How can we talk about phenomena we don’t physically experience?

Imagine that you and I are powerful beings on a world far from Earth.


Now let’s say we’re scientists on our world and we’ve just discovered Earth and we want to conduct some research on its dominant life form, the homo sapien. The only problem is that even in our highly advanced world, funding for basic research is being slashed and so we can only conduct one of three experiments:

  1. We can shrink down to microscopic size and take a tour of a human brain. We’d learn about what causes neurons to fire, how they communicate with each other in a network, and how the brain as a whole reacts to stimuli and develops over time.
  2. We can set up cameras and microphones in the home of a family of four humans, and observe how they act in isolation and as a group.
  3. We can take out a subscription to the New York Times’ International Edition.

Which option should we choose, if our goal is to understand human behavior? (I’m now realizing that only option 1 requires us to be “powerful beings.” Also pretend that all three options cost the same. Those papers don’t deliver themselves!) 

The problem, as any human will tell you, is that these three realms of human life are only tangentially related to one another, and to focus on just one of them out of context would be extremely confusing. 

Let’s consider the top-down experiment 3. We’d get a lot of interesting numbers and histories at this level. Sure, international affairs touch many people’s lives, but you’d have no idea why anybody was behaving the way they were. Big human systems – law, economics, public health – are of little interest until they affect the experience of individuals – in the courtroom, the workplace, or the clinic. (And yes, I know the Times incorporates those things into their reporting, but not to the degree that a dedicated personal narrative would). 

We face a similar problem with experiment 1; learning about the brain in isolation, however beautiful and complex an object it may be, would tell you next to nothing about what a human life really feels like. Maybe, maybe with deep study and insight, you could start to map out the actions of neurons to high precision, and you could theorize about the experiences of the consciousness those neurons gave rise to…

…but wouldn’t it be easier to just ask somebody what they were thinking? So now you’ll say we should pick experiment 2 and observe the one family in their home to get a proper introduction to the human species. And I’d agree with you that this is the best place to start. We’d get to observe a lot of the key components of a human life. Still though, there’s an awful lot missing from that picture: What goes on inside these people’s heads? What happens when they leave the house? How many other people are there? Are their households similar to this one?


When human beings study physics, when we try to describe the world around us, we run into this exact problem: each way of looking at things gives some correct results, but they don’t paint an informative overall picture. Worse, we spend our entire existence inside experiment 2, in the realm of objects whose sizes, speeds, and lifespans are on the same scale as the sizes, speeds, and lifespans of our bodies. But we know that’s not the whole story! It’s just that there are so many things to observe, and it’s very difficult to gather evidence from realms 1 and 3, much less synthesize it with what we know about our own realm.

I’m getting a little ahead of myself. The point of this post is to introduce a concept I refer to* as the three realms. This will guide our conversations going forward, since a lot of the important ideas we need to discuss fall outside the “household” experiences our brains are used to. The three realms are a way of dividing the universe according to relative size scale, and they’re useful because the physics in each of the realms is as different as the three portraits of humanity you’d get in our example above. You can think of them as “versions” of the universe coexisting at different levels of zoom within the same physical space. So there are no distances or hard borders separating these worlds; you can’t leave one and enter another. But you can only have experiences in and make statements about one realm at a time. Let’s get acquainted with them!

We’ll start with the version of the universe we inhabit, which I’m going to call the human or household realm. You can also think of it as the middle. Here, gravitational and electromagnetic forces are the order of the day; by and large, physics answers questions about things we can easily observe and conceptualize. How does smoke spread out and fill a room? What makes certain bodies buoyant? How much weight can a shelf hold? However familiar that sounds, be warned that the household realm is actually pretty inclusive and contains plenty of its own strangeness. I would define it as generally covering the following domain:

  1. Objects in the size range between a single biological cell and the moon
  2. Speeds between zero and a million miles per hour (roughly 50 times faster than a rocket needs to go to escape Earth’s orbit)
  3. Time scales between one second and a billion years

You may complain that some of the above hardly seems like household phenomena. True, microscopes and rocket ships are ways of getting “out of this world,” as it were. So there is a key point to underline here:

We should remember just how narrow the range of our experience is. For example, most humans spend our lives in the same range of roughly 100º Fahrenheit. Let’s say on all of Earth that range is more like 250º from extreme to extreme. But the range of temperatures in the entire universe, from the void of space to the bellies of the hottest stars? Hundreds of thousands of degrees. The same can be said of our lifespans, falling in the range of 0 to 100 years. Compare that to many subatomic particles that pop into being for less than a billionth of a second, or to small, efficient stars that can toil away for over 10 billion years. Now, I’m not trying to make you feel bad for not going out and experiencing all the universe has to offer (quite the opposite, in fact). All I mean to say is that in the grand scheme of things, you have more in common with bacteria or blue whales than you do with atomic nuclei and galactic clusters. Therefore, we’ll lump the former into our domain of reality and sum up by saying the household realm is comprised of things that fit neatly into the perspective of everyday human thought and measurement.

(One related note that I’m sure will come up later, but that I’d like to get on the record now: Physicists are notoriously flippant about factors of 2 or 10 or 100 winding up in their estimates, and that is a good thing. Bear in mind that each of the three realms under consideration are many orders of magnitude apart – a human cell might contain trillions of atoms – and that truly fundamental questions in science are usually resolved by getting the order of magnitude right, as opposed to achieving high decimal precision.)

Now, what happens when we study objects so huge and/or fast that they challenge the scope of our minds? We’ve entered the cosmic or relativistic realm, which I will also refer to as the top. This is the “international section” from our analogy above. Essentially, we approach the cosmic realm whenever we’re dealing with a body that’s massive enough to undergo weird gravitational effects, or anything moving fast enough to even be comparable to the speed of light (if that phrase sounds familiar, the relativistic realm is where Albert Einstein made his most famous contributions, the special and general theories of relativity). The convenient thing about this world is that it is very much in the popular consciousness: we’re talking about outer space! But remember, it’s not the location itself that alters cosmic physics, it’s the scale. It just so happens that such sizes and energy levels are rarely (but not never) found within the surly bonds of Earth (which kind of makes sense, since otherwise stuff would be blowing up all the time). Let’s assign the cosmic realm the following domain:

  1. Objects larger than the moon
  2. Speeds greater than a million miles per hour
  3. A variety of time scales, but often those in the billions of years

I’m excited to talk more about cosmic physics in future posts, but just as a teaser, I’ll just say that this is the realm in which space and time expand, contract, bend, and break. It is the realm in which stars live and die, in which matter and energy trade places with colossal brilliance; this is the big leagues, and our brains are going to need some pretty poignant metaphors to keep up with the data.

Finally, we arrive at the least intuitive part of any physics conversation, a world so small and strange that its title has gone from rigorous definition to sci-fi buzzword. I’m speaking, of course, about the quantum realm, which I will also call the bottom. The quantum realm is an ecosystem whose population is so bizarre that the atoms that make up your body, odd as they may be, are among the most conservative residents. Gravitation holds little sway down here, supplanted by electromagnetism and slippery nuclear forces. Let’s define the quantum realm’s domain:

  1. Objects smaller than a biological cell
  2. A variety of speeds – sometimes near light-speed and other times merely speeds that feel fast to a subatomic particle but that we mammals would sneer at
  3. A variety of time scales, but often those under a second

Why is it called “quantum?” Well, at the bottom, the physics of the everyday begin to brush up against the absolute limits of various quantities. What do I mean by that?

Imagine you and your friend are throwing a ball back and forth. The ball leaves your hand with some kinetic energy, flies through the air, and finally collides with your friend (who absorbs the energy and ultimately transmits it to the ground). Assuming you had extremely fine control over the muscles in your arms, you could in principle give the ball any amount of energy you wanted, and you would see how its velocity through the air changed as a result.

But what if you and your friend were passing a single electron back and forth? It turns out that there is a smallest unit of energy that the electron can have. Let’s call it “h”**, and you can only increase or decrease the electron’s energy by increments of h. So you throw it to your friend with energy 3h. She throws it back to you with energy 4h, you throw it back with energy 2h, and so on. There is no way to throw it with an energy of 3.5h. Here, h is the quantum of energy – the discrete quantity into which it can be divided. You don’t notice the multiples of h when you throw the ball because of the unbelievable number of particles that comprise it (after you reach thousands of trillions, what’s one or two more?). How does this quantization affect the behavior of the electron? Quantum physics holds the answer. It’s the physical equivalent of watching neurons fire inside a brain, and it is truly remarkable stuff.


Phew. We covered a lot of ground here, but I think this is a core concept, and since this blog is going to touch on such broad topics, we need to start from a common understanding. The takeaway is that the laws of physics we commonly observe lose influence as we approach extremes in size, energy, and time. Furthermore, we need to train ourselves to look for scientific explanations at all three levels when necessary. Effects at the bottom propagate up to the top, and often, our daily experiences in the household realm are the manifestations of behavior from above and below.

Thanks for sticking with me through the introductory material. Next week, we’ll dive into the first of the four ideas: time. The usefulness of thinking in terms of three realms should become more clear as we begin that discussion, since we’ll begin with how time manifests in the human realm and then examine its causes and effects in the quantum and cosmic realms.

*As far as I know, this isn’t standard terminology in physics, so when I say “I refer to,” that’s all I mean.

**Strictly speaking, h is actually the lowest energy increment of a photon, or packet of radiation. But electrons also have quantized energy and they’re much more familiar objects to most readers. Don’t worry; we’ll split the hairs on this one in due course.

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