# Antimatter Problem + Arrow of Time

• jacksonwalter
In summary, the conversation discusses the concept of anti-matter and its relationship with the arrow of time. The idea of antimatter being created through time reversal invariance is mentioned, as well as its potential to explain the anti-matter problem. The concept of proper-time is also brought up as a possible explanation for the preponderance of matter over anti-matter in the universe. It is ultimately concluded that moving in the opposite direction in proper-time does not reverse the thermodynamic arrow of time.
jacksonwalter
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.

Welcome to PF!

Hey jacksonwalter ! Welcome to PF!
jacksonwalter said:
… 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. …

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"

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)

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.

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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.

Danger said:
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.

Yup, got that, said that in my post.

tiny-tim said:
No … anti-matter can be created permanently, and without high energy …

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

tiny-tim said:
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 …

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.

tiny-tim said:
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 …

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

tiny-tim said:
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)

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.

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

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.

tiny-tim said:
In an "anti-matter galaxy", everything (including Feynman diagrams) would still work, and entropy would still increase.

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.

jacksonwalter said:
Yup, got that, said that in my post.

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.

jacksonwalter said:
I'm wondering whether the arrow of time can explain the anti-matter problem, or at least how they relate.
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.

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jacksonwalter said:
tiny-tim said:
jacksonwalter said:
… 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. …
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)
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.

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?

(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 )

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.

jacksonwalter said:
"Why don't we see anti-matter galaxies or large amounts of anti-matter in the universe?"
I'm not sure if we can tell the difference between matter and antimatter, just by observing the radiation it emits.

jacksonwalter said:
"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).
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.

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.

tiny-tim said:
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
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

jacksonwalter said:
Could you explain in a little more detail what proper time is and then elaborate on the connection to space?
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:

jacksonwalter said:
Basically, I don't see how the concept of proper time removes preferred direction.
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.

jacksonwalter said:
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.
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

## 1. What is antimatter?

Antimatter is a type of matter that is made up of particles with the same mass as normal matter, but with opposite electrical charge. When antimatter comes into contact with normal matter, they annihilate each other and release a large amount of energy.

## 2. What is the antimatter problem?

The antimatter problem refers to the fact that the universe we observe is made up of mostly matter, with very little antimatter. This raises the question of why there is such an imbalance between the two, as the laws of physics suggest that equal amounts of matter and antimatter should have been created during the Big Bang.

## 3. How does the arrow of time relate to the antimatter problem?

The arrow of time refers to the concept that time only moves in one direction, from the past to the future. The antimatter problem is related to the arrow of time because the laws of physics are symmetrical with respect to time, meaning that they should behave the same whether time is moving forward or backward. However, the observed imbalance between matter and antimatter suggests that the arrow of time may be asymmetrical, which is still a mystery in physics.

## 4. What are some proposed solutions to the antimatter problem?

One proposed solution is that there may be some unknown physical process that favors the creation of matter over antimatter. Another is that there may be a fundamental difference between matter and antimatter that has not yet been discovered. Additionally, some theories suggest that there may have been a period of time in the early universe where matter and antimatter were not equally distributed, leading to the observed imbalance.

## 5. How does studying antimatter help us understand the universe?

Studying antimatter can provide valuable insight into the fundamental laws of physics and the early universe. By understanding why there is an imbalance between matter and antimatter, we can gain a deeper understanding of the processes that govern the universe and potentially uncover new physics. Additionally, antimatter is used in medical imaging and has potential applications in energy production, making it a valuable area of research.

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