# Examples of “Random Access” in real world [closed]

In a Random Access Memory (RAM) an index (usually represented in binary) is used to access a location of memory and retrieve/store a value.

Are there natural physics processes, interactions, reaction, ... that can resemble a "random access" to something ?

In other words, a physic process, interaction, reaction, ... that uses some compact representation to (quickly) access an information or object, ... "stored" in an exponentially "larger list/storage".

As commented by TheGostOfPerdition, an example (the only that comes to my mind) is the radioactive decay

$$N = N_0 e ^{ -t / \tau}$$

in which the current decay rate can be used to "access" the "exponentially larger" age of an artifact (note that, in this case, the information is extracted and used by humans, but nevertheless it is "stored" in the current "version" of the artifact ... sorry for the improper terminology).

(This is my first question on physics.stackexchange, so let me know if I can improve it or it is completely out-of-scope)

## closed as too broad by Kyle Kanos, Norbert Schuch, Ali, John Rennie, CuriousOneJan 12 '16 at 14:40

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Hi Marzio, and welcome to Physics Stack Exchange! I think this is off topic here, but we'll see what other people say. – David Z Jan 12 '16 at 9:55
• I think Radioactive decay would come close to the definition of 'Random access' – Oswald Jan 12 '16 at 9:58
• DNA comes to mind, especially the transscription of DNA to RNA. In a very oversimplified model the transscription of a specific gene is suppressed unless it is activated by an, well, activator. This activator binds to a specific sequence (about 20 base pairs, so fairly unique) which serves as a index. The gene is then read from start to stop codon as in memory. The production of the activator takes the role of the "read memory" command in a computer. In general, there is a close resemblance of some parts of genetic code (on an abstract level) and "human" programming. – Bort Jan 12 '16 at 10:26
• @Qmechanic: I saw you changed the tag from [general-phisycs] to [randomness] ... but I think that "random access" in my question (a synonym could be "direct access") is quite different from the usual randomness in physics – Marzio De Biasi Jan 12 '16 at 10:49
• @MarzioDeBiasi: A list of length 1 is still a list. – Kyle Kanos Jan 12 '16 at 11:20

## 1 Answer

This question is quite difficult to pose. The problem with random or sequential access in computing world comes from a single-thread execution (sequential computation/access) and linear nature of memory. Real world usually doesn't have that, so it's basically all random access unless constraints are put on the "storage" to make it behave in a discrete way that resembles how things are done in computing (taking bottles from a shelf one at a time, having to flip through a book to find a page...). In low-level physics, the interactions don't take turns... they all act at the same time. It's all massively parallelized.

An example: finding a number in an unsorted linear array is O(n) in conventional computing. But when you look at a bunch of numbers on the screen, you can "spot" it without looking at every single one... because your brain is a parallel processor, and processes spatially separated values simultaneously - just like a graphical processor could (in which case this can be O(1)).

Especially in quantum mechanics everything is happening simultaneously -- even in the sense of superposition of states. Defining access complexity requires you to first state your storage model: what are you storing? What is your data and what is the retrieval mechanism? Which physical process are you referring to? What is allowed in this model?

Taking cars from the parking lot can be considered random access, if noone is double-parked, otherwise they are more like a stack... but is this relevant to your question?