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Spinning into the Future

April 15, 2013 MAN & MACHINE No Comments

By Prof. Paul Padley

Department of Physics and Astronomy

Rice University

Electronics works by taking advantage of one of the properties of fundamental particles: electric charge. Particles have many other properties as well, and there is a real possibility that those properties can be harnessed in order to develop new technologies. One such property is called “spin” and harnessing spin could play a key role in the future of electronics.

Nobody really understands the spin of fundamental particles, such as the electron. However, we can routinely measure it and use it. The electron, and other basic particles in nature, act as if they were spinning tops. We can do measurements in which we calculate their angular momentum — or how much they are spinning. We can put electrons in magnets and flip their spins.  Spin

So, why doesn’t anybody really understand that? There are a couple of reasons: To the best of our knowledge, the electron is an infinitely small-point particle. In our current theories, the electron has zero size and, to date, nobody has been able to measure its “size,” experimentally. How is it that something without any size can be spinning?  String-theory attempts to overcome this by postulating that particles are little bits of string in a multi-dimensional space – but, to date, there is no experimental evidence that string theory is correct. In any case, I am not sure that a 10-dimensional string is any easier to think about than an infinitely small, spinning particle.

It gets event stranger. First, I have to explain how to describe the direction of spin. If something is rotating, I can wrap the fingers of my right hand in the direction of the rotation.  If I then stick my thumb out from my hand, I say the direction of my thumb defines the spin.  So, if I am riding my bicycle forward, and I describe the rotation of my wheels in this way, my thumb points to the left.

What is strange about the spin of the electron is that when I describe it this way, my thumb will only point up or down.  It can’t point at an angle; it can’t be tilted.

Otto Stern

Otto Stern

Walter Gerlach

Walter Gerlach

[ … Why can’t the electron spin point at an angle? That is one of the mysteries of the universe. This weird spin of the electron was first measured in the 1920s (by Otto Stern and Walter Gerlach -- http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html ) and has been repeatedly confirmed by experiment, ever since.  It makes my head hurt, and my students heads, too -- this exact question was being asked of me by my students last week (I teach quantum mechanics to junior physics majors).

One of the most important things that makes science different from other ways of knowing is that we have to use what we learn from experiment, whether we understand it or not. So we can write down equations that describe how electron spin will behave.  We can use those equations to predict the electron's behavior so well that we can make electronic (or spintronic) devices, using this description. But we don’t actually know how it comes about or why it is there. So I know the electron will always be measured to be spinning up or down, and not tipped at an angle, but I can’t tell you why. Wish I could (it would get me a Nobel Prize)…. ]

I always measure that the electron is spinning either up or down, no matter how I measure it. In fact, spin is predicted by relativistic quantum mechanics (the combination of quantum mechanics with Einstein’s special theory of relativity). So, perhaps I misspeak when I say nobody understands it  – we can write down the math behind it but, unfortunately, our brains can not picture what it means.

That spin has this property, that it can only take definite directions, is what makes it interesting for electronics. We can use spin to record information and manipulate it to do calculations. There is a whole field of electronics research called “spintronics” that is pursuing this idea. The most likely first application is in memory chips — a technology referred to as “mram,” which is approaching commercialization.  For example, IBM and Toshiba have announced that they are close to producing such chips.

There is an important lesson for the future, here. The concept of spin grew out of work in the 1920s in quantum mechanics. Without the basic science that was conducted almost 100 years ago, the new technologies being developed today would not be possible. The physicists who discovered this amazing property of fundamental particles were not  trying to develop technologies, they were just trying to understand the smallest constituents of matter. Without speculative, basic scientific research, technological progress stops. However, it can be a long time until that basic research bears fruit.

Dr. Paul Padley is professor of physics at Rice University, and a lead physicist of experimental research for the Large Hadron Collider at CERN

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