Light Can Go Backwards Through Time, And This Experiment Proves It 

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That must be why my mother hated it when I used to turn the lights off and on in quick succession. I risked sending the house back in time. That would explain our dated home decor.

Einstein liked boating and in his personal papers often refers to him just lying in the boat and dipping his finger in the water and watching the waves spread out. He also would look at others doing the same and noticed that no matter if he was looking at his wave or someone else’s wave he noticed that the waves produced by him or the other travelled at the same speed no matter how fast either boat travelled. He also noticed that no matter how fast he made the boat travel or how fast he dipped his finger in the water or how big of a drop he made the speed of the wave never changed it remained constant. Shortly after he wrote 2 papers. Photoelectric effect and special relativity. Cool eh!!!

It’s a very popular misconception that light acts different when we are watching it. It doesn’t and it doesn’t care. It acts differently when we MEASURE it because in order to measure light we have to interact with it in some way. It’s this interaction that changes the quantum properties of light not the observation.

the mexican wave explanation is so good! Thank you for your video. Sometimes it's hard to remember core concepts when you're so overwhelmed with all of the formulas and exercices. This has made it intuitive for me again!

There's definitely something deeper going on with light, space, and time that is even more fantastic than we can imagine. Maybe we're on the verge of a new century of physics.

I have often wondered about this when I am in a queue of cars waiting for the traffic light to change. I often see the light changes to green long before the first driver moves.😃

4:41 I was just wondering how slow you could get it to get. WOW 0 THAT'S GODLY!!

Is it possible to replicate this time slit experiment, but with electrons instead of photons ? If yes, this would be fantastic. If not, this would break down matter-wave duality and that's even more a game-changer.

I have to be honest. The quanta phenomena that you are describing is sometimes hard for me to conceptualise. Like light travelling through supercooled sodium atoms, I’ll get there eventually….. What I really love is that our assumptions and understanding is constantly evolving, that there is always something new to discover. As a 62year old who’s lived a life, I can’t tell you how reinvigorated and inspired you make me feel!

I read the paper and a couple explanatory articles, none of them have photons going back in time. There is “diffraction” through the temporal slit, which alters the frequency (and wavelength) of the light reflected off the pulsing mirror. There is also a very tiny time gap between the first and second pulse. To my understanding, the light is “schmeared” through time. The overlap in this “schmear” over the tiny gap in time between the first and second pulse results in interference. Because the diffraction occurs over the frequency, the end result is that some frequencies are constructively and deconstructively interfering. None of this requires backwards in time effects, which would break causality (which quantum physics obeys). As a point about your time-space diagrams, If you position your receivers and mirrors at higher points in the graph, you will see that light doesn’t need to speed up; it can just slow down instead. This prevents the impression of light going into the past, but preserves the ability for inference. Drawing analogy to the original double slit, photons going through the time slit should have a certain amount of uncertainty as to which “slit” they will go through, but the photon must still obey causality.

I found this fascinating. Thanks for sharing this alternative to the double slit experiment in space.

Wow you really explained this soo well! I admit, I had to watch it two times to really get an understanding, but that's nothing considering the complexity of this stuff! Well done.

I'm no scientist myself, but I reckon-given the brevity of a femtosecond-that this interference might be attributable to things like internal refraction within the sensor, or the density of air within the testing chamber. The more refraction is possible, the less it can be ignored…

As a former college physics professor I have a moral quandary over sleeping during a lecture

The Heisenberg uncertainty equation is usually written as (delta-x)(delta-p)>h/2*pi but you can also easily rearrange the Heisenberg equation so that, rather than position and momentum, it instead refers to energy and time. That is (delta-E)(delta-t) on the left side but remember that a photon's frequency is directly proportional to its energy (E=hf). So, in the traditional double slit experiment the delta-x is confined to one of two slits so the uncertainty in the lateral momentum must increase (two probability waves spread out and form a spacial interference pattern). From the (delta-E)(delta-t) point of view if you confine the (delta-t) to two time slits, then a similar thing must happen except now the two uncertainty "waves" are in the E=hf frequency. This creates two interfering frequencies and the associated beat pattern that is observed.

Light has to comply with Heisenberg's Uncertainty Principle, like all other matter, meaning dE*dt >= hbar/(4pi). That even applies to a single "Monochromatic" light pulse, down to individual photons. The more precise the frequency (= E/h for photons), the less precise is its time, meaning it exists over some period of time. If the screen has a shutter speed shorter than the time size of the photon, the photon is "chopped in two" in a way that is pretty similar to a double slit experiment. Fundamentally, this is just one of the consequences of the wave aspect of any elementary particle. Photons do not (when travelling) even HAVE a specific time where they exist. As they travel, they form a wave packet not unlike a single note played on a piano. A photon that has a relatively precise position in Energy-Space will look similar to a drawn out sine wave in Time-Space, since these are Fourier transforms of each other. A single photon will, if it has a precise energy, have a large numbers of periods (peaks) in this sine wave. Removing part of the Time-Space contribution to the photon will, when Fourier transforming it back to Energy Space, cause some kind of clipping effect. In fact, the detector is in effect doing precisely this Fourier transform, as it's built to be sensitive to the energy rather than the time of the arriving photons. For anyone into audio, you can get the same effect by playing a sine wave through an amplifier or speaker beyond it's ability, where it will also "clip" the peaks of each period, which then takes the output from a clean sine wave to multi-frequency noise. Furthermore, the reason we notice this, is because the cochlea of our ears basically does a fourier transform of the sound, taking it from a time-amplitude-space to a frequency-amplitude-space. (We can easily tell a 1000Hz note from a 2000Hz note, but it's near impossible to detect temporal jitter in the order of 0.0005s).

This is probably already somewhere else, but wondering if the photon(s) are spread out enough in time to interfere with photons just before are afterward. In other words, the photon is not an instantaneous blip but a wave packet that essentially extends before and after the detectable blip.

Regarding the time slit stuff, I think this can be easier to understand if you remember that there is a time/frequency uncertainty (much like the position/momentum uncertainty). As a result, the more accurately you know *when* a photon arrives, the less accurately you know it's frequency. Thus if a photon could have arrived via two different narrow time windows, there are two different frequency distributions that it must exhibit and it's those frequencies that (I think) are interfering. (And yes, that's grossly over simplified.)

Nicely visualized. Well spoken as well. Not many physics channels on this level

Thank you! thank you! I heard of this experiment but it was never explained to the point I felt understanding but your explanation was remarkable!

I'm not a physicist, but I've heard on PBS Spacetime in relation to Feynman diagrams that a result of an interaction is a sum of all the infinite number of ways it could go, including both possible and impossible ways (impossible exactly as in faster than light / backwards in time). This seems to be very related.

This was one of your all-time best videos. More like this please; less flash and more fact. Bravo!

seems to be a major oversight in this study, and that is a pretty simple one, indium tin oxide cannot change state faster than the speed of light... the entire phenomenon of the experiment can be explained by light traveling through the indium during is phase change

14:09 this resembles a video of a lightning in a super slow motion - it does the same, various branches search for shortest way to grounded point to make a discharge.

I'm sure it's unrelated, but this reminds me a bit of Feynman diagrams, where every possible path (including ones that travel through time non-linearly) for a particle to reach a destination are valid and need to be summed and averaged.

It's like the prop on an airplane. The light is reflecting off the prop (the slit) and being received by our eyes (the detector), traveling at the same speed regardless of how fast the prop turns. Yet, as the prop changes speed, it appears to our eyes to be rotating at different rates and even backwards. The change in the slit's state (open or closed), happens much slower than the speed of light.

Everything you explained makes so much sense except for the mysterious background music. I think no music at all would not impact the ability of the video to explain what it is intended to, on the other hand, it would enhance it.

I am amazed by this discovery and how it was explained in such a beautiful way. Thank you so much

I've seen so many explanations about the speed of light here on YT. But You explain to me the questions I was so often left with. Thank you from the bottom of my heart - sooooo good!

If a photon is a packet of energy that travels along a medium that's always there, then light (or the medium light travels thru, the fabric of space) is always interfering with itself even when there is no packet of energy to observe.

Wow that analogy with lightning is really helpful! Thank you for an amazing video.

The indium-tin oxide does not stop been reflective at the speed of light. So is not like 1 or 0. What you see when at the end of the experiment is not going faster than speed or traveling back in time, is just that the First pulse is still been partially reflecting at a lower intensity at the moment the second is activated to be reflected.

I'm a first time listener. Fantastic video, great speed of presentation for me as an oldie on a complicated subject matter. I've subscribed to listen to more like this one. Thank you

What incredible is that the amount that light slows down in vacuum vs air is almost 3 times the outbound velocity of the Voyager spacecraft, and yet in RF engineering we basically treat the difference as so negligable as to calculate in air as vacuum.

Dude! This is a great video! I love how you present information, keep it up! This is amazing! Is it possible that the faster light/slower light cancel eachother out? What would that mean if they do?

This was the video that made me subscribe. Looking Glass Universe was also toying around with the speed of light recently too--combining with Grant from 3BlueOneBrown to make this understandable. Great job here from you, Alex. You three need to collaborate to finish this visualization off more, then form a group to talk about stuff. Veritasium, Minute Physics, PBS SpaceTime, sabine hossenfelder, Nick Lucid, and others are all solid, but you three need to combine like Voltron to form a group to catch all of us up from what we missed in high school. Add Eugene Khutoryansky to the mix too--he didn't talk about light speed (that I know of), but bring him in and present the ultimate presentation.

Awesome I loved how you explained how it all worked. Not too difficult to understand but just enough so everybody can understand the workings of how you presented it. Excellent work sir.🎉

I would be very interested in learning the physical processes involved in conducting the single photon double slit experiment. I've always wondered how we actually create the single photons, and how we detect them on the target screen or as they pass thru the slits.

That’s an interesting and good question, Jay. The answer would probably change, every day I look back over my life. You have a lovely looking family in your profile photo. I hope you all have as happy a life, as any of us can.

Having v>c does NOT result in negative time intervals; it results in imaginary time intervals. This would turn an energy gain or loss into an oscillation (and vice versa)... It effectively exchanges the imaginary phase for the real phase in the quantum mechanics wavefunction.

That just blew my mind and I was wondering how far it could ever be blown. Turns out, it was blown larger over a very small interval in time, blowing it even more than it should ha ve!

I'm fascinated by the study and measurement of time. A complete amateur can get glimpses on how physicists see the universe through the ostensibly elementary thing.

I can think of some important concepts we should mention when looking at this experiment? Firstly light (as for any other wave signal) must be a perfect sinewave function to present as a perfect single frequency, or colour in the case of light. This means that for a perfect sinewave to exist (i.e. with no harmonics), it must have started at the beginning of time and gone on to infinity to be totally perfect - and be measured in it's entirety. Anything we do to measure it in our constrained time means that we have no choice but to start and finish the measurement in finite time. This means that our measurement in itself causes an error which will create harmonics. This is why we use 'windowing' in signal processing to reduce the fall out when analysing signal (i.e. fading in and fading out again, by various means etc.). So the introduction of harmonic lines in the light wave after being passed through the switchable reflector can be adequately explained by the light having been subjected to a foreshortening due to the reflector being turned on and off very quickly. BTW this also means that we cannot think of a photon as a discrete 'packet of energy' by dint of wavelengths being 'faded and and out' like his diagram suggests - since this would also cause a frequency spread where no light could ever truly occupy only a single frequency. This simple depiction of bursts of waves must be wrong! The second thing worth considering is that if light is moving at the speed of light (in a vacuum) it does not experience time at all - because of complete time dilation! This means that it apparently ONLY experiences time IF we must think of it as a discrete particle of energy. The only way we can counter this wave / particle duality notion is to state that a photon has no mass, because anything with mass would require infinite energy to reach the speed of light. This means that the photon is apparently the only thing which can transport energy from one place to another that has NO mass! Hmm... Anyway give all of the above - it's not a great stretch of the imagination to understand that gating light 'signals' will cause frequency spread (just like any other signal). And since light as a wave itself does not experience the passage of time at all, the periods in 'our time' the light mirrors happen to be opened and closed is totally irrelevant... Food for thought?

Of all the stuff with quantum physics, I find 'light' to be the most beautiful, difficult, elegant, enigmatic and mysterious. Forget gravity, space-time or subatomic particles. Light is what holds the secrets of the universe.

No joke i feel like i got a fundamental understanding that i have always missed about the size of things in the time axis. i always assumed we were all points in the time axis, but the way light behaves here suggests it is not a point, but rather a small blob. which means it simultaneously exists in the past, present and future, however small slice of those it may be. that's amazing.

It has something to do with the harmonics and frequency spectrum kind of like FFT. If you switch a sound or light on and off fast enough you get extra harmonics that add to the effect iirc.

I believe the group speed remains constant, but the position of the head of the group changes. In other words, relativistic effects occur within the light itself, transitioning between long and short wavelengths, from one location to another through quantum superpositioning.

Brilliant, I wasn't aware of this study. Fascinating. Implications are... yet to be imagined!

0:13: ! Light's behavior as a particle or wave has puzzled scientists, and now it seems that its speed may not be constant either. 3:40: 🌈 Light can be slowed down by interfering with its waves using blocking materials. 4:33: Scientists have been able to slow down light to an astonishing 61 km/h by sending it through a cloud of cooled sodium atoms. 10:33: 🔬 The video explains the time slit experiment and how light interferes with itself. 11:06: Light interferes with itself, causing photons to arrive at the receiver at different times on the time graph. 13:46: 🔬 Light may be testing alternative paths, including paths through time, to find the fastest route. 14:07: These feelers interfere with photons traveling alongside light, as well as photons a little ahead or behind in time. 14:25: We never detect photons taking these alternative paths or traveling slower than the speed of light. Recap by Tammy AI

If the experiment is analogous to the spatial double-slit experiment, that means when actually trying to measure any of these "feelers" by whatever method we might come up with in the future, the interference pattern vanishes as well.

Maybe it's really a continuous wave (not a particle), but it's interacting with something else (the ether) that makes it's activity zero, until it picks back up again, appearing to be a particle. What I'm trying to say is that maybe it's interreacting with something that makes up the fabric of space, so it has to move cyclical. I also find it interesting that theoretically, things can get infinitely small, just as they can get infinitely large.

This effect would be caused by a very simple and universal limitation of light sensors: frequency is detected by overlapping successive waves, and increasing the resolution in the frequency domain requires using longer samples. The experiment being done here stretches the limits of the best sensors available, and I believe the resulting artifacts would exactly match what is described here.

love these. i actually use them to sleep since i have a hard time to, and they help so much. thank you for doing these.

Thanks for this Alex. The idea you present here makes perfect sense while expanding on the original double slit experiment.

“The first gulp from the glass of natural sciences will turn you into an atheist, but at the bottom of the glass God is waiting for you.” ― Werner Heisenberg

This reminds me of another fascinating video I saw where it was explained that we only know the "round trip average" speed of light is 'c' but that it's speed may vary during the trip. The average is always 'c' but the speed to vs the speed from in the round trip journey may differ. There is nothing proving otherwise.

Another fantastic video with beautiful imagery thank you Alex!

You explain things so well. Very well done.

It’s not just light. All energy can travel through time. It’s the reason the electron slit experiment works. It’s the way electrons can change from solid to energy based on the observation by consciousness.

Like how in order for our sight, "sight" doesn't come out from our eyes to the object but the other way around. Light lands on object and bounces off to our eyes. So to measure something, it's like taking a different light and colliding them with normal light which changes the trajectory

Please Mr Astrum, would you consider doing additional videos on the planets? I have watched them all over and over again. I need more!

This is very exciting to learn about. Thanks for posting about this mind-bending experiment. Absolutely amazing!

its so annoying when people say it behaves different when you look at it, it doesn't. in order to "look at it" you need to interact with it in some way, ofcourse you get a different result by interacting with it.

Time isn't a physical entity like distance, intensity, etc. Time is the sequencing of events we need to create in order to perceive the Universe in a single coherent framework. This sequencing is like the act of counting, therefore subject to all the fallacies of the theory of the sets. I'm not surprised to see that, if you exchange space with time in the double slit experiment, the stange link between photons changes place but doesn't disappear...

"Light travels at the speed of light..." One of the great thinkers of our time.

light and other subatomic particles seem to have a kind of retcon/ hash check/edit ability to make up for the breaking of continuity within the world we live in. The ability to shortcut time by being both a wave at the same time as a particle is astounding. every day i'm amazed by our leaps in understanding, and to think mankind not too long ago was living in caves and small huts only being able to make up gods to explain the stars in the sky. If nothing else is worth it in life, this world and beyond it is so fascinating and i'm glad i get to witness it along with each of all you others of the human race. We all are lucky to see such marvels, so make sure you take time out to look back on your life and despite any ugliness just appreciate the marvel that is the universe and even our earth.

This is a good experiment but the first question I have is could the detector still be ringing from the first light pulse, as the second arrives? Need to see the settling time of the photodetector and related circuit to know if the beat mixing is happening in the detector or related electronics or if it had ample time to settle.

Our experience of time is ordered by size. By that I mean that it is both a physical dimension, and a wave in the sense that every particle in the universe is growing/gaining information according to interactions with particles of the same size. Our general experience of particle/wave duality actually stems from interactions with particles that are growing at a rate that regularly overlaps with the rate of growth of the observer - think overlaying rhythms like 2/4 and 4/4 time- and will "sync" up if directly interacted with, at which point they will behave to the observer as a particle. While information is always carried "forward" with an increase in size, under this model interference patterns are possible out of the direction of causality if you allow for multiple instances of the same particle to exist outside of the limited view of what can be directly interacted with by the observer (i.e. the observer's time frame).

The universe is a 3 dimensional spreadsheet with plank sized cells. Each cell contains a value for what's happening in that cell. The speed of light represents how quickly one cell can communicate with it's neighbor to propagate the change across reality. We live in a simulation.

Would this mean that all space/time interval combinations that have the same time total would reach the detector at the same time? The interference pattern would then have itself a pattern along the pulse interval length? This really interesting stuff!!!😮😍

light said... you can 'see' me being slowed down but you'll be shocked that I've already done my job 😂

The Two Slit experiment has been around for a long time. Has anyone ever tried to use a variety of shapes, sizes of slits? Like with two or three s shaped, cross shaped, circle, star, wave shaped slits? Or with slits of variable lengths, widths, or some that are rotating at different speeds, or with different color filters, rotating polarized filters, etc? I am just curious and would not know what to expect, given that several of these light experiments have produced unexpected results.

can we combine the super cooled slow light experiment with the time/reflective double slit ? maybe the testing of routes goes way to fast for us to detect under normal light speed conditions cus it might be a single photon that tests the way or maybe even only a carrier wave without any visible light well many more things to tests and double check in the coming future

Your ability to use analogies is simply genius, particularly the line of Light People doing the wave & lightning seeking the shortest path to earth.. Perhaps photons are something we cannot perceive or conceive of & we should call their structure "wavicles?" Or "partiwaves?" The latter sounds like more fun, at least for the photons.

Yes, is formal speed of collide of reverse light in medium toroid and classical bol of glass can reduct energy with water and magnetism at 0,2mm second particular electron position appareatted and enrolled in movement very pretty experiment

This makes me think that it isn't a problem of light legitimately travelling backward through time, but with our understanding of what light is being flawed in such a way that with our understanding, it appears to arrive faster than light. I don't understand enough of physics or this particular subject to know, but if I were to throw my undereducated idea at it, I would start to think that light is a probabilistically appearing function of something that we just don't know about yet, and that interference patterns are just the natural way that something's made apparent to us with the way these experiments are set up. We just don't know quite what we're looking at.

Great animations, light may travel faster, or slower, and is a wave, and a particle, in a quanta, a packet, being both the wave and the photon particle, travelling by the path of least resistance, and in space, light is actually bent too…compelling, mind blowing, and enigmatic, the story of light is forever fascinating!

Could also be explained by the existence of one (or more) higher physical dimensions, wherein the path through 3D space is actually longer than the path through one of the higher physical dimensions.

i had a brief look at the paper you cited. there is no mention of light going backwards in time or any thing of the sort in other articles about this experiment. i just don't see how the argument that its travel back in time make sense, since speed of light is invariant in all reference frame. i don't know if this logic is sounds, but if you consider the same space time diagram and the fact that light has a constant speed in any frame, to get interference it simply that the distance between the detector and laser is shorten - hence length contraction from special relativity .

Regarding the time slit experiment, how can it be ruled out that a residual vibration in the reflective surface caused by the first light pulse is not causing the interference in the subsequent light wave?

Wow, this is so clearly explained. So much better than all the videos about this topic I have watched so far.

the laser split experiment also needs the reflective surface slits to be looked at as they will also bend the light to some degree and will be a primary reason for the light to become diffuse ... ignoring it you preload your error by at least 5 % ....

I've always wondered if the presumed expansion rate of the universe could be explained not through dark matter, but rather the varying of what we consider constants, either the speed of light or time or both

Really loved the video, only regret this video is made now and not then when I was in 12th standard or in 1st year of my college, at that time if this video was available I wouldn't had to cram these concepts and would have been able to answer all the questions. The fundamental reason of light slowing down in a medium, now I am able to get it. Wish I had known this thing earlier 😢

My feeling is small imperfections/density variation of the instrument environment material (not purely vacuum) through which the light is travelling can cause the light to reach at different point on time and interfere with the previous one specifically when we are talking about accuracy of femto seconds..

i think it needs clarification that 'when light hits at a different angle, its frequency changes' only applies to this diagram. meaning the same light wave but with a faster wave velocity will have its crests/valleys hit the receiver plate more frequently. how light actually behaves when hitting a surface at an angle in geometry is actually complicated. the distance in time between light pulses should be mentioned i think. light has a certain speed through time and speed through space. but the photon experiences no time. like you show its taking the diagonal in this experiment. light either interfering or not interfering with itself would be a function of the delay time between pulses and the distance of the whole apparatus. i think some aspects of light's result on something can be thought of if the whole setup occurred in an instantaneous speed of light. of if the distance it travelled shrunk to zero.

The double split light exp. Always freaks me out . That experiment conducted I think in Europe had 2 parts split by a massive distance & yet both behavoured in tandem.

Is light arriving at different points in the multiverse? What are the practical applications regarding traveling great distances? If various parties leave a destination to arrive at another but experience different factors during the journey, will they arrive at the predetermined location or each a different locale?

what does this imply about the stochastic interference pattern from the traditional double slit experiment though? is the interference pattern for this new experiment also stochastic...? if they repeated this with a single photon shot twice in rapid succession, would it just... feature a random tiny dip in one specific frequency...? (and besides which, I thought lasers operated at one frequency anyways-why is there a range of frequencies to affect in the first place??)

Jesus christ. This gave me Goosebumps. It somehow makes sense and feels impossible

Refraction shows the speed of light is not constant. It changes based on the medium it travels through. In a high refraction index medium like the inner layers of the sun it can take millions of years for a photon to even escape because the photon's direction and speed is being changed so much.

Love this style, hope there are many more to come!

What if, in Lene Hau's experiment, we were to put a person inside a transparent capsule (obviously with oxygen and ideal conditions for the observer) and around that capsule, another container filled with those sodium atoms that stop light? What would the person inside see? Theoretically and hypothetically, could someone travel to the future inside that capsule?

More of these sleep episodes please

This is realy amazing. This experiment opens a new world of investigation

Light is the visible form of energy. Energy is power x time. Time isn't something we typically control. Thus, most of us pay our power bill on-time. Only some of us have mastered the art of calling the power company and requesting more time. These lucky folks have additional time on their hands that we don't!

So so so good... absolutely loved the video ❤ Seems things don't just happen, they are manifested and calculated into reality... 😮

Dude this channel is brilliant.

Light is a pulse wave of photons; therefore it is a particle exhibiting the characteristics of a wave. The photon "packets" speed is reduced when encountering interference from collisions within the medium the packets are traversing. Two beams interseting at an angle to one another will create an interference pattern. Some frequencies will be attenuated while others may be augmented.