This is a supercut of my Weird Light series, so you may have seen some of this content before, however it is now in sequence, with sponsors removed, and all the episodes tied together seemlessly. Enjoy!

The Young double slit light experiment got me hooked on physics and taught me not to completely dismiss things in life that seemed unintuitive.

I woke up at 2am to use the bathroom and go back to sleep. randomly decided to watch a few YT shorts and now here I am, wide awake and fully intrigued in your video. This is the kind of rabbit holes I don’t mind jumping down. 😂😂

This video is truly a gift. I wasn't expecting to watch until the end but got so hooked that didn't want it to finish even after one hour. I learned so much in concepts that I never thought would be able to grasp and were made so comprehensive. Appreciate the effort in doing it!

Incredibly made video, you have explained in simple terms concepts I never thought I could understand. That lightning explanation is such a beautiful analogy for the time slit experiment!

I did a small study on light for my reef aquarium and the symbiotic zooxanthellae algae many corals need to feed on. Coral bleaching is _not_ caused by too warm water, it is caused by lack of light that starves the algae and it doesn't take much light blocking pollution to do it. But the wavelengths of light that was needed for corals tended more towards the blue range as blue light has the most energy. This was before the availability of full spectrum LEDs and we used a blue actinic fluorescent bulb. You look at colors underwater and the first light to go is red as it has the least energy. Have you ever seen water off a boat that looked green, but when you put your hand in the water it was clear? That can tell you the depth of the water you're in is about 30 feet and the light reflected back to you is green as well. 60 feet is about as deep as green goes, then it's all blue and purple is the last color you see. You see the same effect looking at the side of a thick pane of glass. All this about light and yet there are no green stars, but their light is a result of temperature. All those bleached out corals have recovered by the way and are doing fine.

So basically we dont know sh't?

Just wow, this video completely blew my mind. The temporal double slit baffled me entirely and left me with the question if light has his own velocity.

I almost didn't watch it because i foolishly didn't think I'd learn something and didn't want to spend almost an hour to find out. I had not heard of 2 of these experiments, but more important than that, the ones i have heard of were explained here better than I've seen before and i felt i learned something from all of them. Incredibly well presented! Bravo

I think light is like elastic, but instead of stretching in space, it stretches in time. Every photon is tethered at one end at the beginning of time, and at the other end the end of time, and is stretched, like a rubber band, between these two points, but it simultaneously exists along the entire timeline.

Hi Alex, greetings from Oxford. I’m 58, I am fascinated by physics. The duality of particles is mind boggling. Anyone who says they understand quantum physics/mechanics is lying, either they have a superficial understanding, or none at all! Great minds have pondered this perplexing behaviour for decades. Then there is superposition and entanglement. What we do know is that the mechanisms that run our universe are currently incomprehensible to the human mind.

I've got to watch this a few times, and sleep on it before I've got a chance of getting to grips with the problem. A great post, mind blowing.

Chapter Timestamps: 0:00 Prologue 1:18 Intro 2:55 #1 Young’s Double Slit Experiment 5:12 #2 The Photoelectric Effect 7:18 Single-Photon Double Slit Experiment 11:14 #3 Three Polarizer Paradox 14:35 Harmonics & the Probabilistic Nature of Reality 18:15 The Speed of Light? 22:12 #4 & #5 Hau’s Light Speed Experiments 22:45 #6 NEC’s Light Speed Experiments 25:42 #7 Temporal Double Split Experiment 31:14 Startling Implications 33:44 Can Information Travel Backwards in Time? 35:20 Quantum Entanglement 37:28 Fuzzy Properties 38:22 #8 The Bell Experiment 45:52 #9 Delayed Choice Quantum Eraser 50:58 Outro Alex, you can paste these timestamps into the description to create Chapters in the seek bar.

My brother and I don't get to see each other a lot anymore. And our lives have changed a bit so it's harder to find things to talk about/relate too. However, it's often that one of us will watch an episode and find it so interesting we have to call/text and have a conversation about the video and topic.

Awesome video! Truly one of my favorites in a long time now. Thank you for putting together this high quality content. Liked and subscribed to your channel.

Officer: "Do you know how fast you were going?" Electron: "Just a moment, let me check..." Electron: "3,141Km/Hr." Officer: "You just made that up, didn't you." Electron: "So?" Officer: ...

Dude I swear to God, it is 3:37am at the time of writing this, and I just picked up a burrito from a place I go every now and again. I was just about to take a bite of it at the 33:54 mark of the video and I legit stopped and put it down. I'm not eating it. That literally exploded my already exploded brain. That was the most synchroninistic thing I've ever experienced in my life... 😂😂😂😂

Re-watching reference: Warning 0:00 Preface - 1:04 Double Slit Experiment - 3:22 Photoelectric Effect - 5:26 Hey, I can still see the letters :) - 12:51 Interlude - 17:36 Light Speed is not Constant 18:30 Three-Polarizer Paradox - 11:22 Hau Light Speed Experiments - 22:17 NEC Light Speed Experiment - 22:53 Break Time - 33:00 Time Slits Experiment - 26:03 (someone should transcribe the results of this experiment into the visible light range so we can see how the frequency is affected before our eyes) Bell Experiment - 38:26 Delayed Choice Test - 46:14 Btw light might just be the result of EM waves interfering with itself as well as the waves involved with the observance of it, and at these heightened moments of energy the overlapping is perceived by us as light being a particle. Or perhaps it's analogous to a reflection, like when the sun catches you off a car's windshield Is this a reupload? :)

i used to work with cement. its obviously quadrillions of particles. but it also acts like water when theres a large amount of it. it can produce waves when severely disturbed. you can even drown in it if you fell into a silo of it. so perhaps massive amounts of very very small particles can act as waves. that would make photon particles in bulk act as waves. so perhaps light is waves of particles

Viewing all the fundamental particles and light as each being it’s own field takes out all the strangeness honestly. There is non”spooky action” at a distance when it’s just one entire field connected to itself.

Keep in mind that while we observe the Photon to travel at the speed of causality, from the Photons perspective the "speed" is Infinite. no time passes for the photon itself. the photon cant mesure any time between its departure and arrival. departure and arival happen at the same time therefore if you send 2 photons at different points in time that are close enough to each other they can interfere and seem to interefere with the past because WE experience time.

I used to work with radar and I found light to be one of the most fascinating and complicated subjects.

This videos taught me more than any of my school and college physics about light. Brilliant. Keep up the good work.

Amazing explanation. Loved it. Will watch it multiple times again to fully comprehend

There isn’t a word that accurately describes how cool this video is.

Thank you for making this supercut! :) i got stuck on one of them months ago but now, seeing the bigger picture, even i can understand:)

I have seen so many videos on this subject. This one nicely summarises the main experimental outcomes. There is still so much to be understood with quantum mechanics.

This is brilliant! Stunning presentation! Quantum entanglement is truly mystifying, and it is making me wonder if there may be a field that is even more basic than space-time onto which everything else emerges.

You are a great teacher, Alex. I don't know what you do outside of UKposts, but you have a wonderful knack for explaining the absurdity of the universe. I thoroughly enjoy your videos.

Great video, loved how you explained so complicated experiments!

Could you please explain how two random particles are entangled or known to be entangled for the experiment? And can there be entanglement between more than two particles? And which particles are these?

Excellent video, great explanations! Loved it. Many thanks

After doing physics many years ago at school and always had an interest in this stuff I’d recently started to wonder just how long is a photon. I found a great video by HuygensOptics puzzling the same question, he did a fantastic vid about this explaining about the dual slit and duality. Basically the photon is actually huge, it is definitely not a point like particle, it’s a wave that occupies a large area as it propagates through space, I.e. the electromagnetic wave is oscillating in two directions, if this is interrupted it then collapses to a point. This explains why a single photon can pass through both slits and interfere with itself. Watch his video it cleared up a lot for me, but as we all know light is insanely nonsensical

This is one of the best videos. I love this guys grounded yet wondrous infatuation with the universe.

Surface Interaction, is the process by which molecules or atoms on the surface of a material interact with external stimuli, such as light, other particles, or electromagnetic radiation. It is thought to be the prime reason behind light’s behavior, in the double slit experiment.

When I found out about the double slit experiment, my mind was blown so blown like a nuke blown! 🤯🤯🤯

Thank you, Alex! I'm glad I can watch this over and over so it'll sink in. 😊

Thank you so much for all of your amazing AND frequent videos.

Marvelous, well paced explanations about the doings of light thanks for pulling it out!

"Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it." - Terry Pratchett

Delayed choice has been creamating my brain for the past decade. Glad you covered it. Really does trash your morale when you think you can outsmart it.

Finally after many videos of trying to understand what kind of observation is "when observed".... this video clears it up with example and simple language. People just accepted that "when observed".... but I couldn't figure out the who or what observe and how..... Anyway, thanks for the video, could sleep well now. :)

One of my key takeaways is - To test / to detect means to interfere. You cannot look at a light without blocking it. You cannot observe anything by any method without affecting the result. Also particles in space do get affected by the space itself. Particles we fail to detect as of yet are still traveling through our detectable particles and affecting them, potentially causing the entanglement. We're still breaking down particles into their consituents and are hitting serious limits with testing equipment, as to detect increasingly smaller and weaker energy emited by those particles becomes near impossible. I heard that the light appearing to travel back in time was caused by error in testing environment/methodology.

Some of this reminds me of watching extremely high speed footage of lightning or plasma trickling down looking for the path of least resistance. Mind boggling! No soon had I written this comment Alex mentioned lightning as an analogy. This episode is really deep and I absolutely love it!🙃

Wow! This was just great! Thank you so much. Greatly appreciated!

The most attractive explanation (to me) for all of this is the simulation hypothesis. Yes it just kicks the can farther down the road, but what physics doesn’t?

This is one of the best explanations I’ve seen for the polarizing experiment.

There is one experiment which showed the _expectation_ of the person firing the photon affected whether a wave or particle hit the sensor. When triggerd by a random computer, they got 50/50. When intentionally triggered by people they got whatever was expected. The photon seemingly responding to expectations. I don't have access to resources to find it again though. I miss having jStor and Nature.

Wow great video, great references n questions. Easy listening vioce. Thanks for your time

Has anyone ever done a delayed choice experiment concurrently with the time slit experiment? Maybe the frequency shift would also occur when the photon "travels back" through the original splitter?

I like how light and sound is telling me about how light and sound can work and it doesn't even know the full answer itself.

I love this! So as you demonstrate light is both particle and wave , what if it is both, but one is travelling ahead (faster) than the other and you are only recording the average speed for example. So couldn't the preceding segment find a path for the other?

Well said , very clear explanation. Thank you.

Nothing like a litte late night with an Astrum video.

What if we ran the last experiment using the double splitter test slowing down the light particle using the ultracold cloud. Could we observe the test at a slower pace?

many TNX for this video! i am > 50y old and have been fascinated by quantum mechanics from school time on but i am not a physician. Certainly i knew of the double slit experiment, but to be honest i did not know of the experiment with the 3 polarisation lenses. This video is the first explaination that really broadens my understanding of quantum physics slightly, especially when you lalked about not being able to have half a photon and that the prhoton would snap into a discrete state when being observed. Though there is not one single formular or complex maths when trying to imagine that idea, it does make the acceptance of quantum mechanics a lot clearer, at least to me!

I know almost nothing or nothing about just about everything but ever since hearing about Quantum Entanglement many years ago it's just boggled my mind

That last one makes sense to me (the delayed choice). It would be weird if it didn't react to the second beam splitter. Look at it from the light's perspective. Moving at velocity c as light famously does, the lorentz factor is infinite (the universe's divide by zero error). This means light doesn't experience time. From the reference frame of the light, it is emitted and absorbed in the same instant. The splitter being added or removed can't occur between chronologically. It's everything all at once with light.

The way I think of light being a wave function that collapses when it interacts with anything is that I imagine a lightning bolt. When lightning begins to spread from the cloud, it travels in multiple directions at once. If it's heading towards the ground, it will form the familiar reverse-tree-like pattern. However, once it connects, the entire charge rushes through the established path, ignoring all the other branches it created along the way.

Water is also a surprisingly weird thing. That's 2 examples of things we thought were very simple, but it turns out they're not. We should learn from this that maybe DNA and life itself are beyond humanity's ability to comprehend, tinkering with it will always lead to unintended consequences.

I'm absolutely impressed by your video! Well done 👏

Thanks for shedding some.......light on the matter.

As a photographer. Thank you for making this. It explains a lot.

Light seems less strange when you realize photons don't really matter, you're just interested in the electromagnetic or whatever field. Photons are just leaves blowing in the wind, but the wind is what is really guiding the movement of the leaves.

That burrito just threw me off lol like what is that doing in there!😆

I'm usually pretty decent at grasping some of these larger physics concepts, especially considering it's not like I've gone to college and actively studied it. But man that Delayed Choice segment was truly the "No f**king shot!" moment for me. Like nah, throw away everything I thought I knew after that

I love listening to Alex, his gentle, smiling voice is wholesome and relaxing. Much needed after the hectic day is done. Thank you, Alex 🥰

Perfect video to fall asleep! Thank you!

@31:00 Seems like light moves at the speed of time and want's to keep synchronized with it. So you should be able to move faster than the speed of time only if you slow relative time down, you don't need to travel faster than light.

Always seems to me when I am learning about this stuff that space is probably just as relative as time. Nothing knows what its supposed to be until it interacts with something else and what it is seems to be partially determined by what it's interacting with.

this has to be the most exhilarating video I have seen in years. Thank you for making it simple to visualize excellent entertainment for the mind Definitely subscribed

I have long grappled with many of these questions for years....

Really cool video! Thanks so much for making it! How I wish that science was taught like this when I was at school!!!

Fantastic Channel. Just discovered this gem and 51 minutes later I am properly befuddled! Thanks again!

32:40 I think it's not "time" but the "4th dimension" itself where light is sneaking through, an actual physical dimension we humans cannot perceive, yet. Light is merely stepping "sideways" from 3rd to 4th dimension and returning to reach its destination. To us, it appears as light going through time, when in reality is sneaking through another path of less resistance, just like water does.

@38:10 yes it would make a sound. The sound affects the position of so many particles, it would become impossible to separate it's effects from the particles of your brain, even if you never register it through your ears.

Im pretty sure for the one experiment that; up, up, down, down, left, right, left, right, select, start is the correct sequence to give light particles an extra 30 lives!

"Existinial dread" eh? Even though I occasionally heard the word clandestine, when I read it, which was fairly often starting from an early age, I read it as _candlestine._ It wasn't until I was about 26 that I realized the correct pronunciation, and that the word I had been hearing was the same one I was reading─despite also knowing they had the same meaning.

The delayed choice experiment doesn't actually violate causality (go back in time), that's just a common misunderstanding of the experimental setup/results.

Very good video. I wasn't aware of the experiments by Professor Riccardo Sapienza, thank you for making this available to me. I wish more people would be interested in this kind of stuff.

Great work, such a demonstration of what you have practice with the videos you made before … sick sleek and sexy lesson here.

7:48 I would love to know how it’s possible to emit a single photon at a time. Video idea!

As a musician of over 60 years I can attest the essence of a stream of music has the capability to effect its various listeners in a multiplicity of understanding. Succinctly based on how focused the listener is on both watching the performer (light ) as as well as listening (aural) to the performers sound. At this stage both the musician and listener interact as both begins to focus on primarily one an other, thus becoming entangled in the whole of the music. When it began playing as well as to where it ends up. In a live performance. This is the phenom that the musician strives for and the listening desires to experience. Which possibly could be the cause of Pythagoras hyper focusing on the properties of music as many physicists and mathematicians have done in recorded scientific discoveries. Music is the ether the magnetic field of the consciousness of all living things. Ex: Birds chirping etc. the core of language is the contour of a spoken word. A mother hearing a crying baby will walk to the baby and shake a rattle in front of its face while singing a reply like. LOOk At thE RATtle. In a melodic contour of sound rather than in a monotone- LOOK AT THE RATTLE. This is how every living thing learns to communicate when first born. As such light interacts as a single photon ( the initial attack of the sound) to its development the wavelike shape ( contour) to its decay ( silence) . Hence probably the reason many physicists were and are also musicians at some capacity . As some musicians study the physics of sound based on there effect on an audience. The third body principle comes into play as it reveals its magic between both the musicians and the listeners consciousness based on each others level of focus at any moment in time of the piece of music being performed.

Best I can figure when the light is projected, there’s a lot of “dark” (undetected) waves being projected as well and that’s what the light interferes with. The interference only occurs if the waves line up, so when the delayed splitter is added, the light and dark waves line up again, whereas without it they’re traveling in different directions so there’s no interference.

Very well done video, very good at explaining complicated principles and lines of thought, i have to give praise. Very enjoiable learning expirience. I stumbled upon it and i'm glad i did. There's something said in the video i don't undersand though, and it's inconsequential, just bugs me the wrong way: During the bell experiment explanation it's said that classical scientists throw out outcomes and reach a 55/c probability unlike quantum scientist who still describe a 50/c. So I charted the possibilities, reading 3 angles just like descripted in the explination, of the spin of the 2 entangled particles on 2 different angles reading the same direction of spin and the odds came up 50/c if i use all possibilities, But becomes 54.5454545/c if you discard the possibilities when the 3 angles have the same direction of spin. This was a small calculation, and maybe i'm wrong, but it seems like it's falling into a fallacy, you can't discard them just because you know the answer already, they are still a part of the statistic calculation that make up the probability. I hope i'm wrong, that with more data, more charted possibilities, with more angles as variables, the discarded possibilities become negligible, and the results are different from my little analysis. Do you know if my doubts are grounded, or i'm just being to dumb for such intelligent thought?

Great episode : ) The time double slit was performed recently with galaxies and millions of ly's distance/time and same results... as currently hypothesized by some scientists, photons travels millions of light years in the past.... Agree, we don't know what's going on.

16:43 “we are apparently all driven by probability, if you scale things down small enough” seems there may be a requirement for collective observation, particles observing each other, hinting at quantum consciousness

4:41 Whoa this image just reminded me of that show, "Constellation" ~ having a conscious awareness of existing as two separate, but connected entities that personify 'Superposition' (simultaneously being both dead and alive, like Schrodinger's poor cat!) "Bring me the C.A.L."

Newton: calculations with three bodies are causing me serious issues! Young: I'm having lots of fun with two slits.

is it only me that feels the video like a dejavoo. it feels like i've watched it just yesterday...

What if light is in a higher dimension, and it only intersects when it interacts with our physical reality? So its not about how fast lights "move", its about how fast our dimensions can interact with it. Like a framerate of the universe, if you will. It can potentially be anywhere (and maybe it is, in a higher dimension), but our experience of it is limited by the intersection and time of our physical dimensions. It looks like a particle, because that is the intersection. But like a hole in a paper target, the bullet going through it is not actually the shape of the hole. It has a length and a tip, which you cannot tell from the hole in the paper where it penetrated. The same way, our dimensions cannot tell the full shape of the light, because we can't experience more than our 3 dimensions (4 if you count time). Because of time, it looks random. And we can't go back in time, so we can't say if it would be the same if we did the same measurement over. Since time is always passing, every measurement measures a different part.

"Welcome to the planet of fools" - Dmitry Bonch

Great video. The delayed choice experiment is amazing. Why did you show a picture of a burrito at the end?😅

OMG THE WAY YOU EXPLAIN THINGS. Bravo you're really good at teaching and you break things down simply. Subscribing now

Gave me the dreads when you pronounced existential as "existenial" hahaha

Wonderful bro ❤❤ this makes me ponder...does light really travel at a constant speed or is it just the observed light that travels at a constant speed

Thank you for your work

This one is worth watching a few times for me. I’ve been learning from you since Wonders of the Solar System. Thank you for sharing your knowledge and curiosity about our universe with us. ❤ Next time you’re in Michigan feel free to look me up and I’ll teach you all about my tropical plants I grow. I find them endlessly fascinating as well 😌 😉

If the fastest path is back in time we may not be able to perceive it consciously but it has an effect on us in some way. If light can travel back in time then we can possibly get an image of the future

Awesome video! As for the double slit experiment, it makes me curious to know if our eyes act as a double slit and whether our brains experience interference bands or duo our eyes cancel the wave of probability.