Why I stopped believing light is a particle (until now)

[ZapAndersson]

I was screaming this at the Veritassium video but I will tell this to you as well; Please do the same test, but make a small black tube (ideally with vantablack painted on the inside to kill all reflections) and put it at the end of the laser pointer. What results you get with THAT test is the interesting one. As you can see on you video, your eyes can see the bright green end of the laser pointer. The fact that you can SEE it means light is emitted in the direction. By putting a black tube to cover up any side spill, this is eliminated as a false positive.

[brothermine]

We must be careful not to let biases affect our analyses. Feynman and the founders of QM were strongly biased in favor of the particle model, and you can counter their bias by simply substituting the word "wave" for "particle" everywhere in Feynman's "all possible paths" description: it then says a _wave_ travels all possible paths, which is at least as sensible as saying a particle travels all possible paths. Feynman's explanation "cancellation due to phase differences" obviously makes more sense for a wave than for a particle, since phase is a property of wave motion.
As Mithuna pointed out near the end of the video, the only particle-like behavior of light happens when light is absorbed. (For example by an electron in a detector.) That's when it behaves nonclassically... its entire energy quantum is absorbed, whenever & wherever any of its energy is absorbed. But this nonclassical property doesn't imply the light was a particle -- its quantum of energy entirely located at a point or within a very small volume of space -- a moment before it was absorbed, because there's a logical alternative: the quantum of energy is distributed in space (as a wave) while light travels, and the Locality axiom is violated when some of the energy jumps to the point of absorption.

[amontuul]

We may not confuse. A laserpointer isnt a perfect tool, its a 99% or 98% or we dont know exactly laser, including light scattering. Dont confuse that. Further on ask yourself: why do you see the laserbeams. Everyone understands, after a small self-reflection :-) : We see the nicely green beam because it is travelling through your dusty air. And its scattering light around that bright that its apprearing nearly as bright seen through the cam as the point where it meets a white wall:

And there is a 2nd source of possible misinterprtation: The camera setting is adjusted to a dark environment, the laser reflections, in contrast, are all maximally controlled and their relative strength is not differentiated from one another or is not shown truthfully.

The experimental setup for scientific investigation must control all this issues.

For a demonstration its important do show the impact of unexplained side effects explicitely.

Such optic experiments around the natur of light are really important for a better understanding of its nature and it is very important to stay very accurate and self-critical to avoid hasty conclusions. Better ask;`: What could be wrong with my conclusion, what can i do to avoid it.

Best wishes, greetings from germany

[altrag]

7:10 "When Feynman says that a particle of light goes on all possible paths, really what he means is all possible paths where the wave is not zero."
No he really means all possible paths, even including paths that exceed the speed of light. It's just that almost all of them destructively interfere with each other, leaving only the classical path in "normal" circumstances. Adding the black paper and diffraction grating restricts the definition of "possible". I guess impossible paths could be interpreted as zero, but that's a bit confusing as waves have "natural" zeroes as well.

[zdayz]

It's also in an atmosphere full of dust that can bounce the waves in random paths....

[Yoshanuikabundi]

Feynman's like, "light is a particle. But not one particle, every possible particle. And not regular particles, but particles that can cancel each other out. But definitely not a wave!"

[caddywampa]

I don't know how you failed to realize that the off-beam light was coming from scattering: from particles in the air, within the paper, and off the paper and table. And the laser isn't a perfect beam because the optics aren't perfect. Not only do the lenses and mirrors have microscopic imperfections, the light isn't coming from a perfect point source, so it can't be perfectly collimated. You're probably also seeing some light going off-axis due to diffraction from the laser aperture.

[shishkabobby]

The same scattering will come from helium atoms scattering from a crystal surfaces. Helium atoms behave like both particles and waves. In the beam source, you use the expansion of a gas to understand how to produce a monochromatic (high Mach number) source. You then treat the atoms scattering from the surface as waves scattering from the surface electrons, which behave just light scattering from a 2D diffraction grating. We all think of atoms as either a particle or a collection of particles (2 protons, 2 electrons, 2 neutrons). But this two behaves as a wave. As Feynman also said, 'if you think that you understand quantum mechanics, you don't understand quantum mechanics " so enjoy the mystery

[timothymattnew]

I'm studying optics right now, so I find this experiment especially interesting. The fact that the light beam is visible as it falls onto the matte sheet seems the most important to me here. The entire beam line acts as a source of light. Every point on the beam is scattering light in all directions, as a regular point source would. What I'm guessing is happening is that we are seeing two separate images of that beam. This is why hiding part of the beam from the mirror with your hand got rid of part of the "reflection" too. Diffraction grating is crucial here. I'm guessing it could have a similar ability as a Fresnel zone plate: the ability to yield two images of a single point. Afterall, the entire point of the grating is that it splits polychromatic light into several spectra, or a quasimonochromatic light (like in this experiment) into several diffraction points. Thus we would expect the grating to be involved in the production of the two images. But I'm unsure in my hypothesis, because expecting that process to produce an illusion of reflection off a matte sheet just seems a little too good to me.

[marynegro-]

I’m so glad you made this video! I watched the Veritasium one too and immediately hoped all my other favorite channels would cover it to help me understand better—and you did! Thank you!

[zdayz]

Veritassium demo just shows the light from a spread source will bounce off of other things. You can easily see light that isn't in the beam of a laser... you can look at the front and see that it is on.... probably the dominant ring that's being shown is actually a refraction off the lense holder in the end of Veritassium video where the guy shows a laser bouncing off a diffraction grating.

[TheGeoffable]

"...in a vacuum" ;)
Lasers love to scatter in an atmosphere, I wonder how much of that is going on in this very particular setup?

[duelmonitor]

Doesn't that actually show it's more of a wave than a particle? A wave in a pond hit all points and can have cancellations. A particle would have to go in/out of existence based on on all paths being hit and then cancelling out to disappear from some paths. Your laser is also dispersing all over the place after hitting the wall (including from the blackout then reflecting on the table), which will also go back to the mirror. You'd have to have the laser go into a black box and be 100% absorbed inside the box, and then see just how much light is still reflected. Plus you need to be sure the laser at it's source has zero reflectivity in the mirror. If you can see the tip of the laser as green in the mirror then it is clearly not near 0% spread. Great video though!

[scaredyfish]

I want to see a version of this experiment where instead of using a laser, you light up the room with a monochromatic light source, like a sodium vapour lamp. So instead of getting just one spot of reflection, you should get a reflection at an unusual angle. It’s been a long time since I read QED, but I think that’s the experiment that’s actually described in the book.

[FacepalmProduction]

I love that you really broke this down in an easy to understand way, the Veritasium video went through it a bit too quickly and didn't take as much time testing intuitions the way you did. I also appreciate how humble you are, it's so much more relatable knowing that somebody struggles to understand some of this stuff as well rather than pretending like they have an amazing grasp of it.

[busterdafydd]

10:37 you are incorrectly interpreting poor lasers and atmospheric scatter. Please get a better quality laser and a vacuum chamber and repeat the experiment

[boredgrass]

The way you are sharing your thought process "warts and all" goes above and beyond! I think there will be (if there aren't already) many scientists who will look back at your videos as thee influence which made them take the leap! It is a joy to behold! I can't thank you enough!

[slowliving]

I think the path integrals are happening in abstract mathematical space not in actual space time.

[chad]

It’s annoying to me that we don’t consider that fact that space is an active participant in physics. It’s not just “the background” matter IS space in a long standing negotiation or interference pattern. Think of electro magnetism, as a guitar string inverted. Instead of letting the string vibrate, we anchor to our own reference frame and vibrate space, this allows for sensitive materials to phase align and “slide down” the continual unfolding ripple. (Source trust me bro) (please actually think about it)

[PathfinderPhysics]

The photon is simply the quantized matter-field interaction. Think about it: all we see in experiments is interactions with matter.
Look up any table of particles from the standard model. Whats written above the photon? Gauge fields/INTERACTIONS.