Vacuum in Quantum Field theory is not empty

1. Aug 1, 2007

emanaly

I know that the vacuum in Quantum Field theory is not empty, but sometimes I find some people say that the particles are created from nothing because they are created from the vacuum , are those people expression a misleading?

Last edited by a moderator: Mar 7, 2013
2. Aug 1, 2007

bel

When we say vacuum, we mean that it is space, emptied of matter. The particles created in a vacuumn are created by pair production when two beams of energetic (in both sense of the word) electromagnetic radiation collide to form a particle- anti-particle pair in accordance with Einstein's energy mass equivalence. In effect, the particles are created out of energy, not exactly nothing.

3. Aug 1, 2007

Norman

I think the original poster was wondering about pair production in the vacuum with respect to fluctuations due to the Heisenberg Uncertainty Principle. This does not need a reaction to create particles. See for instance the Casimir Effect.

So, the real question is "Can you call the QFT vacuum 'nothing'?"

I don't know. Personally I don't think of it as nothing since I see it as a dynamic system in which these pairs are being created. So in my mind the QFT vacuum is not "nothing."

I think this question may be borderline philosophy however.

4. Aug 1, 2007

emanaly

Yes Norman, you expressed rightly what I mean

5. Aug 1, 2007

humanino

There are philosophical questions you might ponder about in those matters, somewhere else.
However, in QFT the vacuum is well-defined, if not well-known.
The vacuum in QFT is the state of lowest energy possible.

You can work the mathematics out, and see for yourself that indeed, it is not "empty".
It fluctuates and has non-vanishing average values for some observables.

6. Aug 1, 2007

meopemuk

In quantum field theory, the ideas of "vacuum" and "particle" become rather complicated. In the beginning, we define vacuum $|0 \rangle$ as a no-particle state and we define creation operators $a^{\dag}$ which produce 1-particle states by acting on the vacuum

$$|1 \rangle = a^{\dag} |0 \rangle$$

So far, everything is nice and easy. Next, we define the Hamiltonian of our theory, which is an operator expressed as a function of creation and annihilation operators $H(a^{\dag}, a)$.

Now, according to physical intuition, we would expect that our vacuum $|0 \rangle$ is an eigenstate of the Hamiltonian with lowest (zero?) energy. We would also expect one-particle states $|1 \rangle$ to be eigenstates of the Hamiltonian. But this is not true in QFT! It appears that (almost) all Hamiltonians used in QFT do not have $|0 \rangle$ and $|1 \rangle$ as their eigenvectors.

The usual answer to this puzzle is to say that vacuum $|0 \rangle$ is not the real physical vacuum state, and $|1 \rangle$ are not states of real physical particles. It is said that $|0 \rangle$ is a so-called "bare" vacuum, and $|1 \rangle$ are states of "bare" particles. The vacuum and particles we see in reality are called "physical". These physical vacuum $|vac \rangle$ and one-particle $|one \rangle$ states are true eigenvectors of the Hamiltonian, and they are expressed as some complex linear combinations of "bare" particle states. This is the reason why one often hears that vacuum if full of (bare and virtual) particles, and that physical particles are "dressed" by the cloud of (virtual bare) particles.

This situation is a bit paradoxical. In QFT nobody cares about properties of bare particles and states. We want to study physical particles and states. However, our Hamiltonian $H(a^{\dag}, a)$ is expressed through bare particle operators. This makes all calculations and their interpretation very cumbersome.

A great new idea arrived in 1958:

O. W. Greenberg, S. S. Schweber, "Clothed particle operators in simple models of quantum field theory", Nuovo Cim., 8 (1958) 378.

They said (rephrased): "since we don't care about bare particles, there is no reason to keep their creation and annihilation operators in the theory. Let's express our Hamiltonian directly in terms of creation and annihilation operators of "physical " or "dressed" particles. Let's work directly with the full physical vacuum state $|vac \rangle$." Greenberg and Schweber were able to show that "bare" particles can be eliminated and quantum field theories can be reformulated in this "dressed particle" picture without losing anything of importance. In this picture, vacuum $|vac \rangle$ is a no-particle state (there are no "physical" particles in vacuum). One-particle states $|one \rangle$ have just one (physical) particle in them. So, our physical intuition should not be offended.

So, in answering you question about vacuum filled with virtual particles, I should say that this (unfortunately widespread) idea is an artefact of using unphysical (bare) states as our basis in QFT. In fact, there are no "physical" particles in the "physical" vacuum.

Eugene.

Last edited: Aug 1, 2007
7. Aug 2, 2007

Barmecides

Thanks a lot meopemuk for this great answer very clear !
By the way, when we are talking about vacuum in electroweak symmetry breaking for example, are we talking about |0> or |vac> ?

8. Aug 2, 2007

meopemuk

Unfortunately, gauge field theory (including spontaneous symmetry breaking and confinement) is formulated in a language that is quite different from what I presented. I am still struggling to understand its exact meaning. So, I'll let others to answer your question.

Eugene.

Last edited: Aug 2, 2007
9. Aug 2, 2007

Nick666

Cant it be that the fabric of space-time continuum or space is transforming into the virtual particle and vice-versa?

10. Aug 2, 2007

meopemuk

As I tried to explain, virtual (and bare) particles should be understood as mathematical fictions that are characteristic for one particular (perturbative) formulation of quantum field theory. Virtual particles can be met at certain intermediate steps of calculations (e.g., Feynman diagrams), however, they cannot be directly observed in real life.

I don't know what definition of "the fabric of space-time continuum" you have in mind, but I suspect, it is something as fictitious as virtual particles. Can two unreal things transform into each other? Sure. But this is not question about physics, in my opinion.

Physics is about things that we can observe in experiments: energies of atomic transitions, scattering cross-sections, positions and momenta of real particles, etc. If you want to get a satisfactory answer, you should try to formulate your question in this language.

Eugene.

11. Aug 2, 2007

Nick666

Do virtual particles have size? Comparable to planck size. (length)

I heard that when the latest mega particle accelerator will function, they will discover new and exciting things, and new particles, and they might discover that space-time is fabricated of planck size particles.

Last edited: Aug 2, 2007
12. Aug 2, 2007

olgranpappy

Embarrassingly non-vanishing in some cases...

E.g., RMS electric field in the vacuum:

$$<0|\vec E|0>=0$$
Great!

$$<0|E^2|0>=\infty$$
Uh oh!

13. Aug 2, 2007

meopemuk

No.

Most certainly, yes.

Possible

No way.
Eugene.

14. Aug 2, 2007

lightarrow

Sorry, in which region of space such average values are computed?

15. Aug 2, 2007

olgranpappy

I'm not averaging with respect to space, I'm evaluating the expectation value of the electric field with respect to the ground state.

If you are asking me what is the spacial argument of the electric field... it doesn't matter, all points of empty space are equivalent.

16. Aug 2, 2007

meopemuk

Hi olgranpappy,

I wouldn't worry about this infinity at all. First, in QED the "electric field" $\vec E$ is a certain combination of photon quantum fields, or equivalently, a certain function of photon creation and annihilation operators. It is questionable whether this combination has anything to do with real macroscopic observable electric fields. At least, I haven't seen reliable (and comparable with experiment) calculations of macroscopic electric fields within QED. The main purpose of QED is to calculate the scattering matrix (S-matrix) for physical particles and some other properties (e.g., energies of bound states) that are directly related to the S-matrix. And QED does that brilliantly.

Second, it is even questionable whether electric fields are truly observable physical objects. When we "measure" the electric field at point x we, actually, place a test charge at point x and measure its acceleration. The acceleration is created, of course, by interactions of this test charge with other charges around it. It can be debated whether this interaction is transmitted by some independent agent (also known as "field", which, presumably, has energy and momentum of its own) or that we are dealing with "action-at-a-distance". Since fields are not measurable directly, it is, at least, conceivable to think that a theory can be formulated in such a form that electric fields (just as virtual and bare particles discussed in my previous posts) can be eliminated from the theory without any adverse effect. The "dressed particle" theory achieves exactly that.

Of course, I am not talking here about free transverse electromagnetic fields associated with propagating light. They are known to be directly measurable and possess their own momentum and energy. However, within QED these transverse "fields" are better described as collections of large numbers of discrete physical particles - photons. The same description is valid within the "dressed particle" version of QED. And the energy contained in this "field" is just the sum of energies of all photons, so it cannot be infinite. So, there is no contradiction.

Eugene.

17. Aug 2, 2007

olgranpappy

Oh yes, I agree with you. I just find the example I gave amusing.

18. Aug 3, 2007

Nick666

Why dont virtual particles have size(length)?

Is their size (length) zero ?

Why cant they discover that space is composed of particles of planck size (length) or much smaller. or whatever particles of whatever size?

19. Aug 3, 2007

olgranpappy

I hate to say it, but as usual, it gets down to what exactly do you mean by "size(length)?"

But regardless, you can't just go up with a ruler and measure a virtual particle because they only exist as "intermediate states" and not as what actually comes out of, say, a scattering experiment.

no.

Look. That just doesn't make sense. And besides, it's seems to be just a random statement that you plucked out of thin air.

20. Aug 4, 2007

Nick666

Then why do I see physicist saying all the time "the fabric of space-time continuum......bla bla" , "space expands..........bla bla" ?

What exactly is expanding?

What is the fabric of space-time ?

What is it composed of?

Is it composed of nothing?

Is it composed of something?

Do I define it as the quantum vacuum (field), the lowest energy state ?

If so, what could that energy transform into ? (from what I know energy and matter can change into one another.....d`oh)

Last edited: Aug 4, 2007