Welcome to week 19 of Science Friday! Before I begin this week, I have a major announcement to make regarding the future of my series. After this week, Science Friday will no longer be posted on Bungie.net. I made this decision for two principal reasons: one, the formatting options I have available to me on Bungie.net are limited in comparison to the options I have on my new venue. Secondly, when I started Science Friday, I hoped to reach a wide audience to spread my content; the change in venue will help this process by making my content available to more viewers around the web. My new venue is Google Blogger. I have also changed the name of Science Friday to The Science Drop. The principal reason for the name change after months of Science Friday is that National Public Radio already has a weekly podcast called Science Friday (an excellent podcast at that). I know that these may be unfavorable changes to those who have been faithfully following my content on Bungie.net, but they have not been made to [i]change[/i] my audience, simply expand it. On The Science Drop, I will be starting fresh, revisiting many of the same concepts I covered in Science Friday, but massively revised with the benefit of experience. For instance, my first Science Friday post was on DNA. That post was rather short, only elaborating on the history of the nucleic acid. In The Science Drop, I cover DNA again, but the content is completely new with a larger emphasis on form and function rather than history. I encourage everyone who has been following Science Friday on Bungie.net to continue following it.[url=http://thesciencedrop.blogspot.com]You can view The Science Drop here[/url] To those still disappointed with the changes, just know this: all of this will lead to better content in the future. I would not make such drastic changes if that were not the case. Of course, I typed up this week’s Science Friday before making these decisions, so I will post it as the last Science Friday. Thank you very much to those who have read my content and participated in discussions, and I earnestly hope you will join me on the Science Drop to explore the universe. [quote][/quote] Over the past few months that I have been doing Science Friday, I can safely say that I have never garnered more responses than I did last week with my seemingly “controversial” post about 0.999... = 1. It seems that discussion in particular evokes strong emotions on Bungie.net. I do wish that all of the topics I cover attracted similar discussion, but alas, that is out of my control. This week, I am going to talk about the Heisenberg Uncertainty Principle. Some of you may be familiar with this famous law of physics if you are even passingly familiar with the arena of quantum mechanics. Regardless, I hope can confer some new information to even those who have heard of it. Granted, if you are a physics major or graduate student, there is likely nothing new for you here, but my posts have always been geared to those who are interested in science but who are not necessarily scientists. At its most basic level, the uncertainty principle states that it is impossible to know the exact position and velocity (more precisely, momentum) of a particle simultaneously. While this law of physics is most evident on subatomic scales (the reason for which will become clear when I lay out the mathematical formulation of the principle), do not be mistaken; this applies to every single particle in the universe. To get an intuitive feel for why this must be true, let’s consider the following thought experiment. Imagine an electron. Say we want to determine the exact position of this subatomic particle. To do so, we must, necessarily, shine a line on it. But what is light? Light is nothing more than photons, discrete packets of energy that carry the electromagnetic force. When a photon strikes the electron we want to locate, it will be absorbed and confer energy to the electron, thereby changing its momentum. (As a side-note, momentum is the product of an object’s mass and velocity.) The more accurately we wish to ascertain the electron’s position, the higher the frequency of light we will need to use. As a result, as we obtain a more accurate location for the electron, we lose information about its momentum (and therefore its velocity). With this said, it would be easy to reach the conclusion that the inability to ascertain the precise position and momentum of the electron is a result of our experimental limitations. This is not the case. [i]It is a law of physics that we cannot know the precise position and velocity of an electron—or any other object—even in principle.[/i] No advent of technology can overcome this basic property of nature. The uncertainty principle in its mathematical formulation is as follows: ΔxΔp ≥ h/4π So let’s understand this inequality a bit better. Δx is the deviation in the position of the particle and Δp, similarly, is the deviation in the momentum of the particle (p in physics denotes momentum). h is Planck’s constant, a universal constant ubiquitously employed in the realm of quantum mechanics. The value of Planck’s constant (to just a few significant figures) is 6.63 x 10-34 joule per hertz. For this discussion, the units of Planck’s constant are not particularly important. Just notice how small the number h is. The absurdly small nature of this constant is the reason why the laws of quantum physics are evident mostly on subatomic scales despite the fact that they are valid for all scales. We can write the same uncertainty principle, but rather than express it in terms of position and momentum, we express it in terms of energy and time. ΔEΔt ≥ h/4π The energy of an object is related to its momentum, as is position to time, which is why we can reformulate the principle as above. Why did I bother to write this form of the uncertainty principle? Because of one specific ramification: quantum tunneling. Consider an electron that has an energy value of 10 (units are unimportant in this particular discussion). Let’s say there is an energy barrier of 20 that separates this electron from another room. Classical physics asserts that the electron will [i]never[/i] be able to enter the other room since it has insufficient energy. According to the uncertainty principle, however, the electron [i]can[/i] enter the other room. How? The uncertainty principle stated in terms of energy and time implies that a particle can “borrow” energy for a given period of time. Since we have the expression ΔEΔt, the more energy that is “borrowed,” the less time there is for the “energy-debt” to be “repaid.” Therefore, the electron can “borrow” 10 (or any other arbitrary amount, keeping in mind the time to repay the energy debt is inversely related to the amount of energy borrowed) units of energy for a small amount of time and thus be able to cross the energy barrier. This phenomenon is known as quantum tunneling and is the physical equivalent of an ice cube appearing on the other side of a glass without any outside intervention. In fact, this would be a fairly common occurrence if Planck’s number was much larger. That about covers all the science content I have for the uncertainty principle. Before I end this week’s post, however, I’d like to briefly talk about the philosophical consequences the uncertainty principle had on our view of scientific determinism. From the seventeenth to late nineteenth century, scientists held a largely deterministic view of the universe. In other words, they believed it was possible to determine the future state of the universe at any given time if provided the initial conditions at a prior time. Scientific icons such as Newton and Maxwell reinforced this notion through their own discoveries. With the emergent field of quantum physics—largely embodied by the uncertainty principle—in the early twentieth century, this view of physics was completely shattered. No longer was it possible, even in principle, to ascertain the precise state of the universe in the future; we could only calculate the probability of any one state versus an infinite number of states. This uncertainty that quantum physics held at its very core disturbed many prominent physicists of the time, chief of which was Albert Einstein who famously stated “...[God] does not throw dice.” Of course, Einstein was wrong. God does indeed play dice with the universe, as evidenced by quantum physics, the science that is the basis of the information age technology—cell phones, computers, television—we rely on every day. [quote][/quote] There we have it: the uncertainty principle. Once again, please join me at The Science Drop for more content in the future.