THE NATURE OF VACUUM
One of the most amazing things about atoms is that, they are mainly empty space. Almost 99.9% of the atom is empty! what is the nature of vacuum and nothingness ?
If an atom was as wide as your arms space, then the electrons would just be whizzing around in the volume enclosed by your fingertips meanwhile the nucleus would be sitting in the centre and its diameter would be smaller than the width of a single hair.
So all of the atoms that make up you and me and seemingly solid things in the universe are mostly empty space and vacuum, so its of utmost importance we understand what is the nature of vacuum itself.
Now, this is incredible but what is more fascinating is that empty space is not truly empty.
So let us discuss about “nothing”, because it turns out that “nothing’ is one of the most interesting something!!
But the question is, how do we study nothing?
An empty box still contains something in it. Molecules of air, dust , infrared light from its warm environment, ,the ambient electromagnetic buzz from the surrounding environment and a stream of exotic particles from the surrounding cosmic radiation.
But what if we suck out every last molecule of air and chill the jar to absolute zero and shield it from all external radiation?
What do you think?
You would definitely think that the jar now is truly empty in all senses, right?
But it turns out that empty space is far from nothing.
We know it is impossible to reduce any substance to its absolute zero temperature. 0 Kelvin means no motion what so ever, in a substance’s constituent particles.
But that perfect stillness implies that the particles position and momentum are simultaneously perfectly defined and that is impossible according to Heisenberg’s uncertainty principle.
Fix a particles position and its momentum (therefore its motion) becomes a quantum blur of many possible momentas.
This results in a real minimum average kinetic energy called the zero point energy. So the walls of our empty jar will radiate as a faint glow.
But hypothetically what would a perfect empty space look like? Far from the nearest particle of matter and radiation
The answer will bring us closer to understanding the nature of space itself.
Our modern understanding of the quantum nature of vacuum is described by the quantum field theory. In short Quantum Field Theory states that, space itself is comprised of fundamental quantum fields, one for each elementary particle. Those fields oscillate and vibrate with different energies and those oscillations are the electrons, quarks, neutrinos, photons, gluons, etc that comprise the stuff of our universe.
Now these fields are quantum fields which means their oscillations can’t just have any odd energy. They can only be excited in quantum chunks, integer multiples of some baseline energy in each quantum state. So each combination of particle properties is a ladder of energy levels.
A bit like electron orbitals in an atom. Each new line on the ladder represents the existence of one additional particle in the quantum state.
In fact the math of QFT is all about going up and down this particle ladder using the so-called annihilations and creation operators,
The bottom of this energy ladder corresponds to these quantum oscillators having no energy, which simply means there are no particles in the given quantum state.
We call this the “vacuum-state” of the field.
Inside a perfect vacuum, all of the field in all location should be in the vacuum state or exactly at zero energy at all times. But here we run up against the Hisenberg’s uncertainty principle.
We saw that it was impossible to simultaneously fix the particles position and momentum. Well in the same way it is also impossible to simultaneously perfectly define the particles energy and time.
The more tightly we define the time window for the behavior of a quantum oscillator, the less certain we can be about its energy state in that time window.
On extremely short time scales a quantum field can exist as a blur of many energies.
In a vacuum, the most likely state of the blur is the zero energy vacuum state. But sometimes the field finds itself with enough energy to create a particle, seemingly out of nothing.
We call these virtual particles , and they seem to be the machinery under the hood of all particle interactions in the universe, or atleast as described by Quantum Field Theory
For example, the QFT describes the EM force as the exchange of virtual photons between charged particles.
Virtual particles are the links governing all particle interactions in the famous Feynman diagrams. But to properly calculate an interaction of real particles, every imaginable behaviour of the connecting virtual particles must be accounted, which also includes the seemingly impossible behaviour.
These virtual particles play a crucial role in our analysis of the nature of the vacuum.
For example in QFT, virtual particles can have any mass , any speeds, including speeds faster than the speed of light and can even travel backwards in time.
The ambiguous realness of virtual particles seems to grant them some surreal freedom, but there are restrictions,
For example, quantum conservation laws must be obeyed so most virtual particles are created in particle-antiparticle pairs
But the virtual particles can exist only for the instant allowed by the Heisenberg’s uncertainty principle and , higher the energy of the particle the shorter it will exist.
The restrictions, believe it or not is also responsible for the range of the fundamental forces. For example the massless photons can have the tiniest of possible energy and so virtual photons for any amount of time can carry the electromagnetic force over long distances.
On the other hand, it always takes the bottom line chunk of energy to create a gluon, the carrier of the strong nuclear force, because gluons have the mass, that means the limit to how long virtual gluons can exist and travel, which in turn makes the strong nuclear force a very short range force.
Well it can argued that virtual particles are just mathematical tools to describe the dynamic vacuum and no such particles actually exist, or these are only quantum possibility of particles which somehow govern the interaction of real particles, without themselves being burdened with reality.
But how do we verify the existence of these elusive particles?
They live in the interval between measurement of real particles.
By definition, they can only exist when we are not watching. But they do none the less leave their ghostly mark.
The 1st hint of the existence of virtual particles came in the year of 1947 when Wills Lamb
noticed a tiny energy difference between the two electron orbitals that comprised the 2nd energy level of the hydrogen atom.
According to the best existing theory of that time, orbitals should have exactly the same energy.
The slight difference now called the Lamb Shift inspired theorists to dig deeper and it didn’t take them long. In the same year German physicist Hans Bethe successfully explained vacuum energy.
Virtual particles and antiparticle pairs in space between the orbitals in the nucleus align themselves with the electric field. This partially shields the orbiting electrons from the positive charge of the nucleus. The amount of shielding being slightly different between there orbits.
The calculation of the size of the lamb shift is now one of the most accurate predictions in all of physics.
Another way to hunt for virtual particles is, through their bulk effect on the vacuum.
If quantum fields are a buzz with particles popping in and out of existence, then the so called zero point energy of those fields should be zero, empty space should have some real energy. It should have vacuum energy.
In 1948 the Dutch physicist Hendric Casimir came up with a brilliant scheme to detect these virtual particles and vacuum energy.
He imagined two conducting plates brought so close together that only certain virtual photons could exist between the plates. In the same way guitar string of particular length only resonates with waves of certain frequencies, and the only non-resonant virtual photon would be excluded reducing the vacuum energy between the plates.
However on the outer surface of the plate all the frequencies of virtual photons are allowed, the higher vacuum energy outside compared to the inside of the plate should result in a pressure differential that pushes the plates together.
The Casimir effect was only successfully measured in 1996. When separated by less than a micrometer the conducting surfaces were found to be drawn together by a force that matched the prediction of the QFT. Now while there are other explanations for this force, but still this has been taken as the strong evidence that vacuum energy is real.
Now neither the casimir effect not the lamb shift allow measurement of the absolute strength of vacuum energy. They just measure its relative effect inside VS outside the conducting plates or between neighbouring atoms.
So how much vacuum energy is there?
Well there are 2 main ways to estimate this :
- Through observation
- Through theoretical predictions
The observation is that accelerating expansion of the universe. It is hypothesized that dark energy itself maybe vacuum energy. If so, then the amount of vacuum energy required to produce the observed acceleration is tiny, around one-onehundreth of a million erg/cm3.
The theoretical calculation of the strength of the vacuum energy is a little higher than that. Infact it is 120 orders of magnitude higher. This crazy difference between observation and calculation is one of the greatest unsolved mysteries of physics.
A simulation of empty space was made by crunching up calculation from the Quantum Chromo Dynamics, the theory of fundamental particle called quarks, which are the building blocks of protons, neutrons and how they interact with each other.
The above simulation shows the energy density of the gluon field fluctuations. The deeper colours simply indicate more energy density.
what we see above is a bubbling soup of quantum field fluctuation that come and go incredibly quickly. The frame rate of this particular simulation is 10^24 fps.
Now that is truly highspeed!!!
The dimensions of this simulation box is (2.4*3.6*2.4)fm , barely space for two protons to fit.
This simulation goes on to show that , what we think is empty space is actually full of such quark gluon fields.
QFT with its dependence on virtual particles and vacuum fluctuations is one of the most successful theories and yet its prediction on the strength of vacuum energy seems to be so wildly off.
This is actually pretty exciting! It means we still don’t get the entire picture of our universe and provides us with a subtle clue at the next step we need to take.
More articles will be coming soon on The Nature of Vacuum.
please leave a comment below or ask any question you want to regarding this article, “The Nature of Vacuum” .
I would highly recommend a few books that would really help you to know in depth regarding the cosmos:
- THE THEORY OF EVERYTHING
- A BRIEF HISTORY OF TIME
- GEORGE AND THE BIG BANG
- A Brief History of the Universe: From Ancient Babylon to the Big Bang (Brief Histories)
- The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics (Sterling Milestones)
- THE BIG BANG THEORY
- Relativity: The Special and the General Theory (Routledge Classics)
- Black Holes: The Reith Lectures
- The Oxford Companion to Cosmology (Oxford Quick Reference)
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