This Funzing Experience:. Quantum theory tells us there is a whole reality outside our day-to-day experience: the atoms that make up your body are able to exist in two or more places at once, and can even use quantum entanglement to influence each other from across the universe.

Although it is strange and entirely counter-intuitive, we are now putting quantum theory to work, and creating quantum computers, secret quantum codes protected by the laws of physics, and even a quantum internet. In this talk, Michael Brooks will explain the origins and oddities of quantum physics, and why scientists now believe there is far, far more to this world than meets the eye.

Michael Brooks , who holds a PhD in quantum physics, is a journalist and broadcaster. Delve into a totally new subject and challenge your perspectives with like-minded, equally curious people. It's time to try something different - you never know where it may take you.

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- Scientific American (August 2005)!
- Quantum physics - Latest research and news | Nature!
- International Review of Cytology: A Survey of Cell Biology: 164 (International Review of Cell and Molecular Biology);

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Includes your tickets for this talk and immediate access to all activities. Add Promotional Code. See Similar Activities. But there's always a kind of granularity to the underlying reality if you dig into the mathematics, and that's a large part of what leads to the weirdness of the theory.

## Quantum Mechanics for Beginners; A fun-filled introduction to Quantum Mechanics

One of the most surprising and historically, at least controversial aspects of quantum physics is that it's impossible to predict with certainty the outcome of a single experiment on a quantum system. When physicists predict the outcome of some experiment, the prediction always takes the form of a probability for finding each of the particular possible outcomes, and comparisons between theory and experiment always involve inferring probability distributions from many repeated experiments.

There's a lot of debate about what, exactly, this wavefunction represents, breaking down into two main camps: those who think of the wavefunction as a real physical thing the jargon term for these is "ontic" theories, leading some witty person to dub their proponents "psi-ontologists" and those who think of the wavefunction as merely an expression of our knowledge or lack thereof regarding the underlying state of a particular quantum object "epistemic" theories.

In either class of foundational model, the probability of finding an outcome is not given directly by the wavefunction, but by the square of the wavefunction loosely speaking, anyway; the wavefunction is a complex mathematical object meaning it involves imaginary numbers like the square root of negative one , and the operation to get probability is slightly more involved, but "square of the wavefunction" is enough to get the basic idea. This is known as the "Born Rule" after German physicist Max Born who first suggested this in a footnote to a paper in , and strikes some people as an ugly ad hoc addition.

## Quantum physics

There's an active effort in some parts of the quantum foundations community to find a way to derive the Born rule from a more fundamental principle; to date, none of these have been fully successful, but it generates a lot of interesting science. This is also the aspect of the theory that leads to things like particles being in multiple states at the same time.

All we can predict is probability, and prior to a measurement that determines a particular outcome, the system being measured is in an indeterminate state that mathematically maps to a superposition of all possibilities with different probabilities. Whether you consider this as the system really being in all of the states at once, or just being in one unknown state depends largely on your feelings about ontic versus epistemic models, though these are both subject to constraints from the next item on the list:.

A quantum teleportation experiment in action. The last great contribution Einstein made to physics was not widely recognized as such, mostly because he was wrong. In a paper with his younger colleagues Boris Podolsky and Nathan Rosen the "EPR paper" , Einstein provided a clear mathematical statement of something that had been bothering him for some time, an idea that we now call "entanglement.

## Quantum Physics: A Beginner's Guide

The EPR paper argued that quantum physics allowed the existence of systems where measurements made at widely separated locations could be correlated in ways that suggested the outcome of one was determined by the other. They argued that this meant the measurement outcomes must be determined in advance, by some common factor, because the alternative would require transmitting the result of one measurement to the location of the other at speeds faster than the speed of light.

Thus, quantum mechanics must be incomplete, a mere approximation to some deeper theory a "local hidden variable" theory, one where the results of a particular measurement do not depend on anything farther away from the measurement location than a signal could travel at the speed of light "local" , but are determined by some factor common to both systems in an entangled pair the "hidden variable". This was regarded as an odd footnote for about thirty years, as there seemed to be no way to test it, but in the mid's the Irish physicist John Bell worked out the consequences of the EPR paper in greater detail.

Bell showed that you can find circumstances in which quantum mechanics predicts correlations between distant measurements that are stronger than any possible theory of the type preferred by E, P, and R. This was tested experimentally in the mid's by John Clauser, and a series of experiments by Alain Aspect in the early 's is widely considered to have definitively shown that these entangled systems cannot possibly be explained by any local hidden variable theory.

The most common approach to understanding this result is to say that quantum mechanics is non-local: that the results of measurements made at a particular location can depend on the properties of distant objects in a way that can't be explained using signals moving at the speed of light. This does not, however, permit the sending of information at speeds exceeding the speed of light, though there have been any number of attempts to find a way to use quantum non-locality to do that. Refuting these has turned out to be a surprisingly productive enterprise-- check out David Kaiser's How the Hippies Saved Physics for more details.

Quantum non-locality is also central to the problem of information in evaporating black holes, and the "firewall" controversy that has generated a lot of recent activity. There are even some radical ideas involving a mathematical connection between the entangled particles described in the EPR paper and wormholes. Images of a hydrogen atom as seen through a quantum telescope. Credit: Stodolna et al.

Quantum physics has a reputation of being weird because its predictions are dramatically unlike our everyday experience at least, for humans-- the conceit of my book is that it doesn't seem so weird to dogs. This happens because the effects involved get smaller as objects get larger-- if you want to see unambiguously quantum behavior, you basically want to see particles behaving like waves, and the wavelength decreases as the momentum increases. The wavelength of a macroscopic object like a dog walking across the room is so ridiculously tiny that if you expanded everything so that a single atom in the room were the size of the entire Solar System, the dog's wavelength would be about the size of a single atom within that solar system.

This means that, for the most part, quantum phenomena are confined to the scale of atoms and fundamental particles, where the masses and velocities are small enough for the wavelengths to get big enough to observe directly. There's an active effort in a bunch of areas, though, to push the size of systems showing quantum effects up to larger sizes. I've blogged a bunch about experiments by Markus Arndt's group showing wave-like behavior in larger and larger molecules, and there are a bunch of groups in "cavity opto-mechanics" trying to use light to slow the motion of chunks of silicon down to the point where the discrete quantum nature of the motion would become clear.

There are even some suggestions that it might be possible to do this with suspended mirrors having masses of several grams, which would be amazingly cool. Comic from "Surviving the World" by Dante Shepherd. The previous point leads very naturally into this one: as weird as it may seem, quantum physics is most emphatically not magic.