No, particle physics on Earth won’t ever destroy the Universe - Beyond The World
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No, particle physics on Earth won’t ever destroy the Universe

The Large Hadron Collider on Earth accelerates protons to 7 TeV of energy, only 3 m/s shy of the speed of light, before smashing them together and detecting what happens. Many people were afraid about the consequences of researching the unknown before it was turned on in 2008, including the possibility of creating black holes or possibly destroying the Universe. But when it comes to what actually happens in the Universe we live in, nature far outperforms everything we’ve ever constructed or planned to build. This is how we know the Universe is safe.

When you venture deeper into the unknown than ever before, you should be concerned not only about what you’ll find, but also about the type of demons you might meet. That double-edged sword arises in particle physics as we delve deeper into the high-energy Universe. The more we learn about the previously inaccessible energy frontier, the better we will be able to reveal the high-energy processes that created the Universe in its early stages.

Many mysteries about how our Universe began and grew from the beginning can be best investigated using this same method: colliding particles at increasing energy. Accelerator physics can reveal new particles and rare processes at or beyond the present energy frontiers, but this is not without risk. If we can reach the following energies:

  • reveal/show the ability to generate more matter than antimatter (or vice versa),
  • restore the inflationary state and prepare for our hot Big Bang,
  • or move the Universe’s zero-point energy from its “false minimum” condition to another,

certain consequences — not all of which are desirable — could be in store for us all. And yet, just as we know that “the LHC could create black holes that destroy the Earth,” we also know that any experiment we conduct on Earth will have no negative consequences. There are no current or proposed particle accelerators in the Universe. This is how we found out.

The idea of a linear lepton collider has been bandied about in the particle physics community as the ideal machine to explore post-LHC physics for many decades, but only if the LHC makes a beyond-the-Standard-Model discovery. Direct confirmation of what new particles could be producing the observed discrepancy in the mass of the W-boson by CDF may be a task best suited to a future circular collider, which can achieve higher energies than a linear collider. (Credit: Rey Hori/KEK)

On Earth, there are a few distinct approaches to building particle accelerators, with the major differences arising from the types of particles we choose to collide and the energies we can achieve when we collide them. The particles that can collide are as follows:

  • Electrons and positrons are ideal for producing “clean” signals in which as much of the collision energy as possible is transmitted into the formation of new particles (through E = mc2).
  • Electrons with protons, which is the most effective method for exploring the interior structure of the quarks that exist within a proton,
  • Protons with anti-protons, which gives the highest-energy collisions but at the cost of luminosity (the number of collisions per second, as anti-protons are difficult to produce in large quantities) and signal cleanliness (since protons and anti-protons are composite particles),
  • Protons with protons, which have the highest energy collisions but produce more light and mess than protons with anti-protons,
  • Or composite, heavier nuclei with other heavy nuclei, allowing us to create a quark-gluon plasma and study its features.

It may be feasible in the future to collide muons with anti-muons, combining of the electron-positron and proton-antiproton worlds, but that technology isn’t quite there yet.

A potential Higgs event in the ATLAS detector at CERN’s Large Hadron Collider. Even with the obvious signatures and transverse tracks, there is a shower of other particles; this is because protons are composite particles, and dozens of proton-proton collisions occur with each bunch crossing. Examining how the Higgs decays to very high precision is one of the key goals of the HL-LHC. (Credit: CERN/ATLAS Collaboration)

Whatever is up there at the highest energy-per-particle-collision that we get is the item that poses the most “danger” to us. On Earth, the Large Hadron Collider holds the record, as the vast majority of proton-proton collisions result in the gluons inside each proton colliding. Because the proton’s total energy is divided among its constituent particles, only a fraction of the total energy belongs to each gluon when they collide, so it takes a large number of collisions to find one in which a large portion of that energy — say, 50% or more — belongs to the relevant, colliding gluons.

When this happens, however, the greatest energy is available to generate new particles (by E = mc2) or to do other tasks that energy may perform. In physics, we measure energy in terms of electron-volts (eV), or the amount of energy required to raise a sleeping electron to an electric potential of one volt in relation to its environment. The most energetic particle-particle collision possible at the Large Hadron Collider, the current record holder for laboratory energies on Earth, is 14 TeV, or 14,000,000,000,000 eV.

Although no light can escape inside a black hole’s event horizon, the curved space outside of it causes a difference in the vacuum state at different points near the event horizon, resulting in radiation emission via quantum processes. This is where Hawking radiation comes from, and for the tiniest-mass black holes, Hawking radiation will lead to their complete decay in under a fraction-of-a-second. (Credit: The EU’s Communicate Science)

There are things we can be worried about at these highest energies, each with its own possible consequences for either Earth or the Universe as a whole. A non-exhaustive list includes:

  • It may be possible to create small black holes if we achieve high enough energies and there are certain types of extra dimensions. In theory, they should decay via Hawking radiation on extremely short timescales: shorter than the Planck time without extra dimensions, but potentially long enough for them to exist physically.
  • If the matter-antimatter asymmetry formed as a result of a cosmic symmetry breaking at a higher energy, then restoring the symmetry may result in that symmetry breaking again in a different way. Rather than matter “win out” against antimatter at around one part in a billion, it may lose, or win-or-lose by a different amount entirely.
  • If cosmic inflation occurred prior to the Big Bang as a result of particular high-energy conditions being met, then recreating those conditions could result in the inflationary state being recreated. This would cause rapid, exponential expansion of space wherever it occurred, “pushing” our Universe away from it and initiating a new inflationary state.
  • Or, given that the zero-point energy of empty space appears to be non-zero — as evidenced by the presence of dark energy — increasing the Universe to high enough energies may “kick” the energy of empty space out of this condition and maybe send it into another, lower-energy state. This would provide the same conditions as a vacuum decay disaster, resulting in a “bubble of destruction” that destroyed all matter within it as it expanded outward at the speed of light.

Any potential has a profile with at least one point corresponding to the lowest-energy, or “true vacuum,” state. If there is a false minimum at any place, that is a false vacuum, and it is always feasible to quantum tunnel from the false vacuum to the genuine vacuum state, providing this is a quantum field. The higher the “kick” applied to a false vacuum state, the more probable it will depart the false vacuum state and end up in a different, more stable, “truer” minimum. (Credit: Stannered/Wikimedia Commons)

Although all of these scenarios are “bad,” some are worse than others. The birth of a tiny black hole would result in its immediate decay. If you didn’t want it to decay, you’d have to impose some type of new symmetry (for which there’s no evidence or incentive), and even then, you’d just have a tiny-mass black hole that acted like a new, massive, uncharged particle. The “worst” it could do is absorb the matter particles with which it hit and then “sink” to the center of whatever gravitational object it was a member of. Even if you made it on Earth, it would take trillions of years to absorb enough matter to rise to a mass of 1 kg; it’s not threatening at all.

The restoration of whatever symmetry was in place before the Universe’s matter-antimatter symmetry arose is also interesting, because it could lead to the destruction of matter and the creation of antimatter in its place. As we all know, matter and antimatter annihilate when they come into contact, which is “bad news” for any matter that exists around this point. Fortunately, the absolute energy of every particle-particle collision is tiny, corresponding to fractions of a microgram in terms of mass. Even if such a collision produced a net amount of antimatter, it would only be capable of destroying a little amount of matter, and the Universe would be alright overall.

The most basic model of inflation is that we began at the top of a proverbial hill, where inflation continued, then rolled into a valley, where inflation ended, resulting in the hot Big Bang. If that valley does not have a value of zero, but rather a positive, non-zero value, it may be feasible to quantum-tunnel into a lower-energy state, with serious consequences for the Universe we know today. It’s also feasible that a “kick” of the right energy may revive the inflationary potential, entering a new era of rapid, relentless, exponential growth. (Credit: E. Siegel/Beyond the Galaxy)

However, if we were able to reproduce the conditions that caused inflation, the situation would be even worse. If it happened somewhere in space, we’d create the largest cosmic void we could imagine in a fraction of a second. Whereas there is only a small amount of energy inherent in the fabric of empty space now, on the order of a few protons per cubic meter, it was more like a googol protons (10100) per cubic meter during inflation.

If we could achieve those same energy densities elsewhere in space, we may potentially restore the inflationary state, resulting in the same Universe-emptying exponential expansion that occurred 13.8 billion years ago. It would not destroy anything in our Universe, but it would cause an exponential, rapid, and endless expansion of space in the region where those conditions reoccur.

That expansion would “push” the space that our Universe occupies outward, in all three dimensions, as it expands, creating a large cosmic bubble of emptiness that would lead to unmistakable signatures that such an event had occurred. It clearly hasn’t, at least not yet, but in principle, it could.

A quantum field theory calculation is shown as virtual particles in the quantum vacuum. (In particular, for the strong interactions.) Even in empty space, this vacuum energy is non-zero, and what seems to be the ‘ground state’ in one section of curved space will appear different from the perspective of an observer in a different sector of curved space. This vacuum energy (or cosmic constant) must be present as long as quantum fields exist. (Credit: Derek Leinweber)

And finally, the Universe today exists in a state where the quantum vacuum — the zero-point energy of empty space — is non-zero. This is inextricably, although we don’t know how to perform the calculation that underlies it, linked to the fundamental physical fields and couplings and interactions that govern our Universe: the physical laws of nature. At some level, the quantum fluctuations in those fields that cannot be extricated from space itself, including the fields that govern all of the fundamental forces, dictate what the energy of empty space itself is.

But it’s possible that this isn’t the only configuration for the quantum vacuum; it’s plausible that other energy states exist. Whether they’re higher or lower doesn’t matter; whether our vacuum state is the lowest-possible one (i.e., the “true” vacuum) or whether another is lower doesn’t matter either. What matters is whether there are any other minima — any other stable configurations — that the Universe could possibly exist in. If there are, then reaching high-enough energies could “kick” the vacuum state in a particular region of space into a different configuration, where we’d then have at least one of:

  • different laws of physics,
  • a different set of quantum interactions, or
  • a different set of fundamental constants.

Any of these would, if it was a more-stable configuration than the one that our Universe currently occupies, cause that new vacuum state to expand at the speed of light, destroying all of the bound states in its path, down to atomic nuclei themselves. This catastrophe, over time, would destroy billions of light-years worth of cosmic structure; if it happened within about 18 billion light-years of Earth, that would eventually include us, too.

The size of our visible Universe (yellow), along with the amount we can reach (magenta) if we left, today, on a journey at the speed of light. The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. There are an estimated 2 trillion galaxies contained within the yellow sphere drawn here, but that estimate is likely low, perhaps by as much as a factor of 3-to-10. (Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel)

There are a lot of unknowns around these events. Quantum black holes may be just beyond our present energy frontier. It’s possible that the matter-antimatter asymmetry was created only during electroweak symmetry breaking, placing it within current collider reach. Inflation, like the processes that determine the quantum vacuum, must have occurred at higher energy than we’ve ever achieved, but we don’t know how low those energies may have been. Only observations tell us that such an event has not yet happened within our observable Universe.

Despite this, we don’t have to worry about any of our particle accelerators, past, present, or future, producing any of these catastrophes here on Earth. The reason is simple: the Universe is filled with natural particle accelerators far, far more powerful than anything we’ve ever created or even planned here on Earth. Under extreme conditions, charged, moving matter can generate very strong electric and magnetic fields from collapsed stellar objects that spin fast, such as white dwarfs, neutron stars, and black holes. These are thought to be the sources of the highest-energy particles yet detected: ultra-high-energy cosmic rays, which have been observed to have energies many millions of times greater than any accelerator on Earth ever has.

The energy spectrum of the greatest energy cosmic rays detected by collaborations. The results are quite consistent from experiment to experiment, revealing a considerable drop-off at the GZK threshold of 5 x 1019 eV. Nonetheless, many of these cosmic rays exceed this energy threshold, implying that either this image is incorrect or that many of the highest-energy particles are heavier nuclei rather than individual protons. (Credit: M. Tanabashi et al. (Particle Data Group), Phys. Rev. D, 2019)

Whereas we’ve beyond the ten TeV threshold for accelerators on Earth, or 1013 eV in scientific notation, the Universe routinely generates cosmic rays that exceed the 1020 eV threshold, with the milestone set more than 30 years ago by an event known as the Oh-My-God particle. Even though the highest energy cosmic rays are thought to be heavy atomic nuclei, such as iron, rather than individual protons, this means that when two of them collide — a near-certainty in our Universe given the expanse of space, the fact that galaxies were pretty close together in the past, and the long lifespan of the Universe — there are many events producing center-of-mass collision energies in excess of 1018 or even 1019 eV.

  • None of them have ever regained the possibility for inflation.
  • None of them have ever caused the Universe to transition into a more stable vacuum state.
  • And none of them have ever altered physical laws or constants in a way that has persisted to the present day.

This means that any catastrophic, cosmic effect that we could be concerned about is already tightly confined by the physics of what has happened in the Universe’s cosmic history up to the present day.

When a high-energy particle collides with another, it can produce new particles or quantum states, limited only by the amount of energy available in the collision’s center of mass. Although particle accelerators on Earth can achieve very high energy, natural particle accelerators in the Universe can outperform them by a factor of several millions. (Credit: Nicolle Rager Fuller/NSF/IceCube)

None of the possible cosmic catastrophes have occurred, which means two things. The first is that we can likely establish lower limits on where certain cosmic transitions happened. Nowhere else in our Universe has the inflationary state been restored, putting a lower limit on the energy scale of inflation at no less than 1019 eV. This is perhaps a factor of 100,000 lower than where we expect inflation to occur: a reassuring consistency. It also teaches us that it is extremely difficult to “kick” the Universe’s zero-point energy into a different configuration, giving us confidence in the quantum vacuum’s stability and disfavoring the vacuum decay catastrophe scenario.

However, it also means that we can continue to investigate the Universe with confidence. Based on how “safe” the Universe has already proven to be, we can confidently predict that no such catastrophes will happen up to the combined energy-and-collision-total level that has already occurred within our observable Universe. Only when we start colliding particles at energies of 1020 eV or higher — a factor of 10 million higher than the current energy frontier — will we need to be concerned about such events. That would require a much larger accelerator than the entire planet, and so we may reach the conclusion promised in the article’s title: no, particle physics on Earth will never destroy the Universe.

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