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Comment by whiw | Hacker News Reader

> Galactic spectra, which JWST started to send back in earnest at the end of last year, are useful for two reasons.

> First, they let astronomers nail down the galaxy’s age. The infrared light JWST collects is reddened, or redshifted, meaning that as it traverses the cosmos, its wavelengths are stretched by the expansion of space. The extent of that redshift lets astronomers determine a galaxy’s distance, and therefore when it originally emitted its light.

Won't a photon climbing out of a huge gravity well have a huge redshift, thus confounding estimates of distance from us and estimated age?

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The photons that are getting observed are emitted from the accretion disk, which is not very close to the event horizon.

You can estimate how close to the event horizon the disk is based on how broad the spectral lines are. The part of the disk that is coming towards you will be blueshifted and the part of the disk that is rotating away will be redshifted. From that (which is independent of the overall depth of the potential well) you can figure out how fast the disk is rotating. And from that you can figure out how far away the disk is from the black hole.

Well, this is awesome.

Sometimes I despair for humanity [see advertising/social media/politics], but then I see something like this and think, we're doing pretty well for self-taught primates.

That will happen when you are a̶r̶t̶i̶f̶i̶c̶i̶a̶l̶l̶y̶ ̶p̶r̶o̶g̶r̶a̶m̶m̶i̶n̶g̶ ̶h̶u̶m̶a̶n̶s̶ [1] making technology for selling ads every day.

[1] https://youtu.be/ISkAkiAkK7A

It’s pretty fascinating (and scary) to think of the disparity between the brightest among us that are in turn building on the shoulders of giants versus groups of people like outcasts that gather up for the most stupid reasons. No wonder we are facing challenges.

and then we see all of the belief systems to mock, punish, or worse people that are trying to expand human knowledge. looking back at history, you have to wonder where we might be now if there wasn't this attempts at mass eradication of learning. there are times i wonder if we're heading in that direction again. this time, rather than burning the Library of Alexandria or locking everything in catacombs, if we're not just trying to get there on a low-n-slow approach.

The STEM people will just need flashy robes, long grey hairs and market themselves as the magicians that can read the runes, command to the machine spirits of yore, and make the food grow. It's good fun to pester the nerds until the wifi is down.

Fantasy lit is full of stories about the good people rebelling against the evil wizards and their machinations.

Luke fighting Emperor Palpatine? What does Luke or any of the Rebels know about managing an org of Peta-scale population? Or what myriad threats to the galaxy the Emperor was handling with his century of experience, network of millions of loyal agents, and vast magical ability? I'm sure that will all be fine.

The emperor wasn't doing such a great job holding the galaxy together as a political unit, there was a successful overthrow of his rule after just a couple decades in power.

Reminds me of the book "The Last Ringbearer". It tells the story of "The Lords of The Ring" from the perspective of Sauron, Saruman, and the Orks. Turns out, Mordor was a nation in its early industrial stage, of people putting science before magic. Fanatics against science genocide them while painting them as monsters.

I mean it really depends on your personal experience to how your perspective or "where we are now" feels. It's not like everyone is even remotely concerned with or benefitting from this "advancement."

A lot of the so called progress has done nothing but destroyed and distort an enjoyable and peaceful existence.

I guess we might be better off if Pol Pot had killed more smart people than he did. Maybe Trump will win and finish off the rest of us and you can be happy.

We dill die fighting though, good luck

> We dill die fighting though

I agree for the rest but will we? The planet is burning and we don't do shit.

What do you mean? We're solving the world's problems, one opinion on the internet at a time.

Well, that went downhill pretty quickly.

> Won't a photon climbing out of a huge gravity well have a huge redshift

It depends; the key factor is not how "huge" the gravity well is (in terms of how massive the object is), but how close to the black hole horizon the light is emitted. The vast majority of the light JWST is seeing from black holes is not from very close to the horizon. It's from the accretion disk, which is much further from the horizon and so the gravitational redshift is much smaller.

> Won't a photon climbing out of a huge gravity well have a huge redshift,

Yes

> thus confounding estimates of distance from us and estimated age?

Not if you know, or can get a good estimate of, the potential well it's climbing out of.

That said, Brian Cox does sometimes joke that astronomers round π to 1; while I wouldn't know about the reality, it's probably safe to infer at least some frustration on his part about the precision of things in this field.

The joke was way more true at the 20th century. There were many really important measurements where we got unprecedentedly precision, enough to say it's X, 100X, or something in between.

Nowadays astronomy got a lot more precise. But there are disagreements on how much confidence to put on that extra precision.

sometimes, i wonder if astronomy/physics were to only use unsigned numbers, if things would just make more sense. you'd get much more precision, and then you wouldn't have to worry about "but the math says it's possible" issues by taking everything by * -1.

Jesse, what are you talking about?

Well, Mr White, if you look at all of the "weirdness" in physics with theories coming out because "the math says it's possible", then you'd see that it would be much more simple if we were not allowed to have negative numbers because we're only using u64 integers. we'd have more digits for precision as well.

I find that weird outliers are usually evidence that the underlying model is flawed in some fundamental way, but sometimes I think people believe the exceptions are fundamental, and a lot of effort goes to the wrong place.

These are somewhated dated but well worth watching; the professor does all kinds of approximations.

Frontiers/Controversies in Astrophysics with Charles Bailyn: https://youtu.be/ZiK4lg3Tzfs?t=1628

I also don't know how accurate that description is, but this comic comes to mind:

https://xkcd.com/2205/

But I mean, space is really, really big and we are observing from one single spot with (on cosmological scale probably) primitive technology. So of course most of it is guessing and when you "guess" a lot of things, it maybe does not matter a lot, if you have 3.41 Pi or 1, when the data you have are rough estimates anyway. But sure, when you do sloppy math, when you could have precision - that would be just wrong and unscientific.

What if you have 3.14Pi?

Then you're so close to 10 that you can just use 10.

I actually did that on a physics exam once. Somehow I had a value with pi squared. I just replaced it with 10. I don't remember if I got that question right or wrong - I'm hard pressed to think of a situation where pi squared actually legitimately shows up.

Surface area of a Torus?

Not that I remember, or really relevant to undergrad physics - I'm pretty sure it was physics. Fusion and accretion disks are above my pay grade!

That's pretty much pi squared. Which, for weirdly good reason, is close to the value of G in meters per second squared.

only on this particular rock

Nope, on all rocks.

I was confused... the symbol π is actually Pi, but it renders as something that looks like a different alphabet entirely on HN.

> Won't a photon climbing out of a huge gravity well have a huge redshift, thus confounding estimates of distance from us and estimated age?

Of course it will. It will also have a redshift related to the expansion of the space it's been travelling through. Both of those corrections are absolutely part of the model. It's not nearly as simple as "Astronomers forgot about General Relativity!".

But I guess it's true (to be clear: I'm just an amateur in this field) that the ΛCDM model for cosmological evolution that we've all been looking at for the past decade or two isn't holding up well at all. It looked like it was pretty much there and just needed some fine tuning. Then we got a bunch of new data and everything's a mess.

That's kind of exciting all by itself, though it's also leading to a bunch of nattering from the existing iconoclasts (MOND nuts in particular) whose theories are also not working very well to explain JWST observations.

New insight needed, basically. We're all watching for updates.

> Both of those corrections are absolutely part of the model

Is there a way to gain a better understanding of how these parameters are modeled and what the scientific evidence is for the various phenomena in astrophysics? It's somewhat perplexing to me as an outsider of the field to understand how things like mass and distance of stars and planetary bodies are determined when 1) the scales are so outside of conventional experience 2) observation is limited to 2D imaging of the night sky 3) the observations in general are not consistent with our knowledge of gravity and relativity without adding hidden parameters.

I know Terence Tao (yes, that Terence Tao) is working on a book about the cosmic distance ladder, but sadly it's not out yet. (I guess he probably has other projects he's working on.) But he does have some slides from talks he's given: https://terrytao.files.wordpress.com/2020/10/cosmic-distance...

> Won't a photon climbing out of a huge gravity well have a huge redshift

Yes. Though see the sibling comments on why the (remaining) gravity well the emitted photon has to climb out of is not particularly huge here.

> thus confounding estimates of distance from us and estimated age?

Generally speaking: No, because people are not stupid. :) See the integrated Sachs-Wolfe effect for a very similar situation (CMB photons traveling through gravitational wells) -> https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect

My understanding is that a particles, like photons, don't have wave shifts. That's an emergent property of many particles

The conventional Doppler shift and the equivalence principle imply a gravitational redshift for a photon. See page 102:

https://preposterousuniverse.com/wp-content/uploads/grnotes-...

In quantum mechanics, a single particle is also a wave and vice versa. Light is in fact the thing where this observation was first discovered - it had been proven to be a wave for at least a few decades when Einstein discovered the quantum nature of the photovoltaic effect, proving it is also a particle. This discovery was the very start of quantum mechanics, in fact.

"in fact", that was not the very start of quantum mechanics. Einstein's work was preceded by 5-10 years by investigation into the cause of quantized black body radiation. that's why we have Planck's constant and the notion of quantized oscillators. why are you going around talking with such authority? QM and nature is more amazing than you realize. particles are not waves "and vice versa". particles don't exist nor do any classical waves of energy in the way you keep saying as if they're facts. only the wavefunction travels. please be more careful about posting so authoritatively. comments such as yours are why it takes people so long to stumble onto the reality of nature, if they ever are so lucky.

No, individual particles can indeed be redshifted. The particle's wavelength is a fundamental property.

Your comment is a bit strange. Light doesn't travel as photons. Photons exclusively exist at the site and instant of detection of the wave of probability of detection that light really travels as.

When light is redshifted, it loses energy, therefore the wavelength becomes longer.

That is not the current understanding of quantum mechanics, as far as I know. Wave/particle dualism says that different experiments can either view light as a wave or a particle (never both) and that speaking about the nature of light when an experiment is not being performed is non-scientific by definition.

Importantly, light very much behaves like a conventional wave in many real experiments - the interferometer experiment being one of the oldest and most well known. It is not a probability wave in that case, but an actual physical wave (now known to be an oscillation in the electro magnetic field, but long assumed to be a mechanical wave in the luminiferous aether).

Experimentally one never observes waves. Light is detected based on its interaction with electrons and that is always by an electron absorbing a quanta of energy, not via some continuous process as would be the case with waves.

Classically one can imagine that as if electron was hit by a particle. But then we have light diffraction and interference, which classically is described as a wave. So from a classical point of view light travels as a wave but interact as a particle.

As of nature of the light, then consider that there is a reformulation of a classical electrodynamics that eliminates electromagnetic waves all together. There are only electrons that interacts with each other directly with no waves in between. Feynman spent quite some time trying to develop quantum electrodynamic based on that. He failed. Still the point stands that we never observe light directly but only through its effects on electrons and other charged particles. So it could be that what we call light is a theoretical artifact and there is no light in reality.

> So from a classical point of view light travels as a wave but interact as a particle.

And the classical point of view is wrong. Photons resemble classical particles in a few respects, and classical waves in a few others, but at the end of the day they're neither.

> Still the point stands that we never observe light directly but only through its effects on electrons and other charged particles.

This is true of literally everything. "Direct" observation does not exist. Every atom, every cell, every person, every planet, every star - you know them by their effect on your sense-data, or else not at all.

photons dont exist, dude, except in connection with and at the site of the detector. study some qft and then you can go talk about it on the internet with authority.

and yes direct observation exists. that's what measurement is. and that's all you can ever "observe" unless you incorporate the wavefunction, which also doesn't "exist".

> study some qft

I have. Srednicki and Weinberg are sitting on my bookshelf right now.

> photons dont exist, dude, except in connection with and at the site of the detector.

Exactly backwards.

Photons fall out of mode-expanding asymptotic EM field states just like any other particle. It's the interaction picture that can't be rigorously built up out of particle states.

good to know you're informed, but it doesn't mean we're communicating effectively.

what i said is not backwards unless you use the inverse understanding of "exist". and how can photons exist without interaction? they don't. they're localizations by detectors. until then, they only "exist" as the probability wave. that's due to nothing localizing them. that's how i mean "exist". so it's not really meaningful to use that word.

btw "any other particle" doesn't fall out of EM fields.

> btw "any other particle" doesn't fall out of EM fields.

I very obviously meant that XYZ-particle states fall out of mode expansions of the XYZ-field, not that all particles are EM quanta.

> what i said is not backwards unless you use the inverse understanding of "exist". and how can photons exist without interaction?

Exactly the same way any other field configuration can? The state space of the free EM field simply is the photon Fock space.

The state space of an interacting 4D EM field is unknown and may well not exist, hence the need for perturbative approaches and renormalization - in which particle states again emerge as terms in the perturbation series.

I'm pleased to talk with you given that you're familiar with the topic.

> The state space of the free EM field simply is the photon Fock space.

but I'm not sure what your point in mentioning this is. When people hear things like "particles exist between their emission and measurement", they might get the idea that there could be said to be any reality to the existence of particles in transit. They simply aren't particles the way people think of them - they are probability waves. A great example we could discuss is the HBT experiment in which photon bunching was seen. If the bunched photons were really "particles" by any sense of the word, they wouldn't bunch in time of detection merely by the fact they are entangled with each other. That would be like saying entanglement affects the flight of a particle which is simply not able to be said.

> They simply aren't particles the way people think of them

They're not anything the way laymen think of them. They're not classical particles, they're not classical waves, they're not classical anything. They're not really probability waves either, for that matter, or even field configurations - the real object is the algebra of observables. But laymen don't get to dictate the vocabulary of physics, and neither do mathematicians. Fields are the things that would be dual to the observable algebra, were it always equipped with a dual; particles are the things whose creation and annihilation operators would generate it, were there only always such things.

Well, you are still talking about models. The creation and annihilation operators are also mathematical structures. That does not mean, as you seem to imply, that "particle" physics cannot be conceptualized outside of QFT. QFT may be superseded someday and the physical systems we try to describe will still be exactly as they are. I'm not sure if you'll agree with me since I suspect you've got a reason not to.

This comment is needlessly hostile. It's ok to correct someone if you think they're wrong, but this tone isn't conducive to curious conversation.

so it's the tone of my textual words? i think you're mistaking my laughter at their blatant and willful ignorance for hostility, and quite honestly it sounds like you're projecting that hostility. from the start I've been incorrectly downvoted and critiqued while being the primary commenter in this child thread providing a semblance of correct view. to be quite frank i deserve an apology, not some nitpick that comes from your personal assumption about my tone. but i know i won't get one because it's not hostility you care about at all. a lot of you commenters here are pretty hilarious. but not in a good way.

At this point anyone who is as certain about the nature of light as you seem to be may well be right, but is demonstrating a level of confidence the literature does not yet seem to support.

Although there are those who believe that photons only exist at source and at the detector, there is some experimental evidence which contradicts that (for example, this 2013 paper in which researchers 'read' information from a photon without destroying it, which implies its continued existence between creation and observation: https://doi.org/10.1126/science.1246164).

For what it's worth, I am also of the opinion that your tone was hostile, and yes, 'textual words' can and do have a (metaphorical) tone. Regarding downvoting and critiquing, there's an old adage about if you walk into a room and it smells of dog shit, maybe someone in the room stepped in something on the way in, but if every room you walk into all day stinks, well maybe you should check your own shoes.

so, you're trying to tell me that such reasoning about "dog shit" applies to the ignorance of humanity in general? The reality is the very few people actually understood what they were talking about and a lot of people went around operating on what they didn't really know. So while you call me an asshole for correcting a number of incorrect people in this thread who are themselves rather overconfident, I have to again insist that because I am correcting them or destroying their delusions does not make me hostile and there's a very good chance that you're projecting that impression. The definition of hostility includes the intent to do harm or to be unfriendly, which is in fact what the people I am replying to are doing by spreading misinformation, as if they know. If that's too difficult for you to accept, then best that you do not reply to me, because it would be a waste in both of our time. "falsehood travels a great distance in one night before the truth even crosses the threshold of the door". And the masses sentences Socrates to death just because he called them hypocrites. It's a good thing I know these truths or I would have been damaged by you. It still makes me sad though.

and for what it's worth I don't believe the paper you linked says what you think it says.

OK.

you are wrong, and you don't "know". the waves interfere with themselves unless they are measured. then the plate measures interfered waves in the form of singular photons, not waves. look it up.

silly downvoters.

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