Hey Crazies.
I think we’re all due for an existential crisis, so let me ask you something.
What separates a turtle from a rock?
The immediate answer is, of course, the turtle is alive, but why though?
Why is the turtle alive and the rock not alive?
What does it actually mean for something to be alive?
This episode was made possible by generous supporters on Patreon.
In the process of making this video, I realized that it wasn’t going to be complete without your input.
So, I had you take a poll.
Over 3600 of you answered, so we got a great sample size
and the results turned out mostly as I expected, with a few outliers.
A few of you even surprised yourself with your own answers.
In the end though, the obviously living things got lots of votes.
I’m not sure why human isn’t 100%.
Apparently, I’ve got some nihilists in the audience.
But, as the options get less complex, the percentages slowly drift downward.
The obviously living things manage to all stay above 80%.
On the other hand, the obviously non-living things got very few votes.
The only options down there getting above 10% were fire and the Sun.
Now, I expected fire for reasons we’re going to get to later,
but I did not expect the Sun to get so much attention.
That one caught me off guard, but results are results.
You can’t just go throwing them away because they don’t match your expectations.
Anyway, these match pretty close with the opinions of biologists.
As a physicist, I would love to come up with one simple rule that defines life.
But biologists have been working on this problem a lot longer than I have,
so I think we should at least start with what they have to say.
Let’s not overstep too early.
For biologists to considering something to be alive:
It should be made of one or more cells.
It should be able to grow and make copies of itself.
It should also interact with its environment by absorbing energy, maintaining itself,
responding to changes, and adapting to those changes if necessary.
Mind you, this is a list of what life does not what life is,
but that’s actually pretty standard procedure for defining science things.
For example, in physics, we define energy as the ability or capacity to do stuff.
It’s the same technique.
That definition for energy always works though.
The checklist for life only works most of the time.
Let’s run through a few examples and see what happens.
Turtles start as little hatchlings and slowly grow into adult turtles.
That hatchling came from an egg, that came from a mother, so turtles can reproduce.
They’re made of cells, respond to their environment, gain energy by eating, and even evolve over time.
Turtles meet all the criteria.
They’re definitely alive.
What about a rock?
That one works really well too.
Some rocks can grow, but they fall short on the other 6 criteria.
Rocks are definitely not alive.
But not all things are so clear cut.
Several items on this checklist are pretty vague.
What exactly qualifies as a cell?
Is any increase in size considered growth?
What’s the difference between a response and an adaptation?
We can’t effectively use the list unless we answer these questions
and that’s likely to involve a bunch of our biases.
So let’s consider some of the things in the grey area.
Take fire for example. It can grow, reproduce, and respond to its environment.
It even absorbs energy from its surroundings to keep itself going.
That’s 4 out of 7. That’s a majority of the criteria.
So I’m not at all surprised that fire got 12% in the poll,
but it’s still a pretty small result compared to these obviously alive things.
And biologists don’t consider fire to be alive.
So is this just a numbers game then? Is there some kind of threshold? Like 5 or 6?
Not really.
Consider a virus. They can adapt over time and they can reproduce ...sort of.
That’s only 2 of the 7 criteria.
That makes fire a better candidate for life.
Yet 54% of you consider viruses to be alive while only 12% of you considered fire to be alive.
That’s a major discrepancy in the checklist!
You’ll even find biologists arguing over viruses when they all agree fire is not alive.
It’s clear that our instincts don’t always match up with the checklist.
Does that mean our instincts better at this than the list?
Maybe! Let’s test them!
Like, really test them.
All the examples we’ve seen so far are chemistry-based.
Let’s consider something that isn’t, like Conway’s game of life.
John Conway himself didn’t consider his game of life to be particularly interesting,
but we’re going to talk about it anyway.
I do what I want!
Say you’ve got a grid of cells. Each cell can be active or inactive.
The grid changes as time steps forward according to a certain set of rules:
Any inactive cell will become active if it has exactly three active neighbors
and any active cell will only stay active if it has two or three active neighbors.
All other cells are inactive in the next time step.
With such simple rules, you might not expect much to happen,
but that couldn’t be further from the truth.
We end up seeing incredibly complex activity.
Some combinations of cells are even stable.
Many just sit there.
Others will oscillate.
Some even move around.
It’s quite beautiful actually.
It’s what Conway liked to call a zero-player game.
Although, it might not actually be zero-player if these digital things are alive.
It certainly feels to me like they’re alive.
According to the poll, most of you would disagree with me, but hear me out.
These things are made of cells.
They can grow.
They certainly consume the computer’s energy.
They maintain themselves.
They respond and adapt to their digital environment.
And if you find the right configuration, they can even reproduce!
They meet all 7 criteria from the checklist.
All seven!
[off camera] Hold up a second!
Oh no. Here we go.
You’re twisting the rules to suit your narrative.
What do you mean?
That’s not what they mean.
Biologists mean a biological cell with DNA.
Homeostasis implies the system is chemical.
Adaptation means by Darwinian evolution.
Well, that’s a little short-sighted, isn’t it?
Maybe. But the list is designed for them, not for you.
Ok ok.
Fine!
Conway’s game of life is not biological life, so it’s not what the checklist is meant for.
This list is intended to help biologists recognize life wherever they may look on Earth.
It’s the stuff that biological life on Earth has in common.
But there’s no reason to think that life on other planets would also share these qualities.
At least not all of them.
I mean, most worker bees can’t reproduce but they’re still alive right?
Most of you would agree with me on this based on the poll.
So reproduction isn’t actually a necessary criterion for life at least,
at least not on the individual level.
I wonder what other criteria from this list we can throw away.
Let’s see if we can whittle this down to what’s absolutely necessary.
We’ve already ruled out reproduction, but growth doesn’t seem necessary either.
Also, I can’t imagine being made of cells is universally necessary.
Yes, whatever life is, it’s probably some kind of mostly closed system,
but I think that’s covered by these two.
You can’t maintain yourself if your system isn’t mostly closed
and that maintenance requires consuming energy.
These last two are debatable.
Most environments change,
so being able to respond and adapt to those changes certainly seems necessary for long-term success.
That’s why the official NASA definition for life is:
A self-sustaining chemical system capable of Darwinian evolution.
As far as they’re concerned, life doesn’t have be actively evolving.
It just has to be capable of it.
Otherwise, why would a self-sustaining chemical system exist in the first place?
I know my wife agrees with that argument.
Well, yeah. That’s right.
But I’m not sure I’m satisfied with that argument though.
I can imagine an environment that stays exactly the same for billions of years.
It might be unlikely, but it’s possible.
In such an environment, any life that happened to emerge wouldn’t need to be capable of evolution.
Yes, if conditions suddenly changed after billions of years, they’d all die,
but that doesn’t mean they weren’t alive before.
Will life on other planets have the ability to evolve?
Probably.
Does it have to be capable of evolution to be alive?
No.
We can also imagine life that isn’t based on chemistry.
I mean, just look at Star Trek:
The Calamarain colony is a swirl of ionized gas floating around in space.
Their life is based on electricity.
These crystal cells are optical.
Their life comes from beautifully complex patterns of light.
They call us "ugly bags of mostly water."
[off camera] The Star Trek universe is fictional!
I know. It should be real though.
It’s so optimistic about the future.
Make it so, Number One!
Anyway, the Star Trek universe doesn’t have to be real for the point I’m making.
If we can imagine life like this and the laws of physics don’t prevent their existence,
then we have to at least be prepared for the possibility.
We shouldn’t restrict our definition of life to chemistry just because that’s what life on Earth looks like.
Life emerges from complexity.
There’s more than one way to be complex and self-sustaining.
All we really need for that are these two items from the checklist.
Life should consume energy from its surroundings to maintain its own existence
through some kind of complex yet ordered process.
Because of that last bit, Erwin Schrodinger was convinced that somehow
life was a constant fight against the increase in entropy.
Entropy?
Entropy.
I’ve covered it before and will likely again in the future, but here’s the TL;DW.
Every normal-sized system like this glass of water is made of submicroscopic parts called molecules.
But there a lots of different configurations in those molecules that result in the same glass of water.
The entropy of that glass of water depends on one thing:
how many possible configurations there are.
If there are many, the system has a high entropy.
If there are only a few, the system has a low entropy.
It’s that simple.
Sometimes the word "order" is used to describe entropy,
but that’s not meant as the opposite of messy.
It’s meant as the opposite of random.
The second law of thermodynamics tells us,
for any perfectly closed system, entropy tends to increase over time.
The system should become more random.
The entire universe is a perfectly closed system and it obeys this law.
It started in a state of extremely low entropy
and is on a seemingly endless march toward maximum entropy,
toward pure randomness, a perfectly uniform universe.
And, as part of the universe, life cannot break this law, but it does resist.
Consider a single bacterium. It’s a little package of chemicals.
The processes happening inside are complex yet ordered.
There are only a few configurations that result in a functioning bacterium.
The same goes for any other living thing, even us.
We’re all packages of low entropy, of low randomness.
That low entropy must be maintained or we die and decay.
Yet this does not break the second law of thermodynamics.
A bacterium is not a closed system.
Energy goes in. Waste goes out.
Overall, there is an increase in entropy, just not within the bacterium.
It resists the universal march toward randomness.
So what does it mean to be alive?
Well, that depends on who you ask.
Drawing a line between alive things and not alive things is a tough challenge.
It might be impossible.
According to biologists, organic Earth life must have these qualities, at least on the species level.
But that doesn’t leave room for non-organic life or for any life we might find on other planets.
Physicists like me go for a definition that’s a little more universal.
When it comes down to it, maybe life is just matter that’s taken control of its own destiny.
Maybe a living thing is just something that actively prevents entropy from increasing within itself.
A package of self-maintained order, fighting against the endless march toward chaos.
So, what do you think life is?
Let us know in the comments.
Thanks for liking and sharing this video.
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Don’t forget to subscribe if you’d like to keep up with us.
And until next time, remember, it’s ok to be a little crazy.
They say mules can’t reproduce either,
but there’s actually a 1-in-2-billion chance they’ll give birth to horse.
Like, a perfectly healthy horse.
To everyone asking for a deeper explanation of how confinement causes mass,
PBS Space Time already did a great video about it. I can’t do any better.
Link in the doobly-doo.
And thanks for watching!