# Anti photons

1. ### cangus

26
Since an anti photon is a photon travelling backwards in time, is the max speed of an anti photon 186,000miles/sec, or is it -186,000?

2. ### chroot

10,426
Staff Emeritus
Actually, the photon is its own antiparticle. A photon and an antiphoton are exactly the same thing. Further, the concept of time really has no meaning for a photon, since it always travels the speed of light.

Also, a negative velocity has the same meaning as a positive velocity -- it just means the object is moving in the opposite direction.

- Warren

3. ### kurious

653
What about a negative energy photon - does that have momentum in the same direction as it is moving?

4. ### chroot

10,426
Staff Emeritus
There is no such thing as a negative energy photon.

- Warren

5. ### LURCH

2,514
You know, it's never occurred to me before that I've never heard of a negative energy photon. Is negative energy not considered to be quantized?

6. ### chroot

10,426
Staff Emeritus
I'm not really sure I understand lurch; I've never really heard of any "negative energy" particle.

- Warren

7. ### kurious

653
Negative energy could account for the attraction of masses by carrying momentum in the opposite direction to which it is moving and also it could be causing the acceleration of the universe because negative energy would repel positive masses whereas normal positive energy would deccelerate masses.
For more on this issue go to theory development - the mechanism of gravity .

8. ### arivero

3,011
Actually the attractive/repulsive character of a carrier has a lot of its spin. 0 and 2 are attractive, 1 is repulsive (for equal charges, this is).

9. ### LURCH

2,514
I haven't either, that's what I find so strange. Negative energy is known to exist, and I've done a little bit of research into it (though not very much, I'm afraid). It is only in the context of this discussion that I came to realize that I have never heard of a negative energy particle. In fact, I have never heard of negative energy being discussed in any quantized form.

10. ### Tom Mattson

5,538
Staff Emeritus
There are no negative energy photons because photon energies are differences in atomic energy states. The absolute values of atomic energy states have no significance in and of themselves, but their differences do. Since photons are only emitted in transitions from higher states to lower states, you are bound to get a positive photon energy. Of course, you can always adopt the point of view that absorbed photons (those that promote electrons from lower states to higher states) are actually negative energy photons that move backwards in time.

No, the energy levels of the hydrogen atom are En=(-13.6eV)/n2, where n=1,2,3,...

As I said, that's just a matter of convention, and we can just as easily define the states to have positive energies, but there's something appealing about having the zero of energy at n-->infinity. That way, we associate negative energies with bound states, and nonnegative energies with free states.

11. ### ZapperZ

29,986
Staff Emeritus
Maybe this is way too obvious to mention, but in case anyone forgets, the relativistic Schrodinger equation, as solved by Dirac, contains both positive and negative energy eigenvalues. When Dirac solve this, he still had fermions, but with these negative energies. This was the first indication of antiparticles that was verified later.

http://www.phys.ufl.edu/~korytov/phz5354/lecture_D06.pdf

So yes, there can be particles with "negative" energies. It is interesting to note that in condensed matter, these antiparticles correspond to the "holes" in the fermi sea below the vacuum state. So these holes also have negative energies.

Zz.

12. ### Solasis

2
Okay, this is probably going to sound idiotic, but it's 3:30 in the morning, I'm tired, and my mind is mixed up in Nemesis Theory, Evolutionism, Anti-Particles, and a story, but... Photons are particles, correct? And it doesn't make much sense to me to be your own anti-particle; if you are, you can't be annihalated. So, isn't it possible, by some weird anomaly, that anti-photons are not oppositely charged to Photons, but, rather, have the opposite gravitational pull?

13. ### dextercioby

12,314
Nope.In particle physics whe have a clear distinction of "antiparticle" and we have a clear rule of what might actually happens to the quantum field operators in order that that particle coincides with its own antiparticle.

The photon is its own antiparticle,just like the $\pi^{0}$ meson.

Daniel.

14. ### dextercioby

12,314
According to the famous analysis of the irreducible unitary representations of the Poincaré group by E.P.Wigner who published an article [1] in 1939,QFT can accomodate,basically for any value of spin and mass,negative energy particles.

$$\hat{P}^{0}|\vec{p},s_{3}\rangle =\mbox{sign}\left(p^{0}\right) \sqrt{\vec{p}^{2}+m^{2}} |\vec{p},s_{3}\rangle$$

for arbitrary spin massive particles and

$$\hat{P}^{0}|\vec{p},\lambda\rangle =\mbox{sign}\left(p^{0}\right) \sqrt{\vec{p}^{2}} |\vec{p},\lambda\rangle$$

for arbitary spin massless particles.

Upon reading more from [2],i realize that the third axiom of QFT (axiom III.a from [2]) states

"The spectrum of the energy momentum operator $\hat{P}^{\mu}$ belongs to the closed future light cone $\bar{V}^{+}$."

,therefore all negative energy unitary irreds of the restricted Poincaré group do not lead to physical states.

Daniel.

[1]E.P.Wigner (1939),"On Unitary Representations of the Inhomogeneous Lorentz Group",Ann.Math.,40,149.
[2]N.N.Bogolubov,A.A.Logunov,I.T.Todorov,"Introdiction to Axiomatic Quantum Field Theory",Benjamin/Cummings,NY,1975.

Last edited: Jun 22, 2005
15. ### Kruger

219
I have the following definition problem (maybe it's just my teacher's problem):

He said in radioactivity decay (beta+ decay) the positron that gets free isn't an antiparticle. Is that true? In my mind not.

16. ### El Hombre Invisible

I've heard of negative energy in two contexts:

1) the curvature of space - positive energy curves space one way, negative the other... one is well-like, one is saddle-like but I can't remember which way round it is;

2) Hawking radiation, in which a pair of positive and negative energy particles are created on the event horizon of a black hole and separated by its gravity - the negative energy particle is pulled into the hole and reduces its mass and the positive energy particle is expelled away from it. I believe this might be what the OP is referring to, since as I recall it should appear as if the positive particle has come from inside the black hole.

If this 'negative energy' particle is indeed travelling backwards in time, then if it were to be absorbed it would look, forward in time, exactly like a positive energy particle being emitted. This, I guess, demonstrates why the photon is its own antiparticle: a 'negative' photon being absorbed is indistinguishible from a 'positive' photon being emitted and vice versa. So as Tom said, you could talk in terms of emission events being absorption of negative energies and absorption events being the emission of negative energies - convention and everyday experience dictate otherwise.

17. ### dextercioby

12,314
The positron is a particle,the quanta of the positronic quantum field.It's the electron's antiparticle.Under any possible circumstances.

Daniel.

419
The positron IS an antiparticle - the antiparticle of the electron. Are you getting positrons and photons mixed up?

19. ### Kruger

219
My teacher is doctor in Physics. So, might be he is wrong.

20. ### El Hombre Invisible

Don't ask me why I looked at this thread again, but what is the positronic quantum field? I just googled it and got exactly zero matches.