Have you ever thought about what happens to all the information you delete from your computer’s hard drive? You might be surprised to discover that deleted files aren’t actually erased. Even when you delete a file from your computer’s trash folder, your computer holds onto the file while erasing the file name and its listing in any directories. The file itself remains on your hard drive, functionally irretrievable (without special software) until it is overwritten when the hard drive needs that data space. At that point, your data is actually irretrievable. That portion of the hard drive’s disk space has been completely reconfigured to serve the new file’s needs. But what does this mean in terms of the file’s information? We obviously can’t access that information, but does that mean that the information is lost from the universe completely?
If you’re wondering what this line of questioning has to do with black holes, then let’s ask a slightly different question: what happens to all the information that falls into a black hole? It might be tempting to assume that all this information is simply held inside the black hole. Like our deleted file, the information isn’t deleted from the universe, it’s simply functionally irretrievable. Unfortunately, the discovery of Hawking radiation—the slow evaporation of black holes through virtual particle radiation—completely blew this assumption out of the water and introduced the information paradox. One of the tenets of quantum physics is that quantum information cannot truly be destroyed, only transformed. But if a black hole consumes all the quantum information in a piece of matter and spits it back out as indecipherable radiation, then isn’t that information destroyed? The information paradox has been a central problem for theoretical physicists for the past 50 years, and it still has no satisfactory solution.
We’ve already established that falling into a black hole means nearly certain death for you, and all your molecules and atoms, but what about your quantum information? Just like a file, you are made up of information—a set of quantum 1’s and 0’s that encode the past, present, and future states of every atom in your body. If we had perfect knowledge of the universe, we could track each of the particles in your body from the beginning of time to now. Information can’t be lost, because we can always track a particle’s present quantum state back to its past quantum state. Of course, we don’t have perfect knowledge of the universe so we can’t access most of this information. Like our deleted hard drive files, the information exists, but it is unindexed and only accessible with special tools.
Speaking of our computer hard drive, we never answered my original question: is the information from an overwritten hard drive lost from the universe completely? Well . . . no—at least not on the quantum level. The hard drive still possesses all the same particles, they’ve just been rearranged to fit a new file configuration. Again, if we had perfect knowledge, then we could trace the path of those hard drive particles back to their original configuration. The only way to delete a file, in the quantum sense, is to remove those particles from the universe completely.
At any point in time, all the particles in your hard drive are in a particular quantum state—a mathematical property that really just means the set of possible configurations the particle can take. As time moves forward, particles move from one quantum state configuration to another, but the total number of possible quantum state configurations doesn’t change. Think of this as your hard drive capacity. No matter how many files you create, edit, overwrite, or delete, the total storage space of your hard drive stays the same. There’s no way to alter it. Overwriting a file doesn’t delete that space in your hard drive, it just changes it.
But what about the physical particles in your hard drive—is there any way to delete those? Perhaps you’re really dedicated to protecting your quantum privacy (you never know when an omniscient entity is gonna search the quantum states of your hard drive—best to be on the safe side). You might try smashing your hard drive to pieces, but no dice. All the particles and quantum states in your hard drive are still intact, they are just a bit more . . . scattered.
The only way to delete a quantum state is to make two particles fill one state at the same time. In this hypothetical situation, the two particles move from their two separate quantum states into a single quantum state. Information is deleted because knowledge of either particle’s current state doesn’t give us any information about their past states. Imagine that your computer allowed two files to have the exact same file name (and indexing information). This wouldn’t work because any time you reference that file, and your computer tries to call it up, neither you nor your computer will know which file is being referenced.
When our two particles occupy the same quantum state, the total number of quantum states in the universe decreases. Imagine that every time you name two files the same way, a bit of your hard drive’s total storage space is decreased. In reality, the fact that there are two files explains why there is a decrease in the storage space (the space isn’t deleted; it’s just taken by the second file). But since your computer sees both the files as the same file, it seems like that storage space just spontaneously disappears.
According to the laws of quantum mechanics though, it is actually impossible to move two particles into the same quantum state; it is impossible to delete or create quantum states; and it is impossible to destroy information. There is no way to permanently wipe your hard drive on the quantum level . . . or maybe there is. The discovery of Hawking radiation seems to indicate that information that falls into a black hole is destroyed. The past configuration of every particle that falls into the black hole seems to have no impact on the configuration of the outgoing radiation. This is equivalent to if your computer hard drive took all of the 1’s and 0’s stored in it’s memory and shuffled them at random. If you accessed the data stored on your hard drive after this shuffle, all the files would be permanently deleted and overwritten by an unintelligle data stream that can’t be traced back to its original source.
Hawking radiation seems to contradict the conservation of quantum information, hence the paradox. But as paradoxical as the universe can seem, real physical paradoxes cannot exist. Paradoxes arise from holes in our knowledge of the universe. Resolving the information paradox and determining the fate of quantum information inside a black hole has been a contentious issue in physics since the discovery of Hawking radiation. Next week, I’ll discuss some of the competing theories that have arisen in the past 50 years to resolve these issues, and we’ll determine if throwing your hard drive into a black hole is the only way to protect your quantum privacy.
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