Let’s talk about nothing.
We often think of outer space as a never-ending vacuum filled with the occasional galaxy. What we don’t realize is that away from our eyes, this vacuum comes alive. In order to understand what truly happens behind our backs in the vacuum, we must start by examining space itself.
So what is space? Quantum Field Theory tells us that space is composed of fundamental quantum fields, with a separate field for every particle that makes up our universe. Electrons, quarks, neutrinos, and other fundamental particles are just the oscillations of the field with different energies. In specific, they have quantum energy, which exists as multiples of a baseline energy.
You can think of this as a ladder with energy levels. Each rung of the ladder represents the existence of one additional particle in that quantum state. So the bottom of the ladder would be where there is no energy, meaning there are no particles. This is known as the vacuum state.
But as we will see, we cannot actually have zero-energy. Instead, the quantum field gently vibrates randomly. Sometimes this produces enough energy to form particles out of seemingly nothing! The particles arising out of the fluctuation of quantum fields are called virtual particles.
Empty space is teeming with these virtual particles or “wiggles in the field”.
But there is a catch; these particles are created in particle and anti-particle pairs. They live only for a short instance of time until they destroy each other, popping in and out of existence. The higher the energy of the particle, the lesser time it can exist.
Wait a minute. Virtual Particles? That sounds sketchy. Let me show you the proof.
By definition, these elusive particles only exist when we aren’t watching, but their presence can be felt throughout the universe. In 1948, Hendrick Casimir came up with an ingenious idea to observe these virtual particles.
The Casimir Effect
Let’s imagine two conducting plates brought together at a certain distance apart. Between these two plates, only those virtual photons whose wavelengths fit an integral multiple of the gap will be allowed. Sort of like how an organ pipe of a particular length only resonates with waves of certain pitches.
Only certain photons can exist between the plates. This reduces the vacuum energy between the plates, as compared to the outside where all types of virtual photons can exist. The difference in energy results in a pressure differential and pushes the plates closer to each other.
Vacuum Energy Outside > Vacuum Energy Inside
In 1996 this effect was measured, proving that vacuum energy is real!
So we have looked at a real-life measurement of the proof of virtual particles. We can also reason this out logically by considering Heisenberg’s Uncertainty Principle.
Heisenberg’s Uncertainty Principle
This principle tells us that we cannot know both the momentum and the position of a particle at a particular time instant with 100% certainty. If we had “perfect stillness” we would have fixed particles with zero momentum, meaning we know both the position and the momentum. This cannot be true!
Instead, this particle with fixed position would be a quantum blur of many possible momenta. This adds up to a real average kinetic energy which forms Zero Point Energy.
Any “empty space” contains this zero-point energy.
We can also look at a case where there are no particles. Another variation of the Heisenberg Uncertainty Principle tells us that we cannot define both energy and time. If we try to define the time we know for a quantum oscillator, we cannot be certain that there is zero energy at that time window! Again the quantum fields will be a blur of energy, with the most likely energy state being the zero energy vacuum state.
Okay, virtual particles exist. But so what?
The Implications of Virtual Particles
Well, these seemingly insignificant particles have made quite an impact on the universe we know today. Not only do they explain ‘particle-particle interaction’, but they can be traced back to the origin of the universe itself!
The creation of the Universe
Let’s take a journey to the very beginning, all the way back to the Big Bang. You better get comfortable with your neighbors, because we’ve all been squeezed down to a point here.
But don’t worry, we’ll all start to expand steadily, soon. The tiny ‘point’ lasts for the tiniest fraction of a second after which the expansion blows up to 10²⁶ times in an even smaller period of time. Abruptly as it began, it all stops. The universe continues to expand but at a decreasing rate. Fast forward to around 6 billion years, the universe’s expansion rate starts increasing again.
Now let’s tie this back to the idea of those quantum fields we just can’t get to be still. Usually, these oscillations only affect subatomic processes. But back when the universe expanded so rapidly, these tiny fluctuations blew up to the scale of the observable universe!
Without vacuum energy, the matter distribution would have most likely been completely homogeneous (read: incredibly boring).
Incredible right? Without these somethings in nothing, our universe might have truly contained nothing.
But, virtual particles didn’t just shape our universe’s beginning. They continue to alter the universe by affecting even the mighty black holes.
Virtual particles cause black holes, the universe’s vacuum cleaners, to actually evaporate.
Usually, when a virtual particle pops into existence, the particle and its antiparticle exist for a moment and annihilate each other. But when this occurs near a black hole’s event horizon, one particle may fall into the black hole where the other goes into space. This escaping particle causes the black hole to evaporate. This evaporation process is known as Hawking Radiation.
Basically, due to the intense curvature of space-time, virtual particles get boosted to become real particles. The black hole needs to give up a part of its mass in order to create this energy and, in essence, evaporates. But don’t worry! This doesn’t mean all our black holes will go away. It’ll take an insanely large amount of time for a black hole to fade away into nothing.
So we’ve seen that virtual particles contributed to the creation of the observable universe and the evaporation of the black holes. What about zero point energy itself? This energy comes out of, well, literally nothing. Can we harness this “free energy” and use it to ultimately control the universe?
An Excess of Energy?
It turns out we can’t.
Based on quantum field theory, the zero point energy density of the electromagnetic field is estimated to be ~10¹¹² ergs/cm³, an extremely high amount of energy!
This ridiculously high amount of energy has furthered many pseudo-scientists to include it in their calculations of “zero-point energy machines” and even as the negative energy to manipulate warp fields and opening wormholes.
In reality, the second law of thermodynamics tells us that we can harness energy only when we have a difference in entropy. Our energetic quantum field is almost the same everywhere, meaning we are at equilibrium. Sorry pseudo-scientists! Looks like you’ll have to find some other way to travel through time!
This calculation of vacuum energy was derived from Quantum Field Theory, arguably one of the most successful theories in all of physics. But looking at real-life measurements of vacuum energy, something doesn’t add up…
This disagreement between the theoretical and measured values of vacuum energy may be one the greatest unsolved mysteries of physics and is known as “The Vacuum Catastrophe.”
The Vacuum Catastrophe
The measured value of vacuum energy is 10¹²⁰ times less than the prediction made by Quantum Field Theory!
Let’s take a closer look at the case of the virtual photon.
According to QFT and Heisenberg’s uncertainty principle, any field oscillation must be half of Planck’s constant * frequency of the oscillation. That’s a pretty small number. But taking into account that it exists for every frequency mode, it adds up pretty quickly. In fact, it would go to infinity. If we take the cut-off for photon frequency to where photon energy is equal to the Planck energy, we get a total energy density of a whopping 10¹¹² ergs/cm³.
But with this high amount of energy, we should see a great gravitational effect. The universe would be expanding rapidly.
The universe is expanding, just not at that rate. If vacuum energy is the cause for the expanding of the universe, then the absolute density of vacuum energy would be a mere 10-⁸ ergs/cm³, much lesser than the theoretical value!
We can only hope that advancements in science will uncover a solution to this catastrophe. Until then, this problem will continue to perplex physicists into finding a deeper understanding of the cosmos around us.