# Antimatter, matter and time

Today my physics teacher said that "A matter particle going forwards in time is sort of like an antimatter particle going backwards in time."

What did he mean by "sort of"?

mathman
The backwards in time idea is a way of interpreting the mathematics of quantum theory. It does not have a physical meaning.

Staff Emeritus
This may not be quite what you are looking for, but I have found that when a teacher says something I don't understand, it pays to ask him or her what (s)he meant.

This may not be quite what you are looking for, but I have found that when a teacher says something I don't understand, it pays to ask him or her what (s)he meant.

It was my last lesson yesterday and I intend to ask him on monday, but I was wondering if I could find an answer here first.

HallsofIvy
Homework Helper
I believe that observation was originally due to Feynman. Draw something like a large "M" on a sheet of paper. Think of the space axis as horizontal and the time axis as vertical. Take a straight edge, place it horizontally across the bottom of the page and move it slowly up the page. Think of the points at which the lines cross your straightedge as being the positions of particles at the present time. Initially you have two particles that are moving toward each other. Think of the particle on your left as being an electron and the particle on the right as being a positron.

When your straightedge meets the central "V" of the "M", two new particles magically appear out of nowhere! That is a "creation pair"- a particle and its anti-particle, an electron and a positron. Now continue moving the straight edge up the paper. You now have two pairs of particle moving toward each other, one pair on the left, the other on the right. When you finally reach the two "/\" points of the "M" those two pair disappear: an electron and a positron have annihilated each other. The electron that was originally on the left has annihilated the positron that was created in the middle and the electron that was created in the middle has annihilated the postitron that was initially on the right.

Now lift your straightedge and look at the "M" itself. Do you see that you can think of that a single track: as an electron coming in on the left, moving "forward in time" then suddenly is turned "back in time" until, at the "V" it turns "forward in time" again , eventually hits the top "/\" and turns back in time once more. The movement "back in time" is what we, with our limited "one instant" perspective, see as a positron.

You might suspect that it would take some real cataclysm to change motion "forward in time" to "backward in time" and vice-versa. That's exactly right! It is the enormous energy, E= mc2, necessary to create an eletron-positron pair or given up when such a pair annihilates.

I have no idea how "serious" Feynman was when he proposed this but it does answer one basic question: All electrons are identical because there's really only one electron, bouncing back and forth in time!

I believe that observation was originally due to Feynman. Draw something like a large "M" on a sheet of paper. Think of the space axis as horizontal and the time axis as vertical. Take a straight edge, place it horizontally across the bottom of the page and move it slowly up the page. Think of the points at which the lines cross your straightedge as being the positions of particles at the present time. Initially you have two particles that are moving toward each other. Think of the particle on your left as being an electron and the particle on the right as being a positron.

When your straightedge meets the central "V" of the "M", two new particles magically appear out of nowhere! That is a "creation pair"- a particle and its anti-particle, an electron and a positron. Now continue moving the straight edge up the paper. You now have two pairs of particle moving toward each other, one pair on the left, the other on the right. When you finally reach the two "/\" points of the "M" those two pair disappear: an electron and a positron have annihilated each other. The electron that was originally on the left has annihilated the positron that was created in the middle and the electron that was created in the middle has annihilated the postitron that was initially on the right.

Now lift your straightedge and look at the "M" itself. Do you see that you can think of that a single track: as an electron coming in on the left, moving "forward in time" then suddenly is turned "back in time" until, at the "V" it turns "forward in time" again , eventually hits the top "/\" and turns back in time once more. The movement "back in time" is what we, with our limited "one instant" perspective, see as a positron.

You might suspect that it would take some real cataclysm to change motion "forward in time" to "backward in time" and vice-versa. That's exactly right! It is the enormous energy, E= mc2, necessary to create an eletron-positron pair or given up when such a pair annihilates.

I have no idea how "serious" Feynman was when he proposed this but it does answer one basic question: All electrons are identical because there's really only one electron, bouncing back and forth in time!

Thank you very much, that was exactly the kind of answer I was hoping for.

I do however have one question: How do the two new particles "magically appear out of nowhere"?

malawi_glenn
Homework Helper
vacuum energy, the vacuum has energy in physics.

vacuum energy, the vacuum has energy in physics.

Oh, so is the "magical" creation of the particles what they call a quantum fluctuation?

malawi_glenn
Homework Helper
well, these particles are only to exist a certain time interval, the more energy you "borrow" to violate energy conservation, the less time you can have it. Delta E times Delta t = Hbar

But now I think HallsofIvy was referring to pair creation out of photons.

This was the interpretation by feynman and stuckelberg and also the begining in a sense of feynman diagrams.Dirac's theory also predicted particles with negative energy.People at that time couldn't understand what that could mean.You have to think just like when a negative charge is leaving is like having a positive charge coming.that was the case.particles with negative energy travelling back in time are just particles with positive energy traveling forward in time.these are called antiparticles and have same mass but different charge with the original particles.Positrons were found five years after dirac's theory(1927-1932) and i think that feynman was really satisfied with the explanation.

I do however have one question: How do the two new particles "magically appear out of nowhere"?

Once you get comfortable with that idea, consider that may also be how our entire universe started....via a random quantum fluctuation!!!! Seems rather crazy, but NOT improbable.