particles travelling back in time

kurious

In quantum field theory particles are said to travel backwards in time.
I assume this is allowable over quantum distance scales.Over what kind of distance scale does such particle behaviour stop?
And if it happened when the universe as a whole had a radius equal to the quantum distance scale, would a particle travelling backwards in time still be acceptable to theorists? What was the motivation for having particles travelling backwards in time in the first place? And how is this sort of time travel compatible with the fact that I always see a clock travelling forwards in time?

Lama

If I do not mistake then anti-particles are particles that are traveling backwards in time, and by this we save the symmetrical picture between particles and anti-particles.

reilly

Feynman's use of "travelling backwards in time"made a basically technical issue easier to understand. From Dirac's genius we learned that negative energy electrons are, in fact positrons. What Feynman did was to take the minus sign from the energy in the standard exp(i(-E)t - ipx) so that he got exp(iE(-t)-px) , so a positive energy positron is a positive energy electron travelling backwards in time. This ingenious trick made field theory computations much, much easier that they were before his diagrams. His idea of travelling backwards in time is basically a computational aid.

Regards,
Reilly Atkinson

kurious

Reilly said:
"Feynman's idea of travelling backwards in time is a computational aid."

Kurious writes:

Saying that it is a computational aid suggests to me that it is not a valid representation of reality and so is not a technique that should be used.

Lama

Symmetry concept is maybe the most meaningful concept in abstract and non-abstract systems.

By symmetry we sometimes can find the deep connections that exist between so-called different things.

We have learned during the last 100 years that the power of simplicity that is expressed through symmetry can be found in the basis of many interesting abstract and non-abstract systems, for example:

Mendeleyev periodic table (http://www.nfinity.com/~exile/periodic.htm),

Hadrons family (http://www.egglescliffe.org.uk/physi...1/hadron1.html),

Fibonacci series (http://goldennumber.net/links.htm),

Gauge theory (http://www.britannica.com/nobel/micro/228_45.html).

In this address http://math.arizona.edu/~models/Topology/source/2.html
You can see how the symmetry which stands in the basis of a Donut, can be transformed to a Cofee cup, and vise versa.

The physical laws of nature are also described in terms of symmetry and broken symmetry states.