Posted by jlwile on June 5, 2012When you read these words, you are receiving information. Some would call the information “too sciency, too nerdy,” but it is information nonetheless. But what, exactly, is information? Is it a real, physical quantity, or is it some esoteric construct of the mind? Rolf William Landauer spent a lot of time thinking about this question. That’s not surprising, because he was a physicist who worked for IBM, a company that deals with lots of information. In 1961, he wrote a paper for the IBM Journal of Research and Development in which he argued that information is a real, physical quantity that is governed by the Second Law of Thermodynamics. As a result, in order to erase information (such as when a file is deleted from a hard drive), a certain amount of energy must be released.1
It is important to understand what Landauer meant. He didn’t mean that it takes energy to erase information. For example, if you want to erase the writing on a whiteboard, you have to expend energy wiping the markings off the board. That makes perfect sense, but it’s not what Landauer was referring to. He said that in order for information to be erased, energy must be released into the environment. The very act information being destroyed, regardless of the method, requires a physical response: a minimum amount of energy must be released. This is because information is a real, physical quantity and is therefore governed by the Second Law of Thermodynamics.
Now the Second Law of Thermodynamics is misunderstood and misused frequently (you can read more about that here and here), so let’s start with what the Second Law actually says. It says that the entropy of the universe must always increase or at least stay the same. It can never decrease. What is entropy? It is a measure of the energy in a system that is not available to do work. However, a more useful definition is that it is a measure of the disorder in a system. The larger a system’s entropy, the “messier” it is. Using this concept of entropy, then, we can say that the disorder of the universe is always increasing or at least staying the same: the universe never gets more ordered.
Now let’s apply this concept to a computer disk. A computer disk has a bunch of bits, and each bit can have a value of either 0 or 1. On a blank disk, all the bits have the same value. Let’s say it’s 0. However, as you start putting information on the disk, the bits change. Some stay at their original value (0), but others change (they become equal to 1). So as more information gets put on the disk, there are more possibilities for the values of the bits. If you were to erase the disk again, you would set all those bits back to 0. When you do that, the disk gets more ordered. While there was information on the disk, it was possible for many of the bits to have a value of 1. When you erase the disk, that’s not possible anymore – all the bits have to have a value of 0. From the point of view of the disk, then, when you erase the information, the disk gets more ordered.
If the disk gets more ordered when information is erased and nothing else happens, the universe would become a bit more ordered, but the Second Law of Thermodynamics forbids this. Thus, in order to follow the Second Law, the very act of erasing information must release energy. Furthermore, that energy must be large enough to disorder the universe as much as or more than the disk became ordered. That way, the decrease in entropy of the system (the disk) will be offset by the increase in entropy of the disk’s surroundings so that the total entropy of the universe remains the same or increases. Landauer even used the Second Law of Thermodynamics to predict the minimum amount of energy that must be released for each bit of information that is erased.
This idea remained theoretical for more than 40 years, but it was recently tested by experiment, and it seems that Landauer was correct.
In the experiment, Antoine Bérut and his colleagues set up a simple information-containing system. A microscopic bead of silica was held in place by a laser beam that produced two “traps” in which the bead could be found. When the settings of the laser were changed, the bead would switch from one trap to the other. So the two traps represent the two values of a bit: 0 or 1. When the bead is in one trap, the bit has one value. When it is in the other trap, it has the other value. Changing the bead from one trap to another is the same as changing the value of the bit.
The researchers carefully measured the position and speed of the bead as it was moved from one trap to the next. This allowed them to measure the heat released by the bead switching traps. If the bead was moved quickly, it released a larger amount of heat. The more slowly the bead was moved, the less heat it released. This is because the speed of the switch affects the friction in the system. The slower the switching, the lower the friction involved in moving the bead from one trap to another. If it were possible to make the switch infinitely slowly, there would be no friction associated with the bead moving between traps.
If information were not a real quantity, then in the absence of friction, there should be no energy released by the bead’s movement. However, if Laudauer was right, even in the absence of friction, the bead would still release energy. In the end, the experiment showed that Landauer was right. As the switching time was reduced, the energy released got smaller. However, it was not a linear relationship. At first, the energy released decreased a lot as the switching time increased. However, as the switching time was increased further, the energy released started decreasing less and less. The data indicate that even if the switching time were infinitely long, some energy would still be released, and that energy is precisely what Landauer predicted it would be.2 So in the end, it seems that Landauer was correct. Erasing information requires the release of a certain amount of energy.
Now, of course, this research has practical applications in the field of computing. As the number of bits of information we can store increases, the energy released when those bits are erased increases as well. The heat generated by computing is already a problem, and the heat released by erasing bits of information can make that problem even bigger. Thus, engineers will probably have to take this effect into account the larger our data storage capabilities become.
However, this has applications to the creation/intelligent design/evolution issue as well. After all, life is all about information. In order for life to exist in the first place, information has to exist. In order for any physical process to change one life form into a different life form, information must be altered. If information is a real, physical quantity that is subject to the natural laws, then any theories that attempt to explain the origin and diversity of life must take this into account.
1. Rolf Landauer, “Irreversibility and heat generation in the computing process,” IBM Journal of Research and Development, 5:183-191, 1961.
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2. Antoine Bérut, et al.,”Experimental verification of Landauer’s principle linking information and thermodynamics,” Nature, 483:187–189, 2012.
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