# Time in QFT and in special relativity

1. Apr 27, 2012

### exponent137

Special relativity gives that time for a (traveler on) photon do not run. It also gives that every moving elementary particle rest in some inertial system, but photon does not rest in any inertial system.

But how this can be visible in Quantum field theory or in QED? An electron and a photon are too similar in QED.

2. Apr 27, 2012

### strangerep

Not really. Electrons have non-zero invariant mass, whereas a photon's is 0.

3. Apr 28, 2012

### exponent137

This is true, it is seen in propagator like $1/(m^2+p^2)$.

But it is not enough explained, how physics with m and without m in propagator is different?
It is also not enough explained how calculations are essentially different?

4. Apr 28, 2012

### Neandethal00

If photons have no mass then why do we treat them as particles?
A photon probably is a form of energy, not a particle.
The best we can do is give it a theoretical mass as

Energy equivalent of photon mass = m(photon) = hf/(c2).
Will it then run into trouble with Relativity?

5. Apr 28, 2012

### strangerep

That depends on which textbook you're reading. (You didn't which textbooks you've studied).

Advanced treatments like Weinberg give a lot of detail about the differences between massive and massless field representations of the Poincare group.

6. Apr 28, 2012

### strangerep

Photons have zero invariant mass. In modern textbooks, both massive and massless fields are constructed as representations of the Poincare group. The older terminology of "particle" is gradually being replaced by "field".
It's misleading to say that a photon "is" a form of energy. A more accurate picture is that a photon field has both energy and momentum.
What you describe is called the "relativistic mass", which is a distinct concept from "invariant mass". (Both can be useful in different circumstances.)

Relativistic mass changes under velocity boost transformations. But invariant mass is (surprise!) invariant under those transformations.

http://en.wikipedia.org/wiki/Relativistic_mass
http://en.wikipedia.org/wiki/Invariant_mass

Rindler's textbook on special relativity is also quite good.

7. Apr 29, 2012

### Demystifier

It does, but not in a Lorentz inertial system. It is at rest in a light-cone inertial system. The coordinate transformation from Lorentz coordinates x, t to light-cone coordinates x', t' is
x'=x-ct
t'=x+ct