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How do we know that matter and antimatter were created in equal amounts during the big bang (is it not possible for there have been slightly more matter than antimatter at the start)?
Also, why is it that a proton/neutron can be annihilated by either an antiproton or an antineutron while an electron can be only annihilated by a positron?
Original post by BP_Tranquility
Also, why is it that a proton/neutron can be annihilated by either an antiproton or an antineutron while an electron can be only annihilated by a positron?


Source? I would assume that opposite quarks will annihilate so:

Proton + antineutron:

uud + -(udd) --> u + (-d)
Which I think is a pion? A similar case for a neutron / antiproton annihilation I think (you'd get another pion with an opposite charge)
(edited 9 years ago)
Original post by lerjj
Source? I would assume that opposite quarks will annihilate so:

Proton + antineutron:

uud + -(udd) --> u + (-d)
Which I think is a pion? A similar case for a neutron / antiproton annihilation I think (you'd get another pion with an opposite charge)


Oh, thanks :biggrin: It's from a book called Antimatter by Frank Close

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If the sun were about to disappear, it would take about 8 mins before we realised that it had disappeared. However, how long would it take before Earth stopped its centripetal motion and started moving at a tangent (I.e. how long would it take for us to notice that the Sun's gravitational pull disappeared)?

Since gravity is a distortion in space-time, wouldn't the effect of losing the suns gravitational pull be instantaneous since the speed limit (c) only applies to objects moving through space if that makes sense?
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(edited 9 years ago)
Original post by BP_Tranquility
If the sun were about to disappear, it would take about 8 mins before we realised that it had disappeared. However, how long would it take before Earth stopped its centripetal motion and started moving at a tangent (I.e. how long would it take for us to notice that the Sun's gravitational pull disappeared)?

Since gravity is a distortion in space-time, wouldn't the effect of losing the suns gravitational pull be instantaneous since the speed limit (c) only applies to objects moving through space if that makes sense?
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How do you propose making the sun instantaneously disappear without breaking the law for conservation of mass/energy?
Original post by F1 fanatic
How do you propose making the sun instantaneously disappear without breaking the law for conservation of mass/energy?


No idea, I will was being hypothetical-let's assume a universe which is an exact replica of ours without those laws :tongue:

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Why does an orbiting electron radiate light according to Maxwell's theory of electromagnetism? I don't know much about the theory so apologies if it's a silly question

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Original post by BP_Tranquility
If the sun were about to disappear, it would take about 8 mins before we realised that it had disappeared. However, how long would it take before Earth stopped its centripetal motion and started moving at a tangent (I.e. how long would it take for us to notice that the Sun's gravitational pull disappeared)?

Since gravity is a distortion in space-time, wouldn't the effect of losing the suns gravitational pull be instantaneous since the speed limit (c) only applies to objects moving through space if that makes sense?
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Pretty sure the distortions in space-time would not disappear instantaneously- they would disappear at the speed of light, so I think we'd continue to orbit our shared centre of gravity for eight minutes before continuing along a tangent.

The universal speed limit applies to everything I think- at the very least it does apply to information. (If someone better at physics could confirm this, that would be nice.)
Original post by F1 fanatic
How do you propose making the sun instantaneously disappear without breaking the law for conservation of mass/energy?


The question can probably be rewritten in a form that makes some sense

Basically, it's asking whether gravity acts instantaneously, or at the speed of light. I don't know GR well enough to say for sure, but I'm pretty sure the latter is true.
Original post by lerjj
The question can probably be rewritten in a form that makes some sense

Basically, it's asking whether gravity acts instantaneously, or at the speed of light. I don't know GR well enough to say for sure, but I'm pretty sure the latter is true.


Agreed, in Newtonian mechanics gravity propagates instantaneously, but in general relativity it is limited in the same way as everything else by the speed of light and so gravity changes are not felt instantaneously. This then leads to the prediction of gravitational waves.
Original post by F1 fanatic
Agreed, in Newtonian mechanics gravity propagates instantaneously, but in general relativity it is limited in the same way as everything else by the speed of light and so gravity changes are not felt instantaneously. This then leads to the prediction of gravitational waves.


Thanks for the info. Are gravitational waves effectively space time "re-aligning" (for lack of a better word) to the way they should be after some masses move around?

I.e. for a set of masses in certain positions, spacetime has one structure. If the masses move around, spacetime has to assume a slightly different structure. Are gravitational waves the way that spacetime changes it's structure, or are they something else? (You may have guessed that I don't know too much about this topic, I've never studied GR so I'm a bit lost :colondollar:).
Original post by lerjj
Thanks for the info. Are gravitational waves effectively space time "re-aligning" (for lack of a better word) to the way they should be after some masses move around?

I.e. for a set of masses in certain positions, spacetime has one structure. If the masses move around, spacetime has to assume a slightly different structure. Are gravitational waves the way that spacetime changes it's structure, or are they something else? (You may have guessed that I don't know too much about this topic, I've never studied GR so I'm a bit lost :colondollar:).


I have never studied it in great depth either, I went down the small scale rather than the large scale route!

But yes, gravitational waves are ripples in space time. They are still theoretical at this point, one of the last unproven elements of general relativity, although you may have heard of the recent claims to have seen hints of gravitational waves in the BICEP2 experiment, but it still needs to be confirmed by other experiments as there remain some questions about the data quality.
Original post by F1 fanatic
Agreed, in Newtonian mechanics gravity propagates instantaneously, but in general relativity it is limited in the same way as everything else by the speed of light and so gravity changes are not felt instantaneously. This then leads to the prediction of gravitational waves.


Thanks! -that explains it :biggrin:

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Original post by BP_Tranquility
How do we know that matter and antimatter were created in equal amounts during the big bang (is it not possible for there have been slightly more matter than antimatter at the start)?


Source?

Although it is speculated that matter and antimatter existed in equal shares during big bang, there are also some scientists who believe in a nonequilibrium between these quantities, just because all kinds of existence will be disappeared when matter and antimatter would exist and join together in equal shares. That's why some scientists supposed that there was more matter than antimatter in the period of big bang. In their point of view, it was just a tiny little nonequilibrium which has caused the universe and the totalitarian existence. But they don't know anything of this theory exaclty...
(edited 9 years ago)
Original post by Kallisto
Source?

Although it is speculated that matter and antimatter existed in equal shares during big bang, there are also some scientists who believe in a nonequilibrium between these quantities, just because all kinds of existence will be disappeared when matter and antimatter would exist and join together in equal shares. That's why some scientists supposed that there was more matter than antimatter in the period of big bang. In their point of view, it was just a tiny little nonequilibrium which has caused the universe and the totalitarian existence. But they don't know anything of this theory exaclty...


Okay, thanks -I just remember hearing the idea of scientists thinking that there was a balance between matter/antimatter at the big bang in documentaries etc -so you're saying there's no scientific evidence or mathematical models/predictions that support this?

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Does anyone know of any good, informative documentaries on space/astrophysics? I've tried to watch the Universe but I felt that it didn't convey enough information per episode (I.e. were too 'dumbed down')

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Original post by BP_Tranquility
Okay, thanks -I just remember hearing the idea of scientists thinking that there was a balance between matter/antimatter at the big bang in documentaries etc -so you're saying there's no scientific evidence or mathematical models/predictions that support this?


I have nothing found what confirmed our statement. The scientists believes in a nonequilibrium, because anitmatter and matter convert to energy in form of light after Einstein's well known theory of relativity E = mc², if these quantities hit each other. After that matter and antimatter are gone. That's why nothing would exist anymore, if antimatter and matter would exist in equal shares. But if there was more matter than antimatter at the period of big bang, the overbalance of this matter would be left after antimatter and matter hit each other. And this overbalance was the reason in the view of the scientists why everything in the universe exist today.
Original post by Kallisto
I have nothing found what confirmed our statement. The scientists believes in a nonequilibrium, because anitmatter and matter convert to energy in form of light after Einstein's well known theory of relativity E = mc², if these quantities hit each other. After that matter and antimatter are gone. That's why nothing would exist anymore, if antimatter and matter would exist in equal shares. But if there was more matter than antimatter at the period of big bang, the overbalance of this matter would be left after antimatter and matter hit each other. And this overbalance was the reason in the view of the scientists why everything in the universe exist today.


I've heard of this idea where majorana particles were being decayed at the same rate as they were formed, but as the universe cooled down, there was no longer enough energy to produce new majoranas. The majorana particles upon decaying would release neutrinos and antineutrinos which hit the particles/antiparticles causing an imbalance. I can't remember the more specific details but have you heard of this by any chance?


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Original post by BP_Tranquility
I've heard of this idea where majorana particles were being decayed at the same rate as they were formed, but as the universe cooled down, there was no longer enough energy to produce new majoranas. The majorana particles upon decaying would release neutrinos and antineutrinos which hit the particles/antiparticles causing an imbalance. I can't remember the more specific details but have you heard of this by any chance?


I have nothing heard about majorana particles at all. Sorry! The only one what can I tell you is that the charges of particles in antimatter are vice versa in comparison to matter. So an atom of anitmatter consists of positrons instead of electrons, anti-neutrons instead of neutrons, anti-protons instead of protons. That's why antimatter and matter cancel each other.

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