My last post was about travelling faster than the speed of light and going back in time. In it I said that nothing could accelerate past the speed of light. No matter how good your BMW, you’ll never get a car where you just hit the accelerator and break the light barrier. As you zoom faster and faster, light will always be going at 300,000 km/s. It always outpaces you. It always beats you. In a race, you’ll always be Dick Dastardly–light will always win. So travelling in time by going faster than light is never going to work.
Sort of.
There’s a hole in orthodox physics which allows things to travel faster than light. Nothing can accelerate past the speed of light, but things can start faster than the speed of light and never slow down below it. These things are theoretically possible. They’re called ‘tachyons’.
Since you’re currently travelling slower than the speed of light, this means you’ll never go tachyonic. You, yourself, can never travel back in time by moving faster than light.
But that doesn’t mean that tachyons aren’t interesting. If you could find some, and manipulate them, then you could use them to communicate with the past. Because they travel faster than light, and move back in time, you could theoretically send them back in time to a detector and, say, tap out a Morse code message to your earlier self. Imagine it! Looking on Tinder you suddenly get the message:
… .– .. .–. . / .-.. . ..-. –
Consulting what that means, you hastily swipe left…
This method of communication was dubbed ‘the antitelephone’ by Benford, Book, and Newcomb. Benford would also go on to write a famous novel, Timescape, which featured just such a device. Indeed, in fiction tachyons appear all over the place. Sometimes they’re used to communicate with the past (as in X–Files and John Carpenter’s Prince of Darkness). Sometimes they’re just inserted whenever some sci-fi writer needs something suitably timey-wimey and sciencey sounding (such as Star Trek or Babylon 5).
Back in the real world, though, we’ve searched for tachyons. Back in the 70s someone thought they may have found them but they decided that there was nothing there after all—although more recently there’s been some hope for their existence thrown out there. (Indeed, in my own book I argue that the current failures to detect them don’t really cast doubt on their possibility at all–but that’s another story.)
There have been two main methods for detecting tachyons.
One is to look for ‘Čerenkov radiation’ which is emitted when things travel faster than the speed of light in a given medium. Notice that last bit: ‘in a given medium’. Whilst light goes at 300,000 km/s in a vacuum—and nothing can beat that pace—it goes slower when it enters other substances (such as water or those tatty Perspex blocks you use at school to demonstrate refraction). If something passing through such a substance manages to beat the speed that light can do in that substance, it promptly starts emitting the radiation. People thought tachyons would emit the same radiation. When we went looking for it, we found nothing.
But it makes no sense to think that tachyons ever would have emitted such radiation. Think of the world from their perspective. From the point of view of a tachyon, it’s us who are the ones moving faster than the speed of light (and us who are going back in time). It’s long been thought that there’s no sense in asking which of us is ‘really’ moving at such-and-such a velocity. If true, we shouldn’t be able to figure out which of us or the tachyon is the one who is really moving faster than light. But if tachyons emitted Čerenkov radiation then we’d be able to tell which it was––the person going faster than the speed of light would be the guy emitting tonnes of radiation! And since we’re not emitting Čerenkov radiation, it can’t be us going faster than the speed of light. So if you believe in tachyons you’ve got a choice: Give up on that long-standing principle of physics (i.e. that we can’t determine absolute velocities) or give up on thinking that tachyons would blast out radiation as they blitzed around the cosmos.
(Indeed, buried away in the literature is a paper arguing for exactly this sort of thing, claiming that the equations don’t bear out tachyons emitting Čerenkov radiation after all.)
The other way to detect tachyons assumed that they played a role in ‘proton decay’. You’re probably familiar with decay in general. Radioactive atoms decay every now and again, turning into something else––for instance, uranium decays and turns into thorium. When it decays, it emits ‘alpha radiation’ in the form of an alpha particle (and it’s that which makes the uranium radioactive). When the alpha particle shoots off one way, the resulting thorium atom gets ‘kicked back’ and goes the other way.
This ‘kick back’ means that the thorium and the alpha particle spontaneously start moving. But the energy for their movement doesn’t come from nowhere. The mass of the thorium plus the alpha particle is less than the mass of the uranium. The energy for the alpha particle going one way and the thorium atom going the other comes from that difference in mass––by shedding mass, the uranium atom can generate the energy for them to move.
Some people think protons might decay if we left them for long enough. The proton would get ‘kicked back’ by whatever particle it emits when it decays. But here’s the twist: When a uranium atom decays, it ceases to be. But a proton cannot stop existing––that’s because there’s a law called ‘conservation of baryon number’ which means that the number of protons that exist throughout the universe can’t change. And if the proton can’t stop existing, it can’t shed mass to generate the energy to ‘kick it back’. (Similarly, where would the energy come from to create the particle it emits when it decays?)
So if protons decayed, we’d have a quandary. Do we give up on the conservation of energy and say that things can spontaneously start moving? Or do we give up on the conservation of baryon number and say that the proton stops existing? Or what?
The answer is the humble tachyon. I said above that tachyons can’t slow down below the speed of light. Anything that starts off travelling faster than the light barrier stays there! But what happens when a tachyon tries to leave the gravity well of the Earth? Wouldn’t they slow down as gravity pulls them back?
The answer––some people think––is that tachyons will have negative mass. That means gravity works in reverse when it comes to tachyons. They slow down when they approach the centre of the Earth and then speed up when they try and leave Earth’s gravitational clutch! And the negative mass means that, if they were emitted during proton decay, the energy produced would be counterbalanced by the negative mass-energy of the tachyon.
So whilst we might not be able to detect the tachyon itself, some physicists figured that they might see the protons decaying, and moving about spontaneously. That’d then be evidence that tachyons existed.
But when we looked, we did not see any protons decay and didn’t see them spontaneously move about. And so, presumably, they weren’t emitting tachyons.
So tachyons might, in some sense, be allowed by the laws of physics. But even though they are, they’d both be pretty weird (negative mass? what the hell does that even mean?) and, as far as we can tell, they don’t appear to be out there.
CBAs of Time Travel 