spliter is either present in the moment or not. what we experience as paradox can be simply explained by the relativity of simultaneity

Sounds like you're describing the Grand Old Duke of York. _"When he was only half way up, he was neither up nor down."_

Thank you so much for this my entire life I thought spin meant the partical was actually spinning this makes so much more sense now, thank you so much

When I first started studying QM my reaction was "what's going on? this doesn't make any sense" but now it's been 10 years and my reaction is "What's going on? this doesn't make any sense"

Sabine Hossenfelder made a video sometime in the recent past where she explained why the delayed choice experiment is not sending information back in time.

Some of my ex girlfriends seem to possess this ability.

The way you described information travelling back to the first splitter reminds me of the time travel premise in the Primer movie. The box has to be turned on in order to travel back to the moment it was turned on.

This was SUCH an incredible video. Really appreciate you explaining some of the basic principles around quantum physics and entanglement. Before watching this video, I didn't realize that I had the "classical" understanding of quantum physics, and you allowed me to understand things just a little more deeply. As a complete layperson who finds himself fascinated with trying to learn more about this field, that means everything to me. Thank you :)

Loved the slightly longer video, and the topic was very interesting as well. Keep up the good work!

In the delayed choice experiment, the first beam splitter creates two probable paths for the photon quanta to take with the second beam splitter allowing the two probabilistic photons to interfere with its probabilistic wave before being detected as an interference pattern. If you remove the second beam splitter the photon quanta still travels down both paths with oscillating probabilistic wave; whichever detector that is activated is the photon quanta that had the highest probabilistic wave at the time. So if you add the second beam splitter after the photon quanta passed the first beam splitter there is still two probabilistic paths that the photon quanta is taking allowing it to interfere with itself. This doesn't need backward time travel to explain it.

Thank you for breaking things down in an easy to understand way.

the thing that comes to my head is we need to keep in mind that the "time" that we observe works the way we feel only for us, there are relativistic effects happening, like in the twin paradox or lifespan of particles - generally we know that if we go very very fast, a long travel would feel for us like a short one, cause space looks like it shortens a lot (but only for us, not for a stationary observer) - so technically when something moves exactly at the speed of light, the path shortens to infinitely small (zero?) and time taken for the travel does too, so... what if for us the photon took for example 1 nanosecond to travel through the detectors etc, but for the photon everything happened at once, so there was no "time" to "know" that we add or remove detectors - for us it seems like we interact with the photon but from its pov it all happened in one plank's time length, so it "knew" how and if to split or not because it's like it saw the whole situation not like a movie as we observe it, but as A SINGLE FRAME ;o the life we know can't exist without time but if photons can, we have a new phenomenom to understand

I saw this video and immediately decided to subscribe to your channel a couple of years ago. Interesting stuff, as always. Thankyou Alex.

Scary stuff, we should go deeper into it

As usual, I still have no more understanding of Quantaum Physics, thanks to my tiny feeble brain. But, thank you for presenting another incredible video that was entertaining and fun to watch, regardless !

Re. the double 50-50 experiment: have scientists tried starting the experiment with two of those 50-50 mirrors, but remove the second mirror after the photon has entered / begun?

Another fascinating and thought-provoking video. Thank you for using your platform to spread knowledge!

My brain shorted out a little from that ad at the beginning. I actually watch a lot more Warframe content than astronomy/science. :)

You just earned a subscription. Great job! I actually didn't know about the 2006 experiment. That blew my mind!

Thank you so much. Your presentations of Bell's inequality and of the delayed choice experiment are outstanding. I have just learned something fundamental about the strange world we live in.

Way too hard for me to grasp, but loved your explanations anyway!

What explains this is measurement. With the first splitter you’re collapsing the wave function, with the second (if angle of impact is complementary) the light is returned to “close to its original (unaltered) state” which is “wave” A filter, or observer, is merely the elimination of redundant or meaningless and even sometimes contradictory information (like opposing waves that cancel each other out)

Gotta love youtube recomendations, one second you're chilling, the next you're learning about quantum theory.

Thanks Alex. That went some way in improving my understanding about this. Excellent explanation

so you oversimplified the DCQE experiment (because it's too complicated to explain in a video that also covers Bell's Inequality/EPR) to the point that refuting acausal information is impossible. But the answer is no: information does not go backwards in time. What you showed is more like the Elitzur-Vaidman bomb-tester experiment, which is weird--the experiment that is.

omg one of the best videos on the topic iv seen. congrats man, really good analogies and really good explanation when there simple shouldnt be an analogy!

Its not rewriting the past. Its revealing which branch of the past you took to get to the present.

Good video! Also, the Duviri Paradox is going to be incredible!

Awesome video & topic. Thank you for sharing this that was very interesting. I loved it. ❤

I'm really starting to like the idea that particles are activated and deactivated. Meaning that particles themselves don't actually travel but transfer their energy to an inactive particle in an assembly line like fashion, as one particle is activated the previous particle is deactivated and we can only detect the particles while they're active with energy. Using this logic could explain why light looks like a wave to us at certain times and a particle at other times, the wave is the energy activating the particles around it and the particle itself is what interacts with us.

If time stops at the speed of light, it’s not really time travel. From the light’s perspective, it just goes backward and forward in time equally. It’s only us non-light-speed travelers who marvel at photons going back and forth in the timeline. For the photon, it’s just a day at the office.

What if the particles spin in these directions: ↑↑↓↓←→←→ for B and then A starts?

Freaking fascinating. Nicely done.

Entanglement is like going to see a tree in a field at some point in time and establishing the reference that it is standing. The equivalent of you and your friend agreeing on who pays. If you learn later in some way ("observe") that it has fallen, you can deduce (by "entanglement") that it would have made a sound to a nearby observer in that time frame between when you saw it and now ("uncertainty"). A bit abstract but makes the key point that in acquiring the information, nothing "magical" happens, it appears "instantaneous" because the realization happens "now", but the states were and always have been predefined (at creation), you just hadn't observed either of them yet.

Bell's work has additional underlying assumptions, like statistical independence of certain aspects of the events. Those assumptions could be violated instead of locality.

This makes me think we're going to find out that the universe exists in all moments at once, that everything we see sequentially in time is a perception we are bound by.

It's truly incredible that the wave pattern observed in the double slit experiment is due to the interference of the probability of which slit the particle will pass through. Probability interferes with itself creating a wave pattern.

Now when I first heard of the delayed choice experiment, it had a much more, "Spooky action at a distance" affect. I don't remember all of the details as to how the experiment was constructed, however the jist of it was that the "observer" would not get any information until after the photon had reached the screen at the end. If I remember correctly, they may have created an entangled pair of photons at the start. The result was that the implication of the photon going back in time to inform itself that, "IT'S A TRAP"!!, seemed much more apparent. Either way, excellent topic and video. Thank you for sharing.

what you're saying is... quantum entangled waves represent the function or method, showing possibilities, until the output is requested. So we're in a simulation.

One of your best vids so far, wonderful job!

I'm curious how they measure if a particle is entangled or not to know if they are when calculating the Spin of different directions.

Would you consider doing a video on the Milky Way and the Andromeda Galaxies colliding? That would be an Awesome Video, Thank You!

After watching this a second time, I suspect that what's going on in the delayed choice experiment is that the photon is *_always_* going down both paths, but if the second beam splitter is not inserted to recombine the wave, then it can't hit both detectors and "pretends" that it took just one path by only being detected at one or the other!

Well done. I follow a decent bit of this stuff as a layperson but this was trippy for sure.

That would mean the information still persists until it hits the detector, and interacting with that information will cause it to collapse. This makes more sense if the wave and particle are separate like pilot wave theory suggests. Then you have waves radiating away from the point particle in all directions, interacting with that wave anywhere along the path before it hits the detector would change the behavior of the particle. It would also allow a single particle to interfere with itself through a double slit.

I love these educational videos!💚💚💚

Didn’t expect a Warframe sponsor. Not a bad choice of a channel to sponsor

The image of the burrito at the end without acknowledging it was hilarious

Your breakdown of the experiment was very good.

Very interesting. I cant say that I have a great grasp of quantum physics but from the small part I do know and from videos I have watched, its all fascinating. I really think that we are missing quite a lot here. The universe is such a beautiful crazy place.

Point a to b. what happens is a probability wave trees to point b as a wave. When measured at b. The wave is broken and it becomes real. Absolutely mind blowing! Love the stuff!

My brain was fighting to keep up with all the information near the end of the video only to get thrown off by the burrito at 17:50. Something about time traveling, cool video 👍

It doesn't have to choose between taking one path or both paths, right? The photon is always in a superposition of taking both paths no matter what you do before or after it passes the first beam splitter. However, once it arrives at the detectors, the superposition collapses (or expands to encompass the detectors and you as well) so that only one detector detects a photon I don't see the difference to the double slit experiment, there the photon also only hits one spot on the detection screen despite taking multiple paths beforehand

When I was first presented with your three detector positions for the two particles I thought they were arranged on a plane. But I think in order for the other two detectors to still have a 50% chance of each result, they would all have to be at 90 degree angles to each other, as in oriented along the X, Y, and Z axes, respectively. (Otherwise, if they were at something like 120 degrees or just any old random coplanar orientations, I would not expect a 50/50 chance for the other two detectors to get each result after the first particle was "measured".) I'm not sure if I missed you saying that this was the case, or if you did not say it (perhaps assuming it wasn't important or that we'd figure it out on our own), or if I'm simply wrong and *any* difference in detector orientation resets the probability to 50% (which does not seem like it fits with what I know about QM, especially considering how polarization works, etc, and also, how precisely aligned would the detectors then have to be in order to be "sufficiently aligned" to get the "spooky" entangled opposite result? Therefore I doubt I've got this wrong, at least).

Thank you kindly for having non-adsense at/near the very front/back/both of video, rather than interrupting like TV once the flow has set in. It suggests mutual consideration for the creator/viewership relationship :>

Anyway, could a photon's passage through a beam splitter not affect its spin and therefore also the spin of the particle not yet arrived? Would this affect the choice of direction through the splitter for the second particle?

This is your best video yet. Very well put together.

What if observing a particle doesn't create it, but summons a predefined one that exists outside of time... or on the 4th dimension. Much like a circle will pop in and out of a 2d world when a 3d sphere passes through it, a certain version of the particle shows up when it's 'observed', making it seem like it was decided on the spot, when in fact it already existed outside the 3d plane, but it just had no reason to appear. Idk just guessing.

It has been explained by a paper 2008: "Explanation "Causality as an emergent macroscopic phenomenon": The Lee-Wick O(N) model 2008". The explanation is more mindbogglingly strange that first imagined, both spacetime and causality are emergent properties of quantum fields.

There is another explanation that also fits and does not require the future editing the past: super determination. Everything is determined by the interactions that preceded, and “random” is just a quantification of the information that is not available to the observer.

I've had personal experience with "information" being passed back in time without any paradox on multiple occasions. Several times in my life I've experienced strong phantom pain in areas of my body that would months later have a reason to feel like that. The First Time it happened my wrist suddenly spasmed in pain for a few days then about 3 weeks later I broke that wrist in a BMX stunt gone wrong. The last time it happened was the scariest as it resulted in hospitalization and overnight observation with severe chest pain and sharp pains to my inner left groin. Was released clear of heart complications with doctors baffled as to the cause of my chest pain, something that was repeated later down the track. Cue Xmas day and an electric RC car cable of reaching 70km/hr with Broken front suspension and steering. Being unaware of the previously mentioned problems I attempted a manoeuvre that was meant to be driving from the middle of a park to the gravel road Infront of us that was meant to be a slide turn into a donut on the gravel. Instead it plowed into my left ankle tearing my groin in the process flipping me in the air landing on my left side breaking 4 ribs, my hip and crushed the bursa in my shoulder. 2 of the rib breaks were displaced fractures of the 4th & 5th ribs fraction of a millimetre from the bone cartilage join, to close for a normal see. Come to think of it most of the Temporally Referred Pain (TPR) I've suffered from seems to be self inflicted.

How can you insert emove a beam splitter faster than the light traveling between? I accept the results but don't understand how exactly this experiment is conducted. Perhaps a video on that would be helpful?

5 years ago I was working at Caterpillar in NC. I looked at the "accidents since" board: It said 122, but I saw: "288" then it was only 122. I told my manager; he wrote it down. We all forgot about it. Then months later I saw "accidents since" board report ZERO. Caterpillar was entirely focused on how this accident happened and took everyone on a tour of how an employee broke her arm in the parking lot on that day: Day 288. My boss was scared of me and everyone else after that. I get lots of these premonition. The woman was walking in the back lot - there was a 1mm crack on the road: Her shoe grabbed the leveled crack and throw her on the parking lot. They then trained us all on how to avoid us from falling down if you foot hits a 1mm crack in the floor! Gulf War Vet: 1990-1991.

I think the amazing part is light seems to react the whether or not we are measuring it. But if you take that as a given, it actually makes sense that light changes its behavior if it knows the end result is being measured. Light is moving at lightspeed meaning there is no experience of time. There is no before or after for the photon

In other words, if you can control which way the particle spins, you can communicate faster than the speed of light, assuming the other particle is where you want to communicate to. Because it inherently knows what the other particle is doing, all you have to do is change to rotation back and forth in Morse code. I imagine in the future, submarines will use quantum Morse code to communicate. Furthermore, I can’t imagine that you could intercept a quantum message like that making it even more valuable.

Great video explaining those experiments. I have a question. How the heck did they add a beam splitter after the photon was sent? That's some faaast sleight of hand! :-)

16:22 It is generally understood (perhaps erroneously, perhaps not) that a photon does not experience the passage of time. This implies that a photon must be everywhere at once. That is to say, in this quantum physics experiment, the photon exists on all points on the device at the same time, making up its properties on the spot whenever asked by an observer, and the only thing actually being measured is the "light" that the photon generates, not the actual photon. This is important. If the photon is asked about its specific properties as a "particle," then it behaves like a particle, until it is no longer being observed (for instance, the measurement has ended). So, if this is true, and the photon is actually in all points in some kind of quantum state across the entire device all at the same time, and you try to trick that photon by adding a new light splitter while the photon is already on both sides of that splitter (but before the photon's light is visible at that spot), then, yes, I would say the photon has adequate reason to behave strangely under these conditions. I would further go on to say, no, the photon does not send information to the past. The photon, if it actually IS already on every single point on the detector, would instantly know that a 2nd splitter has been inserted, and would change the properties of its "light" accordingly. We need to start asking bigger questions, like, are we actually measuring the photon, and do we actually even understand the photon, or are we just looking at the tip of the iceberg (the light of the photon)? Or I might be crazy.

I think the beam splitter is acting like a double-slit. Think of it. In splitting the beam, it makes the wavefunction go to both detectors, causing an interference pattern.

Alex, you are GOLD. Such high quality voice, presentation, script, delivery and visuals. You should be on mainstream TV, your show would make you a very wealthy man indeed - or perhaps you already are?!.. You are a gift to us, and by far one of the best channels covering and explaining these complex subjects for people with basic to intermediate knowledge. Thank you man.

but why is this such a big deal? Aren't quantum particles supposed to travel at the speed of light? If so, they don't experience time, so from their perspective, no time has past whatsoever.

Yes, time travels in both directions at same time. Take a key ring with key on it. Put another key in, but before continue put key already on ring in also. Then complete. One goes on and one comes off, yet both traveled exact same path in the exact same way. Phone signals internet, electric power lines, tire pushes against road, road pushes back. Energy flows both ways.

As a Tenno, I thought I accidentally clicked on a Warframe related video when Warframe logo popped up haha

if they are making it up on the spot then by rechecking the particles repeatedly you should get different results with each check. you can't ignore exceptions as they are still results.

What this suggests to me is that the particles whos spin is measured are BENEATH Some foundation of physics. Like how dark matter doesnt interact with baryonic matter a similar situation occurs with entanglement.

We must keep in mind that electricity and light are exited states for electrons moving in a field. Light is not the photon, electricity is not the electron, Both are the flux of energy mediated by electrons. The same as a wave moving through water, the wave is not the water, the wave is energy.

It's almost as if there's some kind of conspiracy, some kind of intelligence that's keeping us from discovering or accurately measuring things at the quantum level. But that would be crazy.

In the past humans have often believed our sight was a sense that threw a mold over the world and allowed us to perceive it, like something invisible until you throw paint on it, revealing its location. It'd be interesting to know for certain that theres truly no offect in us looking at a particle

Wow this is crazy. Imagine what we could do with this.

A coin has 2 sides. It's in a quantum state until I flip it and it lands either heads up or tails up. As the sides are entangled ( opposites) it makes no difference if the coin is a thin dime or a thick nickel. Once it is known which side landed up, we can predict with high certainty which side is facing down.

Quantum Theory sounds a lot like time and perception. "So long as it is being perceived, it is moving forward." but that leads to the question, "what is needed to perceive time?" And this theory might just be telling us that WE are not the answer but a byproduct. We perceive time as only moving forward because we are "attached" to this universe at this time. Or the second experiment is just acting similarly to what lightning is doing: Every bolt has branching paths but the only path that matters is the one that hits first, which causes all other paths to go backwards to strike with the bolt, all happening faster than the eye can see.

It happens. He started all of this. Give him credit. We learn new things all the time

If you crash two billiard balls together the total momentum before is the same after. if you look at the momentum of one ball after the collision you know what momentum of the other without looking at it. What's the difference are the billiard balls entangled 😂

Thank you for the video. There is a hypothesis - a single picture of the universe: When moving and fluctuating in a vacuum, the electromagnetic field in the nodes - Forms quanta of gravity - Which carry the speed of light. Suppose - Can be detected using a mobile, new hybrid - the experience of Michelson Morley, if it is in motion relative to the DGP - the dominant gravitational field, for example in 🚆, as in Einstein's mental experience.

Communicating with the past is doable-- so long as you could aim a coherent particle beam at where your target WAS. Sincs the Earth spins, orbits the sun, and the sun moves, inside a galaxy that moves, you would have to find a VERY precise point in space where your particle beam target WAS. Obciously this makes one hour ago magnitudes easier than one year ago....

Extremely fascinating! From another of my topics of great interest, I draw a parallel. Virtually all those who have experienced a near death experience (NDE) (and let's just take the hundreds that can be documented as having truly died and then verified information they could not possibly have sensed during their deaths - like detailed events outside the operating theatre) indicate that time and space are illusions and that this 3D world we "live in" is itself, an illusion. They all insist that this world serves as a sort of simplified stage on which we play a role for a short time in order to learn. When you think about it, learning can only happen when there are consequences, and these can only be experienced in a realm with time. If these NDE'ers are right, then our detection of sub-atomic particles is nothing more than us detecting bits whose real nature is beyond our own limited 3 dimensions and time. Much like a 2D person from flatland (in Abbott's great analogy), when encountering a 3D sphere passing through his world, see a dot becoming a circle, growing, then shrinking to a dot and then disappearing and thinking it utterly amazing. The only reason we think it strange when subatomic particles seem to be independent of time or space is that we are constrained to our 3 dimensions and time. We know only that world. But I have a hunch there is much more... If only science would borrow from other areas of study instead of focussing on their own siloed topics and methods. NDEs are now so well documented that they are the becoming the popular study material of numerous doctors and philosophers - something untouchable a mere 20-30 years ago.

Einstein: God does not play dice God: 🎲🎲🎲🎰

If a tree falling in the woods doesn't make a sound, then it never fell.

Thank you for breaking things down in an easy to understand way.. Thank you for breaking things down in an easy to understand way..

Quantum mechanics to me is the physical proof that this world is created by an intelligent designer. Call it god or the simulation programmer or whatever.

What's really crazy is that all these geniuses at colleges across the world can KNOW all these intricate theories and yet can't give you the definition of a man or a woman. Really makes you wonder.

According to our current understanding of physics, the transmission of information backward in time is not supported by any known physical laws. The concept of information traveling backward in time raises significant paradoxes and contradictions, such as the famous "grandfather paradox" where one could potentially alter the past in a way that prevents one's own existence. In classical physics and in our everyday experience, causality flows forward in time, meaning events in the past influence events in the future, not the other way around. While some theoretical physics concepts like closed time-like curves or wormholes have been proposed as potential mechanisms for time travel, they remain speculative and highly theoretical, lacking empirical evidence or practical application. Thus, as of our current understanding, information traveling backward in time remains purely hypothetical and is not supported by the principles of modern physics.

When I first heard of this family of delayed choice experiments, and the apparent implications for time, I was quite worried it appeared obvious to me that particles aren't travelling backwards in time and only something like super-determinism would make sense of this apparent time travel. Fortunately it turns out that there *are* quantum physicists who "clearly do understand what is happening here", and I would strongly recommend @SabineHossenfelder 's video "The Delayed Choice Quantum Eraser, Debunked" for anyone who wants to get their head around the subtlety of what's going on.

The backwards time aspect on the second experiment might be due to the photon traveling at the speed of light, maybe that one theory of photons existing for the entirety of their path is true, so something altering the previous section of their path could affect future travel across that path, since it's still within the timeframe of the photon's existence.

I tried googling what a 'detector' was in quantum mechanics, and it broke my mind

Thank you for explaining how infinite improbability drive works. Remember don't panic.

How to travel "faster than light": Create a kugekblitz black hole. Encase the bh in a box. Attach bh box to your starship. Feed small amounts of matter into the bh to generate heat and thus power. The gravitational effect of the bh slows relative time inside the starship, allowing the crew and vessel to travel to another star system in what appears to be a very short period time.