Friday, July 19, 2013

Joys of Noise: Technologies that Rely on Randomness

From Nautilus:

Engineering: The reliability of some technologies depends on just the right amount of randomness.
In engineering, uncertainty is usually as welcome as sand in a salad. The development of digital technologies, from the alphabet to the DVD, has been driven in large part by the desire to eliminate random fluctuations, or noise, inherent in analog systems like speech or VHS tapes. But randomness also has a special ability to make some systems work better. Here are five cases where a little chaos is a critical part of the plan:

Stochastic Resonance
Scientists who make sensitive detectors often go to extreme lengths to eliminate noise. If they are trying to spot neutrinos, for example, they’ll build their detector at the bottom of a mine to stop the results from being swamped by regular cosmic radiation. But there are times when adding noise is the only way to pick up a weak periodic signal.

This phenomenon is called stochastic resonance, and it works something like this: Imagine you’re trying to count the number of waves at the seashore, and your detector is a wall built across the middle of a beach. The height of the wall represents the threshold of detection: Only if water washes over the top of the wall will it be registered. But our imaginary wall is high enough that the swell of the water never quite rises to the top of the wall. Adding noise is like adding some rapidly changing wind—it whips up waves in a random pattern. With the right amount and right variation of wind, when the wave comes in the water will splash over the top of the wall and be detected. If there’s too little wind, the calmer waves will never make it over the top; too much wind and the water level may stay over the wall for long stretches, drowning out the signal of the waves.
There are times when adding noise is the only way to pick up a weak periodic signal.
And stochastic resonance doesn’t just apply to scientific instruments: There’s evidence that our own nervous systems use it to detect signals between cells, and that it also plays a role in our perception of sight, touch, and hearing. For example, the balance of elderly people can be improved by fitting their shoes with insoles that produce “noisy” vibrations below the threshold of sensation. This improves the seniors’ sense of touch in their feet, which leads to better balance. Researchers believe this works because the sub-threshold stimulation primes sensory neurons to fire when a foot contacts the floor. The stimulation has to be somewhat random because otherwise the sensory neurons would adapt to, and ultimately ignore, the additional stimulation.
Codes and ciphers are a case where being predictable can literally get you killed. The goal of cryptography is to turn a message—the “plaintext”—into a meaningless jumble—the “ciphertext.” Ideally, the ciphertext should be indistinguishable from a random string of letters or numbers: If code-breakers discern any pattern in the ciphertext, they can use it to help reveal the plaintext.

For example, during World War II, Germany relied on a code machine called Enigma. An operator would push a button on its keyboard, and a letter on a panel would light up, as determined by a system of rotating wheels inside. Crucially for the Allies, the setup was such that a letter couldn’t be encrypted as itself; that is, a “b” could be encoded as any letter except “b.” This might sound like a good thing—shouldn’t all the ciphertext be completely different from the plaintext? But in fact, it was a critical weakness, reducing the number of possibilities code-breakers had to consider.

Modern cryptography encrypts messages by combining plaintext with randomly generated digital keys using various algorithms. The security of the system depends on the algorithm chosen, the length of the keys, and the keys being truly random. The algorithms and keys used today are so good that it should take longer than the current age of the universe to break a properly encrypted message. Nonetheless, some security-conscious individuals and organizations are worried that new code-breaking techniques may be found. Consequently, researchers have created and deployed some quantum encryption systems, which rely on fundamentally random subatomic processes and, in theory, can never be broken....MORE