This post is going to explore loneliness from a physics point of view. Looking at the grandeur of the physical world, with space that's possibly infinite in extent, and with multiverses containing other worlds you'll never reach, it's easy to feel small and especially alone.

It's quite possible that studying too much physics can leave you feeling alone, so let's take a close, rational, logical, look at the problem. Let's ask ourselves what loneliness is, and whether there truly is a rational reason to feel that way.

We've all felt loneliness at some point or another, so we have an intuitive idea of what it is. We might differ on the specifics, but we should all agree it has something to do with other people and our interactions with them. I think there's value in knowing exactly what we're talking about, so let's try to put that intuition on firmer logical ground.

Be warned: I'm leaping off the diving board of science into the abyss of philosophy and speculation.

Loneliness isn't lack of material wealth

Suppose one day you wake up, and there are no humans left on Earth except you. Fortunately, this happens long in the future, where all of our critical infrastructure has been automated robustly enough to last decades without maintenance. You don't have to worry about the power going out or the water shutting off or even running out of food. You can easily live out the rest of your life.

In this scenario, you have access to all the material wealth of the world. There's no one to stop you from stealing the most luxurious car you can find, driving it to the aiport, stealing a plane, and learning how to fly. Once you get good enough, you can walk into a military base and steal an F-22. How fun! After you're done flying, you can come home to the billion-dollar Mansion your favorite celebrity used to live in.

This would be a life of happiness and adventure, depending on who you are and what you decide to do. But what would happen to your feelings of loneliness? Would they go away? You might be a lot happier with all of your new material wealth, but I think most would intuit that your loneliness would not go away. At best, you would be too busy having fun in your world-sized sandbox to think about it.

Conversely, if someone is well loved, has lots of great friends, and feels no loneliness at all has all of their material possessions taken away, you would not expect them to feel lonely. They will most certainly feel sadness and anger, but they will not be lonely; all of their relationships are still intact.

So, we've established that loneliness is separate from material wealth. I'm sure you already knew that; this is the easy one. The next conclusion we'll reach about loneliness is more subtle.

Loneliness isn't lack of interaction with others

When we think of loneliness the first thing that comes to mind is being with other people. Being away from others makes you feel lonely, and being with others makes you feel less lonely. Surely, then, loneliness must be determined by how much you interact with other people.

Actually, I think that is wrong. The correlation we observe between interaction and loneliness is indeed there, but I believe interaction is just a signal of something deeper. First I'll give you the reason that's wrong, and then I'll tell you what I think loneliness actually is.

Suppose a benevolent dictator wants to completely rid their country of loneliness. To do so, they find people who are currently lonely and currently not. They observe the correlation we all know to exist: the lonely people have lower levels of interactions with others, and the less lonely have higher levels of interactions with others.

If loneliness is your level of interaction with others, the dictator reasons, to get rid of loneliness we have to increase everyone's level of interaction. The dictator decides on a standard minimum amount of human interaction everyone needs to get. In order to enforce this in a practical way, the dictator decides who will spend time with who, and anyone who refuses is killed.

In this country, everyone has plenty of interaction with other people. But, I would argue, loneliness will still exist. Sometimes people don't like each other. Sometimes people hate each other. Suppose that just by chance, the only people you get to spend time with end up being people that hate you. You know these people are only spending the time with you because if they don't, they'll be killed. Of course you will still feel lonely.

So simply spending time with others, and nothing else, is not a cure for loneliness. Loneliness is therefore not identical to the time you spend with others.

We'll see soon enough that time spent with others is indeed a crucial part of not feeling lonely, but it is not the ultimate cause.

What I think loneliness is

In order to explore what loneliness really is, let's look at some of the symptoms of being lonely. Here are some things a lonely person might say:

• I have no friends.
• Nobody likes me.
• I'm not fun to be around.
• Everyone thinks negatively about me.
• Nobody cares about me.
• I feel irrelevant.
• ...

We just have to flip these around to find the things a not-lonely person would have:

• Friendships.
• People like them.
• People enjoy their company.
• People think positively about them.
• People care about them.
• They don't feel irrelevant.
• ...

Each of these things has something in common. If you have the thing, then you've improved the quality of someone else's life.

If you have friends, they're still your friend because keeping you in their life makes their life better. If you're liked and people like being around you, you're obviously improving their lives even if it's only for a short while. If people care about you (above the baseline level at which the person cares about you simply because you're a living human), it's likely because you've had some positive impact on their life in the past.

So if you're not lonely, you've had a positive impact on other peoples' lives. Does that mean that if you've done so, then you won't be lonely? Not at all. Loneliness is a judgement about yourself, so if you don't know that you've had a positive impact, you won't have any reason not to feel lonely. If you've made major improvements to others' lives but haven't been recognized for it, that's no good. If you do get recognized for it, then you ought not to feel lonely.

So we can say: Not being lonely is equivalent to having made positive impacts on others' lives as well as being recognized for it. In other words, loneliness is when you either haven't made enough of a positive impact on others' lives, or you have done so but you are unaware of it.

There are certain circumstances where you know for certain you've made a huge positive difference in someone's life, but haven't been recognized for it. Would you be lonely then? I personally don't know because I've never been in that situation. If you were, then this theory needs to be adjusted.

Something else to consider is that loneliness may have a biological component. There may be a physical need for social interaction, touch, or sex. If that's the case, then we have to adjust our description of loneliness to include those physical needs being satisfied.

I may not have arrived at the correct formulation of loneliness. You might think of loneliness as something completely different than what I've worked out here. But for me, personally, this is my working definition.

Irrelevance and Insignificance

Feelings of irrelevance and insignificance are closely related to loneliness.

Thinking physically, it's easy to be trapped up in these feelings. Every physical theory says something about how alone you are in the world, and how big you are compared to everything else that exists. In the next sections, we'll consider the philosophical extremes, and then re-use the arguments to tackle loneliness and insignificance closer to home.

Solipsism

The most extreme viewpoint is that you are all that exists in the universe. There are no other people in the universe you inhabit. It is all just a hallucination.

There are some plausible ways this could happen. For example, you could be a Boltzmann brain. The universe is really just filled with gas of particles, randomly moving, and they happened completely by accident to assemble into a self-aware structure that experiences the world as you experience it. The entire thing could fit into a tiny volume of space. None of the experiences are real, they just feel real, because all the right "accidents" happened to make your lonely brain have those experiences.

If you're truly convinced that the universe is like this, then by the definition of loneliness we've worked out, it's undefined if you should feel lonely or not. There's nobody around to help, so you can't meet the criteria of improving others' lives, but at the same time there's nobody around to help, so you shouldn't feel bad about not being able to. It's like you're living under the benevolent dictator described above, except instead of people threatened with death, it's all a hallucination.

I think this kind of a universe is unlikely. The usual argument in favor of the hypothesis is that one thing is simpler than many things. A universe in which only you exist and everything is a hallucination is simpler than a universe in which you exist as well as the rest of the universe you observe.

I don't think that argument works.

In mathematics, general things seem to come before more specific things. We construct the integers before we narrow them down to prime numbers. The idea of the whole set of real numbers easily fits into our heads, but most real numbers require an infinite amount of information to describe. A universe where only you exist is a special case, and a universe where everything actually exists is more general, and is thus more likely.

A universe described by a set of laws of physics has less information content than a universe that only contains you, where you believe those laws of physics apply. The reason is simple: Either way, the description of the universe has to contain those laws of physics. In the first case, you could say the universe is those laws of physics and nothing more. But in the second case, the universe's description contains the laws of physics (since you believe them to apply they are encoded in your brain), and must contain additional information to describe you.

Another argument against solipsism, which Max Tegmark makes in his book "Our Mathematical Universe" in favor of multiverses, is that when something exists we look for a process that created it. Cars are made in factories, rabbits are made by natural selection. A property of all the processes of creation that we know of, both natural and artificial, is that they make more than one thing. Car factories make lots of cars, and natural selection makes lots of rabbits (and other species too!). Therefore, it would be extremely strange for whatever created you to have not created another one. You're probably not the only thing of your kind in your universe.

Given these arguments, I don't think you have to worry too much about being truly alone in the universe. Actually, I'm certain nobody reading this is alone, since I know I exist. Sorry, but I can never prove it to you.

The Multiverse: Huge Inaccessible Populations

At the other extreme is the multiverse, where pretty much everything possible happens somewhere at some point in time. Unlike solipsism, the multiverse is physically plausible, and there is even some evidence for it.

Theories of inflation, which describe the process by which our Big Bang formed, seem to require that our observable universe is not alone. The space we live in is predicted to extend out infinitely, similarly peppered with galaxy-forming and planet-forming matter. If you look far enough away (really far away!), you ought to eventually find an exact copy of our observable universe, or at least something that looks so similar that you couldn't tell the difference.

Quantum mechanics also predicts a multiverse, if you interpret it in the right way. If the multiverse interpretation is correct, the "wave function collapses" that you've probably heard about don't actually happen. Instead, you, the observer, see one outcome, but an exact copy of you sees the other outcome. Everything that could happen by the laws of quantum mechanics does happen, it's just a matter of probability which universe you'll find yourself a part of.

Given that multiverses of at least two different kinds are implied by mainstream theories, we have to take them seriously when considering feelings of irrelevance and insignificance.

This can be difficult to grapple with for some: You're just one of an infinite number of people, some exactly like yourself, others just slightly different from you. Most of them are so far away that you'll never influence them, and they'll never influence you.

(need to get better imagery, include causal separation, you can never meet these people no matter how hard you tried, and they will never know you exist, the vast majority of everyone who lives will never know about you no matter what you do)

fix: shift your viewpoint to relative, you are very real to the people in your local neibhborhood, and the impacts you have are just as real as your feelings of loneliness.

The Dead Universe: Huge Amounts of Unconscious Matter

The Earth: Huge Accessible Populations

TODO: adapt the multiverse argument to earth (your effective universe + economic background noise which is more like unconcious physics from your perspective.) difference: you can change the size/shape of your universe. counter: what about judging people by the size of their universe?

Implications

Let's think through some easy implications of this view of loneliness.

Jealousy is ok

Say you're completely in love with another person, but that person decides to leave you for someone else because they make them happier than you can. On top of the pain coming directly from the loss of that person in your life, you will feel jealous.

We tend to think of jealousy as a bad thing, but under this interpretation of loneliness, it's a perfectly rational response to what's happened. You've lost the ability to make a positive impact on your love's life, and the knowledge that someone else is better at making them happy is evidence that your ability to make a positive impact on other people in the future is lower.

So you shouldn't feel guilty about feeling jealous. It's kind of surprising, but under this view of loneliness, it's totally rational.

Don't get tunnel vision

A common occurrence of loneliness is when someone you respect greatly leaves your life. It's easy to think that after a breakup, or the end of a friendship, you'll be alone forever. But you shouldn't lose hope. It is true that you have lost a means by which you can impact a person's life in a positive way, but remember that one-on-one relationships are not the only way to do that.

To academics, artists, programmers, and essentially anyone of any craft whatsoever: Remember that the work that you do can affect thousands or millions of people in positive ways. Your research gives other researchers something to build on, your art inspires others, and your code makes others' lives easier. Don't discount these contributions.

You play both sides

As a final note, I'd like to point out that you're a player on both sides of the loneliness game. When you're lonely, it's easy to feel like you're the only one with those feelings. Remember that you aren't. Other people feel lonely too, and reducing their loneliness will improve their lives and, if all goes well, reduce your own loneliness in turn.