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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
midtskogenDate: Sunday, 31.07.2016, 08:05 | Message # 646
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I don't think homogeneity or isotropy would have to be abandoned. And of course we don't know how a different perspective would be. It's hard to describe a new insight beforehand. :)

Let's imagine that the geocentric view of antiquity survived the renaissance. That our minds were so stuck in the idea that earth is the centre of the universe and that everything in the sky can only be described as spheres or circles. As observational accuracy improved, we would have added epicycles, and would congratulate ourselves with our progress every time we see how remarkably well our models would fit observations, further convincing us that divine circles exist, though their physical properties are obscure. I suppose it can be mathematically proven that any irregular orbit can be described as a sum of circles (cf. Fourier), so we could be doing this forever, and the proof would just serve to us as a proof of the divinity of circles.

Of course, abandoning the geocentric view and the idea of divine circles would dramatically reduce the complexity of the theories, though they were accurately matching observations.

I'm not suggesting that this would be a direct analogy of dark energy/matter. I only say let's not rule out the possibility that a different approach would simplify our theories, specially if theories start out as or correspond to something ad hoc. But for now we can be perfectly happy using dark energy and dark matter to describe what we observe.





NIL DIFFICILE VOLENTI


Edited by midtskogen - Sunday, 31.07.2016, 08:06
 
WatsisnameDate: Sunday, 31.07.2016, 09:40 | Message # 647
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Watsisname, I always wondered, does dark energy and dark matter. are so hard to detect maybe because they dont react to any force exept gravity? and gravity is the weakest force, and we havent even detected graviton yet.


Kind of. smile We know of them because of their large scale gravitational effects.

Dark matter is just like regular matter except "invisible". Its only meaningful interaction is by gravity. So it collapses into galaxy and galactic cluster sized regions, increasing the gravitational field there, which we can measure by its effect on the motions of things we do see, and the lensing of light.

Dark energy can also be thought of as a gravitational effect, except a repulsive one. Again we don't measure it directly, but infer its existence from how it affects the large scale universe.

Detecting dark matter directly is a problem of particle accelerators. There are possibly some indirect clues from observational astronomy, such as radiation from its own annihilation.

Figuring out how to directly detect dark energy is... challenging. Despite being the most significant form of mass-energy in the universe (at least since the last few billion years, and ever more so in the future as the universe expands), on the local scale it is totally dominated by regular matter. If the mass of dark energy were as protons, then its density is only a few protons per cubic meter. But... dark energy isn't protons. It's not even particles, in the usual sense. To directly detect it, we must probe the vacuum itself. This is weird. It's super strange, super mind-bendy, and super exciting.

midtskogen,

If we adjust our geocentric model of planetary motions with epicycles, telling ourselves the model agrees with observations, then I agree this is rather unsatisfactory. Of course it would agree with the observations. We're forcing it to. That was the motivation for making them. It's ad hoc in the extreme. And that's where the power of the model ends. There's no convergent evidence to favor those adjustments. The model is not corroborated by new predictive power.

That's what's so different with dark matter and dark energy. They make further predictions for which there is no real reason to expect them to agree with observations. There's no reason to expect that dark matter and dark energy would work out to produce the correct form of the angular power spectrum. But they do. There is a variety of independent convergent evidence that favors the ΛCDM model. This is why cosmologists take it so seriously and act like they are sure these things are real, despite not having directly detected them.

Look, I get why this is so hard to swallow for most people. I really do. We're saying we are confident in the existence of invisible, mysterious entities and forces that we can't detect directly. It feels ad hoc and model-tweaky. It feels like there must be some simpler explanation from another angle that we're just not seeing. But you really have to go through the motions and see how these things are figured out.

Consider the model of the atom as a dense, positively-charged nucleus surrounded by electrons, developed by observation of the scattering and deflection of alpha particles fired at thin metal foils. Prior to these experiments, the favored model was "plum-pudding", where it was thought the positive and negative charges were mixed together in some kind of substrate. Imagine how surprising it was to discover massive, high-velocity alpha particles usually scattering by a couple degrees as they pass through (makes sense for the alpha interacting with the charges), but occasionally deflecting straight back. "Like shooting a cannon-ball at a piece of tissue paper and having it bounce back and hit you." So Rutherford proposes there is a dense nucleus with the same type of charge as the alpha particle. This explains why, very rarely, the alpha bounces right back. But it's ad hoc. A fix of the model to explain observations. It works, but why is it satisfactory? Maybe there's a simpler explanation?

Maybe there is. But this one makes further predictions. And those predictions happen to work very well with what we observe with the alchemy of radioactivity, and the further development of nuclear physics.





 
HuesudoDate: Sunday, 31.07.2016, 10:49 | Message # 648
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Quote Watsisname ()
This is units of Earth surface gravities (9.8m/s2) per meter.


So if you were at a point where you felt 1 G from the black hole and mover one meter towards it, you would feel 2 Gs? surprised
 
WatsisnameDate: Sunday, 31.07.2016, 11:15 | Message # 649
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Not exactly. The difference in the pull of gravity over that one meter is 1g. You might be falling in with an acceleration of thousands of g's. It's not the acceleration that your body feels (you don't feel freefall), but the difference. If you fall in feet-first, it feels like your feet are pulled downward and your head upward.

This is why being close to stellar-mass black holes is very bad for you. The difference in the pull of gravity over your body is huge. It easily tears you apart. surprised





 
spacerDate: Sunday, 31.07.2016, 11:19 | Message # 650
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Quote Watsisname ()
This is why being close to stellar-mass black holes is very bad for you. The difference in the pull of gravity over your body is huge. It easily tears you apart

i heard that we could survive falling to super massive black hole.
i guess i know understand why smile





"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer
 
FastFourierTransformDate: Sunday, 31.07.2016, 16:27 | Message # 651
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i heard that we could survive falling to super massive black hole.
i guess i know understand why smile


Yes. Because tidal forces are not so great from that distance to the singularity (if you are above the event horizon of a very very massive black hole). But you would be equally disrupted after passing the horizon.

Alow me to continue with the stellar encounter issue smile
Quote Watsisname ()
Alek, thanks for checking that out! It seems that an expected close encounter would have affected its orbit in a measurable way (if we could measure it), but not so much as to greatly affect the solar system's dynamics. That's pretty neat!


Yeah! that's totally awesome. If we finally discover planet 9 we could find clues of a close stellar encounter with the solar system. And maybe with the Gaia data we could find the star that once invaded our system. Maybe we exchanged material with that star (asteroids and comets) and we would know that there are captured objects of the other system in our solar system to explore to find different chemical composition.

Well, considering that the galactic year is about 250 million years (not so much) it's probable that this star could be everywhere in the galaxy, totally undetectable or not in the gaia census. So I may be so wrong about my expectations.
 
spacerDate: Sunday, 31.07.2016, 16:33 | Message # 652
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this star could also be the reason for mass extinction events on earth.
also there is good probabilty that a planemo pass our system. and who knows?
maybe planet 9 is a planemo that cought by the sun gravity smile





"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer
 
midtskogenDate: Sunday, 31.07.2016, 17:04 | Message # 653
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Quote Watsisname ()
Look, I get why this is so hard to swallow for most people. I really do. We're saying we are confident in the existence of invisible, mysterious entities and forces that we can't detect directly.

It's not hard to swallow that dark matter and energy exist. It's the best descriptions that we have. And it doesn't matter that it can't be detected directly. Most exoplanets can't either, but exoplanets are familiar things, the same kind of stuff that we see and touch in our own solar system, so there is no reason to think that these things are different from what they look like, no reason to suspect that we're describing shadows in Plato's cave.





NIL DIFFICILE VOLENTI
 
WatsisnameDate: Monday, 01.08.2016, 00:20 | Message # 654
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Quote FastFourierTransform ()
Well, considering that the galactic year is about 250 million years (not so much) it's probable that this star could be everywhere in the galaxy, totally undetectable or not in the gaia census. So I may be so wrong about my expectations.


We have the same problem with trying to identify the Sun's "sibling" stars, which formed with it out of the same molecular cloud. We can identify them by their compositions, ages, and tracing back their orbits, but over 4.5 billion years they've drifted a long way and are hard to find. We finally just found one a couple years ago, which was a nice surprise. smile

With stars that had very close encounters with the solar system long ago, another challenge is that the precision of measurement of their motion may be too weak to constrain the distance and geometry of the encounter, if we're dealing with only thousands of AU. I think it'd be really hard to connect a distant star to a precise change in solar system orbits, without knowing what the orbits were before the encounter.





 
HuesudoDate: Monday, 01.08.2016, 10:48 | Message # 655
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Quote midtskogen ()
the same kind of stuff that we see and touch in our own solar system

Our solar system has a bunch of peculiarities. The innermost planet is a stripped iron core, and the two outer gas giants are in places where planet formation shouldn't be possible.

And unlike most other solar systems discovered, ours doesn't have any hot jupiters or super earths surfing just above the star's surface. surprised
 
midtskogenDate: Monday, 01.08.2016, 11:18 | Message # 656
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Quote Huesudo ()
Our solar system has a bunch of peculiarities. The innermost planet is a stripped iron core, and the two outer gas giants are in places where planet formation shouldn't be possible.

Why is this peculiar?

Quote Huesudo ()
And unlike most other solar systems discovered, ours doesn't have any hot jupiters or super earths surfing just above the star's surface.

That's because it's those kind of planets we can currently discover. You can't base ideas of typical planet distribution on that.





NIL DIFFICILE VOLENTI


Edited by midtskogen - Monday, 01.08.2016, 11:18
 
WatsisnameDate: Monday, 01.08.2016, 12:04 | Message # 657
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It's also possible that systems where gas giants migrate close to the host stars are less likely to produce habitable-zone terras.




 
midtskogenDate: Thursday, 11.08.2016, 16:54 | Message # 658
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Where are the new particles?




NIL DIFFICILE VOLENTI
 
spacerDate: Thursday, 11.08.2016, 17:05 | Message # 659
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need more time. if we didnt find it now, doesnt mean we wont find any forever.
there is missing piece of the pazzle, i hope we find it someday. if not today.... there is always tomorrow.
smile





"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer
 
WatsisnameDate: Thursday, 11.08.2016, 20:52 | Message # 660
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The obvious answer is that they have gone off into hiding, from the awful particle physicists who would smash them to bits. tongue

This looks like bad news for super-symmetry. Multiverse aficionados are probably loving it. I was reading a paper in the same vein the other day about the lack of sterile neutrinos found by IceCube, which was also a bit of a disappointment.





 
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