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Question: How many thoughts or ideas can a person's brain hold?
This is a tough question. I don't think anyone has a definite answer. But just saying so wouldn't be enough for "Ask a Scientist," so let's try some estimates using what is known about our brain.
First, we need to think about how to count thoughts or ideas. How "big" is an idea anyway? How many thoughts were included in a journal you wrote in school yesterday, in a book you read last night, or in a TV news report you watched this morning? Thinking this way, you realize that the definition of "thoughts" or "ideas" is very fuzzy, so it is difficult to identify them as a crisp, countable entity. We need to use something different, more concrete, to measure the amount of thoughts or ideas.
So let's use the notion of "information" instead. Information is also a fuzzy word used in various ways. But in science and engineering, there are rigorously defined units for measuring its amount. Have you ever heard "bit", "byte", "kilobyte" or "megabyte"? They are all units of information. 1 bit is the amount of information that allows you to make a decision between two options. For example, the sex of a person (female or male) can be described using 1-bit information. Similarly, if some information allows you to choose one of four options, the amount of that information is 2 bits, because you can single out your final choice by halving the set of options twice.
People also use other units to represent larger amount of information: 1 byte is usually defined as 8 bits, 1 kilobyte is about 1,000 bytes, 1 megabyte is about 1 million bytes, 1 gigabyte is about 1 billion bytes, etc. An average-length English sentence has about 100 bits (~13 bytes) of information. A high-quality digital picture contains about 1 to 2 megabytes of information. A standard DVD can hold as much as 4.7 gigabytes of information.
Now let's go back to the original question, restated as: "How much information can a person's brain hold?"
The brain is a huge network of interconnected nerve cells, called "neurons", communicating with each other using chemical and electrical signals. A human brain is made of about 100 billion neurons. Each neuron is connected to 1,000 to 10,000 other neurons. Therefore, the structure of the network, by itself, already holds a lot of information. But how much? To calculate this, we need to know about combinatorics, a mathematical theory about counting many possibilities. I don't dare to talk much about it here as it is rather complicated. Anyway, the result is, if we assume each of 100 billion neurons connects to 1,000 other neurons on average, a specific structure of a network has 1.4 quadrillion (=1,400 trillion) bits of information.
In addition, a brain can learn and forget things by changing the connections of neurons, called "synapses". Each synapse has a level of connection strength, which changes dynamically. If we represent a synapse's connection strength on a scale of 1 to 100 (this roughly corresponds to 7-bit information), the amount of information stored in synapses is about 350 trillion bits in total.
Moreover, each neuron has its own state, or a level of excitation, that also changes dynamically. This change is caused by signals coming from other neurons and takes place much faster than the changes of connection strength. Such a fast, dynamic state change of neurons on the network causes reasoning and consciousness in a human brain. Again, if we represent a neuron's state on a 100-point scale, the amount of information stored in neurons is about 0.7 trillion bits, which is very small compared to the numbers we got earlier, but still quite huge.
Let's get the bottom line. The total amount of information a person's brain can hold is estimated to be about 1.4 quadrillion + 350 trillion + 0.7 trillion = 1.75 quadrillion bits. Or 218,000 gigabytes. Or 46,500 DVDs. Can you imagine how big it is?
Note that we made a couple of assumptions to simplify the calculation when developing this estimate. The true amount of information stored in a brain may be way larger. Of course, not all of that information storage in your brain is used just for your study at school; it is mostly used for more critical things that keep us alive, such as controlling respiration, digestion and other physiological mechanisms. Nonetheless, your brain can learn and process A LOT of things in such a small volume. So much so, that no other machines can compete with it.
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