**Ask a monkey a physics question thread**

[MadMojoMonkey]

What's the latest on the quantem enigma?

What's that?
(google search)
Oh... I'm pretty sure that's a misunderstanding of what physics says because it's being interpreted by non-physicists and the physicists didn't make it clear what their definitions are.

Is there any progress or viable theories on why matter behaves differently based on whether or not you're watching it?

I'm guessing the confusion stems from the typical quantum physicist's use of the word "observe" to be interchangeable with "interacts with."

The behavior of matter has nothing to do with whether a person or consciousness is watching it.

It simply states that when matter isn't interacting with anything, it behaves in one way, and when it is interacting with other stuff, it behaves in another way. Both patterns of behavior are well defined within the statistical limits of observation and theory.

[BananaStand]

What's that?

The behavior of matter has nothing to do with whether a person or consciousness is watching it.

Hmm, that's not how it was explained to me. Caution: Netflix-documentary style science ahead.

Imagine a solid wall. A few feet in front of it, you have another wall with two holes cut out of it. You stand in front of both walls and throw baseballs. Some of your baseballs will go through the holes in the outer wall, and hit the solid wall behind it. If we marked the points of contact, the solid wall in back would have two hole-shaped regions where it was struck by baseballs, and the rest of the wall would be untouched.

Now imagine instead of baseballs you launched something that travels in a wave, like sound, toward the walls. The sound waves would hit all over the outer wall, and some of the sound wave would make its way through the holes. Since there two holes, the sound wave is now emanating from two points on the outer wall and travelling toward the solid wall. Again, if you could mark the points of impact where the sound wave meets the solid wall, you'd have a completely different pattern than the baseball example.

So particles traveling through holes in the outer wall will only impact the solid wall in certain hole-shaped regions that align with the outer wall. Waves travelling through the holes will split up, emanate through two holes, and the two new waves will overlap each other somewhat before they hit the solid wall. And if you could mark their impacts with the solid wall, it would make a different pattern than the baseballs. Regions where the waves overlapped got double-impact, and places that only got hit by one wave had less impact. On the show it looked like vertical lines.

So what should light do?

It's not a wave, it's photons. Particles with a finite observable size and shape. Logic tells you that they should behave like the baseballs, and only make impact with the solid wall in the regions that align with the holes on the outer wall.

Except that's not what happens. Light hits the solid wall and makes a pattern as if it were a wave

So scientists saw this and said "What the fuck??" Let's slow this all down, and watch individual photons and see what they do.

So they built some kind of science-y apparatus that let's them see individual particles and they watched what happens when photons are fired through this outer wall with holes in it.

Well jiminy cricket.....when they watched the photons move through the holes, they impacted the solid wall in only specific regions. Just like baseballs.

Then they turned around and ran the experiment again, without watching, and found that the light acted like a wave again.

So they tried again, this time watching, and......baseball pattern

Close your eyes and try again......wave pattern

Now open them and do it one more time.....baseball pattern

What's up with that?

EDIT: This link explains it a little better than I did
https://motherboard.vice.com/en_us/a...ssical-world-2

[MadMojoMonkey]

So what should light do?

Weeee!

It's not a wave, it's photons.

Wave = particle = wave = particle
Observations to the contrary reveal a fault in human perception (which I share).
The experimental confirmation of wave-particle duality is ridiculously well established.

Just today, I demonstrated the photoelectric effect which shows the particle nature of photons. Tomorrow I'm demonstrating that exact double-slit experiment you described, showing the wave-nature of photons.

I have Geiger counters which detect Beta radiation (emitted electrons from nuclear decay) as individual particles. I also have a demonstration which shows electrons diffracting like waves when a beam of them is passed through a crystal.

I mean... none of this should really convince you, as it's all hearsay... on the internet, no less... but it's actually not very hard to do some experiments yourself. It takes some equipment and a DIY spirit, but you don't really need expensive stuff to tease out the physics.

photons. Particles with a finite observable size and shape.

Photons are hardly alone on the list of particles which have no known measurable size or shape.
As always with these kind of measurements, it's impossible to prove a 0. There is always measurement uncertainty. You can keep pushing down the maximum size, given that you keep measuring 0, but your resolution isn't infinite. There's always a cutoff where you have to admit that it could be there, but smaller than X.

Quantum particles have fuzzy edges... their positions and momenta are probability fields manifest in moving matter, which obey various uncertainty relations. Even for particles whose size and shape we can measure, e.g. protons and neutrons, these properties are described as probability density functions. I.e. there's a sphere which we call the proton radius, but it really represents some statistical percent chance of finding a proton in that volume, and not the size of a ball which is the proton occupying that volume.

Logic tells you that they should behave like the baseballs, and only make impact with the solid wall in the regions that align with the holes on the outer wall.

When flawless logic yields absurd results, then at least one of the "given" statements of the logic must be false.
Algebra is awesome!

In this case, it's the assumption that "particle" and "wave" are somehow not the same.
The difficult to accept reality is that they are the same, or at least, aspects of the same thing.

Except that's not what happens. Light hits the solid wall and makes a pattern as if it were a wave
So scientists saw this and said "What the fuck??" Let's slow this all down, and watch individual photons and see what they do.
So they built some kind of science-y apparatus that let's them see individual particles and they watched what happens when photons are fired through this outer wall with holes in it.
Well jiminy cricket.....when they watched the photons move through the holes, they impacted the solid wall in only specific regions. Just like baseballs.

"Watched" is in need of defining, here. I don't know what exact experiment you're referencing, but they are all similar in the important respects.

They setup some way to detect which hole the photons passed through. In so doing, the photons' superposition of passing through both holes collapses to a single hole, and it propagates accordingly thereafter.

The "watching" isn't being done by scientists... it's being done by whatever they setup as a detector. All detectors detect by interacting, so it's the detector watching the photons that matters, not the conscious minds which set it up.

Then they turned around and ran the experiment again, without watching, and found that the light acted like a wave again.
So they tried again, this time watching, and......baseball pattern
Close your eyes and try again......wave pattern
Now open them and do it one more time.....baseball pattern

What's up with that?

When the wave function approaches a hole or slit commensurate with its wavelength, weird things happen.
The photons don't technically have to pass through a hole, they just have to pass closer than their wavelength to something to diffract off of it.

The wave function diffracts off of the edges of the hole, and if the hole is small enough, the waves will diffract throughout its opening. However, the quantum weirdness is that no matter how the position wave function spreads out, we never observe the energy of the photon spread out. We only observe single bumps (quanta) of energy, equal to the energy corresponding to that wavelength of light.

It's weird. It defies intuitive understanding.

The diffracting causes the photon's wave function to exist as a superposition of more than one position/momentum pair.
Which means that until it interacts with something, it is spread out over many places, and its momentum is spread out to match how it got there.

If it interacts with the wall, then the wave function collapses will only be where the wave function is not completely destructively interfering with itself. The wave functions are collapsing at the wall, which captures the amalgam of many individual wave function collapses, and the probabilistic nature of their superposition manifests as wave interference they experience at the moment of their observation (by the wall).

If it interacts with something else, prior to the wall, then the spread-out wave function collapses at that interaction and it no longer exists in the particular superposition of states which was set up in passing through the hole.

EDIT: This link explains it a little better than I did
https://motherboard.vice.com/en_us/a...ssical-world-2

This experiment is a little different in that they discuss measuring the photons' positions just prior to one of the slits.
It raises the question of why does the photon which doesn't pass through the detector's slit still not behave as though it passes through both slits?

The answer is the same. Prior to being detected, the photon was in a superposition of states with a spread in it's position and momentum functions. Whether or not the photon was detected at the detector, certain portions of its wave function always passes through the detector. A lack of detection can still have caused a wave function collapse. The portions of the superposed wave function which describe the wave passing through the detector collapsed to nothing. The measurement was made. The photon wasn't there. The rest of that photon's wave function continues on, uncollapsed, and passing definitively through the undetected hole.