We’re taught that exponents are repeated multiplication. This is a good introduction, but it breaks down on 3^1.5 and the brain-twisting 0^0. How do you repeat zero zero times and get 1?

You can’t, not while exponents are repeated multiplication. Today our mental model is due for an upgrade.

## Viewing arithmetic as transformations

Let’s step back — how do we learn arithmetic? We’re taught that numbers are counts of something (fingers), addition is combining counts (3 + 4 = 7) and multiplication is repeated addition (2 times 3 = 2 + 2 + 2 = 6).

Repeated addition works when multiplying by nice round numbers like 2 and 10, but not when using numbers like -1 and sqrt(2). Why?

Our model was incomplete. Numbers aren’t just a count; a better viewpoint is a *position on a line*. This position can be negative (-1), between other numbers (sqrt(2)), or in another dimension (i).

Arithmetic became a way to transform a number: Addition was sliding (+3 means slide 3 units to the right), and multiplication was scaling (times 3 means scale it up 3x).

So what are exponents?

## Enter the Expand-o-tron(TM)

Let me introduce the Expand-o-tron 3000.

Yes, this device *looks* like a shoddy microwave — but instead of heating food, it grows numbers. Put a number in and a new one comes out. Here’s how:

- Start with 1.0
- Set the
*growth*to the desired change after one second (2x, 3x, 10.3x) - Set the
*time*to the number of seconds - Push the button

And shazam! The bell rings and we pull out our shiny new number. Suppose we want to change 1.0 into 9:

- Put 1.0 in the expand-o-tron
- Set the change for “3x” growth, and the time for 2 seconds
- Push the button

The number starts transforming as soon as we begin: We see 1.0, 1.1, 1.2… and just as finish the first second, we’re at 3.0. But it keeps going: 3.1, 3.5, 4.0, 6.0, 7.5. As just as we finish the 2nd second we’re at 9.0. Behold our shiny new number!

Mathematically, the expand-o-tron (exponent function) does this:

or

For example, 3^2 = 9/1. The base is the amount to grow each unit (3x), and the exponent is the amount of time (2). A formula like 2^n means “Use the expand-o-tron at 2x growth for n seconds”.

**We always start with 1.0 in the expand-o-tron to see how it changes a single unit.** If we want to see what would happen if we started with 3.0 in the expand-o-tron, we just scale up the final result. For example:

- “Start with 1 and double 3 times” means 1 * 2^3 = 1 * 2 * 2 * 2 = 8
- “Start with 3 and double 3 times” means 3 * 2^3 = 3 * 2 * 2 * 2 = 24

Whenever you see an plain exponent by itself (like 2^3), we’re starting with 1.0.

## Understanding the Exponential Scaling Factor

When multiplying, we can just state the final scaling factor. Want it 8 times larger? Multiply by 8. Done.

Exponents are a bit… finicky:

You:I’d like to grow this number.

Expand-o-tron:Ok, stick it in.

You:How big will it get?

Expand-o-tron:Gee, I dunno. Let’s find out…

You:Find out? I was hoping you’d kn-

Expand-o-tron:Shh!!! It’s growing! It’s growing!

You:…

Expand-o-tron:It’s done! My masterpiece is alive!

You:Can I go now?

The expand-o-tron is indirect. Just looking at it, you’re not sure what it’ll do: What does 3^10 mean to you? How does it make you feel? Instead of a nice tidy scaling factor, exponents want us to feel, relive, even smell the growing process. Whatever you end with is your scaling factor.

It sounds roundabout and annoying. You know why? **Most things in nature don’t know where they’ll end up!**

Do you think bacteria *plans* on doubling every 14 hours? No — it just eats the moldy bread you forgot about in the fridge as fast as it can, and as it gets more it starts growing even faster. To predict the behavior, we use how fast they’re growing (current rate) and how long they’ll be changing (time) to figure out their final value.

The answer has to be worked out — exponents are a way of saying “Begin with these conditions, start changing, and see where you end up”. The expand-o-tron (or our calculator) does the work by crunching the numbers to get the final scaling factor. But someone has to do it.

## Understanding Fractional Powers

Let’s see if the expand-o-tron can help us understand exponents. First up: what does at 2^1.5 mean?

It’s confusing when we think of repeated multiplication. But the expand-o-tron makes it simple: 1.5 is just the amount of time in the machine.

- 2^1 means 1 second in the machine (2x growth)
- 2^2 means 2 seconds in the machine (4x growth)

2^1.5 means 1.5 seconds in the machine, so somewhere between 2x and 4x growth (more later). The idea of “repeated counting” had us stuck using whole numbers, but fractional seconds are completely fine.

## Multiplying exponents

What if we want to two growth cycles back-to-back? Let’s say we use the machine for 2 seconds, and then use it for 3 seconds at the exact same power:

Think about your regular microwave — isn’t this the same as one continuous cycle of 5 seconds? It sure is. As long as the power setting (base) stayed the same, we can just add the time:

Again, the expand-o-tron gives us a *scaling factor* to change our number. To get the total effect from two consecutive uses, we just multiply the scaling factors together.

## Square roots

Let’s keep going. Let’s say we’re at power level a and grow for 3 seconds:

Not too bad. Now what would growing for half that time look like? It’d be 1.5 seconds:

Now what would happen if we did that twice?

Looking at this equation, we see “partial growth” is the square root of full growth! If we divide the *time* in half we get the *square root* scaling factor. And if we divide the time in thirds?

And we get the cube root! For me, this is an *intuitive* reason why dividing the exponents gives roots: we split the time into equal amounts, so each “partial growth” period must have the same effect. If three identical effects are multiplied together, it means they’re each a cube root.

## Negative exponents

Now we’re on a roll — what does a negative exponent mean? Negative seconds means going back in time! If going forward grows by a scaling factor, going backwards should shrink by it.

The sentence means “1 second ago, we were at half our current amount (1/2^1)”. In fact, this is a neat part of any exponential graph, like 2^x:

Pick a point like 3.5 seconds (2^3.5 = 11.3). One second in the future we’ll be at double our current amount (2^4.5 = 22.5). One second ago we were at half our amount (2^2.5 = 5.65).

This works for any number! Wherever 1 million is, we were at 500,000 one second before it. Try it below:

## Taking the zeroth power

Now let’s try the tricky stuff: what does 3^0 mean? Well, we set the machine for 3x growth, and use it for… *zero seconds*. Zero seconds means we don’t even use the machine!

Our new and old values are the same (new = old), so the scaling factor is 1. Using 0 as the time (power) means there’s no change at all. The scaling factor is always 1.

## Taking zero as a base

How do we interpret 0^x? Well, our growth amount is “0x” — after a second, the expand-o-tron obliterates the number and turns it to zero. But if we’ve obliterated the number after 1 second, it really means any amount of time will destroy the number:

0^(1/n) = nth root of 0^1 = nth root of 0 = 0

No matter the tiny power we raise it to, it will be *some* root of 0.

## Zero to the zeroth power

At last, the dreaded 0^0. What does it mean?

The expand-o-tron to the rescue: 0^0 means a 0x growth for 0 seconds!

Although we *planned* on obliterating the number, we never used the machine. No usage means new = old, and the scaling factor is 1. 0^0 = 1 * 0^0 = 1 * 1 = 1 — it doesn’t change our original number. Mystery solved!

(For the math geeks: Defining 0^0 as 1 makes many theorems work smoothly. In reality, 0^0 depends on the scenario (continuous or discrete) and is under debate. The microwave analogy isn’t about rigor — it helps me see why it *could* be 1, in a way that “repeated counting” does not.)

## Advanced: Repeated Exponents (a to the b to the c)

Repeated exponents are tricky. What does

mean? It’s “repeated multiplication, repeated” — another way of saying “do that exponent thing once, and do it again”. Let’s dissect it:

- First, I want to grow by doubling each second: do that for 3 seconds (2^3)
- Then, whatever my number is (8x), I want to grow by
*that new amount*for 4 seconds (8^4)

The first exponent (^3) just knows to take “2″ and grow it by itself 3 times. The next exponent (^4) just knows to take the previous amount (8) and grow it by itself 4 times. Each time unit in “Phase II” is the same as repeating all of Phase I:

This is where the repeated counting interpretation helps get our bearings. But then we bring out the expand-o-tron: we grow for 3 seconds in Phase I, and redo that for 4 more seconds. It works for fractional powers — for example,

means “Grow for 3.1 seconds, and use that new growth rate for 4.2 seconds”. We can smush together the time (3.1 × 4.2) like this:

It’s different, so try some examples:

- (2^1)^x means “Grow at 2 for 1 second, and ‘do that growth’ for x more seconds”.
- 7 = (7^0.5)^2 means “We can jump to 7 all at once. Or, we can plan on growing to 7 but only use half the time (sqrt(7)). But we can do that process for 2 seconds, which gives us the full amount (sqrt(7) squared = 7).”

We’re like kids learning that 3 times 7 = 7 times 3. (Or that a% of b = b% of a — it’s true!).

## Advanced: Rewriting Exponents For The Grower

The expand-o-tron is a bit strange: numbers start growing the instant they’re inside, but we specify the desired growth at the *end* of each second.

We say we want 2x growth at the *end* of the first second. But how do we know what rate to start off with? How fast should we be growing at 0.5 seconds? It can’t be the full amount, or else we’ll overshoot our goal as our interest compounds.

Here’s the key: **Growth curves written like 2^x are from the observer’s viewpoint, not the grower.**

The value “2″ is measured at the *end* of the interval and we work backwards to create the exponent. This is convenient for us, but not the growing quantity — bacteria, radioactive elements and money don’t care about lining up with our ending intervals!

No, these critters know their *current, instantaneous growth rate*, and don’t try to line it up with our boundaries. It’s just like understanding radians vs. degrees — radians are “natural” because they are measured from the mover’s viewpoint.

To get into the grower’s viewpoint, we use the magical number e. There’s much more to say, but we can convert any “observer-focused” formula like 2^x into a “grower-focused” one:

In this case, ln(2) = .693 = 69.3% is the instantaneous growth rate needed to look like 2^x to an observer. When you enter “2x growth at the end of each period”, the expand-o-tron knows to grow the number at a rate of 69.3%.

We’ll save these details for another day — just remember the difference between the grower’s instantaneous growth rate (which the bacteria controls) and the observer’s chart that’s measured at the end of each interval. Underneath it all, every exponential curve is a scaled version of e^x:

Every exponent is a variation of e, just like every number is a scaled version of 1.

## Why use this analogy?

Does the expand-o-tron exist? Do numbers really gather up in a line? Nope — they’re ways of looking at the world.

The expand-o-tron removes the mental hiccups when seeing 2^1.5 or even 0^0: it’s just 0x growth for 0 seconds, which doesn’t change the number. Everything from slide rules to Euler’s formula begins to click once we recognize the core theme of growth — even beasts like i^i can be tamed.

Friends don’t let friends think of exponents as repeated multiplication. Happy math.

## Leave a Reply

146 Comments on "Understanding Exponents (Why does 0^0 = 1?)"

what about irrational exponents?

Great question. At an intuitive level, I see an irrational exponent as a different amount of time, in between known rational numbers, and ultimately approximated by a rational number.

When we write “3 * sqrt(2) = 4.24” we “know” that we’re really taking an approximation, and that 3 * sqrt(2) goes on forever.

Similarly, “3^sqrt(2) = 4.72” is just an approximation and the real decimal result goes on forever.

There is a small problem with your 2^3^4 = 2^12 example: exponentiation is (usually?) understood to be right-associative (ie top-down), therefore: 2^3^4 = 2^(3^4) = 2^81

I’m pretty sure your “repeated exponentiation” section is not quite right – when there are repeated exponents like that, in the standard notation, we work from right-to-left. Thus, 2^3^4 means 2^(3^4) which is neither 2^(3*4) nor 2^(4^3)…

2^3^4 is in fact 2417851639229258349412352 while 2^(3*4) is 4096 and even 2^(4^3) is only 18446744073709551616.

@Steven, sabik: Thanks for the comments! My mistake, I didn’t realize it was right-associative. I’ll add in the parentheses to make this more clear.

Amazing work once more Kalid. You make learning fun again.

@Paul: Thank you — I find math enjoyable when I’m working with analogies that make sense to me.

Forgive me if I ask a dumb question. It’s been awhile since I’ve been in any sort of math class.

If I put 3 in the expand-o-tron for 0 seconds, I get 3 because there was no time expanding. If I put it in for 1 second, shouldn’t I get 9 since it spent one second expanding? But I though 3^2 was 9. Maybe I’m reading it wrong.

@Todd: Great question — I should make the operation more clear. Normally, we start off with 1.0 and see how it grows, so with a setting of 3x power for 2.0 seconds, we’d get 1.0 * 3^2.0 = 9.0.

If we started with 3.0 and ran it for 1 second, we’d get: 3.0 * 3.0^1 = 9.0.

Most of the time we want to see what happens to a “unit” amount.

It’s great to see another article. As with many of your articles, it helped me realize that I have not understood something I’ve been using for so long.

About evaluating i^i: it appears that I have the problem half-way done.

So, I put 1 in with i times growth for i seconds. Meaning, each second I rotate 1 counter-clockwise 90 degrees in the complex plane. However, how do I look at i seconds intuitively and thus complete the computation?

Nobody: Unfortunately the “^i” operation is a little confusing. You should start by understanding e^i and e^(a*i) where a is a real number. Then remember that x = e^(log x), and i = e^(log i)

so i^i = (e^(log i))^i = e^(i*log(i))

Analogies only go so far!

@Nobody: Great question! (Tomer, thanks for the details).

It’s hard to think about “i” seconds, just like it’s hard to think about -1 seconds — it’s easier to think about what the transformation does. Negative numbers flip the direction, and i brings items into the complex plane. When you take i as an exponent, it changes the direction in which you are growing (instead of growing in the real dimension, you start growing in the imaginary dimension). There’s a lot more to say, but that’s the intuitive approach I take with it :).

I really appreciated this article, and just want to suggest that you make it more clear at the top that you ALWAYS start with one.

I really struggled with your explanation of 0^0 until I got down into the comments and saw that that was the case. Thanks.

@Andrew: Thanks for the comment! I’ll make that more clear, really appreciate the feedback.

When you take i as an exponent, it changes the direction in which you are growing (instead of growing in the real dimension, you start growing in the imaginary dimension).Umm, what? When you have i as an exponent, you go in circles, getting neither larger nor smaller.

Is there a Nobel prize for explaining math :D

@Sabik: It’s tricky — consider i^i. It “seems” like it should stay on the unit circle (that’s where it started, magnitude of 1), but it doesn’t. I should be more clear: i as an exponent changes your _instantaneous_ rate of change by 90 degrees.

@NF: Heh, I think the rumor is Alfred Nobel hated mathematicians so there’s no prize for pure math :).

Thanks for the great visualization of how exponents work. I’m a huge fan of your articles.

The step I can’t intuitively grasp is in the multiplying exponents section: “What if we want to two growth cycles back-to-back? Let’s say we use the machine for 2 seconds, and then use it for 3 seconds at the exact same power”.

To me, if I put something in the microwave for 2 seconds, then again for 3 seconds, intuitively this would be like adding together two doses of microwaving:

x^2 + x^3 instead of x^2 * x^3 (i.e. running the microwave for 2 min, and then again for 3 min is 2 + 3 = 5 min, not 2 * 3)

I’m not sure how to picture what x^2 * x^3 means in microwave terms…maybe something like increasing the wattage by 3 times.

Anyway, keep the great articles coming!

yet another “a-ha” moment when reading this…=)

This certainly explains exponents better then I’ve ever encountered explanations before.

(In essence it doesn’t differ much from the repeated multiplication explanation but introducing “time” in the Expand-o-tron 3000 simplifies imagining non-integer powers and their effect on outcomes.)

@CL: Great question! I had the same thoughts myself, I may need to go back and make that section more clear.

Exponents act like multiplication, but the amount you multiply by has 2 inputs: growth rate and time (vs regular multiplication, which is just growth rate).

You can combine regular multiplication: Multiplying by 3, and then multiplying by 15 is the same as multiplying by 15 (3×5) all at once.

With exponents, using x^2 and then x^3 is the same as multiplying by x^(2+3) = x^5 all at once.

The tricky thing is to remember that the time is being combined, but it’s happening in the exponent you raise it to.

Hope this helps!

@mike: Glad it happened :)

@Theo: Thanks! That was the goal — to help expand our insight so things like fractional (or even negative) exponents can make sense. Negative counting is confounding.

[…] Kalid at Better Explained likes to find intuitive ways to explain difficult mathematical concepts like, Why does 00 = 1 ? […]

Thank you Kalid. Another reminder that math concepts are easy to teach and very hard to explain. Your analogies and patient decomposition of the problems are wonderful to read.

Excellent article! You are good. You are on my RSS feed.

@Stuart: You’re welcome — I agree, I think the difficulties in understanding math is more to due with complex explanations, not complex ideas. Thanks for the kind words :).

@CJ: Glad you liked it!