- The concepts of ‘false vacuum’ and ‘real vacuum’ states are vastly different. Here are some of the reasons why we don’t want to live in the former.
- The vacuum is defined as empty space’s zero-point energy: how much energy-per-volume remains after all physical quanta have been eliminated.
- This value may be 0, but it isn’t: it is positive and non-zero.
If we live in a phony vacuum rather than a real one, the vacuum might degrade, resulting in disastrous effects for the Universe.
One of the major existential concerns among theoretical physicists is that the vacuum of space may not be in its actual vacuum condition, but rather in a false vacuum. If you were to clear a big area of space of anything you could conceive, including:
- external magnetic and electric fields,
- and any gravitational sources or curvature of spacetime
You’d be left with nothing but empty space, or as near to a physical definition of “nothing” as we can get. You’d think that if you drew an imaginary box around this “nothing” zone and measured the total amount of energy within, you’d find it to be exactly zero. However, even after removing all known quantum and classical sources of matter and energy, we discover that there remains a positive, non-zero quantity of energy inherent in space itself. What does this entail for the nature of quantum vacuum, particularly the distinction between “real vacuum” and “false vacuum”? Eric Mars is curious about this, and he has posed the following question:
“Could you kindly clarify what the terms false vacuum and genuine vacuum entail, as well as their consequences for the universe’s existence?”
It’s an excellent question, and it necessitates that we begin with the concept of zero – specifically in physics.
This artist’s rendering portrays how the foamy structure of spacetime may seem, with tiny bubbles quadrillions of times smaller than an atom’s nucleus that are continually shifting and lasting just fractions of a second. At the quantum scale, spacetime possesses intrinsic fluctuations, which most likely equate to a non-zero zero-point energy, rather than being smooth, continuous, and uniform. (Credit: NASA / CXC / M. Weiss)
Zero is a number in mathematics that denotes the lack of either a positive or negative amount of any quantity. However, in physics, there is another way to define zero: a system’s zero-point energy, or the lowest conceivable energy state that it can acquire while still being the same system we started with. There will be at least one arrangement for each physical system we can imagine that has the lowest overall amount of energy in it. There is always at least one lowest-energy arrangement for each physical system you can think of.
- A black hole is the lowest-energy configuration of a collection of masses isolated from the rest of the Universe.
- The lowest-energy configuration for a proton and an electron is a hydrogen atom in the ground (n=1) state.
- And it is to have totally empty space in the absence of any internal or external fields or sources for the Universe itself.
The zero-point energy of a system is the lowest-energy arrangement. If any system’s zero-point energy were specified as zero, it would make sense — and many of us would simply intuit that it is. However, this is not the case.
An electron orbits an atomic nucleus in this artist’s conception, where the electron is a fundamental particle and the nucleus may be split down into even smaller, more fundamental elements. The most basic atom is hydrogen, which is made up of an electron and a proton linked together. However, the lowest-energy form, in which the electron simply sits motionless in the proton’s core, never happens. (Credit: Nicole Rager Fuller / NSF)
Consider the hydrogen atom, which has only one electron circling a single proton. If you think about it in terms of classical physics, the electron might orbit that proton at any radius, from huge to small. You’d assume that a negatively charged electron could orbit a positively charged proton at any distance, based only on the speed of the orbit and the balance of kinetic and potential energy, much as a planet may orbit a star at any distance based on their mutual masses and relative speeds.
However, this overlooks a very essential aspect of nature: the fact that the Universe is inherently quantum mechanical, with quantized energy levels being the only ones permissible for an electron circling a proton. As a result, a physical system like this can have a lowest possible energy state, which does not correlate to the electron lying at rest squarely atop the proton (that is, the lowest imaginable energy state). Instead, there is a physically permissible lowest-energy state, which corresponds to an electron circling a proton in the n=1 energy state.
Even if you chill your system to absolute zero, your system will still contain a finite amount of non-zero energy.
The influence of binding energy and the link between the electron and the proton in quantum physics are demonstrated through electron transitions in the hydrogen atom, as well as the wavelengths of the generated photons. The n=1 state corresponds to hydrogen’s lowest energy state: a ground state with a limited, positive, non-zero quantity of energy. (Credit: OrangeDog and Szdori / Wikimedia Commons)
The concept of a zero-point energy in any quantum mechanical system dates back to Max Planck in 1911, and it was extended to fields by Einstein and his partner, Otto Stern (the same Stern who created the infamous Stern-Gerlach experiment), in a 1913 publication. We now know that our Universe is governed by a mix of General Relativity, our law of gravity, and quantum field theory, which describes the other three basic forces, more than a century later.
Both General Relativity and quantum field theory mention the concept of a zero-point energy in the fabric of space, but in very different ways. The curvature of space governs the future travel of matter and energy through the Universe according to General Relativity, whereas the presence, distribution, and mobility of matter and energy influence the curvature of space. Matter and energy direct the curvature of spacetime, and the curvature of spacetime directs the movement of matter and energy.
What makes you think this is only “nearly” true? Because, as everyone who has done an indefinite integral (from calculus) knows, you may add a constant to your answer: the dreaded “plus c.”
The curvature of space is determined by the existence of matter and energy in General Relativity. Quantum field theory contributions will lead to the same net impact in quantum gravity. You can also include a constant: a cosmological constant in General Relativity, which is equivalent to the sum of all loop diagrams for the vacuum in quantum field theory. It’s possible that quantum gravity contributions to space’s zero-point energy are to blame for the dark energy we witness today, but this is only one of several plausible scenarios.(Credit: SLAC National Accelerator Laboratory)
This constant is used as a cosmological constant in General Relativity, and it can have any positive or negative value we choose. When Einstein attempted to build a static Universe, he added a positive constant to prevent his toy model of the Universe from collapsing — one in which masses were uniformly distributed indefinitely throughout space; the cosmological constant would oppose gravitational attraction. There was no justification for him to give this constant the positive, non-zero value he did. He merely stated that it had to be thus, because the Universe could not remain static otherwise. The constant was no longer needed with the discovery of the expanding Universe, and it was ignored for more than 60 years.
Quantum field theory, on the other hand, exists. The creation/annihilation of particle-antiparticle pairs as intermediate stages, radiative corrections, and any other set of interactions not banned by quantum physics are all encouraged by quantum field theory. However, it goes on to say something that most people aren’t aware of. It claims that, in addition to these interacting fields in the presence of matter and energy, there are “vacuum” contributions, which describe how quantum fields function in space without any particles.
Virtual particles in the quantum vacuum are seen as part of a quantum field theory computation (specifically, for the strong interactions). Even in empty space, this vacuum energy is non-zero, thus what seems to be the “ground state” in one section of curved space will appear to be different from the perspective of an observer in a different region of curved space. This vacuum energy (or cosmic constant) must be there as long as quantum fields exist.(Credit: Derek Leinweber)
Now comes the awkward part: we have no idea how to compute space’s zero-point energy using these quantum field theory methods. Each individual channel that we can compute can contribute to this zero-point energy, and the method we determine individual contributions is by calculating what we call their vacuum expectation value. The difficulty is that each of these channels has a massive vacuum expectation value: almost 100 orders of magnitude too big to be feasible. Positive contributions are made by certain channels, while negative contributions are made by others.
We made an erroneous assumption since we couldn’t perform a rational calculation: that all of the contributions would cancel out, summing to zero, and that the zero-point energy of space would be exactly zero.
Then, in the 1990s, things began to shift once more. Observations of the Universe began to show that something was accelerating the expansion of the Universe, and that “thing,” whatever it was, was consistent with a positive, non-zero quantity of zero-point energy to the fabric of space itself, rather than any kind of matter or radiation. We’d just calculated the value of the vacuum energy that pervades space, and it was modest but significant: it was larger than zero.
The Universe’s predicted destiny (top three images) all point to a Universe in which matter and energy collaborate to combat the original expansion pace. A form of dark energy, which has yet to be described, causes a cosmic acceleration in our visible Universe. The Friedmann equations, which connect the expansion of the Universe to the various forms of matter and energy contained inside it, regulate all of these Universes.(Credit: E. Siegel / Beyond the Galaxy)
This sparked a flurry of inquiries.
- Was this type of energy, which we now refer to as dark energy, a cosmological constant or not? (At least to the accuracy with which we can measure it, the answer is yes.)
- Did it stay the same over time, or did it get stronger or weaker? (The explanation is that being a perfect constant is consistent.)
- Could we possibly expect to calculate it using quantum field theory knowledge? (The answer is that we don’t know, but we’re no closer now than we were 20 years ago.)
- And, more importantly, is the zero-point energy we’re seeing the genuine vacuum of space, or is it just a phantom vacuum? (We have no idea.)
Why should we be concerned about the previous one? Because the most significant attribute of the vacuum of space isn’t the exact value of the zero-point energy; rather, it’s crucial to the stability of our Universe that the vacuum of space has a constant zero-point energy. A Universe in a false vacuum will retain the capacity to transition to a genuine vacuum (or a lower-energy but still false vacuum) state, much as a hydrogen atom in any excited state may transition to a lower-energy state on its journey down to the zero-point state.
Any potential will have a profile with at least one point corresponding to the lowest-energy, or “real vacuum,” state if you draw it out. If there is a false minimum at any place, it may be called a false vacuum, and it will always be feasible to quantum tunnel from the false vacuum to the genuine vacuum state, providing this is a quantum field. (Credit: Stannered / Wikimedia Commons)
You may imagine beginning a ball atop a mountain and allowing it to roll down — and down, and down, and down — until it came to a stop. If your slope is smooth, you can picture how easy you might roll all the way down into the valley beneath the mountain, where it would settle. That is a real vacuum state: the lowest energy level in which no transition to a lower energy state is physically feasible. You’re already as low as you can get in a genuine vacuum.
However, if your hillside is jagged, with pits, divots, moguls, and glacial lakes, your ball may end up anywhere else than the lowest spot conceivable. Any other location where it may stay indefinitely is not the genuine minimum, but rather a fake one. Anything other than the lowest conceivable state of the Universe is a fake vacuum state, if we’re talking about the vacuum state of the Universe.
Given that our Universe’s cosmological constant has a positive, nonzero value, it’s quite plausible that we’re living in a false vacuum, and that the genuine vacuum, whatever it is, exists in a lower-energy form.
A high, thin, but limited barrier separates a quantum wavefunction on one side of the x-axis from another in this typical example of quantum tunneling. While the majority of the wavefunction, and therefore the probability of the field/particle it represents, reflects and remains on the original side, there is a limited, non-zero chance of tunneling through to the opposite side of the barrier.. (Credit: Yuvalr / Wikimedia Commons)
It’s also possible that this isn’t the case; we might be in a genuine vacuum. If that’s the case, there’s no way to get back to a lower-energy level, and we’ll be stuck here for the rest of our Universe’s existence.
But what if we’re living in a fictitious vacuum? In a quantum Universe, there is always a finite, greater-than-zero probability of quantum tunneling from a higher-energy to a lower-energy state, regardless of how large the distance between the false and true minimums is, how high the barrier separating the false and true minimums is, or how quickly or slowly the quantum mechanical wavefunction describing your state spreads out.
This is known as the vacuum catastrophe because there is no reason to think that the rules and/or constants that govern the Universe would stay unaltered if we quantum tunnel to a lower energy state. Atoms, planets, stars, and, yes, human beings will all be killed wherever this vacuum decay happens. This “destructive bubble” will spread outward at the speed of light, which means that if it happens right now, somewhere within 18 billion light-years of us, we will be killed by it. This may be supported by our best measurements of fundamental particle characteristics, which show that the electroweak force, one of nature’s fundamental forces, is intrinsically metastable.
Based on the masses of the top quark and the Higgs boson, we could either live in a region where the quantum vacuum is stable (true vacuum), metastable (false vacuum), or unstable (where it cannot stably remain). The evidence suggests, but does not prove, that we are in a region of false vacuum. (Credit: T. Markkanen, A. Rajantie and S. Stopyra, Front. Astron. Space. Sci., 2018)
It’s a scary concept, especially because we wouldn’t be able to predict it. We’d just wake up one day to a wave of devastation approaching us at the speed of light, and then we’d all be gone. It is, in some respects, the least painful option we can think of, but it is also one of the saddest. Our cosmic heritage — all that has ever been, is, or will be — would come to an end in an instant. All of the effort done during 13.8 billion years of cosmic evolution to build a Universe brimming with the components for life, as well as perhaps innumerable realizations of it, would be wiped away forever.
Yet, with the conclusion of cosmic inflation and the commencement of the hot Big Bang, it’s likely that something comparable has already happened. A transition from a presumably extremely high-energy vacuum state to a considerably lower-energy one, albeit a fundamentally different form of transition than quantum tunneling, ended inflation and filled our Universe with matter and radiation 13.8 billion years ago. Nonetheless, the idea that we are living in a false vacuum should serve as a reminder of how transitory and delicate everything in our Universe is, and how dependent everything is on the stability of the rules of physics. Every minute of existence may be our last if we live in a fake vacuum condition, which we could.