Lightweight overview “Zero Point Energy” or “Quantum Foam”

Version : 0.9
Date : 15/03/2010
By : Albert van der Sel
Type of doc : just an attempt to decribe the subject in a few simple words. But, I am not sure if it is going to be any good.


Contents:

1. Heisenberg and Quantum Foam
2. The Dirac equation and the “Dirac sea”
3. Other current theoretical idea’s (or problems)
4. Vacuum polarization and related
5. Higgs and the Vacuum
6. Are practical ZPE applications possible?

One fascinating phenomenon in physics, is the “Zero Point Energy”, or “Vacuum fluctuations” or “Quantum Foam”.

This is related to the following intruiging question: Is "empty space" really just empty, or is it a sort of bubbling ocean
with the creation and destruction of virtual matter? Let’s take a look at this question, from a few different angles.

1. Heisenberg and Quantum Foam:


If we were able to ‘zoom in’ deep enough into the space-time fabric, we might observe a wild “sea”
of particle creation and destruction.
The trick is, that the overall “net” result stays zero. That is, on a somewhat
larger scale, the fluctuations tends to “cancel out”.
Only at extreme brief moments, particles exists.
This is the creation of particle-antiparticle pairs, or virtual particles.

Quantum fluctuations produce a foam of erupting and collapsing,
virtual particles, which on the leftside figure, is visualized as a topographic distortion
of the fabric of space time.

There is a relation of this to one of the Heisenberg uncertainty principles, which
states that ΔE.Δt ≥ h/2, meaning that energy and time considered together "at close inspection",
are intrinsically “fuzzy”. But the equation itself, implies a Energy and time relation
in the absolute vacuum, where the Energy “in any timeframe” just cannot be ‘0’,
because that would violate the equation.
So, if you think that Quantum Theory is any good, it's hard to escape from Vacuum energy.

The effect should apply to all fields, but since the Electromagnetic field is so strong, the
virtual particle pairs in question are commonly considered to be electron/positron pairs.

There are quite a few experimental results that points in the direction that the fluctuations
are a real property of the vacuum. For example, the famous Casimir effect seems to be
a direct result of this phenomenon. See the main page for links describing the Casimir effect.

Besides this, many modern theoretical studies have linked this phenomenon to the
accelerating expansion of the universe, to “dark energy”, and the cosmological constant Λ.

2. The Dirac equation and the “Dirac sea”:


In relativistic quantum mechanics, Dirac's equation permits both positive and negative energy states.
This is indeed a fact that cannot be ignored.
To prevent the fact that positive energy particles directly radiates away energy,
Dirac further proposed that all negative states are already occupied, thereby creating a "sea" of negative
particles or states. If a negative energy electron is promoted to a positive energy state,
the "hole" is perceived as a positron.
This prediction of the positron, was (as we all know) succesfully confirmed later

Although the Dirac’s “sea” of filled negative energy states, satisfied the equation, it certainly didn’t satisfy all other physicists at that time.

And today's interpretations varies quite a lot. It depends on what sort of physicist you are talking to.
For example, it seems rather convincing to have a "fermion" sea (half spin like leptons), while it does not
seem to account good "enough" for bosons (whole spin).
Actually, it's rather the bosons which can all occupy the "same state", while fermions will obey Pauli's
exclusion principle. That's why in the Dirac sea, all states are occupied.

Anyway, the Dirac sea still remains a fascinating and valid interpretation for the vacuum.
Some "lattice like" interpretations (varying in some aspects) from the Dirac sea have emerged,
and may, at least, be considered as candidate scientific theories.

3. Some other current theoretical idea’s:

3.1 Photon interaction with the "vacuum"

Suppose we have a photon travelling through the "vacuum". Now, since the vacuum has fluctuations,
there is a good chance the photon will interact with such a electron/positron fluctuation from the vacuum.
What will happen?
After interaction, a new photon might result, on it's way to the next interaction.
What does it mean? Suppose there was no Dirac sea, or Fermion sea, would then light travel faster?
One thing is for sure. What we see as the "true photon", is the net result of the bare photon with
all vacuum interactions already accounted for.
It won't conflict with Einstein, because the speed of light is constant all moving frames of reference,
withouth that Einstein is actually specifying a specific number.
There is a serious argument, that when the speed of light is measured in a sort of Casimir cavity, it will be
a very tiny bit faster than it travels outside. This is compliant with our idea of the vacuum, because within
the Casimir cavity, the number of waves is lower (just as what is the regular explanation of the Casimir effect).
There are some experiments done, pointing the effect to be true. But the scientific community seems still
somewhat divided in this area. There is for example a difference in the speed of light at the wavefront,
as compared to transmitting "true information" as such. Secondly, very good and convincing results seems not
to have been produced yet.

3.2 The accelerating universe.

According to the latest insights, the universe is expanding in an accelerated rate.
That's an incredable phenomenon by itself ofcourse, but in relation to the modern thoughts
about the vacuum, we might have a "crisis". Quantum field theories predict a huge cosmological constant
from the energy of the quantum vacuum.
Now, it's fair to say that science still do not have figured out exactly what is going on.
One startling point is the idea that the acceleration seems to be fairly recent compared to
what is seen as the total age of the universe.
The following is somewhat speculative, but Dark energy — the unknown substance that is causing
the expansion of the universe to accelerate — could be due to quantum fluctuations of the vacuum.
That is a serious scientific consideration.
Indeed, my description is incredably vague, but the only purpose here is to point out that the vacuum energy and structure
is a serious component in modern cosmology too.
But the structure of the vacuum has been revised. We already had the classical concept of fluctuations, and,
superimposed, a new "field" has been "designed", which is called "quintessence", which will vary over space and time.
This then might be the driving force behind the acceleration.
Others argue that the "Cosmological constant" (a measure of the energy in vacuum), is driving the expansion.
But.. that is just one of the theories. Other explanations exist as well


4. Vacuum polarization and related:

If the vacuum produces short-lived charched "virtual" particle-antiparticle pairs, then they would on average
"reposition" themselves in the presence of a field or charged particle.
In a way, the vacuum is in effect counteracting the field, or charged particle.
This effect is called "vacuum polarization".
Has this been observed in experiments? In several ways, yes.
For example, the small "Lamb" shift
in spectral lines of Hydrogen (and other elements), is attributed to the interaction with the vacuum.
Similar to the former effect, vacuum polarization is very likely to be experimentally observed in muonic atoms.


5. Higgs and the Vacuum:

Waves and "fields" are not exactly the same. With "fields", virtual particles pop into and out of existence constantly.
We already have seen the photons assiociated with EM fields

A hypothetical "Higgs" field may exists, with has associated Higgs particles. This field has a certain value in all
space, and "interacts" with many elementary particles, giving rise to their observed mass.

Now, some theories propose that this Higgs field itself, may exist in a "false vacuum" state, and that
might be responsible (as some folks say) for the driving force behind the accelaration of the universe.

Certainly the following is a well-know hypothesis among cosmologists: As spontaneous symmetry breaking occurred after
the inflationary period, the Higgs field takes on different orientations in unconnected regions of space.
Along the boundaries, between these differently oriented regions of space, the vacuum energy (or phase)
for that "region" is trapped, and a defect is formed. In effect, the universe could consist of a whole array of these "bubbles".
Each of those domains would have different "vacuum" conditions.
Interrestingly, "cosmic strings" may have formed along the domain 'walls', but that is another subject.

Even if no separate domains were formed during the early universe, the theory is that at a certain fase,
Higgs rolled down the potential giving rise to a large mass to the Higgs "particle" itself ( believed to be 122-197 times
the mass of the proton).

There is much to learn from Higgs, and thats one reason why the coming LHC experiments are quite exiting:
will the Higgs particle be found?
If it does get found (or even if it does not get found), that will have important conseqences for our understanding
of the Vacuum (and the universe as a whole).

6. Are practical ZPE applications possible?:

Since everything "sits" in the vacuum, and since the vacuum is interacting with "everything",
in a sense, everything we do or create, is already effected by the vacuum.
If somebody would say that the vacuum in a way effects everything, I would not immediately disagree.

Ofcourse, what is meant by "practical applications", is the question if we can utilize the vacuum energy
for a practical purpose like energy production, or as a source for propulsion.
A great application would be ofcourse "clean" and cheap energy for all.
One other facinating application would be to use ZPE as way to propell a spacecraft.
Unfortunately, some theoretical studies have shown that, generally speaking, the resulting acceleration
is very little. But it is likely that there is still a lot to learn on optimizing materials, orientation,
and applying some catalizing substance, or energy.
Anyway, I would not be too surprised if a true practical ZPE device would be presented one of these days.