# What is reversible computing?

## Explained in 200 seconds

In the world of computers, everything is based out of logic gates at the fundamental level. In short, logic gates are the building blocks of any processor. Think of them as tiny blocks that take in the binary code (1’s and 0’s) as input and give out some output, once again in binary.

There are different types of logic gates- AND, OR, NOT, NAND, NOR, etc.

Let us take an example of a 2 input AND gate. It basically implements the logic of getting a HIGH ( binary 1) output when both the inputs are HIGH. Else, the output is LOW.

All the combinations of this logic would be as follows (this is known as a truth table)-

Observe that when inputs are both ‘1’, the output is 1.

A question for you.

Imagine, the output = 0. Can you tell me what the inputs are?

Looking at the truth table, you can conclude that it can be any of the first three combinations. You can also be sure that one of the inputs is definitely a zero. What about the other bit? It’s lost. You have no way to determine it from the output you have.

I can restate this by saying that the information (aka entropy) is lost in this logical operation. It implies that heat is dissipated to perform this, and there is no way to reconstruct the inputs from the output we have.

So, all the modern-day computers are based on logic gates like that. This is what is known as “irreversible computing”.

Now consider a NOT gate. It implements a simple logic of giving you the opposite of input, i.e if the input is one, the output will be zero (and vice-versa). That means you can be sure that the input is definitely a ‘one’ if your output was ‘zero’. Right?

This is an example of reversible logic where you can deduce the inputs from the output. NOT gate is a reversible logic gate.

And if you are doing computation using such reversible gates, it is reversible computing.

It is theoretically possible to build a computer completely composed of such gates, and such a system will not dissipate any heat as per thermodynamics (at least the heat generated due to irreversible computing). Many scientists, in fact, believe this to be the future of computation.

I hope you got a fundamental understanding of this topic. Of course, we have just scratched the tip of the iceberg. To explore more, you can refer to the links below.

Explained in 200 seconds is a new content series I am trying out where I give short & concise explanations of various technologies out there in a way that is easy to understand and remember (I hope). I will mainly be focusing on analogies, personal anecdotes, and high-level understanding from the context of the business aspect, what the tech is, and why we should care about it.

This series will also be the medium via which I will document my own exploration of different technologies and possibilities. I also hope to provide rich and creative insights (or starting points to explore) to entrepreneurs, tech evangelists, and other stakeholders via the same.

Feel free to let me know any feedback for this series or if your find any errata in the content.

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