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Andromeda is made of antimatter. Am I wrong? Why?

Of course I do not know that Andromeda is made of antimatter.
_but____ I do not know that Andromeda is made of matter.

Does anybody know what is the correct sentence? Why?

I used Andromeda as a substitute for any non-local galaxy.
Lets put some geometry (due to the first two comments):
Antimatter galaxies (AMGs) have in general AMGs in the neighborough. MGs only MGs.
For instance suppose the hurricanes in the northern hemisphere rotate X way and in the southern rotate anti-X way, and they dont meet ;).
That is to say, it is not mandatory that AMGs colide at all with MGs.

I suspect that we have no way to decide if the light we receive is originated in an anti-atom or from an atom.

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The answer---that we'd see the annihilation light from the inter-galactic medium at the boundary---is embedded in the answers to Experimental observation of matter/antimatter in the universe which is not a very good duplicate. – dmckee Jun 28 '11 at 2:41
@dmckee: And we've seen that large galaxies are built up from collisions of smaller ones, and we don't have any evidence of matter-antimatter galaxy collisions. So at least at the granularity of galaxy clusters, we don't have mmajor agglomerations of antimatter. – Omega Centauri Jun 28 '11 at 3:32
@dmckee seems to me since the answer is the same, it is a duplicate question that can be joined. Or is it worth answering how we would know if a spectrum came from collisions of anti atoms on atoms? – anna v Jun 28 '11 at 4:44
@Helder This Q is a total mess -- can you please smooth it a little (like rewrite)? And maybe remove the statement form? – mbq Jun 28 '11 at 5:46
@all I dont want to be disrespectful, by no means. I made the assertion in the title, but in the body I used both M or AM in equal foot. I used a more dramatic form only because we are so used to think that there exists an asymetry of M/AM that I had to be radical. Lets admit that due to a hidden property of the universe the M/AM are set apart at born, in general. So AMGs and MGs do not collide at all. We can be blinded by our deep convictions. I'm only trying to surpass any form of 'blindness' of my mind. We do not know. We have no theory. It can be done and ,imo, it will be done. When? – Helder Velez Jun 28 '11 at 10:18

I believe the current modern theory is that there cannot exist anti-matter galaxies unless anti-matter is discovered to have a repulsive interaction with matter via gravity (as opposed to the normal attractive force), which would have interesting applications to the structure of the universe. However, to my understanding, this is not generally theoretically supported as most models have anti-matter interacting normally, and also if Andromeda was an anti-matter galaxy then it would not be colliding with the Milky Way anytime soon.

If there were anti-matter galaxies with anti-matter behaving like matter gravitationally then we would occasionally see HUGE explosions across the sky as anti-matter galaxies collide with matter galaxies and anti-matter stars collide with matter stars.

Just some quick napkin calculations: lets take two solar mass stars, one matter and one anti-matter and see how much energy is released when they annihilate. $2M = 4 \times 10^{30}$ so since $E=mc^2$ we have $E= 3.6 \times 10^{47}$ joules which is about four times the energy released by the most intense gamma ray burst. Although the dynamics would be interesting as I wonder how much a repulsion effect annihilations with inter-stellar medium would be, as well as wether or not when the two stars start to annihilate they would "explode" away from each other...

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We know that they are not colliding. If they are set in a non collisional route, at born, by some property (of the universe) that we do not have derived yet. I also understand that we have no 'trace' of this possibility until now because an anti-atom radiates exactly as an atom. – Helder Velez Jun 28 '11 at 12:50
Helder Velez With such assumptions of initial conditions, i.e. that the universe is totally divided and there is no interface where matter and antimatter meet in space to annihilate and give the characteristic radiation of e+e- annihilation and p antip one, you could maintain that the demoted planet Pluto is made of green cheese because we cannot go there and find out if this is true or not. The question with apriori no interface allowed in the domains of matter and antimatter is non sensical. – anna v Jun 28 '11 at 13:54
@anna in this WP Baryon_asymmetry I see that you cant provide an explanation for your preferred M/AM asymetry. IYO the [Alpha Magnetic Spectrometer experiment guys] ( (delivered at ISS in May 19, 2011) are spending M$s seeking green cheese. You are aimed to seek, or just waiting, the explanation for the asymetry. I'm aimed to understand why we can not see the symetry.May be the case that my goal is easier to achieve. Yesterday I saw a way to why 'Chirality',rooted is the same ground, IMO. – Helder Velez Jun 29 '11 at 0:09
@Helder the difference with your speculation is that they are taking data. One can then speculate on theories that explain data. Constructing models of the universe such that no data can even be extracted are pure speculation and belong to science fiction, imo. – anna v Jun 29 '11 at 4:30
There is lots of evidence for stellar collisions:… The article you cited even says in the final paragraph even says that where there are higher densities of stars going faster (say, in collision of globular clusters of an anti-matter galaxy and a matter galaxy), collisions will be much more common. – Benjamin Horowitz Jun 30 '11 at 6:56

IF Andromeda were antimatter, or any galaxy in general, we would be able to notice the huge amount of energy from colliding matter and antimatter galaxies. As we don't, the only explanation is (while still thinkin' "Andromeda is antimatter") that matter and antimatter would repel each other instead of having normal gravity. However, Andromeda IS going to collide with milky way and if the un-gravity theory were true, it would not be happening.

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from WP Interacting_galaxy: "Due to the extremely tenuous distribution of matter in galaxies, these are not collisions in the normal sense of the word, but rather gravitational interaction." Also on Earth the Northern hurricanes can not collide with the southern anti-hurricanes and both can exist. – Helder Velez May 10 '13 at 11:23

One way of looking at this is thru time. Andromeda is either "made of matter" or "made of antimatter" if we experience it using a direction of time we exhibit on our galaxy. If we experience it in reverse time, it changes from matter to antimatter and vice versa.

So the question is whether the direction of time is absolute. If it's not, Andromeda is either made of matter or antimatter, depending on the direction of time of the observer.

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We can remember that an Anti-atom was conserved alive by 15 minutes of normal time not long ago. By no means I can 'eat' the anti-time possibility. I know that in almost all equations where time appears (by conservation laws) we can substitute t by -t but it is only in the equations not in the universe. Time units are derived from the atomic properties and 'c' speed. Time concept reflects only the ordering of events. The same event can not happen and rehappen, and rehappen; even if they appear to be the same event they are distinct events and one proceed the other. – Helder Velez Jun 28 '11 at 11:14
So your opinion is that direction of time is absolute? (based on your sentence "By no means I can 'eat' the anti-time possibility") – Mayoneez Jun 28 '11 at 12:30
Yes indeed. But the time unit is derived from atomic size and constant 'c' speed (property of space). All system of units are 'atomic units'. Construct a world with doubled atoms (physical laws holding the same ) and the atomic time unit will be halved. After Einstein we know that the local time unit can also change due to motion, mass. ( see more in the paper mentioned in this PSE-answer – Helder Velez Jun 28 '11 at 12:41
According to QED, anti-particles are identical to their (real)-particle counterparts with the only exception being that the direction of time has been reversed. I would therefore suggest that our universe has both entities that experience it in forward time and entities that experience it in reverse time. If you say that the direction of time is absolute, there cannot be any anti-matter and your original question becomes obsolete. – Mayoneez Jul 1 '11 at 6:28

protected by Qmechanic May 9 '13 at 18:13

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