# Scientists Say: Imaginary Number

## These weird numbers can multiply themselves into reality

**Imaginary number** (noun, “Ee-MAH-juh-neh-ree NUM-ber”)

An imaginary number is a kind of number that allows us to solve math problems that involve taking the square root of a negative number. They’re called “imaginary” because they don’t count or measure things, the way real numbers do.

All numbers you use to count or measure things in a typical day are known as real numbers. Simple numbers such as 1, 2 and 3 are real numbers. So are fractions and negative numbers. You can place real numbers on a number line.

But imaginary numbers are different. You can’t point out where an imaginary number would lie on a standard number line. Despite that, imaginary numbers aren’t make-believe. You can multiply, divide, add and subtract imaginary numbers just like real ones.

Imaginary numbers come into play when you need to take the square root of a negative number. Taking the square root of positive numbers is straightforward. A number’s square root is the number that, when multiplied by itself, gives the original number. For example, the square root of four is two. That’s because 2 × 2 = 4.

But taking the square root of a *negative* number is a little different. Consider -4, for instance. No number exists that you can multiply by itself to get -4. You might think that -2 multiplied by -2 should give you -4. But following the rules of mathematics, if you multiply two negative numbers, your result is a positive number. You cannot take the square root of a negative number.

To solve this problem, mathematicians hundreds of years ago defined a new type of number — imaginary numbers, represented by the symbol *i*. The symbol *i* represents the square root of negative 1, or -1. Mathematically, that looks like this:

*i* = *√*-1

Here’s the important part: if you multiply *i *by itself, you get -1.

*i* × *i* = -1

And -1 is a real number, not an imaginary one. So, equations using imaginary numbers can sometimes give non-imaginary answers.

We can use* i* to make an imaginary number answer for the square root of -4:

*2i* = *√*-4

That 2*i* is an imaginary number. It’s a placeholder, of sorts. But multiply it by itself and you get -4. That’s a real number.

Imaginary numbers make much of today’s technology possible. That’s because they play an important part in modeling the behavior of waves — such as light waves and sound waves. These kinds of waves transmit the information we receive through devices such as cell phones and other wireless technologies.

#### In a sentence

Some math problems in quantum physics can only be solved by using imaginary numbers.

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