If someone told you that there was a simple way of making sure you live forever, you would consider them crazy. If this person then went on to tell you that the very same method of doing this – which is incredibly easy to do – would allow you to prove to yourself that an infinite number of universes exist at the same time all around us, you would surely laugh nervously and walk away swiftly in the opposite direction.

But suppose you stayed and listened. Suppose the apparent madman told you that performing this remarkably cheap and easy experiment would be the biggest gamble of your life as well as the most controversial of all possible activities. Would you do it? Would you do it if you knew the man telling you about it was a physicist?

The man's name is Max Tegmark¹. He is a physicist – at the University of Pennsylvania to be more precise. Below is the practical guide to performing his controversial experiment, but first, an explanation, and then, a dire warning.

The Explanation

A brief word about parallel universes and quantum mechanics is perhaps necessary. Quantum mechanics is a highly successful theory that describes the motions and interactions of particles at a fundamental sub-atomic level. This means that it describes – mostly in a mathematical way – how particles such as electrons and photons (photons are basically particles of light) behave. In short, this description is unusual to say the least. One of its predictions is that particles can be in multiple places or multiple states of existence simultaneously. This is called superposition.

In fact this concept has been proved by the simple 'double-slit' experiment. This is where one fires individual electrons through a sheet of a material with two vertical slits in it, and onto a blank white screen. There is a 50-50 chance that the electron will go through any one slit.

When two electrons meet each other, you see, they stop behaving like particles and start behaving like waves. This means that they produce an 'interference' pattern. These interactions are analogous to those that occur between two water waves or two sound waves. When an individual electron is fired at the two slits, an interference pattern still emerges almost immediately on the screen.

If you have not heard of this experiment before, the result should be surprising. How can one electron produce a pattern that can only form when two electrons mix? The answer is that the electron interferes with itself – that is, it has existed in two places at the same time. But more precisely, what has happened is that the universe has given birth to a new universe, one where the electron goes through the other slit. The electron in one universe interferes with the electron in the other.

Of course, physicists cannot currently be certain whether this 'many worlds' interpretation of the double-slit experiment is correct. Many people - including the most esteemed scientists - have taken this as proof that an infinite number of worlds exist in parallel to ours, playing out all the possibilities. For example, there is a world where you are not reading this article, a world where the magnolia plant never evolved, a world where JFK was not assassinated, and a world where the Titanic didn't sink. And there would be worlds that have different laws of physics altogether.

The way to imagine these universes is to envisage the universe we are in at the moment as a two-dimensional sheet of paper. All the other universes exist likewise, and they are stacked on top of our universe like a pack of cards. So, if you assume the true definition of a two-dimensional entity - ie, one that has no thickness whatsoever - the sheets of paper are all no distance from each other at all. In other words, if the parallel universe theory is correct, all of the universes are existing right next to us now, and all around us, except that instead of being two-dimensional sheets, they are three-dimensional entities stacked up in a fourth dimension that we humans are incapable of imagining².

The suspense must be torture now, so we shall move on.

The Warning

Like all typical insurance deals, the experiment proposed by Max Tegmark comes with a catch. It is purely a gamble. Nobody, including Tegmark, can assure you that it will go smoothly, because nobody has actually attempted it yet. There are two possibilities for the outcome of this experiment: either you live forever and you are satisfied that parallel universes exist, or - and this is the controversial part - you die.

Don't give up reading yet. This experiment is as grounded in scientific fact as any other modern theory of physics. It may sound like temptation from the devil, but it is an investigation in quantum mechanics like no other. Understand that if the experiment succeeds, you would have a hard time convincing your friends that parallel universes exist. So without further ado, we present...

The Practical Guide to Immortality

There are many variations of this experiment. All of them will produce essentially the same effect, but some are cheaper and some are more difficult to get right because they may require precision instruments or access to a physics laboratory. Nevertheless, none of them require technology that is not already in existence today, which is what makes this experiment so simple in comparison to those that NASA might spend billions of dollars constructing. Variation B is the one that Tegmark proposed specifically.

Step One

Variation A. Find a radioactive atom from the nearest store of chemical elements. Uranium and plutonium are among those that are feasible. Connect the atom to a Geiger counter³. You may need a physicist to help you with this step. As you may recall from a physics lesson in the distant past, a Geiger counter measures the radioactive decay of the element.

The reason we use a radioactive element is that its decay is wholly random. It is top priority that the variation of the experiment you use involves a reliably-random element. This is why computers are no good. You cannot simply use the random function present in most spreadsheet applications to make the decision. Computers are developed by humans and as such can never produce totally-random events because the random-number-generating algorithm, when untangled and simplified, is always predictable. You will discover why this is such a priority later on.

Variation B. It is necessary for this variation to understand a quantum mechanical concept known as spin. 'Spin' is a loose term used by physicists that has no fixed definition, pertaining to the angular momentum and symmetry of a particle. One unit of spin is half of a Planck unit⁴. So, if a particle has spin 3/2, then it basically means that after rotating it one and a half times, it will look the same as its original state (this is the same as rotational symmetry). In other words, its order of rotational symmetry is 1.5.

Spin is a strange property in this way, because particles can have negative spin and 'spin 2' as well. If you rotated a particle with spin 2 around 360 degrees, it would not look the same as how it started; you would need to rotate it 360 degrees again!

For this variation all you will need is a sub-atomic particle such as an electron. The electron will have a random spin, which we can refer to in simple terms as being either 'up' or 'down'. The chance of its being one or the other is totally random, and again, this is crucial. In order to detect which state the particle is in, you will also require a superconductor. The process of this detection is described in the corresponding entry about superconductivity.

Variation C. There are many other quantum mechanical ways of producing random results. You could, for example, set up a 'pure'⁵ parallel electric circuit in a vacuum and use the current at different points in the circuit for your random event. While you probably think this is predictable, electrons (which are the substance of the 'flow' of electricity in the wires) always take a random route through the wires. When they reach a junction in the parallel circuit, it makes a random decision, which is why it is almost always the case that half of them go one way and half go the other.

Step Two

Step two involves finding a reliable means of dying. Please don't shy away at this point. If the experiment goes precisely according to plan, you won't feel a thing. Of course there are many ways that you can set up a death trap for yourself, but in this case you must be certain that the method you choose fits two criteria:

1. The method should be capable of killing you – not just producing serious injury. If there is any chance that the method you choose will fail to kill you (even though we do not plan to die in the experiment), choose a different method.
2. The method should be certain of killing you almost instantaneously. At no point should you be aware you are about to die, if indeed that is the case, and we hope it isn't, of course.

From this we can successfully rule out the use of drugs, unless they are given to you after anaesthetic. We can also rule out a method involving fire, or long drops, or electrocution.

Step Three

It is now necessary to link together the random event generator created in step one with the death machine set up in step two. For this you will benefit from a willing assistant and a computerised device. Suppose you chose variation A in step one, and a machine gun for step two. To link them together, a simple computer program will need to measure the Geiger counter output against a set of values that it has been provided with, so that it can determine whether the atom has decayed or not. If it has, it should fire the gun, which should be partially concealed so that any robotic trigger-firing component is invisible to the witness in order that they cannot tell when the trigger is being pulled.

All you need to do is stand with the barrel of the gun pointing at your head and wait. The random decision will be made by whatever method you chose, and either the gun will fire or it won't. If it doesn't, it would be helpful to fix it so that the computer in charge of determining the random event produces a 'click' or, if you prefer, the lyrics from the chorus of your favourite song, which would lighten the atmosphere considerably.

Making Sense of the Result

If everything goes as it should, you can repeat the experiment ten times and still survive. Even though - from the point of view of your assistant - there is a 99.9% chance that you will be dead, you will have survived from your point of view.

The reason? It's so simple. There is a 99.9% chance that after ten repeats of the test you are dead in this universe - the one that you started out in. However, there is a 100% chance that you survive in a parallel universe, because in the many worlds theory all possibilities are enacted, no matter how improbable. To make sense of the experiment, here is a scientific view of it:

After the first go with the experiment, the universe splits in two. In one universe you survive, in the other you are dead. In the second go, the universe where you survive splits again, producing another two universes with similar circumstances. And so it goes on. The fact is, you don't know of the existence of the universes where you are dead, because you cannot experience anything when you have died.

In the words of Tegmark himself, speaking of someone that might attempt his experiment, 'he would have no awareness of these realities [the ones that he survives] since he would be dead. The only realities in which he would continue to be aware of would be the ones in which he survived.'

So this means that you can perform the experiment as many times as you see appropriate and - so long as the theory of parallel universes is correct - you can be certain of surviving. In effect, you can be certain of immortality.

An Assumption

It may be on the tip of your tongue to make the assumption that quantum mechanical events are what make up every single process in the universe, and thus when we die it will also be a random event because it is based on things that occur at the sub-atomic level. You would be right to say that quantum mechanics ultimately governs everything, but you cannot be so hasty in assuming that you will live forever just because your death is a quantum mechanical event at a fundamental level.

If you had been right, then we will all unquestionably live forever because as soon as it comes to the point where we would most probably die, the universe will split and we will only be aware of the split universe in which we survive. Everybody else, of course, will be aware that we are dead, which is why we perceive the concept of death as it is. But as individuals, in this case, we would all live to the end of the universe, and even then there is a slight possibility that we escape and carry on living in an alternative reality.

Tegmark, however, is not so optimistic. In the experiment above, we made sure that three criteria were met: that the event in question was wholly and unquestionably random; that the method of the termination of our life happened without us knowing beforehand that it would happen; and lastly, that we certainly die from the event and not just get injured very badly. As you can probably work out, most death circumstances do not follow these precise rules. If you are diagnosed with ischaemic heart disease, for example (the most common cause of death worldwide), then you might be given a certain amount of time to live. Therefore you know you will die and the quantum mechanical randomness of it is destroyed.

What is the Point?

Proving the existence of parallel universes is not just a neat party trick that you can demonstrate in front of your friends. From their point of view, they need only repeat the experiment an eleventh time and the probability that they will see you dead is overwhelming. Thus, you can never convince them.

But knowing that the many worlds theory is correct has implications in other areas. You probably know of the paradox of time travel. If you go back in time to kill your grandfather before your parents were born, then how could you exist to do it? If we consider this from the point of view of parallel universes, there is no paradox to consider. When you go back in time, you simply enter another universe that exists at the period of time you think you are going back to. In this universe, it doesn't matter to your existence if the grandfather dies at that point, because he is not your grandfather. He is an alternative version of your grandfather, and as such, will only affect the existence of the alternative version of his grandson.

Parallel universes are also the guiding principle behind explaining quantum computation in a meaningful way. Quantum computers work by performing different parts of a calculation in different universes, so that the answer can be reconstructed almost instantaneously at the computer. If parallel universes did not exist, then computers would be limited to the resources of one universe, and as such the processing power of computers would forever be restricted.

What if the Many Worlds Theory is Wrong?

If it transpires that parallel universes do not exist, which is possible of course, then somebody performing the experiment described above would die. But it would also prove another theory - the opposite of the many worlds theory. This is called the Copenhagen Interpretation, which was the 'standard interpretation' of quantum mechanics before the many worlds theory arrived and began gaining great popularity.

The Copenhagen Interpretation presents an alternative explanation for questions posed by the double-slit experiment, also described above. It states that it is not possible to predict the interactions of quantum mechanical entities before they occur and it also describes the nature of a process called 'decoherence'. It must be understood that particles that are smaller than atoms are constantly misbehaving. They never stay in one place, or even remain the same type of particle, and they tunnel from place to place without travelling in the space between. And, just like a room of rowdy school children when the teacher walks in, they calm down when they are observed. On observation, they suddenly retain just one identity and one location. In other words, they lose the ability to be in a superposition.

It probably sounds odd. For example, how does a particle know when it has been observed? Unfortunately, physicists can't pretend they have an answer, but again, there are different interpretations of this concept. The definition of an 'observation' in most views does not need to be a conscious being looking at the particle; the observation can be a photon (a carrier of light) that bounces off it, taking 'information' away with it. In stronger versions of the Copenhagen Interpretation, conscious beings such as humans are what cause decoherence and are thus the reason that objects only appear to exist in one place or one state at any given time.

Conclusion

This 'quantum suicide' experiment will remain controversial. Even if the experiment is enacted, the person doing it will not be able to communicate the results. Only those brave enough to attempt it will know the outcome: either they die and the Copenhagen Interpretation is correct; or they live on, alone in the knowledge that an infinite multiverse surrounds them.

More from the Edited Guide

Fascinated by particle physics? Take a decidedly un-random stroll through the Edited Guide with your fellow Researchers:

• The Standard Model of Particle Physics
• The Sub-Atomic World of Kaons, Axions, J/Psi, Sigma and Xi Particles
• Symmetry and CP Violation
• The Higgs Boson
• The Solar Neutrino Problem
• Physics and the Knowledge of Ignorance
• The Problem of Free Will
• Schrödinger's Cat

Related BBC Links

• Are the frontiers of science moving too fast for you? Keep up with them at BBC Science.