Ask HN: What scientific phenomenon do you wish someone would explain better? • Hacker News Board

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#101

Post by leni536 » Sun, Apr 26, 2020, 10:04 PM UTC

pjungwir wrote: post #68

Quantum spin. Electrons aren't really spinning, right? But why do we call it spin? I know it has something to do with angular momentum. What are the possible values? Is it a magnitude or a vector? Is there a reason we call it "spin" instead of "taste" or some other arbitrary name? How do you change it? What happens to it when particles interact?

Electrons have both angular momentum and magnetic momentum, these have fixed magnitudes and they are always parallel.

The situation is somewhat similar to a classical spinning charged sphare, although this similarity easily breaks down.

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#102

Post by stevebmark » Sun, Apr 26, 2020, 10:04 PM UTC

knzhou wrote: post #90

stevebmark wrote: post #88

The LIGO detector. I've never heard a logical explanation for it. If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. If your explanation doesn't involve higher spatial dimensions (not time) then you don't understand it. If you haven't even con…

> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?

I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#103

Post by contravariant » Sun, Apr 26, 2020, 10:05 PM UTC

qubex wrote: post #56

I find most explanations of the Equivalence Principle that lies at the foundation of General Relativity to be very lax. To wit, the idea is that you cannot distinguish whether you are in an accelerated frame or in a gravitational field; alternatively stated, if you’re floating around in an elevator you don’t know whether you’re freefalling to your doom or in deep sideral space far from any gravitational source (thoug…

Yeah the problem is that that the equivalence principle is a _local_ property that cannot really be expressed precisely in standard English.

Clearly it will fail given a big enough lift to experiment in, since a big enough lift would essentially include whatever object is creating that gravitational pull (or enough to conclude its existence from other phenomena). However these effects are nonlocal, you need two different points of reference for them to work (like your two baseballs). In fact most Tidal forces are almost by definition nonlocal.

The precise definition involves describing curved spacetime and geodesics, but that one is really hard to visualize as a thought experiment. The thought experiment does offer insight though, as it is possible to imagine that, absent significant local variations in gravity, you cannot distinguish between free-fall and a (classical) inertial frame of reference without gravity. This insight provides the missing link that allows you to combine gravity with the laws of special relativity and therefore electromechanics, including the way light bends around heavy objects, which provided one of the first confirmations of this theory.

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#104

Post by knzhou » Sun, Apr 26, 2020, 10:07 PM UTC

stevebmark wrote: post #102

knzhou wrote: post #90

Earlier quoted context omitted.

> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?

I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense

Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations.

Maybe you'll find this paper helpful: https://aapt.scitation.org/doi/10.1119/1.18578

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#105

Post by ShamelessC » Sun, Apr 26, 2020, 10:07 PM UTC

sciolizer wrote: post #48

lpellis wrote: post #37

Bell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me at least) of just how it works.

Yudkowsky's explanation[1] is the first one that worked for me. I later found Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing. 1: https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-the... 2: https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...

I started to read the first one but his insistence that Many Worlds is true was too frustrating. Many Worlds Theorem seems specifically useful at saying "the variables aren't hidden because everything before wavefunction collapses actually plays out in different worlds.

But, we specifically have no way of proving that theory. So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#106

Post by baron_harkonnen » Sun, Apr 26, 2020, 10:08 PM UTC

pjungwir wrote: post #72

Entropy. Sometimes you read that it's a measure of randomness; sometimes, information. Aren't randomness and information opposites?

Entropy is measured in bits (they exact same bits we talk about in computing), this can help unify the ideas of information and randomness:

Which file can contain more information: a 1.44 MB floppy disk or 1 TB hard disk?

Which password is more random (i.e. harder to guess): one that can be stored only 1 byte of memory or one that is stored in 64 bytes?

Information theory deals with determining exactly how many bits of it would take to encode a given problem.

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#107

Post by epmaybe » Sun, Apr 26, 2020, 10:09 PM UTC

qqqqquinnnnn wrote: post #15

epmaybe wrote: post #10

Earlier quoted context omitted.

Are you talking epigenetics? It's relatively closely related to mendelian inheritance. At a high level view, if genes are present in your genome that aren't "on" in a previous generation, but due to changes in behavior become more active, then in a future generation that gene could be expressed more. It's not that the genes have changed their code, they are just flipped on or off.

No, more things that are modifications without any genetic basis - like myopia. Doesn't seem to have any genetic/epigenetic basis. It's purely a physical use case, where the eye matures improperly if it's only exposed to near-field work.

ah, I see what you're saying. Luckily, I've looked into this specific topic!

Myopia is definitely hereditary, especially the pathologic variants that can lead to retinal tears and the like.

That being said, there is the process of refractive development that occurs early on in life. The eye develops at a frighteningly fast pace, and you achieve near-adult globe size after about 18 months. The genes that drive this refractive development could be hereditary, if that is what you're trying to figure out.

Now, we can make a claim that this adaptation during infancy could eventually affect our genome, but I have not delved into the epigenetic literature to determine if that has been borne out or not.

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#108

Post by abdullahkhalids » Sun, Apr 26, 2020, 10:09 PM UTC

pjungwir wrote: post #68

Quantum spin. Electrons aren't really spinning, right? But why do we call it spin? I know it has something to do with angular momentum. What are the possible values? Is it a magnitude or a vector? Is there a reason we call it "spin" instead of "taste" or some other arbitrary name? How do you change it? What happens to it when particles interact?

If you take some ball of charge and actually spin it, and then place in inside a magnetic field, it moves in a certain way. If you take a single electron and place it in a magnetic field, it moves in that same way as the ball of charge. Ergo, its natural to call the relevant intrinsic property of electrons "spin".

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#109

Post by neilk » Sun, Apr 26, 2020, 10:12 PM UTC

When I hear explanations like “space is expanding like the surface of a balloon” it’s always confusing. Because a surface is an object, separate from anything on it, but space is the thing we’re all embedded in, so we’re like drawings on the balloon.

If space is expanding why aren’t the radii of fundamental particles and their orbits and molecules also expanding? And if that were the case we couldn’t notice space expanding.

Re: Ask HN: What scientific phenomenon do you wish someone would explain better?

#110

Post by contravariant » Sun, Apr 26, 2020, 10:12 PM UTC

pjungwir wrote: post #68

Quantum spin. Electrons aren't really spinning, right? But why do we call it spin? I know it has something to do with angular momentum. What are the possible values? Is it a magnitude or a vector? Is there a reason we call it "spin" instead of "taste" or some other arbitrary name? How do you change it? What happens to it when particles interact?

Annoyingly I've always struggled with the precise definition of spin, but to my understanding angular momentum is only conserved if you take spin into account as well. Being quantum mechanical there is some ambiguity in the direction, but in principle it is a vector (except it's a superposition of several vectors, and this superposition is also equal to superpositions of different vectors, I think there is usually one 'pure' vector, but that might just be after measurement).