Space and time are interchangeable at short distances; antiparticles come from this

If we have a relativistic electron, then the uncertainty in its energy E is of the same order of magnitude as the uncertainty in the momentum p.

We can combine the Heisenberg uncertainty principles into a spacetime uncertainty principle:

       Δ sqrt( t^2 + x^2 ) ΔE ≥ h / (4π).

Time and space appear completely symmetric in the relation and we can use the euclidean metric.

This is probably the reason why a "natural" wave packet of a Dirac electron always contains also negative frequencies, that is, the positron. The positron is an electron traveling back in time. If time and space are interchangeable, we cannot ban such paths, just like we cannot ban paths that go to the negative x direction.

We cannot ban superluminal paths either. The path integral for the electron should contain all kinds of paths, zigzagging in any direction in spacetime.

Richard P. Feynman in his 1949 papers stressed the symmetry of his diagrams when t and x are switched.

Large objects are, for some reason, confined inside the light cone (why?), but particles are not when the distance is less than their Compton wavelength. This allows the wave functions of particles be smooth in spacetime, which in turn may remove the need for regularizarion in the vacuum polarization loop of QED.

If a particle has a zero rest mass, or m + V is zero, then p = E, and the Heisenberg spacetime uncertainty principle is perfectly symmetric in time and space. Photons, too, can zigzag in time. Is there zitterbewegung of photons?

The symmetry of space and time at short distances explains why particles have to have antiparticles: there is no mechanism which prevents a particle from colliding in a way which sends it back in time, that is, it becomes an antiparticle.