Hot answers tagged antimatter
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So, what is antimatter?
Even from the name it is obviously the "opposite" of ordinary matter, but what does that really mean?
As it happens there are several equally valid ways to describe the difference. However, the one that I think is easiest to explain is that in antimatter, all of the electrical charges on all of the particles, at every level, have ...
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To the best of my knowledge, most physicists don't believe that antimatter is actually matter moving backwards in time. It's not even entirely clear what would it really mean to move backwards in time, from the popular viewpoint.
If I'm remembering correctly, this idea all comes from a story that probably originated with Richard Feynman. At the time, one of ...
14
If I ruled the world, I would ban the phrase "pure energy" in contexts like this. There's no such thing as pure energy!
When particles and antiparticles annihilate, the resulting energy can take many different forms -- one of the basic principles of quantum physics is that any process that's not forbidden (say, because of violation of some sort of ...
14
The ultimate goal is to be able to do precision spectroscopy of antihydrogen, to make sure that the energy states are the same as in ordinary matter. If there are differences between the energy levels of ordinary hydrogen and antihydrogen, that would violate "CP" symmetry, which says that if you change the sign of all the charges in some system, and invert ...
12
The Dirac equation implies negative energies as well as positive. This is due to energy-momentum relation $E=\pm \sqrt{m^2+p^2 }$. If we replace $E$ and $p$ by operators $E\to i\frac{\partial }{\partial t}$ and $p\to -i\nabla$ we get the Klein-Gordon equation $(\Box+m^2)\phi=0$ for scalar (spinless) fields $\phi$. The problem with this equation is that it ...
12
Well, they do and don't. Depends on your point of view. Here's the story.
Quantum field theory requires for consistency reasons that every charged particle has its antiparticle. It also tells you what properties will the anti-particle have: it will have the same characteristic from the point of view of space-time (i.e. Poincaré group) which means equal mass ...
8
This refers to Feynman's 1949 theory.
See http://www.upscale.utoronto.ca/PVB/Harrison/AntiMatter/AntiMatter.htmllink text
From there:
"Feynman's Theory of Antimatter
In 1949 Richard Feynman devised another theory of antimatter.
The spacetime diagram for pair production and annihilation appears to the right. An electron is travelling along from the lower ...
8
Dear Chad, you misinterpret the statement that "the known sources of CP-violation are not enough to explain the matter-antimatter asymmetry in the Universe."
You seem to think that the statement means that the known CP-violating parameter (namely the CP-violating phase in the CKM matrix) and the processes based on it are qualitatively insufficient to ...
8
It depends on your definition of annihilation. But microscopically all processes are described by Feynman diagrams such as these
of which last one describes electron positron annihilation (if it weren't for the typo in the out-going photon). But as you can see it's all a simple matter of how you turn your head around and the very same diagram represents ...
8
Black holes and "anti"-black holes are the same objects. A black hole resulting from the collapse of normal matter, and a black hole resulting from the collapse of antimatter, are indistinguishable. Recall that black holes only have charge, mass, and spin and there is no way to tell that a black hole originally was matter or not (e.g., we can't measure B or ...
8
Charge is only the most familiar of the properties that are inverted between a particle and its antiparticle, but it's not the only one. So you should not consider "same mass and opposite electric charge" to be a definition of what an antiparticle is; it's merely a plain-English explanation.
A list of properties in which particles and antiparticles differ ...
7
The do not disappear with zero energy.
Their energy (both that originating from rest mass and any kinetic energy) appears somehow. As photons, a spray of other (lighter) particles, etc.
For instance, when an electron meets a positron (that is, a anti-electron) the most common result is a pair of gamma rays each of 511 keV (in the center of momentum (CoM) ...
7
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 ...
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You'll find Dirac's 1933 Nobel lecture on the Nobelprize.org website. The pdf is quite brief (5 pages long) and speaks on the antiproton at the end (p4). The argument is the following :
In any case I think it is probable that negative protons can exist, since as far as the theory is yet definite, there is a complete and perfect symmetry between positive ...
7
From the very basic understanding that they are created out of nothing mutually and collide to annihilate each other seems to indicate this happens due to an attraction.
Why? this just means that if two of them are nearby, they can annihilate. Remember that particles are waves, and thus are quite spread out. They don't have to be directed to collide ...
7
That other question mentioned in the question-comments also discussed annihilation of particles and neutralization of electric charge inside the event horizon. In either question, the no-hair theorem trumps all. If GR is the end of the story, particle identity is destroyed by the singularity. Even if post-GR theories of gravity rescue the Universe from the ...
7
The anti-particle corresponding to a neutron is an anti neutron!
The neutron is made up of one up quark and two down quarks. The anti-neutron is made up of an anti-up quark and two anti-down quarks. Both have zero charge because the charges of the quarks within them balance out.
You are correct that elementary particles with no charge are often their own ...
6
In quantum field theory and its extensions including string theory, the electric charge is a generator of a $U(1)$ symmetry which should be promoted to a local symmetry i.e. gauge symmetry.
In string theory, the $U(1)$ symmetry and the gauge field often appear as parts of the low-energy effective action. This could be enough to answer the question: we ...
6
The anti-particle for any particle is obtained by charge C and parity P conjugation. C is the operation that interchanges positive and negative charges and P is the operation that reflects in a mirror. The combined operation of CP must produce a particle of the same mass. This is a theorem of relativistic quantum field theory due to CPT symmetry. This other ...
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So there are two different things you might be talking about which often get conflated in popular level discussions of the topic:
Real particle/anti-particle pair production in reactions. An example reaction (one among many) would be $ p + \gamma \rightarrow p + e^{-} + e^{+} $ (a proton and a gamma ray collide to produce a proton and electron positron ...
6
From the article to which you linked:
The cloud shines brightly in gamma rays due to a reaction governed by
Einstein’s famous equation $E=mc^2$. Negatively charged subatomic
particles known as electrons collide with their antimatter
counterparts, positively charged positrons.
So you see, the very reason that we know it's a cloud of antimatter is ...
5
Antimatter is in every precise meaningful sense matter moving backward in time. The notion of "moving backward in time" is nonsensical in a Hamiltonian formulation, because the whole description can only go forward in time. That's the definition of what the Hamiltonian does--- it takes you forward in time a little bit. So if you formulate quantum mechanics ...
5
There is one technical inaccuracy in saying that antimatter moves back in time (whatever it might mean). In quantum field theory we get positive energy solutions (usual particles) and negative energy solutions. Negative energy solutions behave in time as if they were propagating backward in time. But they are not the antiparticles, they are just the ...
5
Never seen any good reason to think that antimatter is affected any differently.
Light and anti neutrinos (from supernovas) seem to be affected by gravity the same as regular mass, and the theory of gravity (general relativity) doesn't distinguish mass and any other energy.
If we beam a gamma photon down the gravitational well (from space to ground, for ...
5
Yes, photons are the same if produced by matter or antimatter. A simple way to see this is that electromagnetism is CP, C and P invariant. Then, changing particles by antiparticles in any process one should obtain exactly the same "amplitude" (i.e. energy levels, probability of absortion emmission, etc...).
Weak interactions are neitehr CP nor C or P ...
5
1) Does antimatter-matter symmetry exist? Yes there is a CP violation and the whole Nobel prize thing. On the other hand there is CPT symmetry which is very protected. So call it what you want. As for the popsci articles... I would express my thoughts, but this is a family site.
2)Does CP violation explain matter-antimatter imbalance? It's certainly ...
5
Good question! There actually isn't a term for this that I know of. The most common use of such a term would be to classify a particle, for example "the 'polarity' of the electron is matter-polarity," but in that case most physicists would just say "the electron is a matter particle."
There is a mathematical operator called the charge conjugation operator, ...
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