What are pseudo-random numbers? What’s wrong with them?
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> What are pseudo-random numbers?
They are numbers generated by algorithms that are completely deterministic (i.e. if you know what state the algorithm is currently in, you can work out what numbers it will produce next) but have properties similar to random numbers.
> What’s wrong with them?
Nothing, really. Some are poorly designed and are very un-random, like the infamous RANDU. The ones that are used nowadays in scientific simulations have been very heavily studied and pass batteries of tests that look for deviations from what you would expect from random numbers (for example, some numbers being more likely to show up than others). There are also cryptographically secure random number generators (CSPRNGs), for which it is very difficult to predict the next output given the previous outputs – these are used for stuff like generating random passwords. It’s always possible that there are problems with the way a CSPRNG is designed or implemented, allowing someone to guess the next output. However, true random number generators can have the same problem. For example, there are true random number generators that work by detecting radioactive decays – if the detector develops a fault, then it might start producing predictable numbers.
True random number generators are often used for security applications, though, and they’re also often used for is initialising the state of a pseudorandom number generator, so that it doesn’t produce the same numbers every time you run the program. Most computer systems essentially have a basic true random number generator built into them that generates numbers based on things like the timings of mouse movements and keypresses.
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