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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
spacerDate: Wednesday, 22.06.2016, 23:35 | Message # 571
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Quote Watsisname ()
A detector with the mass of Jupiter, placed next to a neutron star, could expect to see a graviton about once a century!

whaaaa. is gravity is that weak!!?
and we feel it very strong in our daily life and observe it! :o
but it can change in the future as we get more advanced! we may find things that will lead us to gravity secrets!





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

-space engine photographer


Edited by spacer - Wednesday, 22.06.2016, 23:54
 
WatsisnameDate: Thursday, 23.06.2016, 01:45 | Message # 572
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whaaaa. is gravity is that weak!!?


I was waiting for this. biggrin It's one of my favorite realizations in all of physics.

Consider the following:

With a magnet just the size of my fist, I can produce a field that supports my weight against the gravitational pull of the entire planet!

That's pretty insane!

Or suppose I have two protons: one which I fix down in place, and another that I release from rest 10cm away. How fast will its acceleration be when I release it, due to the electric repulsion of the other proton?
Answer: Slightly more than 1g (~14m/s2)

Ignoring the electric repulsion, how fast will its acceleration be due to the gravitational attraction between the two?
Answer: ~10-36g. I don't even know a name for a number that small offhand. Google tells me that's an "undecillionth". I'll write it as 0.000000000000000000000000000000000001.

The gravitational force is by far the weakest of all the fundamental forces. This can be seen by comparing their coupling constants. If the strength of the gravitational interaction is defined as "1", then the strength of the electromagnetic interaction is 1036 times stronger. The strong force: 1038. How about the so-called "weak" force? 1025.

This is pretty weird. The other three fundamental forces are within 13 orders of magnitude of each other (still a big difference), but gravitation is 25 orders of magnitude weaker than the weakest of them! Why should that be? We don't actually know. There are a lot of ideas for it. But this is strongly related to why the graviton is so hard to detect directly. Gravitation itself is weak. It only becomes relevant when you have a huge amount of stuff brought together. On a particle-by-particle basis, other forces totally dominate.

In fact -- counter-intuitively -- the whole reason gravitation becomes important on astronomical scales is precisely because it is so weak. After the Big Bang, charged and nuclear particles met up and became buddy-buddy very quickly, because they are so strongly attracting. And since there are positive and negative charges, they neutralize on large scales, so those forces don't become important astronomically.

Gravity on the other hand always attracts. But it is very weak. So it took a very long time for matter to begin to clump together to form structures within the expanding universe.

Quote spacer ()
but it can change in the future as we get more advanced! we may find things that will lead us to gravity secrets!


We can always hope. smile When black holes were first realized out of the field equations, few thought it possible that they would really exist in nature. Then when we started to learn that they do, and figure out their properties and appearance, people said "it's important that we get this right -- we will never be able to observe this." I love claims like those -- they usually end up being wrong, given enough time. Next year, we will have an actual image of one. How cool is that?





 
spacerDate: Thursday, 23.06.2016, 02:11 | Message # 573
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Quote Watsisname ()
Next year, we will have an actual image of one. How cool is that?

yeah and we also just detected last week more gravity wave!
also, which black hole we will try to image? sagittarius a? or some other massive one?





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

-space engine photographer


Edited by spacer - Thursday, 23.06.2016, 02:13
 
WatsisnameDate: Thursday, 23.06.2016, 03:23 | Message # 574
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Sagittarius A* for sure, as it has the largest angular size from Earth. (This is still absurdly small: ~50 microarcseconds. To put this in perspective, this is like if you are in Israel, and Midtskogen held up a coin in Norway [and if you could see over the curve of the Earth], then you could not only see the coin he is holding, but even read the inscriptions off of it!)

The team also hopes to observe M87 -- about 1600 times more massive but also about 2000 times farther away, so its apparent size is the second best at about 40 microarcsec. And these are calculated with consideration of the gravitational lensing, which increases the apparent size of the black hole shadow a bit.





 
midtskogenDate: Thursday, 23.06.2016, 05:25 | Message # 575
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50 microarcseconds is also close to the angular size of the International Space Station if it were in orbit around Jupiter!




NIL DIFFICILE VOLENTI
 
WatsisnameDate: Thursday, 23.06.2016, 05:57 | Message # 576
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WOW. That's a good way of putting it!




 
midtskogenDate: Thursday, 23.06.2016, 10:24 | Message # 577
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whaaaa. is gravity is that weak!!?
and we feel it very strong in our daily life and observe it! :o
but it can change in the future as we get more advanced! we may find things that will lead us to gravity secrets!

Yes. This leads me to my favourite passage in Newton's Principia (1726), where he insists on no speculation. Strictly observe and induce. I found this translation:

Quote Isaac Newton
Hitherto we have explain’d the phaenomena of the heavens and of our sea, by the power of Gravity, but have not yet assign’d the cause of this power. This is certain, that it must proceed from a cause that penetrates to the very centers of the Sun and Planets, without suffering the least diminution of its force; that operates, not according to the quantity of surfaces of the particles upon which it acts, (as mechanical causes use to do,) but according to the quantity of the solid matter which they contain, and propagates its virtue on all sides, to immense distances, decreasing always in the duplicate proportion of the distances. Gravitation towards the Sun, is made up out of the gravitations towards the several particles of which the body of the Sun is compos’d; and in receding from the Sun, decreases accurately in the duplicate proportion of the distances, as far as the orb of Saturn, as evidently appears from the quiescence of the aphelions of the Planets; nay, and even to the remotest aphelions of the Comets, if those aphelions are also quiescent. But hitherto I have not been able to discover the cause of those properties of gravity from phaenomena, and I frame no hypotheses. For whatever is not deduc’d from the phaenomena, is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferr’d from the phaenomena, and afterwards render’d general by induction. Thus it was that the impenetrability, the mobility, and the impulsive force of bodies, and the laws of motion and of gravitation, were discovered. And to us it is enough, that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies, and of our sea.





NIL DIFFICILE VOLENTI


Edited by midtskogen - Thursday, 23.06.2016, 10:31
 
midtskogenDate: Thursday, 23.06.2016, 11:01 | Message # 578
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Quote Watsisname ()
After the Big Bang, charged and nuclear particles met up and became buddy-buddy very quickly, because they are so strongly attracting. And since there are positive and negative charges, they neutralize on large scales, so those forces don't become important astronomically.

Gravity on the other hand always attracts. But it is very weak. So it took a very long time for matter to begin to clump together to form structures within the expanding universe.

That was a clear way to explain the structure of the universe at both the small and large scale, and why the scales and physics are so different. Thanks.

EDIT: Were the relative strengths of the forces equal in the early universe, that is, do they have any ties to the expansion of space?





NIL DIFFICILE VOLENTI


Edited by midtskogen - Thursday, 23.06.2016, 12:47
 
WatsisnameDate: Thursday, 23.06.2016, 21:54 | Message # 579
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Quote midtskogen ()
That was a clear way to explain the structure of the universe at both the small and large scale, and why the scales and physics are so different. Thanks.


Thank you. :)

Quote midtskogen ()
Were the relative strengths of the forces equal in the early universe, that is, do they have any ties to the expansion of space?


At least some of them become equal in strength and symmetry very close to the Big Bang -- this is the unification of forces. We know the electromagnetic and weak interactions unify at sufficiently high energy and are able to demonstrate it in the lab. Grand Unified Theories predict a unification with the strong interaction at even greater energy, and unifying gravitation would produce the so-called "Theory of Everything".

Unification definitely has an effect on the expansion of the universe at those energy scales. As the universe expands and cools the forces separate. This is akin to a "phase change", releasing energy and changing the character of the space. It is thought that these transitions explain the power of the Inflationary Epoch, and perhaps even the Big Bang and the formation of space-time itself.

A big question right now is what drives the accelerating expansion of the universe. We have a name for it, and a mathematical description from the field equations, but the exact physical mechanism remains unknown.

At some point when I stop being lazy I hope to make a big post explaining the cosmic expansion in detail -- what exactly it is, how we know it happens, and how does it work?





 
spacerDate: Thursday, 23.06.2016, 23:16 | Message # 580
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Quote Watsisname ()
I stop being lazy I hope to make a big post explaining the cosmic expansion in detail

but but...you wrote so much! tongue





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

-space engine photographer
 
WatsisnameDate: Thursday, 23.06.2016, 23:44 | Message # 581
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Yeah, I'm known for that. tongue But making a post for explaining expansion (and I want to actually go through the equations, show where they come from and what they mean) takes a lot more work. smile Actually I already have the derivations worked out by Newton's Laws -- I just need to write.




 
steeljaw354Date: Friday, 24.06.2016, 15:26 | Message # 582
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Is terraforming the sun actually possible and realistic?
 
AlekDate: Friday, 24.06.2016, 17:55 | Message # 583
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Quote steeljaw354 ()
Is terraforming the sun actually possible and realistic?


I doubt it. The gravity on the surface alone would be enough to crush you and thats if there was even something to stand on and the temperature wasn't high enough to melt practically any metal you can think of





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
WatsisnameDate: Sunday, 26.06.2016, 22:38 | Message # 584
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Moved the last page or so of posts to the Terraforming Thread, since this has developed into a nice on-topic discussion. smile




 
WatsisnameDate: Thursday, 30.06.2016, 07:16 | Message # 585
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A physics exercise for anyone interested, by the good people at PBS Space-Time. This was actually one of their easier ones (for a given value of 'easy'), and the idea behind it is interesting -- a description of how radioactive decay works via quantum tunneling. Here we are calculating the probability that the particle successfully tunnels or "teleports" out of the atomic nucleus. Nuclear/quantum physics is neat.



Can you figure out the answer? It is very small. smile (Requires only algebra and some physical intuition. I won't give away the solution or method here, but if anyone calculates a number I'll tell you if you are right.)





 
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