Who is correct here? We don’t know, you don’t know, it is uncharted territory. Would you bet the entire human history and the existence of our solar system on it?
–from a user comment on my old website on the topic of the Large Hadron Collider
Back when all the hype and hoopla concerning the LHC startup had people afraid that the Large Hadron Collider would create a black hole that would eat and destroy the Earth, I tried to quell some of these fears. At my old site, I wrote three articles explaining why the Earth was safe, and why — even if the worst-case scenario came true — we’d be completely fine. And I had forgotten about it, until last night’s Daily Show came on.
|The Daily Show With Jon Stewart||M – Th 11p / 10c|
|Large Hadron Collider|
I had completely forgotten that Walter Wagner — the crackpot at the center of the media firestorm — participated in a spirited discussion on my old site. Thankfully, John Oliver was able to show how absurd this man and his contentions are.
But even with that, I was unprepared for this fallacy of an argument that came out of the interview:
John: So, roughly speaking, what are the chances that the world is going to be destroyed? One-in-a-million? One-in-a-billion?
Walter: Well, the best we can say right now is a one-in-two chance.
Walter: Yeah, 50-50… It’s a chance, it’s a 50-50 chance.
John: You come back to this 50-50 thing, what is it Walter?
Walter: Well, if you have something that can happen and something that won’t necessarily happen, it’s going to either happen or it’s gonna not happen. And, so, it’s kind of… best guess at this point.
John: I’m… not sure that’s how probability works, Walter.
Yikes. Let me recap some major physics points, so that you understand what’s going on, and why there’s nothing to be afraid of. Let’s take a look at our current understanding of physics. We have:
- three spatial dimensions,
- an extremely good and well-tested understanding of quantum mechanics,
- probed and established the known laws of physics up to energies 120 GeV (the equivalent of temperatures of 1,400 trillion degrees Celcius), and
- a very good understanding of how large black holes are formed and grow.
Most of the work establishing the particle physics understanding was done at Fermilab, a place where I’ve visited, spoken at, and even worked at for a brief time.
Now, let’s go through what would need to happen at the LHC to create this doomsday scenario. First off, the honest truth: the LHC is going to achieve higher energies than Fermilab ever did. There are some very speculative papers out there that say that it is not completely ruled out that it’s physically impossible to have extra dimensions at those energies. It is extremely unlikely and there’s no evidence for it, but it hasn’t experimentally been ruled out, because we haven’t done the experiment. I wouldn’t say there’s a one-in-two chance that it’s possible, but there’s a small but finite probability that there are extra dimensions accessible at these energies.
So if there are extra dimensions, it is conceivable that they could be of a specific type allowing the (again, very rare, but plausible) formation of a microscopic black hole. This black hole will have, at most, a mass equal to the energy of the proton-proton collision, or 2.5 x 10^-20 grams. Wow, that’s a tiny, tiny, tiny black hole! Again, not a one-in-two chance, but not completely impossible.
But, even if you have extra dimensions of the right scale, and of the right type, and you make this black hole, you still have a problem: it’s unstable. Due to the laws of quantum mechanics, this black hole is going to decay by a process known as Hawking radiation. For a black hole of mass 2.5 x 10^-20 grams, the decay time is about 10^-84 seconds, which is not even enough time to exist! For physics to be meaningful, we need a time of about 10^-43 seconds or longer. Translated into black hole mass, we’d need it to be at least 0.00002 grams to have even a chance of existing. To overcome this, you have to throw out the known laws of physics. These laws are so well-established that it’s inconceivable that they’d be wrong — it would be like waking up tomorrow and seeing the Sun rise in the West. This is where the zero-probability argument comes in. But, for argument’s sake, let’s assume there are some new laws of physics that we haven’t conceived of yet that could make these black holes stable. Let’s assume we make them, and they just fall through the center of the Earth and start eating whatever matter it encounters. What will happen?
Well, you can compute the rate at which this black hole will eat matter. Assuming it eats every proton, neutron, or electron that it comes in contact with — and also taking into account its gravity, to see what it attracts — it will eat about 66,000 protons and neutrons per second. That rate will be constant until the black hole becomes quite large; at about one billion metric tonnes, the black hole will start to grow faster. Capturing 66,000 nucleons per second, how long will it take to get the black hole up to even one kilogram? Three trillion years, which is much longer than the lifetime of the Sun or even the age of the Universe. So even if you make a black hole, and even if the laws of physics that we know are wrong and it lives forever, it is still harmless. So the Earth will still be okay.
So now you know what happens in the worst-case scenario, and now you know why we’re safe. Moreover, you know that there is a zero probability that the Earth will be destroyed and eaten by a black hole. And a special thanks to my loyal reader Landon — in an earlier time zone than I am — for urging me to watch the Daily Show last night!