No, particle physics on Earth won’t ever destroy the Universe – Big Think

Anytime you reach deeper into the unknown than ever before, you should not only wonder about what youre going to find, but also worry about what sort of demons you might unearth. In the realm of particle physics, that double-edged sword arises the farther we probe into the high-energy Universe. The better we can explore the previously inaccessible energy frontier, the better we can reveal the high-energy processes that shaped the Universe in its early stages.

Many of the mysteries of how our Universe began and evolved from the earliest times can be best investigated by this exact method: colliding particles at higher and higher energies. New particles and rare processes can be revealed through accelerator physics at or beyond the current energy frontiers, but this is not without risk. If we can reach energies that:

certain consequences not all of which are desirable could be in store for us all. And yet, just as was the case with the notion that The LHC could create black holes that destroy the Earth, we know that any experiment we perform on Earth wont give rise to any dire consequences at all. The Universe is safe from any current or planned particle accelerators. This is how we know.

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 causing CDFs observed discrepancy in the W-bosons mass might be a task best suited to a future circular collider, which can reach higher energies than a linear collider ever could.

There are a few different approaches to making particle accelerators on Earth, with the biggest differences arising from the types of particles were choosing to collide and the energies were able to achieve when were colliding them. The options for which particles to collide are:

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In the future, it may be possible to collide muons with anti-muons, getting the best of both the electron-positron and the proton-antiproton world, but that technology isnt quite there yet.

A candidate Higgs event in the ATLAS detector at the Large Hadron Collider at CERN. Note how even with the clear signatures and transverse tracks, there is a shower of other particles; this is due to the fact that protons are composite particles, and due to the fact that dozens of proton-proton collisions occur with every bunch crossing. Examining how the Higgs decays to very high precision is one of the key goals of the HL-LHC.

Regardless, the thing that poses the most danger to us is whatevers up there at the highest energy-per-particle-collision that we get. On Earth, that record is held by the Large Hadron Collider, where the overwhelming majority of proton-proton collisions actually result in the gluons inside each proton colliding. When they smash together, because the protons total energy is split among its constituent particles, only a fraction of the total energy belongs to each gluon, so it takes a large number of collisions to find one where a large portion of that energy say, 50% or more belongs to the relevant, colliding gluons.

When that occurs, however, thats when the most energy is available to either create new particles (via E = mc2) or to perform other actions that energy can perform. One of the ways we measure energies, in physics, is in terms of electron-volts (eV), or the amount of energy required to raise an electron at rest to an electric potential of one volt in relation to its surrounding. At the Large Hadron Collider, the current record-holder for laboratory energies on Earth, the most energetic particle-particle collision possible is 14 TeV, or 14,000,000,000,000 eV.

Although no light can escape from inside a black holes event horizon, the curved space outside of it results in a difference between the vacuum state at different points near the event horizon, leading to the emission of radiation 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.

There are things we can worry will happen at these highest-of-energies, each with their own potential consequence for either Earth or even for the Universe as a whole. A non-exhaustive list includes:

If you draw out any potential, it will have a profile where at least one point corresponds to the lowest-energy, or true vacuum, state. If there is a false minimum at any point, that can be considered a false vacuum, and it will always be possible, assuming this is a quantum field, to quantum tunnel from the false vacuum to the true vacuum state. The greater the kick you apply to a false vacuum state, the more likely it is that the state will exit the false vacuum state and wind up in a different, more stable, truer minimum.

Although these scenarios are all bad in some sense, some are worse than others. The creation of a tiny black hole would lead to its immediate decay. If you didnt want it to decay, youd have to impose some sort of new symmetry (for which there is neither evidence nor motivation) to prevent its decay, and even then, youd just have a tiny-mass black hole that behaved similarly to a new, massive, uncharged particle. The worst it could do is begin absorbing the matter particles it collided with, and then sink to the center of whatever gravitational object it was a part 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; its not threatening at all.

The restoration of whatever symmetry was in place before the Universes 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 upon contact, which creates bad news for any matter that exists close to this point. Fortunately, however, the absolute energy of any particle-particle collision is tiny, corresponding to tiny fractions of a microgram in terms of mass. Even if we created a net amount antimatter from such a collision, it would only be capable of destroying a small amount of matter, and the Universe would be fine overall.

The simplest model of inflation is that we started off at the top of a proverbial hill, where inflation persisted, and rolled into a valley, where inflation came to an end and resulted in the hot Big Bang. If that valley isnt at a value of zero, but instead at some positive, non-zero value, it may be possible to quantum-tunnel into a lower-energy state, which would have severe consequences for the Universe we know today. Its also possible that a kick of the right energy could restore the inflationary potential, leading to a new state of rapid, relentless, exponential expansion.

But if we instead were able to recreate the conditions under which inflation occurred, things would be far worse. If it happened out in space somewhere, wed create in just a tiny fraction of a second the greatest cosmic void we could imagine. Whereas today, theres only a tiny amount of energy inherent to the fabric of empty space, something on the order of the rest-mass-energy of only a few protons per cubic meter, during inflation, it was more like a googol protons (10100) per cubic meter.

If we could achieve those same energy densities anywhere in space, they could potentially restore the inflationary state, and that would lead to the same Universe-emptying exponential expansion that occurred more than 13.8 billion years ago. It wouldnt destroy anything in our Universe, but it would lead to an exponential, rapid, relentless expansion of space in the region where those conditions occur again.

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 has not, at least, not yet, but in theory, this is possible.

Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. (Specifically, for the strong interactions.) Even in empty space, this vacuum energy is non-zero, and what appears to be the ground state in one region of curved space will look different from the perspective of an observer where the spatial curvature differs. As long as quantum fields are present, this vacuum energy (or a cosmological constant) must be present, too.

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 dont 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 its possible that this isnt the only configuration for the quantum vacuum; its plausible that other energy states exist. Whether theyre higher or lower doesnt matter; whether our vacuum state is the lowest-possible one (i.e., the true vacuum) or whether another is lower doesnt 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 wed then have at least one of:

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). The limit of the visible Universe is 46.1 billion light-years, as thats 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. However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. Any catastrophe that occurred within 18 billion light-years of us would eventually reach us; ones that occur today at distances farther away never will.

There are tremendous uncertainties connected to these events. Quantum black holes could be just out of reach of our current energy frontier. Its possible that the matter-antimatter asymmetry was only generated during electroweak symmetry breaking, potentially putting it within current collider reach. Inflation must have occurred at higher energies than weve ever reached, as do the processes that determine the quantum vacuum, but we dont know how low those energies could have been. We only know, from observations, that such an event hasnt yet happened within our observable Universe.

But, despite all of this, we dont have to worry about any of our particle accelerators past, present, or even into the far future causing any of these catastrophes here on Earth. The reason is simple: the Universe itself is filled with natural particle accelerators that are far, far more powerful than anything weve ever built or even proposed here on Earth. From collapsed stellar objects that spin rapidly, such as white dwarfs, neutron stars, and black holes, very strong electric and magnetic fields can be generated by charged, moving matter under extreme conditions. Its suspected that these are the sources of the highest-energy particles weve ever seen: the ultra-high-energy cosmic rays, which have been observed to achieve energies many millions of times greater than any accelerator on Earth ever has.

The energy spectrum of the highest energy cosmic rays, by the collaborations that detected them. The results are all incredibly highly consistent from experiment to experiment, and reveal a significant drop-off at the GZK threshold of ~5 x 10^19 eV. Still, many such cosmic rays exceed this energy threshold, indicating that either this picture is not complete or that many of the highest-energy particles are heavier nuclei, rather than individual protons.

Whereas weve reached up above the ten TeV threshold for accelerators on Earth, or 1013 eV in scientific notation, the Universe routinely creates cosmic rays that rise up above the 1020 eV threshold, with the record set more than 30 years ago by an event known, appropriately, as the Oh-My-God particle. Even though the highest energy cosmic rays are thought to be heavy atomic nuclei, like iron, rather than individual protons, that still means that when two of them collide with one another a near-certainty within our Universe given the vastness of space, the fact that galaxies were closer together in the past, and the long lifetime of the Universe there are many events producing center-of-mass collision energies in excess of 1018 or even 1019 eV.

This tells us that any catastrophic, cosmic effect that we could worry about is already tightly constrained by the physics of what has happened over the cosmic history of the Universe up until the present day.

When a high-energy particle strikes another one, it can lead to the creation of new particles or new quantum states, constrained only by how much energy is available in the center-of-mass of the collision. Although particle accelerators on Earth can reach very high energies, the natural particle accelerators of the Universe can exceed those energies by a factor of many millions.

None of the cosmic catastrophes that we can imagine have occurred, and that means two things. The first thing is that we can place likely lower limits on where certain various cosmic transitions occurred. The inflationary state hasnt been restored anywhere in our Universe, and that places a lower limit on the energy scale of inflation of no less than ~1019 eV. This is about a factor of 100,000 lower, perhaps, than where we anticipate inflation occurred: a reassuring consistency. It also teaches us that its very hard to kick the zero-point energy of the Universe into a different configuration, giving us confidence in the stability of the quantum vacuum and disfavoring the vacuum decay catastrophe scenario.

But it also means we can continue to explore the Universe with confidence in our safety. Based on how safe the Universe has already shown itself to be, we can confidently conclude that no such catastrophes will arise up to the combined energy-and-collision-total threshold that has already taken place within our observable Universe. Only if we begin to collide particles at energies around 1020 eV or greater a factor of 10 million greater than the present energy frontier will we need to begin to worry about such events. That would require an accelerator significantly larger than the entire planet, and therefore, we can reach the conclusion promised in the articles title: no, particle physics on Earth wont ever destroy the Universe.

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No, particle physics on Earth won't ever destroy the Universe - Big Think