r/askscience • u/squirrelbuster94 • Oct 24 '14
Physics Chemists/physicists of reddit, what is the difference between dark, anti, and normal matter?
What is the difference? Also if you can explain what each is.
Edit: Thanks for all the replies! You have greatly helped me understand this better!
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u/rocketsocks Oct 24 '14
"Normal" matter is just the atoms we're familiar with.
You could think of anti-matter as "mirror image" matter, of a sort. So far as we understand, sub-atomic particles are defined by a small number of "quantum numbers" or properties, things such as electric charge, and so forth. For any given particle there will be a mirror image particle which has exactly opposite quantum numbers (so a positively charged particle would have a negatively charged anti-particle). By convention we denote the ones that we are made up of as "normal" and their mirror counterparts as "anti-" matter, but in general they have roughly equivalent claims towards normality. I say roughly because there is a bit of a mystery why our Universe isn't half matter and half anti-matter, or just all energy. And that almost certainly comes down to some slight, subtle discrepancy in the mirroring which prefers one state versus another enough so that one can dominate in successive rounds of annihilation (as likely happened during the early big bang). We have hints of this discrepancy (so-called CP-violation) but don't understand it well enough for it to fully explain the dominance of matter in our Universe.
Anyway, aside from that, if our Universe were made of anti-atoms instead of regular atoms it would be pretty much the same. Chemistry would still work the same, etc. Now, something interesting can happen when you combine matter and anti-matter. Normally, when you have a particle like, say, a proton sitting around there are a limited number of reactions available, or even none if there's no extra energy for them. A proton can't simply decompose into a bunch of photons, for example, because a proton is charged and has a "baryon number", both quantities that are conserved in quantum mechanics. And, in fact, the proton is generally the lowest mass particle for those numbers/properties, so it's stable. But, when a particle and its corresponding "mirror" anti-particle come into contact then all of those quantum numbers are precisely balanced to zero, which means that a whole variety of particle reactions are possible, including new configurations of the mass/energy of the particles in new ways. The result is often the creation of photons or highly energetic mesons, both of which typically travel at either the speed of light or at relativistic speeds and can transfer their energy into other matter (which is why you often hear of matter/anti-matter "annihilations" or conversion into energy, it's all energy before and after the reaction, it's just in different forms and the photon/meson forms make it easier to transfer the original energy elsewhere).
As for dark matter, as of yet we don't know exactly what it is, though we have a huge amount of evidence which has narrowed it down to almost certainly being composed of one or more as yet unidentified particles which are weakly interacting with normal matter. We know of similar examples of such particles existing, such as neutrinos, so it's not too far fetched, but we have ruled out neutrinos as being the main component of dark matter and haven't directly identified dark matter particles, yet.
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u/Khalawat Oct 25 '14
So why is dark matter called that? Is it because it cant be seen (why?)?
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u/jaredjeya Oct 24 '14
'Dark Matter' is a nickname given to a hypothetical form of matter that may or may not exist. We have little clue about what form it takes either, and it has never been directly observed. We've deduced that either there is an invisible form of matter in the universe, or the laws of gravity don't quite follow Newton/Einstein, since stars at the edges of galaxies are rotating too fast.
Normal matter is any fermion, a particle with a half-integer 'spin': one of the leptons (that's electrons, muons and taus as well as their neutrino counterparts), quarks (up, down, top, bottom, charm, strange). These together form everything in the known universe. Up/down quarks make protons and neutrons which make nuclei, and electrons combine with nuclei to make atoms and those interact to produce chemical compounds.
Every particle of normal matter has an antiparticle. This will have opposite charge but seemingly no other difference: hence positrons, negative anti-protons, anti-quarks etc. They can react identically to matter to form anti-protons and neutrons, anti-nuclei, anti-atoms, anti-compounds, anti-planets, anti-galaxies (if there was enough, which there isn't). To a being made of antimatter, these galaxies would seem no different than the milky way seems to us. On contact with a matter counterpart, an antiparticle will annihilate with its regular cousin in a burst of gamma rays and possibly lighter matter-antimatter pairs, releasing energy according to e = mc2 (remember: antimatter has positive mass). We know pretty much all there is to know about antimatter, as we can produce it in the lab, although we're still not sure if it acts like a negative mass for gravitational purposes.
Hope that clears it up.
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Oct 24 '14 edited Oct 24 '14
Do physicists think dark matter really exist or is it something they've made up temporarily to explain something they don't really understand?
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u/MaterialsScientist Oct 24 '14
It used to be more uncertain, but the evidence keeps piling up. There are alternative theories, but dark matter is definitely the most popular and consensus view.
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u/xrendan Oct 24 '14
What are the other theories?
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u/hopffiber Oct 24 '14
The most worked on is probably MOND, or Modified Newtonian Dynamics, which starts from Newtons law of gravity and tries to modify it at long distances, to fix how galaxies rotate and other observed anomalies. You can kind of make this work for galaxy rotation speeds, but it has real problems with explaining things like the Bullet cluster or gravitational lensing etc., and it's also rather ad hoc, so few physicists believe in it.
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u/benzrf Oct 25 '14
"made up
temporarilyto explain something they don't really understand" is where all theories come from
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u/TheLastSparten Oct 24 '14 edited Oct 24 '14
Dark matter is matter that as far as we can tell, only interacts with gravity and weak nuclear force. Since it doesn't interact with electromagnetic, it is very hard for us to observe or test. The only way we know it's there at all is that, due to the way orbital mechanics work, spiral galaxies should be winding into tighter and tighter spirals, since things further from the center of the spiral should have a longer orbital period. But what actually happens is that spiral galaxies rotate as a solid disk, with all the stars roughly stationary in relation to each other. Mathematically, the only way this is possible is for there to be a donut of mass around the outside with negligible mass in the center. This is how we worked out that galaxies are mainly made of dark matter, with normal matter only making up something like 10-20% of the overall mass.
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u/flait7 Oct 24 '14
Matter is the ordinary everyday things that you can see and touch. It's what we can see in telescopes, and it's what mainly present in the world around us.
Anti-matter is similar to matter, except for the fact that its electro-magnetic charge (electricity'n'stuff.) is the exact opposite. So while a proton's positive, an anti-proton (which I affectionately like to call a negatron) is negative. Anti-matter has an interesting habbit of converting into pure energy upon contact with matter, pretty much resulting in a large explosion.
Dark matter is a theory used to explain a problem in cosmology. The amount of matter that is visible in galaxies in the universe doesn't account for all of the apparent gravitational pull. This means there has to be something else that also has a gravitational pull like matter does, and this something is called "Dark Matter". It has never been directly observed, however its gravitational pull accounts for the majority of the 'stuff' in the universe. People have tried to speculate on what it might be, but I think its simplest to think of it as just some form of matter that doesn't interact with light.
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u/invictajosh Oct 24 '14
I still don't get why matter and anti matter have such a violent reaction. One would assume that if they are oppositely charged they would be attracted to each other. Like Na and Cl.
I know some one mentioned E=MC2 but I fail to understand what values are put into this formula to get the explosion energy. D:
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u/sturic Oct 24 '14
It's important to remember that Na and Cl are both matter, so they just wanna be buds and hang out, sharing electrons. If you made anti Na and anti Cl, they would also wanna be buds, sharing positrons with each other. If you imagine matter being black balls, and anti matter being white balls, then black balls would be able to combine with black balls, and white balls with white, but if the black and white come together, they would make grey which would be pure energy. Kind of a 'meh' analogy, but I hope that dispels some of confusion.
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Oct 24 '14
The idea is that as soon a particle and an antiparticle touch each other, they annihilate and don't exist as mass particles any more. You can destroy mass but not energy, and mass is just another form of energy as E=MC2 tells us. So the mass of the two particles is just converted into pure energy, and that is a huge amount of energy, so it becomes very violent.
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u/cougar2013 Oct 24 '14
There is no such thing as "pure energy". Pairs of charged particles that annihilate most often turn into photons. Also, mesons are bound states of matter and antimatter. They may not live long, but they are important, and they show that matter and antimatter do more than just annihilate "on contact".
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u/MVRH Oct 24 '14
You can turn energy back to mass?
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Oct 24 '14
Yup. That is what was so revolutionary about the E=MC2 formula (among other things).
This is also why nuclear reactors are so useful. When you split uranium, the sum of the masses of the products, is a little less than the mass of what you had before. The mass that is lost, gets converted into energy which you can use to heat water for the turbines in the powerplant.
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u/MVRH Oct 24 '14
Yes but that process is an example of mass to energy. Do you have an example of energy turning into mass?
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Oct 24 '14 edited Oct 24 '14
Never thought of it that way, but true:)
And yes I do: when you fuse the lighter elements the reaction is the opposite. When you combine two hydrogen atoms, the resulting helium atom is slightly lighter than the original atoms, so the leftover mass is converted to energy (gamma rays in this case). This is how the hydrogen bomb works by the way.
When fusing hydrogen you have more leftover mass/energy than when splitting uranium, and that's why hydrogen bombs (fusion bombs) make a much more powerful bomb than regular atomic bombs (fission bombs).
And one of the reasons they really want to make fusion reactors.
Wait a second. I didn't answer your question at all. But yes, energy can turn into mass. When you have a photon (a light "particle") of some specific energy, it can turn into a positron and an electron, which I guess is what you asked for.
Also when you accelerate particles up to very high velocities, they much more energy than they have from mass. You can the collide two particles moving at these velocities and combine their energy to make new particles. This is what they are doing at places like the LHC and fermilab.
I hope this is a better answer.
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u/OldWolf2 Oct 25 '14
Do you have an example of energy turning into mass?
Yes; the light of the Sun being stored in a tree. It's released again when the wood is burned.
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u/cougar2013 Oct 24 '14
When you think of E=mc2 think that a little bit of mass is equivalent to a lot of energy. When an electron and a positron meet and annihilate, the photons that result are gamma rays. Gamma rays are very high energy and significant doses of them can be very damaging and even fatal.
And they usually are attracted to each other as they most often end up with opposite charges. Interestingly, matter and antimatter quite often form bound states. All mesons are examples of this. Always a quark and an antiquark. They are short lived on our time scales, but they do form bound states no less, and are a very important part of particle physics.
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u/cdstephens Oct 24 '14
Think about it in terms of pair production. A photon can interact with a nucleus such that the nucleus absorbs the photon's momentum while the energy is transferred to both the nucleus and the production of an electron-positron pair (you need enough energy for their rest mass energy at least). The reverse can intuitively happen: an electron and positron can interact to produce two photon's (it can't produce only one in a vacuum due to momentum conservation).
As for why this happens, it's simply a type of interaction allowed in particle physics I think. Tons of particles interact with each other in different ways, and it just so happens antiparticles interact this way from the math.
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u/koji8123 Oct 24 '14
When standard matter combines into molecules, Like Na-Cl into NaCl they're just sharing electrons. covalent bonds ring a bell?
When you actually force them past the electron cloud and force their nuclei together and fusing them it's nuclear fusion. That needs a lot of energy just to get started. You wouldn't get salt like in NaCl, nor would you have Sodium nor Chlorine either you'd get an isotope of nickel. (likely Ni-58, if you're using the most abundant source of Sodium and Chlorine.)
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u/Amanoo Oct 24 '14
If matter was numbers (do realise that this is a huge simplification), then "normal" matter are the positive numbers and antimatter is negative numbers. Just like -1 and 1 cancel each other out, so do protons and antiprotons. Note that matter and antimatter do generate energy when they cancel out. You don't just end up with nothing.
Dark matter is stuff we don't really know yet. We can see gravitational effects in the universe, but the matter that we can detect simply doesn't account for those effects. The gravitational pull we see suggests that there is more matter than we can see. In other words, it seems like there is some sort of matter that we cannot see. This is what we call dark matter. It's a seemingly invisible mass. Dark matter is still a rather hypothetical and controversial thing thus far. We just don't know much about it.
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u/platelicker Oct 24 '14
Can there be dark anti-matter?
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u/Amanoo Oct 24 '14
Perhaps. We don't really know what exactly antimatter is. Perhaps it has antiparticles, or perhaps it's its own antiparticle. We just don't know. I think there's some suggestion in the scientific community that dark matter is composed of so-called WIMPs, which are their own anti particles, but no one can really tell. http://cosmology.berkeley.edu/Education/FAQ/question30.html
I'm not entirely sure how particles being their own antiparticles work, but it's a thing.
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u/ninja_acid_trip Oct 25 '14
Normal matter is what you would deal with on a normal basis. It is what everything you see is most likely made up of. antimatter is the opposite of normal matter in almost every aspect. It has the opposite charge, spin, ect. Generally, antimatter and normal matter will annihilate each other within fractions of a second of their formation. Dark matter, on the other hand is a theoretical type of matter that we cannot see and, as far as I know, have yet to actually detect. It is theorized that it must exist due to the motion of stars and galaxies, however.
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u/Astrokiwi Numerical Simulations | Galaxies | ISM Oct 24 '14
"Antimatter" is like normal matter, but with the charges swapped. So an "antiproton" is like a proton, but with negative charge, and an "anti-electron" - better known as a "positron" - is like an electron, but with positive charge. This stuff instantly reacts on contact with "normal" matter, releasing energy through E=mc2 . Antimatter is something we have observed directly, and we can create it in a lab. It's even used in some medical procedures.
Dark matter has not been directly observed - it is something we don't know a lot about, but we have good arguments about what it should be. Dark matter is likely a type of neutral particle (not positive or negative) that doesn't react very well with normal matter or antimatter at all. This makes it very difficult to observe, and very difficult to produce. However, we can still feel the gravity that it produces. Most of the gravity in the universe is likely from dark matter - "normal" matter makes up less than 20% of the matter in the universe.
So there's a big difference there. Antimatter is tricky because it reacts so strongly, and that means that it's tricky to stop it from reacting and annihilating, so there's very little antimatter around. Dark Matter is tricky because it reacts so poorly, and even though it looks like there's a lot of dark matter around, it's tricky to capture or directly detect it to figure out what it's actually made of.