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Antimatter Problem + Arrow of Time

  1. Dec 11, 2009 #1
    Hey all,

    First post so I should introduce myself. I'm a freshman physics+math major at Virginia Tech, probably headed towards particle physics. I do some undergrad research w/ one of my professors, pretty lame though as I only build photomultipliers, but nontheless my foot's in the door. I'll probably be spending some time here while I'm in school.

    I apologize if this has been posted before, I looked a little and I don't think it has. I'm wondering whether the arrow of time can explain the anti-matter problem, or at least how they relate. It seems that the universe is made out of normal matter. Anti-matter obviously can be created for short periods of time in high energy collisions or as virtual particles one would see in a Feynman diagram. One way to think of an annihilation event in a Feynman diagram is to picture an electron going forward in time, emitting a photon, and then heading backwards in time. Entropy is increasing (second law), time seems to be going forward on large scales, so does this have anything to do with the preponderance of matter vs. antimatter? I've always been unsatisfied with the "Well, there was one extra proton at t=0, so everything annihilated and now it's all matter." Thanks.
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  3. Dec 11, 2009 #2


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    Welcome to PF!

    Hey jacksonwalter ! Welcome to PF! :smile:
    No … anti-matter can be created permanently, and without high energy …

    the positrons created for example in positron emission tomography have an infinite lifetime (well, as infinite as the electron's), and no high energy is involved …

    an unstable nucleus will undergo beta decay, which for an excess of neutrons in the nucleus means electrons are produced, but with an excess of protons, positrons are produced …

    see http://en.wikipedia.org/wiki/Beta_particle" [Broken] :wink:

    I don't understand where anti-matter comes into your example of a Feynman diagram … it only has two electrons (alternatively, it has an electron and a positron, both going forward in time) :confused:

    Anyway, the choice of what to call matter and anti-matter (in a Feynman diagram or anywhere) is purely arbitrary.

    In an "anti-matter galaxy", everything (including Feynman diagrams) would still work, and entropy would still increase.
    Last edited by a moderator: May 4, 2017
  4. Dec 12, 2009 #3


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    Welcome aboard.
    I'm the uneducated mouthpiece of PF, so I always defer to those who have gone to school. My understanding was that antimatter can be thought of as matter traveling backward in time. The term that I saw was 'time reversal invariant'. It was based upon the fact that if you film a particle interaction going forward, that same film run backward will describe an antimatter interaction. There is no violation of conservation laws.
  5. Dec 12, 2009 #4
    Yup, got that, said that in my post.

    Yup, that's fine I was just giving an example.

    That's cool, I realize they don't decay by themselves. I meant they'll probably interact with matter fairly soon after they're created.

    Right, I understand there are many processes in which positrons are produced, really not the issue I was trying to get at.

    What are you talking about? It does not have two electrons, that was half the point of my post. The whole diagram has one electron that is going forwards in time and then goes backwards in time.

    Right, understandably you could call the positron an electron and vise versa, you could call the space axis time and the time axis space, really not the point at all.

    Okay, this is the only somewhat relevant bit. If entropy would still increase if everything was going backwards instead of forwards in time, then could you explain why rather than just throwing that out there.
  6. Dec 12, 2009 #5


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    Actually, you didn't. Your reference was to an individual particle going forward in time, emitting a particle, and then backing up through time. That's not the same thing at all.
  7. Dec 12, 2009 #6


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    I have a similar idea, where arrow time refers to proper-time: Matter and antimatter advance in opposite directions along the proper-time dimension, while light doesn't advance along the proper-time dimension at all. Unlike the coordiante-time dimension in Minkowski space time, the proper-time dimension is very similar to the spatial dimensions.

    I explain space-propertime in post #1 and the interpretation of matter/antimatter within it in post #2 of this thread:

    But I agree with tiny-tim that moving in a opposite direction in proper-time, does NOT reverse the thermodynamical arrow of time: Entropy in an antimatter galaxy would still increase. As I said: the proper-time dimension is very space-dimension-like: it doesn't have an distinguished positive/negative direction. Anti-matter is not moving "back" along the proper-time dimension, just in the opposite direction than matter.
    Last edited: Dec 12, 2009
  8. Dec 12, 2009 #7


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    hmm … that's just quibbling over how we count them … do we say that there are two electrons, or just one electron?

    but whether there are two or one, I still don't understand how you make any deduction about anti-matter …

    the diagram can be described as one electron that is going forwards in time and then goes backwards in time, but it can also be described as one positron (anti-matter) that is going forwards in time and then goes backwards in time …

    so how can that be used to distinguish between matter and anti-matter? :confused:

    (and let's remember that a casual observer simply sees matter and anti-matter going forward in time and annihilating each other to produce a photon … with the matter and anti-matter on equal footing :wink:)
  9. Dec 12, 2009 #8
    Clearly I need to rephrase my idea a little.

    Yes, I agree, entropy would still increase in an anti-matter galaxy.

    Danger, I don't know how you don't see your first post as redundant. It looks like some Wikipedia paraphrasing and didn't really add much to the discussion.

    What I really wanted to do was roll up the question into "Why don't we see anti-matter galaxies or large amounts of anti-matter in the universe?" into "Why does time have a preferred direction?" (i.e. we seem to be moving 'forward' along it and there doesn't seem to be a mechanism to move 'backwards' along it). I think these are two aspects of the same question.
  10. Dec 12, 2009 #9


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    I'm not sure if we can tell the difference between matter and antimatter, just by observing the radiation it emits.

    Why do you think, that we seem to be moving 'forward'? We just defined it to be 'forward'. An anti-matter-alien would claim that he is moving 'forward' and we 'backwards' (and he would call us 'anti-matter-aliens'). The only thing that is clear, that we move in the opposite direction than he is.

    If you consider space-propertime, you'll find that the propertime-dimension is equivalent to the 3 space dimensions: there is no preferred direction along it. There is no absolute 'back' & 'forward' in time, just like there is no absolute 'up' & 'down' in space.
  11. Jan 16, 2010 #10
    Could you explain in a little more detail what proper time is and then elaborate on the connection to space? From what I've read it seems almost like 'rest time' in the sense that it's the time measured in the same reference frame as the observer, akin to rest mass, mass measured in an inertial frame. Basically, I don't see how the concept of proper time removes preferred direction.
  12. Jan 16, 2010 #11
    The positron lifetime in ordinary matter is a few picoseconds or nanoseconds, depending on whether it forms a singlet or triplet atomic positronium state with an electron. In an absolute vacuum, its lifetime is forever. Positrons from nuclear decay are used in PET (positron emission) tomography, and in the process is annihilated and emits two opposite 510-Kev gamma rays. Reversing the annihilation process requires a lot of energy in the lab.
    Bob S
  13. Jan 16, 2010 #12


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    Proper time is what a clock measures. Coordinate time is what a clock at rest measures. So if you observe a moving rocket with an attached clock, this clock shows the rockets proper time. While your wrist watch shows coordiante time (or the observer's proper time).

    The connection to space is just a possible geometrical interpretation of Relativity, where aging (accumulating proper time) is equivalent to moving along a proper time dimension, which is orthogonal to space:

    The idea is that clocks and entropy measure the absolute value of movement along the proper time dimension. But they don't tell you which direction along the proper time dimension the clock moves. By convention we take the positive square root when computing proper time, yet for antimatter-clocks we could just assume the negative one. But antimatter-aliens would probably call our direction in proper time the negative one.

    There is no absolute 'forward' in poper time just like there is no absolute 'left' in space.
  14. Jan 16, 2010 #13
    My preferred proper time definition is always going forward in time in a Feynman diagram. If there is an annihilation vertex, there is a positron and electron going into it, not an electron going into the vertex, scattering, and then leaving by going backwards in time.

    Theorists may disagree.

    There is also (for experimentalists) the proper time definition in a fixed relativistic Lorentz reference frame (as stated in previous posts).

    Bob S
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