## Science fleeting thoughts

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

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ijuin
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### Re: Science fleeting thoughts

A disc can be topologically equivalent to a sphere as long as you are able to ignore the differences in angles and distances.

Eebster the Great
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### Re: Science fleeting thoughts

Angles and distances are geometrical, not topological. The topological difference between a sphere and a disk is closure. The disk is open because it has a boundary, but the sphere is closed because it does not. Both are compact, simply connected 2-manifolds. If the disk were also closed, it would be (homeomorphic to) a 2-sphere.

ijuin
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### Re: Science fleeting thoughts

Yeah, just trying to head off any claims that topological similarity implies that the Earth (or any other body commonly held to be a spheroid) really is a disc or other shape possessing a surface of non-positive curvature.

Sableagle
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### Re: Science fleeting thoughts

You used more syllables than a flat-earther can process.
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Eebster the Great
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### Re: Science fleeting thoughts

Flat Earther proof that all manifolds are planes: looks flat = is flat.

p1t1o
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### Re: Science fleeting thoughts

Flat earthing isnt a science cognition problem.

These people will take a paracetamol (a drug resulting from the understanding of complex biochemical pronciples, themselves derived from chemistry and physics as we know them) but can discount any concept they feel like by saying "science is a conspiracy, dont believe it".

Its a societal problem not one of scientific comprehension.

Discussing how they would reject this or that evidence is like discussing how a crocodile would handle a shopping list.

Its not an issue of the crocodiles reading skills, its a fricken crocodile.

Similarly, the scientific skills of a flat earther are not at issue - they dont even come into play. Its more about do they trust what the world around them is telling them, and many of them are deciding not to. The literal shape of the earth is a target ideally suited for rejection, the evidence against is remote and second hand (they arent going to start measuring shadows) and the consequences of being wrong are zero.

Honestly I think flat eartherism might be a sign of minor mental illness.

Eebster the Great
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### Re: Science fleeting thoughts

There is a degree of some sort of ineptitude in there though, maybe spatial reasoning. People don't usually start out thinking the world is flat. They start out with a variety of other conspiracy theories that they believe, while aware of others that they find silly. But they watch some videos by confident people making arguments they find convincing and change they mind. So there is a period of time within which they could be dissuaded by their own observations.

The observation of a sunset clearly shows that if the Earth is flat, the Sun is moving in a big circle around it, and you the observer are near the center of the Earth. This is not a difficult conclusion to draw--it's exactly what things look like. Clearly the Sun rises and sets.

But the currently accepted flat earth model does not look anything like that, because of course, it doesn't fit the large world we know where it is always day somewhere. Clearly the Sun never passes below the Earth. So instead they have this idea of the Sun circling overhead and say the appearance of rise and set is due to perspective. This explanation can only make sense to people who cannot visualize perspective. Who can look at a projection and really not understand what's going on. That's not something that takes much education to understand, just a functioning intellect and eyes. This is the equivalent of insisting that an American football field is only 15 yards long, and the NFL is lying to us. You go to a game and look at the field, and if you can really think that field is 15 yards long, something is wrong with you.

p1t1o
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### Re: Science fleeting thoughts

All I can think is that such severe deficit of perception ought to have significant overt symptoms in day-to-day life, such as inability to tell distance or negotiate corners. They'd be total trainwrecks. To not understand perspective is to not know that things appear larger with proximity. If you dont grasp that fundamentally, then walking around the world has got to be a terrifying experience. I just dont buy that all flat earthers are that special.
I think it is something more complex than "they are too stupid to get it", although I'd bet it correlates with the lower end of the intelligence spectrum.

Xenomortis
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### Re: Science fleeting thoughts

The last I heard is that they use "Optics" as a sort of cop-out
Because of course, lenses and such are sufficiently magical that we can use them to dodge awkward statements like "what you're saying does not fit with what I can actually see".

Sableagle
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### Re: Science fleeting thoughts

I think they can be divided into the fakes doing it for giggles, the utterly crazy (have you seen the one who insists the Atlas Mountains are the remains of an ancient dragon?), the dishonest (they're the ones with the livestreams with "superchat" donations, the patreon tip jars and the merchandise) and people who don't understand some aspect of science-based solar system modelling.

The aspects most likely to not be understood seem to be scale and the third dimension. I've seen a video in which someone said the globe model couldn't be right because Polaris was in the same place over the same roof every night of the year, and his diagram was a perfect explanation of why any star along the ecliptic wouldn't. Polaris, though? At pretty much a right angle to the ecliptic. Straight up from the sheet of paper. As for scale, well, it'd be kind of hard to draw a scale diagram of how a total or annular solar eclipse happens on a normal classroom ... blackboard? Greenboard? Whiteboard? Chalkboard? Inkboard? Projector screen? I don't know what they use in classrooms these days, but if you want a digital image of it you may need to get your resolution up to 5 km per pixel, which makes your image 3.1 million pixels wide and 280000 pixels high to include the Sun and Earth at aphelion, and if you want to project that at 1 mm per pixel (1:5000000 scale so a total eclipse path 70 miles wide is shown almost an inch wide and can probably be seen from the back of a classroom), that would make the image 3.1 km long and 280 m high, which is going to make drawing straight lines from parts of the Sun past the surface of the Moon to the Earth kind of difficult, even if you can find an adamantium ruler 3.15 km long to draw them *with*. This means people probably never get a sense of the scale of things in the basic curriculum.
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ucim
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### Re: Science fleeting thoughts

p1t1o wrote:...and the consequences of being wrong are zero.
I think that's it in a nutshell. Find me one flat-earther who is making actual decisions with consequences which depend directly upon xis being right about the shape of the earth.

A lot of them believe it to the extent of opening their pie-hole and staking their reputation on it, but I wonder how many would (or are even in a position to) set a course at sea and stake their life on this alternative geometry.

Jose
edit: tag mustard
Last edited by ucim on Tue Jul 30, 2019 2:08 am UTC, edited 1 time in total.
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cyanyoshi
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### Re: Science fleeting thoughts

ucim wrote:A lot of them [b]believe [/i] it to the extent of opening their pie-hole and staking their reputation on it, but I wonder how many would (or are even in a position to) set a course at sea and stake their life on this alternative geometry.

Given the vast numbers of people who have staked their life on their religion being correct, I suspect quite a few flat-earthers genuinely believe the Earth is flat to that same degree.

Eebster the Great
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### Re: Science fleeting thoughts

p1t1o wrote:All I can think is that such severe deficit of perception ought to have significant overt symptoms in day-to-day life, such as inability to tell distance or negotiate corners. They'd be total trainwrecks. To not understand perspective is to not know that things appear larger with proximity. If you dont grasp that fundamentally, then walking around the world has got to be a terrifying experience. I just dont buy that all flat earthers are that special.

I think they believe something special happens over long distances. They also say objects disappear if they get too far away "because of perspective" or "because of diffraction" or whatever. But the problem is, once you get to this point where you have to defend why the sun appears to set if it actually doesn't, you have already lost. People get sucked in in part because they believe the world looks flat. Once they realize it actually does not look flat, and they have to invent half-baked explanations for why looks are deceiving, they should normally realize that their whole premise was wrong to begin with. It looks round. But by then they're sucked in and now they have all this evidence from the dozens of videos they watched purporting to show some flaw in the round Earth model. Besides, they are very smart, and they can't have fallen for something like that if it weren't true.

My point is just that there must have been a point at first where they really did think the motion shown by the sun and moon in the model made sense, and it really didn't occur to them that things just don't look like that. They couldn't mentally put themselves on such a model and see the receding sun and think "this is stupid, that's not what happens." They don't get it until someone else points it out, which is a pretty serious deficit in spatial reasoning.

But no, they probably don't go all Dougal from Father Ted.

Sableagle wrote:classroom ... blackboard? Greenboard? Whiteboard? Chalkboard? Inkboard? Projector screen?

Most classrooms use SMART Boards or white boards now. There are still a lot of overhead projector screens too (in rooms without smart boards), but not as many chalkboards as there used to be and practically no transparencies.

ucim
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### Re: Science fleeting thoughts

cyanyoshi wrote:Given the vast numbers of people who have staked their life on their religion being correct...
They've staked their soul on it. Subtle but important difference.

Jose
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p1t1o
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### Re: Science fleeting thoughts

ucim wrote:
p1t1o wrote:...and the consequences of being wrong are zero.
I think that's it in a nutshell. Find me one flat-earther who is making actual decisions with consequences which depend directly upon xis being right about the shape of the earth.

A lot of them believe it to the extent of opening their pie-hole and staking their reputation on it, but I wonder how many would (or are even in a position to) set a course at sea and stake their life on this alternative geometry.

Jose
edit: tag mustard

What is it they actually say about our total inability to locate an edge?

Where do they think water...you know what, why am I even asking. The mind boggles at the ability to reject rationality to such a degree and still get on with life.

Maybe if I stopped expecting people to be so rational, the world would make more sense.

ucim
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### Re: Science fleeting thoughts

p1t1o wrote:What is it they actually say about our total inability to locate an edge?
The usual stuff. (You do know that there's an ice wall, protected by the military, so that nobody without clearance goes there....)

p1t1o wrote:The mind boggles at the ability to reject rationality to such a degree and still get on with life.
Where rationality (or its lack) has no actual impact on their lives, it's easy to ignore. For most people, unless they are active in that particular field, the wonders of science are taken on faith. You learn about fusion in school, but you never actually build a fusion reactor. Other people do, and you read about it. Ditto microbes, space travel, biodiversity, and even the existence of foreign countries and lizard people. (Nobody's stopping you from going to Atlantis or Tazmania, but unless you go there, you're relying on third party reports). So, there could "easily" be a grand conspiracy to keep these things secret.

What gets me is how easily they believe accept that such a conspiracy is, in fact, easy.

Jose
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p1t1o
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### Re: Science fleeting thoughts

ucim wrote:
What gets me is how easily they believe accept that such a conspiracy is, in fact, easy.

Jose

Simple. Any piece of evidence that has ever led you to believe that a conspiracy would be difficult to achieve for any reason, has been fed to you by the conspirators.

ucim wrote:For most people, unless they are active in that particular field, the wonders of science are taken on faith. You learn about fusion in school, but you never actually build a fusion reactor. Other people do, and you read about it. Ditto microbes, space travel, biodiversity, and even the existence of foreign countries and lizard people.

This is sometihng I think about from time to time. The reliance of scientific progress on faith. Faith is in fact a huge part of scientific progress, religion doesnt have even a majority hold on the concept of faith.

Even a researcher on a lab doing meticulously planned and recorded experiment, *absolutely must* take a great deal on faith.

Yes, he will calibrate his instruments.

But he wont, for example, spend 3 weeks doing the practical work and writing up a physical proof that his vacuum distilator works.

Or finding out the melting and boiling points of all of his reagents from scratch, no - he will look those up in a book, and use his vacuum distillation equipment as his education has taught him to.

We all take things on faith, all the time, its no excuse for making things up at all

Its all I can think about when I hear a religious person say something about "I pity those people who live away from god, without faith"

All I can think is "Im an agnostic chemist and I could teach this guy more about faith than he has ever preached in his life".

Pfhorrest
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### Re: Science fleeting thoughts

There's an important difference between faith in the sense of trust that other scientists have done their work right, and faith as used in religion. In the former case, nobody is asking anybody to just accept the truth of something on someone's indubitable word. Everything is still supposedly proven, later researchers just refer to earlier work for the proof of the things that they've already proven, but in principle someone sufficiently skeptical could start from the latest research, go look up all the research whose conclusions that research starts from, go look up all the research whose conclusions that research starts from, and so on to replicate the entire chain of knowledge. Nobody actually does that, but the system is set up so that that's possible in principle and you can do as close to it as your skepticism says is warranted. But faith in the religious sense asks you to take someone's word for it and just stops there, no possibility of digging deeper if you want to, you just either accept it or you don't.
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Eebster the Great
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### Re: Science fleeting thoughts

ucim wrote:For most people, unless they are active in that particular field, the wonders of science are taken on faith. You learn about fusion in school, but you never actually build a fusion reactor. Other people do, and you read about it. Ditto microbes, space travel, biodiversity, and even the existence of foreign countries and lizard people.

This is true for most conspiracy theories, but it really isn't true for Flat Earthism. This is a rare case of a conspiracy theorist where a sufficiently motivated and dispassionate person could very easily demonstrate to themselves that the world is flat. Admittedly, to show that the world is not a flat disk with them in the middle and the Sun circling in a plane perpendicular to it, you would have to travel a couple hundred miles or call someone who lives at least that far away in any direction. But that's all. It doesn't require precise measurements either, just a trust that what you see with your own eyes very roughly corresponds to what is actually happening. That's all it takes. That's what startles me so much about it. Very, very few conspiracy theories can be rejected this way. No personal observation can ever prove to you that we landed on the Moon, but it can prove that the Earth is round.

ucim
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### Re: Science fleeting thoughts

Eebster the Great wrote:This is true for most conspiracy theories, but it really isn't true for Flat Earthism. This is a rare case of a conspiracy theorist where a sufficiently motivated and dispassionate person could very easily demonstrate to themselves that the world is flat.
Are you sure? I personally checked and found that the data is consistent with its being round. The data was also consistent with it being banana shaped if I measured the right axis.

Typos aside, it's the "sufficiently motivated..." part that lets this thing continue. Most flat earthers are not sufficiently motivated. They have no actual stake in the actual truth or falsity of it.

p1t1o wrote:Even a researcher on a lab doing meticulously planned and recorded experiment, *absolutely must* take a great deal on faith.
Yes, he will calibrate his instruments.
But he wont, for example, spend 3 weeks doing the practical work and writing up a physical proof that his vacuum distilator works.
It's kind of like open source vs proprietary software. I'm not going to personally take linux apart to ensure there's no funny business going on. But it's important that it be possible. I have faith that there are enough geeks in the world that somebody is going to be doing it, and funny business will come to light.

Perhaps <*koff* heartbleed *koff*> the faith is misplaced. But it's a darn sight better than proprietary stuff.

Scientists think the world is open source. The religious think it's proprietary.

Jose
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Sableagle
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### Re: Science fleeting thoughts

Part of the problem is that someone did very easily demonstrate to himself that the Earth is flat:

"Eeeeh, this duck-pond looks very flat."

That was all it took. It was that easy to convince him.

Then he stayed convinced. He noticed the glowy ball thing in the sky and that it wasn't always in the same place, and tried to explain that based on the unquestionable knowledge that the Earth is, in its entirety, as flat as that duck-pond looked.

It's called "zetetic astronomy." Its core tenet is that, having made up your mind about something (like "Earth is flat" or "apples ripen in August") you then have to explain everything else in ways that don't challenge what you've already decided. A similar process can be observed in the investigations conducted by any teacher convinced that a particular child is to blame for whatever just happened.
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### Re: Science fleeting thoughts

Parody superheroes have to deal with the Earth's shape.
Pfhorrest wrote:I do think that I recall reading something about some kind of radiation experienced by objects moving at relativistic speeds which was basically a consequence of vacuum energy doing weird relativistic things, so that seems like it would be one possible option, as each part of this "stationary" sphere would be "moving" at relativistic speeds relative to the expanding space. But it's past 2AM here and I can't remember much about that right now.
The problem is that objects from far enough away to be heated, would also be heavily red shfted, reaching a maximum temperature of 10^-30 K, which is also so cold that photons would be able to fit in our universe anymore.
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### Re: Science fleeting thoughts

Apparently the flerfs are having a meet-up on 4 Sep at the Globe pub in Brighton. Anyone near Brighton who feels like filming a globe proof in Brighton that day, go have a laugh.
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### Re: Science fleeting thoughts

Pfhorrest wrote:I do think that I recall reading something about some kind of radiation experienced by objects moving at relativistic speeds which was basically a consequence of vacuum energy doing weird relativistic things, so that seems like it would be one possible option, as each part of this "stationary" sphere would be "moving" at relativistic speeds relative to the expanding space. But it's past 2AM here and I can't remember much about that right now.
The problem is that objects from far enough away to be heated, would also be heavily red shfted, reaching a maximum temperature of 10^-30 K, which is also so cold that photons would be able to fit in our universe anymore.

What's redshifted relative to what?

Dust flowing out through the sphere isn't redshifted relative to the part of the sphere it's approaching, and the different parts of the sphere aren't redshifted relative to each other because they're not moving relative to each other.
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### Re: Science fleeting thoughts

gmalivuk wrote:
Pfhorrest wrote:I do think that I recall reading something about some kind of radiation experienced by objects moving at relativistic speeds which was basically a consequence of vacuum energy doing weird relativistic things, so that seems like it would be one possible option, as each part of this "stationary" sphere would be "moving" at relativistic speeds relative to the expanding space. But it's past 2AM here and I can't remember much about that right now.
The problem is that objects from far enough away to be heated, would also be heavily red shfted, reaching a maximum temperature of 10^-30 K, which is also so cold that photons would be able to fit in our universe anymore.

What's redshifted relative to what?

Dust flowing out through the sphere isn't redshifted relative to the part of the sphere it's approaching, and the different parts of the sphere aren't redshifted relative to each other because they're not moving relative to each other.

I was about to say that parts of the sphere aren't coming from anywhere, but then I thought: isn't anything coming from anywhere going to be blueshifted?
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### Re: Science fleeting thoughts

gmalivuk wrote:What's redshifted relative to what?
The number I gave was relative to a point at the center of the sphere, although distant portions of the sphere's surface would also be red-shifted relative to each other. Specifically with regard to the Unruh effect heating the shell

The Cartesian coordinate frame in which the sphere in stationary is (in a sense) a fiction. No part of the surface is following a geodesic, and as such no part is truly staying in the same place. This is an effect of general relativity, and as such simply checking (in any coordinate system) if there's motion can be deceptive. The situation is very analogous to an object hovering just outside a black hole (or a spherical shell constructed just outside of the event horizon.
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### Re: Science fleeting thoughts

gmalivuk wrote:What's redshifted relative to what?
The number I gave was relative to a point at the center of the sphere

By which you mean, the sphere is redshifted relative to the center, right? (You missed the first "what".)

The Cartesian coordinate frame in which the sphere in stationary is (in a sense) a fiction. No part of the surface is following a geodesic, and as such no part is truly staying in the same place. This is an effect of general relativity, and as such simply checking (in any coordinate system) if there's motion can be deceptive. The situation is very analogous to an object hovering just outside a black hole (or a spherical shell constructed just outside of the event horizon.

Surely something a billion light years away would be blue-shifted if it were moving toward us (i.e. its proper distance to us were decreasing) fast enough, right?

So at what rate-of-change-of-proper distance would there be zero blue- or red-shift? You seem to be saying "zero" is an incorrect answer, but you're basing that on what?
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### Re: Science fleeting thoughts

Let me back up, I think my last explanation sucked. You may want to ignore what I said about no object not following a geodesic being stationary.

There are three kinds of red-shifting: Doppler, gravitational, and cosmological.

If we do just special relativity, the shell and the observer at the center aren't moving relative to each other. There is no red or blue shifting from the Doppler effect. An object not attached to the shell would naturally be moving away from us and could be expected to have a contribution from the Doppler effect.

Gravitational redshift is caused by the curvature of space and is discussed in terms of objects without relative motion (for example a person at the base of a mountain and on top of the mountain). Going up redshifts and down blueshifts.

Cosmological Redshift is the combination of both. When we calculate cosmological distances based off redshift assume there's a combination of those two factors to get the recessional speed.

The proper distance between the shell and the observer in the center is constant, so there is no Doppler redshift. Loose stuff "falling" thought the shell does have proper motion relative to the center and as such will be more redshifted.

The shell will still be red-shifted from the gravitational effect, and as such it will be more redshifted the further out it's placed.
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### Re: Science fleeting thoughts

Total redshift is a combination of cosmological, gravitational, and peculiar velocity redshift (i.e. Doppler shift). I'm pretty certain you can't treat cosmological as a combination of gravitational and peculiar velocity, though. (Why would a more distant sphere have more gravitational redshift, for example?)

---

Cosmological redshift depends only on how the scale factor of the universe has changed since the light was emitted. Peculiar velocity is by definition the difference between actual recessional velocity and the Hubble flow.

Where z = redshift, and assuming negligible gravitational component (i.e. the light emitter isn't near a black hole), the linked page has the equation:
1 + zobserved = (1 + zpeculiar)(1 + zcosmological)

Using the specific equations here:
1 + zobserved = sqrt((1+v/c)/(1-v/c))*a(now)/a(then)
Where a(t) is the scale factor, usually taken to be 1 at present.
a(then) = sqrt((1+v)/(1-v)) (in velocity units where c=1)

If H is constant (a good approximation in cosmologically short time spans, or in the very distant future), then a(t) = exp(H(t-t0)) (where t0 is "now", the time for which you've set a=1). The comoving recessional velocity at distance d is H*d, so that is also the inward peculiar velocity something would need in order to maintain proper distance to an observer at d=0.

In those same units (where c = 1), light emitted T time ago from T distance away (e.g. years and lightyears) from an object with zero recessional velocity gives the equation:
1 + zobserved = sqrt((1+HT)/(1-HT))exp(-HT)

This does increase without bound as HT approaches 1, it's true, but a constant H gets to be a worse and worse approximation as you go back in time.
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### Re: Science fleeting thoughts

I think the reason a photon can be redshifted even though the emitter and receiver are at constant distance is because the space it travels through between those points is expanding the whole time. Which would make some intuitive sense if the aether existed and was being stretched out as photons travel through it, but is hard to wrap my mind around knowing that isn't the case.
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cyanyoshi
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### Re: Science fleeting thoughts

Yeah, it's really weird how that works out. According to this video, the photons we see as the CMB were once higher-energy, but much of their energy was legitimately destroyed since they were emitted. I don't know enough about GR to say what's going on here. However I can't help but notice that i) the edge of the observable universe is a horizon, ii) the event horizon of a black hole is also a horizon, and iii) light emitted from close to a black hole gets redshifted by the time it reaches us. Maybe there's some deep connection between horizons and redshifing?

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### Re: Science fleeting thoughts

Mathematically speaking, in the limit as the source of light reaches the horizon, the photon redshifts to 0 frequency. This is one way of looking at the reason black holes are black.

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### Re: Science fleeting thoughts

Eebster the Great wrote:Mathematically speaking, in the limit as the source of light reaches the horizon, the photon redshifts to 0 frequency. This is one way of looking at the reason black holes are black.

Right. I guess the more interesting question is if there can be a horizon which does not cause things near it to be redshifted to oblivion.

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### Re: Science fleeting thoughts

Not if spacetime is continuous, I wouldn't think.

Beyond a horizon are events not in the causal past of any point in your future. That suggests that events on this side affect you farther and farther into the future as they get closer to it, with a limit of infinity.

Ergo time dilation and redshift.
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### Re: Science fleeting thoughts

gmalivuk wrote:Total redshift is a combination of cosmological, gravitational, and peculiar velocity redshift (i.e. Doppler shift). I'm pretty certain you can't treat cosmological as a combination of gravitational and peculiar velocity, though. (Why would a more distant sphere have more gravitational redshift, for example?)
Okay, so I think I may have made mistake somewhere. I was assuming that the situation of an object falling into a black hole was analogous to an object "falling" out of the universe. Hawking radiation is described mathematically as if it were from a fixed shell of ideal blackbody material arbitrarily close to the horizon.

In that case, the approachinginfinite acceleration produces an approachinginfinite temperature with approachinginfinitely hot black-body radiation, which is than red-shifted approachinginfinitely. Canceling out the limits produces a finite observed temperature that is very low for stellar black holes and ridiculously low for an observable universe sized mass.

But if they were analogous, either the rigid shell just outside of a black hole would be blue-shifted from it's "motion". Or galaxies (with zero peculiar velocities) near the cosmic horizon would be doubly red-shifted.
cyanyoshi wrote:Right. I guess the more interesting question is if there can be a horizon which does not cause things near it to be redshifted to oblivion.
Well a pretty good definition of a horizon is "place where things are redshifted to oblivion". It doesn't take too abstract thought to understand an association between "almost at thing that redshifts to oblivion" and "almost redshifted to oblivion".

If we cut out the almost, that means the free-fall velocity jumps to c (and the proper velocity to infinity), which mean the surface gravity is infinite at some spot, which means infinite tidal force, et cetra.

This may not seem a big difference since regular black holes still have infinities, but these are very different kinds of infinities. Physics has infinite limits and that situation has actual infinities.

"Can a guy who can move any rock move a rock that can't be move by any guy?" That question doesn't make any sense because the definitions of the guy and the rock contradict each other. This is what happens with actual infinities.

"Can a guy move a rock?" That question makes sense. "If we keep making the guy stronger, can he move the rock?" and "if we keep making the rock bigger, can the guy move it?" also make sense. "If we keep making the guy stronger and the rock bigger, can the guy move the rock?" is complicated, but also makes sense; we can figure out the answer by looking at the guy and the rock, and how they are being improved.
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### Re: Science fleeting thoughts

Quizatzhaderac wrote:I was assuming that the situation of an object falling into a black hole was analogous to an object "falling" out of the universe.
There are ways these are analogous, such as redshift approaching infinity, but I don't know why you'd assume the precise values would match in any particular way.

But if they were analogous, either the rigid shell just outside of a black hole would be blue-shifted from it's "motion". Or galaxies (with zero peculiar velocities) near the cosmic horizon would be doubly red-shifted.
I mean, most things are in a sence "triply red-shifted" (or blue-shifted), in the sense that to get the total observed redshift you have to consider doppler shift, cosmological shift, and gravitational shift.
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### Re: Science fleeting thoughts

gmalivuk wrote:
Quizatzhaderac wrote:I was assuming that the situation of an object falling into a black hole was analogous to an object "falling" out of the universe.
There are ways these are analogous, such as red-shift approaching infinity, but I don't know why you'd assume the precise values would match in any particular way.
It's not so much the precise values, but the list of what factors contribute and where, whether they are zero or not. And this is after controlling for other factors.

There are four situations under consideration:

Code: Select all

`Rigid or free  Near                  Proper velocity  experiences acceleration  corrected distance  Doppler red-shiftRigid          black hole            Zero             Yes                       Changing            NoFree           black hole            Non-zero         No                        Constant            YesRigid          Cosmological horizon  Zero             Yes                       Changing            YesFree           Cosmological horizon  Non-zero         No                        Constant            No`

Notes on sepcifics
Spoiler:
By "corrected distance" I'm generalizing the concept of comoving distance. Specifically, I mean adjusting out effects of the geometry of space-time, such that the distance between two objects doesn't change if neither of them experiences acceleration. Also,that this quantity is equal to proper distance in flat space-time. In situations with significant cosmological distances and no nearby significant masses, this is co-moving distance.

All situations have red-shift due to the geometry of space-time (gravitational or cosmological).

In the free black hole case, I'm assuming the proper velocity is exactly opposite the escape velocity; as if the object had fallen from stationary infinity. The "Corrected" distance can be extended to a "corrected" velocity, in which would be zero in this case.

In the rigid black hole case the "corrected" velocity would be non-zero, analogous to a peculiar velocity in the rigid cosmological case.

In the free cosmological case, I'm assuming zero peculiar velocity and no nearby objects able to produce significant gravitational effects.

In the rigid cosmological case, the peculiar velocity is necessarily exactly opposite the recessional velocity.
What I was expecting was for the rigid situations to be like to each other, and for the free situations to be like each other. Right up until we ask if there's a Doppler red-shift, they are.
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### Re: Science fleeting thoughts

As you fall into a black hole, your acceleration does not approach infinity from any perspective, at least not until you reach the singularity. The gravitational redshift does. Hawking radiation is not a result of this infinite redshift being cancelled out.

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### Re: Science fleeting thoughts

Code: Select all

`Rigid or free  Near                  Proper velocity  experiences acceleration  corrected distance  Doppler red-shiftRigid          black hole            Zero             Yes                       Changing            NoFree           black hole            Non-zero         No                        Constant            YesRigid          Cosmological horizon  Zero             Yes                       Changing            YesFree           Cosmological horizon  Non-zero         No                        Constant            No`

Why do you think the rigid cosmological horizon case experiences acceleration?

It would have to be sped up to get a peculiar velocity equal and opposite to the comoving recessional velocity, but after that's done, it's inertial again.

If you want a better analogy to a black hole's event horizon, I think you're looking for the Rindler horizon.
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### Re: Science fleeting thoughts

Eebster the Great wrote:As you fall into a black hole, your acceleration does not approach infinity from any perspective, at least not until you reach the singularity. The gravitational redshift does. Hawking radiation is not a result of this infinite redshift being cancelled out.
Yes, it does. The coordinate acceleration adjusted for time dilation approaches to infinity. Otherwise, one would be able to escape the event horizon with a finite thrust. That's what the event horizon is, the point where not finite outward acceleration can cause you to escape.

The proper speed between the distant observer and the in-falling observer increase asymptotically as (from the distant observer's perspective) the in-faller approaches the horizon; the proper velocity and the proper distance between the two approaches infinity over an infinite amount of time.
Why do you think the rigid cosmological horizon case experiences acceleration?
Recessional velocity is proportional to proper distance. Since the free falling object distance is increasing, it's recessional velocity must be increasing to maintain that constant relationship; a change in velocity is an acceleration. As the falling object is accelerating relative to the center, and the frame is not, therefore there is an acceleration between the two. This is also true if we start from looking at the change in proper or comoving distance between the shell and the falling object.

As the two are in the exact same position, we can rule out both comsological expansion and any kind of gravitational effect. The shell is the only to have a non gravitation force acting on it.
If you want a better analogy to a black hole's event horizon...
Not really, I'm only talking about black holes because I wanted analogy to the cosmological horizon and the Unruh effect, and the Unruh effect as it relates to black holes is well described.
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