Will LHC Kill us All?

You have probably heard rumblings about the sci-fi dangers of the Large Hadron Collider, but didn’t know what they were about. Well, let me break it down for you:

A strangelet is a hypothetical particle that has an up quark, a down quark, and a strange quark — or at least equal numbers of them in that ratio. The worry is that the LHC will create a strangelet in one of its experiments. If a strangelet comes in contact with a lump of ordinary matter such as Earth, it could convert the ordinary matter to strange matter.

This doomsday scenario is as follows: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter.

Wormholes

In physics, a wormhole is a hypothetical topological feature of spacetime that is fundamentally a 'shortcut' through space and time. Wormholes allow faster-than-light travel by ensuring that the speed of light is not exceeded locally at any time. If two points are connected by a wormhole, the time taken to traverse it would be less than the time it would take a light beam to make the journey if it took a path through the space outside the wormhole. However, a light beam traveling through the wormhole would always beat the traveler. As an analogy, running around to the opposite side of a mountain at maximum speed may take longer than walking through a tunnel crossing it. You can walk slowly while reaching your destination more quickly because the distance is smaller.

A wormhole could allow time travel. This could be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back; relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer, similar to what is seen in the twin paradox, in which a twin who makes a journey into space in a high-speed rocket will return home to find he has aged less than his identical twin who stayed on Earth.

For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2005 while the stationary mouth's clock read 2010. A traveler who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2005, in the same region but now five years in the past. Such a configuration of wormholes would form a closed loop in spacetime, known as a closed timelike curve.

Time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry. There is a catch though: a time traversing wormhole cannot take you back to before it was made--which might explain why there's no time travelers popping into your local McDonald's.

Multiple Universes

In 1972, Hugh Everett formulated the many-worlds interpretation of quantum mechanics. In layman's terms, this means that there is a very large, perhaps infinite, number of universes since everything that could possibly have happened in our past (but didn't) has occurred in the past of some other universe(s). Parallel universes might provide a way out of paradoxes because all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. Thus time travel then involves not moving forward and backwards through a single universe, but moving horizontally between similar universes.

A curious consequence of many-worlds is an experiment known as the quantum suicide machine has been proposed by cosmologist Max Tegmark. It examines the Schrödinger's Cat experiment from the point of view of the cat.

For example, a man sits down before a gun, which is pointed at his head. This is no ordinary gun; i­t's rigged to a machine that measures the spin of a quantum particle. Each time the trigger is pulled, the spin of the quantum particle is measured. Depending on the measurement, the gun will either fire, or it won't. If the quantum particle is measured as spinning in a clockwise motion, the gun will fire. If the particle is spinning counterclockwise, the gun won't go off. There'll only be a click.

Nervously, the man takes a breath and pulls the trigger. The gun clicks. He pulls the trigger again. Click. And again: click. The man will continue to pull the trigger again and again with the same result: The gun won't fire. Although it's functioning properly and loaded with bullets, no matter how many times he pulls the trigger, the gun will never fire. He'll continue this process for eternity, becoming immortal.

Go back in time to the beginning of the experiment. The man pulls the trigger for the very first time, and the particle is now measured as spinning clockwise. The gun fires. The man is dead.

But, wait. The man already pulled the trigger the first time -- and an infinite amount of times following that -- and we already know the gun didn't fire. How can the man be dead? The man is unaware, but he's both alive and dead. Each time he pulls the trigger, the universe is split in two. It will continue to split, again and again, each time the trigger is pulled, and become quantum immortal.

The idea behind quantum immortality is that the experimenter will remain alive in, and thus remain able to experience, at least one of the universes in this set, even though these universes form a tiny subset of all possible universes. Over time, the experimenter would therefore never perceive his or her own death.

Schrödinger's Cat

This is a follow up to the Double Slit experiment

Schrödinger's cat is a thought experiment devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of quantum mechanics being applied to everyday objects. The thought experiment presents a cat that might be alive or dead, depending on an earlier random event.

Schrödinger wrote: "One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts."

The purpose of the thought experiment is to illustrate an apparent paradox: our intuition says that no observer can be in a mixture of states, yet the cat, it seems from the thought experiment, can be such a mixture. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer?

The Double Slit Experiment

Voted by Physics World readers as “the most beautiful experiment in physics”, the classic two slit experiment was first conducted by Sir Geoffrey Ingram Taylor in 1909. It involves firing tiny individual particles at a thin plate with two parallel slits and watching the pattern they make on the wall behind it.

The outcome of this simple experiment is quite shocking because it suggests light behaves differently when being observed. But to understand the science of the Double Slit Experiment, first take a look at the way particles and waves behave in different circumstances.

Firing light at a plate with a single slit in the middle always leads to the same result: a vertical line on the back wall. This is because some light reflects off the plate, while some go directly through the slit and land on the wall predictably. Adding a second slit should therefore create the same result: two vertical lines on the back wall.

Waves are a little more complex; so think of them as like ripples in a pond. When the ripple collides with the plate, only the apex passes through the slit and radiates out. It strikes the back wall with the most intensity in the middle – directly in line with the single slit. This is similar to the single vertical line created by firing particles.

But, sending waves through two slits has a completely different effect. As the top of one wave meets the bottom of another wave, they cancel each other out. This creates an interference pattern on the back wall. There are now lots of vertical lines across the wall where the many little waves hit it with the highest intensity.

In the Double Slit Experiment, electrons are fired at the slit in much the same way. A single slit causes a single vertical line of electrons along the back wall. But when the electrons are fired at two slits – and here comes the kicker – the result is an interference pattern. Why are these tiny bits of matter, fired individually at the wall, suddenly behaving like waves? What are they interacting with?

For some physicists, the conclusion was inescapable. The tiny electron arrives at the plate as a single particle, becomes a wave of potentials, goes through both slits, and interferes with itself. Mathematically, this theory is even more bizarre. The electron goes through both slits – and neither. It goes through just one – and just the other. By this reckoning, the Double Slit Experiment suggests that every possibility actually occurs in parallel worlds.

But this was just a theory, so physicists put up a measuring device – an observer – next to the plate to see which one the electron really went through. Amazingly, the electrons returned to behaving like particles again, creating two vertical lines on the back wall. The act of observing the quantum world actually changed the outcome! In short, the electron "decided" to act differently, as if it were "aware" it was being watched. The observer collapsed the wave function simply by observing.