# Antimatter and Matter

• December 2nd, 2005, 06:34 PM
The_Science_Geek
Antimatter and Matter
Okay, so I have heard everywhere that the difference between matter and antimatter was charge, but in that case neutral elementary particles would not have an antimatter opposite. In that case they would be Majorana particles, but I have heard many times that neutrinos can not be Majorana particles, because they "lack vector interactions".

I have also heard about neutrinos being "right handed" and antineutrinos being "left handed", but in that case, how would you originally tell if neutrinos were "right handed" (how would you know that it was neutrinos that were right handed and not antineutrinos)?

Neutrinos and antineutrinos have the same spin as well 1/2. How can an elementary particle have to be rotated 2 full times before it looks the same as it originally looked?

And if charge is the only difference between antimatter and matter, what causes them to annihlate each other? If it were charge it would just cancel out, right?

Sorry if these questions seem basic, but I am a newbie, and am only 12 and in 7th grade.
• December 5th, 2005, 11:37 AM
Neutrino
I'm not sure WHY they annihilate. When I did a fairly quick search online for that info, I didn't get any reasons - just a bunch of statements that they DO annihilate but not why.
What I think is particularly interesting is that mathematically, particles are indistinguishable from their anti-particle travelling backwards in time. Ie, an electron travelling backwards in time is exactly the same as a positron.
• December 5th, 2005, 04:20 PM
The_Science_Geek
I was thinking maybe the conservation of energy (the charge would dissapear, causing it to lose energy), so it had to annihilate, but I am not sure if the charge is energy.

Quote:

What I think is particularly interesting is that mathematically, particles are indistinguishable from their anti-particle travelling backwards in time. Ie, an electron travelling backwards in time is exactly the same as a positron.
Thats neat, but I thought mathmatically it was impossible to travel backward in time?
• December 5th, 2005, 04:54 PM
mitchellmckain
Quote:

Originally Posted by The_Science_Geek
I was thinking maybe the conservation of energy (the charge would dissapear, causing it to lose energy), so it had to annihilate, but I am not sure if the charge is energy.

Quote:

What I think is particularly interesting is that mathematically, particles are indistinguishable from their anti-particle travelling backwards in time. Ie, an electron travelling backwards in time is exactly the same as a positron.
Thats neat, but I thought mathmatically it was impossible to travel backward in time?

Nothing in mathematics or physics says that it is impossible to travel backward in time. It is only logic and common sense that there cannot be a means of traveling into the past that we can control.

But the idea that Neutrino is explaining is that if you play a movie of a particle backwards you get a movie that makes perfect sense if it is the anti-particle instead. This doesn't in any way mean that it is possible to reverse time for real. It just that there is a symmetry in the equations of particle physics that if you replace time t by its negative -t and you replace particles by their anti-particles, then you get back to the same equation you started with.

You may wonder how some thing like a positive charge can be changed into a negative charge simply by playing the movie backwards. But if M-theory or string theory is correct and the different particles are just different vibrational modes of branes or strings in a ten-dimensional space-time then it is not so difficult to see how this could be.
• December 5th, 2005, 08:54 PM
The_Science_Geek
Quote:

Nothing in mathematics or physics says that it is impossible to travel backward in time.
Okay, I wasn't sure about that. I jsut remember my Physical science teacher saying that we had the math to go into the future, but not the past.

Quote:

You may wonder how some thing like a positive charge can be changed into a negative charge simply by playing the movie backwards. But if M-theory or string theory is correct and the different particles are just different vibrational modes of branes or strings in a ten-dimensional space-time then it is not so difficult to see how this could be.
I don't know much about the string theory, but from what you are saying, it sounds like different types of the same particle are on different strings (opposite charge on one, opposite spin on another). Am I right?

I thought that it was 11 demensions though?
• December 6th, 2005, 03:34 AM
mitchellmckain
Quote:

Originally Posted by The_Science_Geek
I don't know much about the string theory, but from what you are saying, it sounds like different types of the same particle are on different strings (opposite charge on one, opposite spin on another). Am I right?

I thought that it was 11 demensions though?

11 dimensions were used in Supergravity theory (A kind of super-symmetric adaptation of General Relativity) which has been abandoned.

The point of what I said about string theory is that all the properties of the different particles can be reduced to how these strings or branes vibrate in these ten dimensions, so that running the movie backwards can change the motion that represents a positive electric charge into the mothion that represents a negative electric charge.