As particles travel through the Universe, there's a speed limit to how fast they're allowed to go. No, not the speed of light: below it.
If you want to travel as fast as you can through the Universe, your best bet is to pump as much energy as possible into as small a mass as you can find. As you add progressively more kinetic energy and momentum to your particle, it will travel through space more quickly, approaching the ultimate cosmic speed limit: the speed of light. No matter how much energy you manage to add into the particle in question, you can only get it to approach the speed of light — it will never reach it. Since the total amount of energy in the Universe is finite, but the energy required for a massive particle to reach the speed of light is infinite, it can never get there.
But in our real-life Universe — not the idealized “toy” version we play with in our heads — we don’t simply have arbitrary amounts of energy to give to particles, and we also have to accept that they’re traveling through the space that actually exists, rather than what we imagine as a complete, perfect vacuum. While the Universe is capable of imparting far more energy to particles through natural accelerators — like neutron stars and black holes — than we can ever give them on Earth, even at state-of-the-art machines like CERN’s Large Hadron Collider, the fact that “the vacuum of space” isn’t a perfect vacuum is far more limiting than we often care to admit. Rather than the speed of light, the actual speed limit of particles is below that: set by what we call the GZK cutoff. Here’s what truly limits our motion through space.
Any cosmic particle that travels through the Universe, regardless of speed or energy, must contend with the existence of the particles left over from the Big Bang. While we normally focus on the normal matter that exists, made of protons, neutrons, and electrons, they are outnumbered more than a billion-to-one by the remnant photons and neutrinos. (Credit: NASA/Sonoma State University/Aurore Simmonet)
There are two facts that, when taken together, teach us that reality is not as simple as Newton intuited. Those facts are:
The particles that rapidly travel through the Universe are largely protons, electrons, heavier atomic nuclei, and occasionally positrons or anti-protons. All of these particles, detectable here on Earth and in space as cosmic rays, are electrically charged.
Light, which exists from many different sources, including stars, galaxies, and even the Big Bang itself, is an electromagnetic wave, and can easily interact with charged particles.
While even today’s modern physicists often automatically default to Newtonian-like thinking, we have to be careful to think of things as mere masses moving through the Universe, accelerated only by the forces that other particles and fields exert on them. Instead, we have to remember that the Universe is composed of physical quanta: individual energy packets with properties of both wave and particle, and that those quanta, unless somehow specifically forbidden from doing so, will always interact with one another.
A combination of X-ray, optical, and infrared data reveal the central pulsar at the core of the Crab Nebula, including the winds and outflows that the pulsars care in the surrounding matter. Pulsars are known emitters of cosmic rays, but the rays themselves don’t simply travel unimpeded through the vacuum of space. Space is not a perfect vacuum, and particles traveling through it must reckon with everything they encounter. (Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech)
There are plenty of things left over from the Big Bang, including:
stars
gas
dust
planets
stellar corpses
However, all of the items we just listed only compose about 2 to 2.5% of the total energy budget of what’s present in the Universe: only about half of the normal matter. There’s also dark matter, dark energy, neutrinos, photons, and a sparse, tenuous, ionized plasma present in space, with the last being known as the WHIM: the warm-hot intergalactic medium.
However, the biggest hinderance to charged particles traveling freely through the Universe is actually the least energetic component of all of these: the photons, or leftover particles of light from the Big Bang. While starlight is copious within an individual galaxy, there are places in the Universe — such as the remote depths of intergalactic space — where the only substantial quanta present are the photons left over from the Big Bang: the cosmic microwave background radiation, or CMB. Even today, in our Universe that’s expanded and cooled to be 46.1 billion light-years in radius, there are still around 411 CMB photons per cubic centimeter of space, with an average temperature of 2.7 K.
When cosmic particles travel through intergalactic space, they cannot avoid the leftover photons from the Big Bang: the cosmic microwave background. Once the energy from cosmic particle/photon collisions exceed a certain threshold, the cosmic particles will begin to lose energy as a function of the energy in the center-of-momentum frame. (Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP)
Now, let’s imagine that we’ve got a natural particle accelerator like a neutron star or a black hole, creating electric and magnetic fields that are unheard of on Earth. In these extreme environments, millions of times the mass of Earth exists in a volume of space no larger than a few kilometers in diameter. These astrophysical locations often can achieve field strengths that are millions, billions, or even trillions of times in excess of the strongest electromagnetic fields ever generated in laboratories on Earth.
Any particle accelerated by these objects will be sent on an ultra-relativistic journey through the Universe, where it will inevitably encounter all sorts of particles. But it will particularly run into the most numerous of all particles: the CMB photons that are present. With around ~1089 CMB photons filling our observable Universe, they’re the most abundant and evenly distributed type of quanta present in our cosmos. Importantly, there’s always a probability for a charged particle and a photon, regardless of what the relative energies of the particle and the photon are, to interact.
In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays. Photons are also produced. Processes such as this may be responsible for the generation of the highest-energy cosmic particles of all, but they inevitably interact with the leftover photons from the Big Bang. (Credit: IceCube collaboration/NASA)
If there were no other particles — if we could activate our “toy” vision of an empty Universe where particles simply traveled unimpeded in a straight line until they reached their destination — we could imagine that only the field strengths of these astrophysical environments would place a cap on the total amount of energy a particle could possess. Apply a strong electric field in the direction it’s moving, and it’ll go faster and become more energetic.
In fact, you’d expect there wouldn’t be a limit at all. If this were how the Universe worked, you’d expect there’d be some sort of energy distribution of particles: where large numbers of particles had low energies and a few outlier particles had higher energies. As you looked to higher and higher energies, you’d keep finding particles, but they’d be fewer in number. The slope of the line might change as various physical processes became important at certain energies, but you wouldn’t expect particles to simply stop existing at some energy; you’d just expect there to be fewer and fewer of them until you reached the limit of what you can detect.
Why The Cosmic Speed Limit Is Below The Speed Of Light
Reviewed by Explore With Us
on
February 27, 2023
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