“Alas! must it ever be so?
Do we stand in our own light, wherever we go,
And fight our own shadows forever?” -Edward Bulwer-Lytton
Ever since it was conjectured that the speed of light was the ultimate speed limit of the Universe, we have tried — with the most powerful of our tools — to push as close to it as possible.
And we know the speed of light in vacuum — in perfectly, completely empty space — exactly: 299,792,458 meters-per-second. That’s what we refer to as c, or more casually, the speed of light. And that’s really exact, 299,792,458.00000000… (etc.) meters-per-second.
And until the recent hullaballoo about neutrinos, in which one experiment claims that they can go about 0.002% (or 6,000 m/s) faster-than-light, we’ve never observed matter moving at or above the speed of light.
The previous “best measurement” we had on neutrinos was from this supernova, above, which confirmed that neutrinos move at speeds indistinguishable from the speed of light, to an accuracy of ±0.62 meters per second.
But we’ve spent generations trying to accelerate matter as close to the speed of light as we possibly can under extremely controlled laboratory settings. And although it was never the goal of any of these experiments, it’s one of those serendipitous results you get for free. Here’s how.
One of the most basic properties of charged particles is that if you place them in an electric field (E-field), they experience a force and accelerate. Positively charged particles accelerate in the same direction as the E-field, while negatively charged ones accelerate with the same magnitude, but in the opposite direction.
Charged particles, once they’re moving, also respond to magnetic fields.)
If you orient a magnetic field perpendicular to a charged particle’s velocity, it will bend its orbit into a circular shape, with positively and negatively charged particles bent into opposite directions (clockwise/counterclockwise) from one another.
And if you combine these two properties in the right configuration, you can accelerate these particles in a circular path!
This is a primitively designed cyclotron, predecessor to the great ring-shaped particle accelerators of the modern day.
In this early design, you’d have a magnetic field that bent your charged particles into a circular path, but every time they passed through the middle of your cyclotron, you’d engage an alternating electric field to give them a “kick” and accelerate them. As the particles move faster and faster over time, the circle that they make gets larger and larger, as the magnetic field — a constant quantity — has a harder time bending the particle into a circle: it’s harder to alter a particle’s motion when it’s got more momentum!
In our more modern design, however, we don’t use a magnetic field from a permanent magnet and an increasing, spiral path. Instead, we line our particle accelerators — giant rings — with electromagnets, capable of producing just the right amount of magnetic field to keep your fast-moving particle within the ring. This is true whether you’re moving at 99%, 99.99%, 99.9999%, or any other percent of the speed of light, so long as you’ve got your physics right.
So the way this works is you inject your particle into the accelerator ring, and tune your electromagnet to just the right level for its speed. Then the particle enters a small region where an electric field can accelerate it — cleverly named an accelerating cavity — which gives it a kick, bumping up its speed by just a little bit. In order to keep the particle moving in the same ring, you’ve got to have the next electromagnet tuned slightly more powerfully, or else this particle will smash into the side of your ring!
So you ramp your electromagnet’s field strength up each time you accelerate your particles just a little bit more, and as you get progressively closer and closer to the speed of light, you adjust your magnet to keep your particles where they need to be. And whether your particle is moving at 299,492,093 m/s or 299,492,108 m/s makes all the difference in the world as to whether it stays in the ring or slips out, slamming against the side of the accelerator.
Older accelerators, like the Tevatron (above), accelerated particles in one direction and antiparticles (with the same mass but opposite charge) in the opposite direction. Over the span of thousands and thousands of trips around the ring (with each trip taking just microseconds), they would be accelerated up to their maximum speed, with the electromagnets consistently ramping up towards their maximum field strength to accomodate them!
While the point of the science was to see what interesting things pop into existence when these high-energy particles and antiparticles collide, you needed to carefully calibrate the electromagnets and accelerating cavities to operate at just the right frequencies to account for these particles as they sped up along their way to as close to the speed of light as they could possibly get! In fact, like many undergraduate interns, it was one of my very first jobs (back in 1997; hi, Roger and Erik) to make sure the electromagnets that bent the particle beams at the Tevatron were optimized to achieve the particle energies (and luminosities) we wanted, up to 99.999956% the speed of light!
Today, the energy record is held by the Large Hadron Collider at CERN, with protons achieving individual speeds of 299,792,447 meters per second, just 11 meters per second shy of the speed of light! (Go ahead and calculate it for yourself!) After the LHC’s impending energy upgrade, it’ll get even closer: up to 299,792,455 m/s, or a tantalizing 99.9999991% the speed of light.
Protons don’t move at or faster-than the speed of light; we’d actually be able to tell, from our electromagnetic adjustments of the accelerator, if they were. But they’re not even the fastestparticles we’ve created!
Because even though electrons and positrons at LEP — the Large Electron-Positron Collider that was dismantled to make room for the LHC — only got up to a maximum energy of 104.5 GeV, or just 1/33rd of what the LHC gets, a proton has nearly 2,000 times the mass of an electron! What does this mean for speed?
It means LEP’s electrons and positrons reached maximum speeds of 299,792,457.9964 meters per second, or a whopping 99.9999999988% the speed of light, just millimeters per secondslower than light in a vacuum.
And yet, they were slower than light, and we can tell! So whatever the verdict winds up being with these neutrinos, rest assured that all the protons and electrons that make up you and the world you love are still bound by the laws of special relativity. And that’s how we know!