Babies Are Stronger Than Adults

Recently, I've been blessed with becoming a father. I didn't know much about babies before, so I didn't expect to see much in their earliest phases. Yes, they sleep and drool and cry, but there's not much else to see there, right?

But after spending time with one such specimen of my own making, all I can say is - boy, was I wrong.

I'd like to explain it to you, but I don't think words can do the job. So let's use numbers instead, to paint at least one aspect of it - how freaking strong babies are.

Adults

Let's first see how strong an average human adult is. In the EU, the average means 1.80m tall, 80kg heavy, 44.5-year old, probably mostly sedentary with no or minimal exercise. Daily, that person expends around 2000 calories - more or less, but it's a common round figure to use, so let's stick with it.

Now, when we say calories we actually mean kilocalories (kcal). It's important to remember that kilocalories are just a measure of energy, and as such we can convert them to other units to build an intuition on how much it actually is. The official SI unit for energy is Joule, and we can convert kilocalories to Joules by multiplying by 4184. The math tells us that our 2000 kcal of energy is equivalent to 836,800 Joules of energy.

But Joules, while "official", never really caught on in everyday life, so it's not helping us build intuition on how much that is. Luckily, another unit did - watts.

Now really, a watt is a unit of power, and power is not energy, but rather how fast you can spend that energy - it's energy over time. But OK, we can get the energy then by multiplying watts by time, which in common usage gets us a unit of watt-hour, which is a unit of energy. For now let's just remember that calories and watt-hours basically express the same thing - energy.

To build an intuition around this, take this example. Your baking oven might be rated at 2000 watts - that's power. That oven is gonna expend 2000 watt-hours per hour - that's energy, and that's what your electric company will bill you for - watt-hours, not watts.

The point of this apparent digression is that kilocalories and watt-hours are the same thing and we can convert them willy-nilly. The factor here is 1.16, so 2000 kcal ends up being 2320 Wh. You can see here that you spend as much energy daily as your oven does in one hour. This was the kind of intuition we needed.

Conversely, because watts tell us how much energy you expend over some time, to calculate the power we need to divide said energy by time. Daily you're expending 2330 Wh, so if we divide it by 24 hours we get that hourly that is around 97 Wh/h. You can notice the funny form of having watt-hours per hours here - the hours get cancelled and you're left with 97 watts - which is, finally, your power. Let's round it up to 100 watts for easier calculation.

You can stumble on that fun fact online saying that a human body is like a 100 W space heater. This is how they came to that number. It hinges on a fact that all energy you spend eventually ends up as heat - either from your metabolism which literally burns sugar to power you, or other sources like internal friction between the muscles and joints - but eventually all of your 100 watts of power end up as heat.

But I digress once again. We're here to talk about small babies.

Babies

Now let's see what the numbers look like for babies. I found this formula that says babies expend 100 kcal per day per kilogram. This means that a two-month-old baby weighing 5 kilograms expends 500 kcal per day. Converted, that's 581 watt-hours per day. Divided by 24 hours, it coincidentally comes to 24 watts, which is how strong a baby is.

While 24 watts might be peanuts to your 100, remember that you're comparing yourselves with a 5-kilogram baby here. That baby certainly couldn't beat anyone in hand-to-hand combat, you say, not even you, a hypothetical sedentary person with no or minimal exercise. And that would be true - it would be an unfair comparison. But we could use a different metric that's often used to compare power across different weights - the power-to-weight ratio.

The power-to-weight ratio is used in some automotive or aeronautical contexts as a more fair metric on how strong something really is. It's power normalized across weight and expressed as watts per kilogram of weight - so how strong something is relative to how much it weighs. Let's calculate it for you first.

It's simple - your 100 watts of power divided by 80 kilograms of weight - your power-to-weight ratio is 1.2 then.

Now for this two-month-old baby: 24 watts divided by 5 kilograms is a power-to-weight ratio of 4.8.

So a little newborn baby has a four times bigger power-to-weight ratio than you do. If this doesn't sound like a big difference, please think again, or I'll be forced to show you an example of how much that really is. Still not getting it? Ok, you really can't tell people anything. An example it will be then.

If you peruse a Wikipedia article on power-to-weight ratio, you can find many examples, so let's pick something common and everyday you might be seeing around. Here's is a Mini Cooper 1.6D, a 109-horsepower car you might see on a road.

Sorry for using horsepower here - like calories, they're also just another unit to express power and can be converted to watts instead, so let's do it - it comes to 81 kilowatts of power. So 81,000 watts, some 810 times more powerful than you.

If we calculate this car's power-to-weight ratio, we get 68 W/kg. We can see that even this small car packs 50x more punch than you do. But let's not compare ourselves with machines, but with babies instead. We said before a baby has a 4 times bigger power-to-weight ratio than you do, so let's see - if you were an ordinary road-legal Mini Cooper with a power-to-weight ratio of 68 W/kg, what would a six-month baby be in comparison?

That drooling, babbling six-month-old baby would be this 520-horsepower Porsche 911 GT2 to your Mini Cooper. Packing 271 W/kg, this is what a four times higher power-to-weight ratio means. This is how strong that baby really is when compared with you.

And if we wanted to be proper capitalists and put a monetary value on everything, we could, just to drive the point home. You can get a Cooper for around $5000, while this Porsche will set you back around $500,000. Having a four times bigger power-to-weight ratio will cost you one hundred times more. This is how much more it actually is.

The How and The Why

Now, why don't we keep this kind of strength in adult years? Energy expenditure, probably. If you were to keep a baby's 4.8 W/kg power-to-weight ratio, on your 80 kilograms of weight this would mean having a power of 400 watts. Going through the math from the beginning of the post in reverse, it would mean you'd have to eat 8000 kcal of food per day to survive. That may have been impossible for the life humankind has lived for the majority of its existence, so we settled on a more sustainable form factor of having a 100-watt bodies.

Having that power would also mean that instead of being an equivalent of a 100-watt space heater, you'd then be a 400-watt one. You'd be a heat equivalent of four people glued together. Imagine surviving the summer like that.

This thing expands more generally in the form of Kleiber's law - as an animal increases in size, its power does not increase equally fast, but a bit slower than that. An animal that is ten times bigger is not ten times stronger. It's maybe six times stronger at best.

I believe this is just biology-speak for a more fundamental square-cube law. As you grow, your body volume increases more than your skin surface does. We're mostly cooling ourselves through our skin, either through evaporation or radiation, so as you grow in size, your skin surface can't grow as fast to cool you enough. So you have to lose relative power to be able to not die from heat exhaustion.

So why are babies so strong? Simply because they can be. And for the rest of us, well - the physical constraints of living in a three-dimensional universe just don't allow us to remain that way.