**Decoding Reality: The Universe as Quantum Information**

*What is reality? Is the universe, ultimately, no more than bits of information? Physics, and Quantum Theory in particular, have grappled with the fundamental structure of nature’s basic building blocks for decades, but the answers remain elusive. Quantum physicist and Ceasefire columnist Sebastian Meznaric takes a look at a new book on the topic and finds it full of intriguing and original insights.* by Sebastian Meznaric

What is information and how do we quantify it? Can we, through genetics, use the concept of information to describe living organisms? How about the stock market and social changes? Decoding reality makes a compelling case that the answer to all of the above is yes, and goes even further by introducing the reader to quantum mechanics in the context of the Information Theory. In addition, it deals with many of the philosophical implications of modern physics, such as the question of whether nature is fundamentally unpredictable and random (also known as the determinism question), whether something can come from nothing and, indeed, if nothing can again arise from something. As such, the book deals with many topics and so in this review I will address some of them in more detail, namely: Information Theory, Quantum Mechanics and determinism.

Information theory is a branch of mathematics that deals with quantifying information. So how does one go about quantifying a concept that appears vague and ambiguous at best? Imagine the toss of a fair coin, where the probability of it landing on heads or tails is exactly equal. Then one bit of information is defined to be exactly the amount that you learn if I tell you that the coin will land on either heads or tails. On the other hand, if one knows with certainty and in advance that the coin is going to land on heads, then learning about the future outcome from someone else imparts exactly zero bits of information. In between the two extremes, the amount of information one gains depends on the probabilities one assigns in advance to the different outcomes.

But of course, one might argue, reality cannot be described simply as a collection of coin tosses. The events in the real universe often have multiple possible outcomes, sometimes infinitely many. And the outcomes themselves might not even have equal probabilities of occurrence. The book informs the reader, in a simple to grasp way, about how information about such events might be quantified and further postulates that reality as a whole can be described using such quantities.

Notice that the concept of uncertainty is essential to the above discussion. Indeed, if one knows with certainty everything that is going to occur, then one has effectively ‘decoded’ the universe. However, the modern physical theory of Quantum Mechanics has a few surprises for us here. In fact, the standard interpretation of quantum mechanics tells us that it is in fact impossible to predict all future events with certainty. Imagine a spinning atom. In our everyday understanding of the world, the atom can spin either to the left or to the right. However, this is not so in quantum mechanics – the atom can exist in a state whereby it spins both to the left and to the right at the same time!

This concept seemed so incredible and counter-intuitive to the physicists at the dawn of quantum mechanics that many rejected the idea outright. However, experiments have been conducted that demonstrate that such states indeed do exist and are, in fact, very common in the world of things as small as atoms.

Now imagine one has a device that is capable of finding out only whether the atom is spinning to the right or to the left. It is not capable of measuring anything in between. According to the standard interpretation of quantum mechanics, it is impossible to predict whether the device will find the atom spinning to the right or to the left. The only thing that we may predict are the probabilities of either one or the other. Furthermore, the act of measurement will disturb the atom in a way that will force it to spin either to the left or to the right immediately after the measurement.

Does it then follow that the world can never be decoded, and that our future is unknowable? Well, another group of physicists believes that it is in fact possible to predict whether the device will find the atom spinning to the left or to the right. Their crucial argument lies in the idea that quantum mechanics does not only apply to things as small as atoms but also to large things, like footballs, cars and indeed everything. Now, when the atom is undisturbed by large measurement devices, its state changes in a way that is completely predictable. It then stands to reason that if the entire universe is governed by quantum mechanics then so are the devices which we use to measure the way the atoms spin. And since the universe cannot be disturbed by anything external to itself, it is then entirely predictable. These two viewpoints have been argued over for many decades so that the problem has even been given its own name – the measurement problem – and the final resolution of the argument has not yet been achieved.

However, determinism is not important merely as a philosophical issue. On a daily basis, many bankers attempt to predict stock market fluctuations in order to stay ahead of the competition. Their calculations lead them to a probability that the price of the stock will go up or down by tomorrow and by how much. The information theory can then tell us what is an optimal portfolio based on these probabilities.

Equally, information theory can be used in biology. Our genetic code (DNA) is composed of a sequence of bases – adenine (A), cytosine (C), guanine (G) and thymine (T). A group of three bases forms an amino acid which means that our DNA can encode 43 = 64 amino acids.

However, only 20 of these possibilities are found in living organisms. In other words, there are more than three times as many symbols (amino acids) than we need to encode. In other words, nature has implemented a natural redundancy so that, in case an error is made in some of the bases, the DNA is still usable. Information theorists have developed a very similar error-toleration system for computers and, in fact, such systems are still a topic of active research in quantum computation.

Vedral illustrates many more other uses of information theory and provides an introduction to the subject that is both illuminating and useful. But the core of the book is undoubtedly dedicated to the Quantum Physics side, where the strange and non-intuitive ways of nature reveal themselves in all their glory. The questions of determinism and the workings of nature are most likely among the deepest questions that humans have considered. Their answers, as illusive as they may have been, may just now be coming into the realm of modern scientific understanding and this book provides a thought-provoking, and timely, introduction.

Decoding Reality by Vlatko Vedral

Oxford University Press

£16.99

Sebastian Meznaric is a theoretical physicist and doctoral reseracher at the University of Oxford. His areas of interests include the study of information theory in quantum mechanics. He is also a keen observer of politics and current affairs.