Wednesday, January 25, 2017

What is Physics? [Video]

I spent the holidays watching some tutorials for video animations and learned quite a few new things. The below video collects some exercise projects I did to answer a question Gloria asked the other day: “Was ist Phykik?” (What is phycics?). Embarrassingly, I doubt she learned more from my answer than how to correctly pronounce physics. It’s hard to explain stuff to 6-year olds if you’re used to dealing with brainy adults.

Thanks to all the tutorials, however, I think this video is dramatically better than the previous one. There are still a number of issues I'm unhappy about, notably the timing, which I find hard to get right. Also, the lip-synching is poor. Not to mention that I still can’t draw and hence my cartoon child looks like a giant zombie hamster.

Listening to it again, the voiceover seems too fast to me and would have benefited from a few breaks. In summary, there’s room for improvement.

Complete transcript:

The other day, my daughter asked me “What is physics?”

She’s six. My husband and I, we’re both physicists. You’d think I had an answer. But best I managed was: Physics is what explains the very, very small and very, very large things.

There must be a better explanation, I said to myself.

The more I thought about it though, the more complicated it got. Physics isn’t only about small and large things. And nobody uses a definition to decide what belongs into the department of physics. Instead, it’s mostly history and culture that marks disciplinary boundaries. The best summary that came to my mind is “Physics is what physicists do.”

But then what do physicists do? Now that’s a question I can help you with.

First, let us see what is very small and very large.

An adult human has a size of about a meter. Add some zeros to size and we have small planets like Earth with a diameter of some tenthousand kilometers, and larger planets, like Saturn. Add some more zeros, and we get to solar systems, which are more conveniently measured with the time it takes light to travel through them, a few light-hours.

On even larger scales, we have galaxies, with typical sizes of a hundred-thousand light years, and galaxy clusters, and finally the whole visible universe, with an estimated size of 100 billion light years. Beyond that, there might be an even larger collection of universes which are constantly newly created by bubbling out of vacuum. It’s called the ‘multiverse’ but nobody knows if it’s real.

Physics, or more specifically cosmology, is the only discipline that currently studies what happens at such large scales. This remains so for galaxy clusters and galaxies and interstellar space, which fall into the area of astrophysics. There is an emerging field, called astrobiology, where scientists look for life elsewhere in the universe, but so far they don’t have much to study.

Once we get to the size of planets, however, much of what we observe is explained by research outside of physics. There is geology and atmospheric science and climate science. Then there are the social sciences and all the life sciences, biology and medicine and zoology and all that.

When we get to scales smaller than humans, at about a micrometer we have bacteria and cells. At a few nanometers, we have large molecular structures like our DNA, and then proteins and large molecules. Somewhere here, we cross over into the field of chemistry. If we get to even smaller scales, to the size of atoms of about an Angstrom, physics starts taking over again. First there is atomic physics, then there is nuclear physics, and then there is particle physics, which deals with quarks and electrons and photons and all that. Beyond that... nobody knows. But to the extent that it’s science at all, it’s safely in the hands of physicists.

If you go down 16 more orders of magnitude, you get to what is called the Planck length, at 10^-35 meters. That’s where quantum fluctuations of space-time become important and it might turn out elementary particles are made of strings or other strange things. But that too, is presently speculation.

One would need an enormously high energy to probe such short distances, much higher than what our particle accelerators can reach. Such energies, however, were reached at the big bang, when our universe started to expand. And so, if we look out to very, very large distances, we actually look back in time to high energies and very short distances. Particle physics and cosmology are therefore close together and not far apart.

Not everything in physics, however, is classified by distance scales. Rocks fall, water freezes, planes fly, and that’s physics too. There are two reasons for that.

First, gravity and electrodynamics are forces that span over all distance scales.

And second, the tools of physics can be used also for stuff composed of many small things that behave similarly, like solids fluids and gases. But really, it could be anything from a superconductor, to a gas of strings, to a fluid of galaxies. The behavior of such large numbers of similar objects is studied in fields like condensed matter physics, plasma physics, thermodynamics, and statistical mechanics.

That’s why there’s more physics in every-day life than what the breakdown by distance suggests. And that’s also why the behavior of stuff at large and small distances has many things in common. Indeed, methods of physics can, and have been used, also to describe the growth of cities, bird flocking, or traffic flow. All of that is physics, too.

I still don’t have a good answer for what physics is. But next time I am asked, I have a video to show.


Michael Fisher said...

I liked it & I'm a big kid!

David Schroeder said...

Just a small typo. "What is phycics?", should have been "What is physics?".

Stuart said...

The question "What is physics?" is philosophical. It is therefore advisable to consult philosophers and of course, their answer will probably fill at least 3 encyclopedias.

Sabine Hossenfelder said...


It's not a typo; she mispronounced it. As the next sentence emphasizes. I didn't add this to the video because I felt it would be too much distraction. (Not to mention that she doesn't speak English.)

daviberto said...

I'd say that the very different objects of study that we now consider part of physics can be compressed with the question: What is the best way to study this object? If the answer was by using mathematical models then that became Physics. So, Thermodynamics and Electromagnetism yes, but not Psychology and even Chemistry (at least historically). It also explains why with more mathematical tools and computing power, some new areas of study became part of Physics, as Biophysics and Econophysics.

mlmcl said...

I always enjoyed this kind of question from my kids. A child's curiosity forces one to clear away the cruft and consider the practical answer.

Place two identical balls on the floor.
Push the first ball so it hits the second ball.
Note how far the second ball moves.

Now repeat, replacing the first ball with a smaller ball. Before rolling the smaller ball, ask her what will happen. Let her know she's using physics to answer the question (using current knowledge to predict an outcome, performing an experiment, refining knowledge).

Tell her that using physics it's possible to predict what happens if somebody throws the planet Venus at Mars.

Because all she really needs to know is that physics is cool... ^^

Subbi2 said...

A friend's kid (roughly 6 yrs old at that time) ones ask me (Physicist) how fridge magnets work. Whoa, how do you explain it without feeling completely unsatisfied?

Bill said...

Dr. B -- You're blessed to have inquisitive children, and they are blessed to have physicist parents. Me, I'm still trying to pronounce the word in German -- I keep saying Phew-SEEK. But I've got Mathematik and Gymnasium down pretty good.

I would have answered Gloria's question with "Physicists are like detectives who use a lot of math and science to uncover deep mysteries about the world we live in." But maybe that's too much for a six-year-old.

Matthew Rapaport said...

Although physics strictly speaking has specialized and thus split into many disciplines, broadly speaking it is a study of the entire physical world using a technique (methodological naturalism) that properly limits its search space to that world. Since it is the foundation of all other science, we philosophers often use the word 'physics' to stand for 'science' broadly conceived.

TheBigHenry said...


I am sure you understand that your post is too sophisticated for a six-year-old. But I think it is a fine introductory summary for an adult who, at a minimum, is familiar with scientific notation for large and small numbers.

Thanks for your handy reference, which I intend to use when I am asked, by a non-physicist adult, what I did before I retired. I have been asked such questions many times, and I am tired of waving my arms around so much :)

JimV said...

Physics is studying how everything works and figuring out their rules so you can predict what is going to happen in the future due to actions now and in the past. Such as, if you kick a soccer ball hard enough to sink your shoe into it two centimeters, how far will it go? If you learn enough physics, you will know how a car works, and a TV, and a phone, and everything you see around you.

That would be my try, because when they want to learn you have to try to answer, although you know at six their brains aren't fully developed yet.

Sabine Hossenfelder said...


Yes, I know it's not suitable for 6-year olds. It's just that Gloria's question got me thinking about it and I thought it would make a nice wrapper. I believe for what she is concerned my answer was close enough. (She already had a few lectures on atoms and galaxies and photons and protons, so it kinda made sense to her I think.) Best,


Klavs Hansen said...

Here is my take:
Physics is describing Nature; what is it composed of and what the components do.

M&M said...

I like to define physics in contrast to the other sciences which are more readily comprehended.
Biology is the study of Life - plants, animals, their behaviour and composition, etc.
Chemistry is the study of Stuff - substances and materials, their composition and reactions, etc.
It might seem that this doesn't leave much else to study. But there are all the intangible processes and phenomena which we can't see or hold and might take without question. For example:
Heat - physics asks what makes an object hot or cold, how is heat transferred, how can we quantify it, etc?
Light - what is light, where does it come from, where does it go to when you turn the lights off, what is the difference between red light and blue light, etc?
Motion - why/how do things move, why do they stop moving, how do we quantify this
Magnetism - what produces the invisible force we feel pulling or pushing on magnets, how is it transmitted?
Gravity - why does everything fall down, why at different speeds?
And so on. The study of these phenomena reveals the patterns, the regularity, the predictability of nature. So roughly speaking:
Physics is the study of Intangibles

Nikola Milovic said...

Physics is an area of ​​learning phenomena and their properties, and these phenomena are formed before us. That's why we're trying to find out how it all took place.
But there is even greater problem in knowing the true causes of the phenomenon, and in us and in the universe. This is a consequence of ignorance and contempt of existence the organization of the universe. So far, science and most of its participants do not want to accept that the primary creator of all that is formed in the material universe, the energy entity (MEEU) Spiritual Entity of the universe (SEU).
The fact that the majority of scientists is trying to elevate ourselves above others and even ourselves presente as the Creator, they are only living creatures with a very low level of awareness, a consciousness of the power of creation and understanding of the true causes of the phenomenon. All this talk about BB, expansion of the universe, multiterzumi some relativity and the like, these experts represent a mirage in which their consciousness is contaminated enough to ignore and to understand who they are, why are there and how they arise, especially when the one who it is formed.
 These theories and images obtained using the model as a "cartoon", intended for those who need more of you stupid, to be manipulated with them, such as drug sellers manipulate junkie.

David Schroeder said...

Actually, Bee, your drawing skill is quite good, at least from the perspective of someone like myself very lacking in artistic talent, that finds drawing a decent circle, without tools, challenging. Perhaps such artistic talent runs in your family.

Growing up in a family of eight (two parents, two sets of twins, two single birth), about half of us had artistic talent. One member of each set of the twins could draw very well. My twin brother just had a natural knack, and one of my twin sisters did also. Of the single births, my older sister had a really good drawing ability, as did our dad.

APDunbrack said...

My own summary tends to be: "Physics studies simple questions," where by "simple" I mean "with at most O(10) parameters."

One atom, physics. Many atoms, chemistry (or materials science). One wire, physics. Many wires, electrical engineering. A few rigid objects, physics. Many deformable objects, engineering. Biological/geological/etc. systems all have many objects involved and have to consider it more precisely than physics would (see: jokes about "spherical cows").

I mean, it's not perfect, but it also captures the "large and small scales" pretty effectively: small scales because there are fewer objects to look at, large scales because one simplifies massively to consider only the bulk properties (e.g., cosmology just about reduces GR to the scale parameter, at least on a first pass).

fizzycist said...

Just to comment on the previous post, I disagree with the classification of simple. The simplest viable SUSY models have something like 100-and-something parameters. And string theorists, if I remember correctly, can't even work with less than 11 dimensions, but I think I get your point. It's really about minimizing the number of parameters necessary to describe the physical world.

I'm an experimentalist and have worked at most scales, starting with cold atom trapping (at BEC scales), to particle astrophysics, studying phenomena at the 10^11 GeV scale. I've also worked on a terrestrial collider in between, what's still called high energy physics, but after working in astro-particle physics, it feels like low energy physics to me. I've also dabbled with econo-physics (as a hobby) and even started out with bio-physics as an undergrad. I have trouble focusing. :)

By the way chemo-physics exists already too. It's just traditionally called P-Chem. I took a *lot* of chemistry in undergrad...did I mention I have trouble focusing?

So I do think the best explanation is the author's original "Physics is what physicists do."

To illustrate, let's consider a new field called poli-physics (short for political-physics) and if we had to get it kick-started how would we do it? First off, I would try to come up with a mathematical model of the average voter. Then we'd need to account for interactions ("How do some voters influence others? How are they influenced by what they read?" etc...). Then what metrics am I going to track? In normal physics these might be momentum, energy, time, but in this case we haven't decided what space we're working in yet. Think about what else they track in plasma physics or condensed matter systems, but in this case we might start with probability of voting and who are they going to vote for. Then I would look for evidence in the real world that lets me know whether my model is correct or not, updating parameters or even model assumptions as new information comes in. I'd be happy getting close at first, 'cause after-all all physicists solve every problem using perturbation theory. It's become so ingrained, I use it to cook, bake, fight fires ("Great! I knocked the fire down. That's my zeroth-order solution. Let's go for higher order corrections now...")...pretty much everything. The real world is too complicated for analytical solutions.

By the way, this field does exist already. I've known several physicists who have gone into data analytics applied to politics. It's just that no one is calling it poli-physics yet. Just like no one called it econo-physics when Wall Street used quants to model the stock market.

So I still think "Physics is what physicists do." is still a great answer. Sometimes I say "It's applied math combined with experiment." Both cover traditional physics along with bio-physics, econo-physics, and whatever new physics fields that might come along.

Grrr...I just read through more of the comments and realized that daviberto said pretty much exactly the same thing, just more succinctly. Oh well. :)

Giotis said...


Isn't theoretical Physics constrained Mathematics?

As for the video, I think your avatar does not do justice to your beauty, I would prefer a live appearance in that sense:)

Kaleberg said...

I remember learning what physics was; it was the science of how pool balls bounced around when you hit one with the cue. You could just as easily say that physics is the study of what happens when your foot hits a football. That gets you energy, momentum, velocity and so on, all in a way anyone can understand. The scaling stuff, the different forces, the probabilistic behavior and so on are just icing on the cake. The heart of physics is much simpler.

Sabine Hossenfelder said...


Well, yes, but that would have required me to explain what mathematics is and that's even more challenging...

Unknown said...

A physicist, is someone who learned her electromagnetic theory from Jackson. (I didn't; I'm just an astronomer. However, I did TEACH Jackson, for three years!)

Sabine Hossenfelder said...

I didn't learn electrodynamics from Jackson, though it sits on my shelf. I learned it from a local prof's lecture notes, which I still have and still use (thought they're out of print - a shame, they were really good).

Unknown said...

I remember an occasion (long ago) when my small son asked me what I was doing ( I was writing physics lectures).
"What is physics?"
Well, am I ever glad that kid asked!
I took my pen, and I said, "watch it drop," and I dropped it, all the way to the floor.
I asked him, "Did it speed up as it fell, or, did it fall with constant speed?"
"It didn't speed up."
I said, "OK, I hear you, but now let's TEST your conclusion, THAT is physics!"
I'd never done this before in my life. (Have you? if not, try it right now!)
I held the pen, now, just six inches above the table by my chair, and I dropped it, saying out loud (so as to time the fall), "Elephant one" (saying that takes about one second).
Result: about half a second to fall six inches.
THEN, I held the pen beside that table, so it would fall the MUCH LONGER distance to the floor, and I released it again.
I was utterly flabbergasted: it again took about half a second!
That pen was accelerating like mad! And yet the acceleration is invisible to the human eye!
The human perception of the fall is (I've tried this subsequently on large audiences) almost always of a fall with constant speed. A small percentage of the audience always says that they can see some acceleration.
Our brain has evolved to compensate our sense of the rate of passage of time to TAKE OUT the acceleration. Presumably that aids our survival in some way.
I've never gotten around to publishing this, but someone should. It isn't physics, but it is important.

Uncle Al said...

@Unknown: Free fall velocity in a medium: Bodies do not accelerate as they fall (aside from an initial perturbation).

8c7793aa-15b2-11e5-898a-67ca934bd1df said...

Your circular representation of the cosmic scale is awesome, Bee. This is the best description of the reality of physics and the universe I have ever seen. Great job.

Unknown said...

A wag might also say:

"a physicist is someone who thrived & flourished through 2 years and 3 volumes of the 'Feynman Lectures on Physics' "


-- TomH

MarkusM said...

In my opinion physics is the act of cherry-picking done by certain certain people (a.k.a. scientists), who only care about the part of reality which they manage to describe in terms of mathematics. (Some are said to even wonder why mathematics is so effective in what they do). If they fail to do so, they often do funny things, like assuming that a cow is spherical instead of asking a biologist ( :-)

Uncle Al said...

@Unknown: Physics, definition - Viam sapientiae mundi, per quam pervenitur.

@MarkusM: Said cow is also homogeneous, isotropic, uncharged, irrotational, and fermionic. You got bosonic cows, you got bad troubles in your milking shed.

MarkusM said...

@Uncle Al Yes, in particular on cold days, then spherical bosonic cows could form a cow condensate and would be indistinguishable. What one should also add to the list is that they should have neither hair nor quantum hair because that would make the problem intractable for most physicists :-)

Chris Mannering said...

Only on the testimony of personal experience, your career in physics or whatever, is not 'out-of-the-box' advantageous in your child's challenge to learn it. Talking here, we could spend as much time as we wanted unpacking all the reasons why that is, but for the most part the reasons are intuitively obvious, so I'm going to skip to the also intuitively obvious solution: To be an advantage always begin your answer (which in practice is or becomes a long running dialogue) with the standard definitions in the school books for the age group, or year above (or below, or wherever it needs to be).
And then consider making the fact the schoolbooks are limited or simplistic standing where you are, not about the schoolbooks, but about a basic feature of reality itself which your kid will already have across, which is how whatever it is we do, it always becomes much more than what's on the box.
Then you'll find you get back not only interest, but some aspect of that interest that your child can feel like a participating equal. On some levels we are all equal (no one gets out alive - The Doors). You also get a lot of latitude that you wouldn't have had, to make the situation into a container for yourself that you can explore in a child like way, what physics is, and that your child won't get confused that you don't know.

Chris Mannering said...

Hi Unknown - FWIW Natural Selection probably doesn't work by screening out but screening in. The human perception is that of screening out because, for example human perception itself, involves conscious decisions about filtering, which is something that as a subject to talk about tends to come up much more frequently in contexts where the focus of the subject is negative (about screening something out)

akidbelle said...

Hi Sabine, I'll play the bad guy with this one:

Physics is the art of wording ignorance in the form of principles.


Marcus Blackfellow said...

Bee, your English is very strong. Please raise your kids to be as comfortable with English as they are German. It's not too late and they will thank you for it one day.

Phillip Helbig said...

I agree. One needs pretty good fluency to teach children a language, but I'm sure Bee's English is good enough. It might take some determinism to switch so late, though.

Sabine Hossenfelder said...

I've tried to teach the girls English, but they're not interested. And I'm not interested enough in teaching to push hard. They'll learn it soon enough.

Having said that, my English used to be pretty miserable, but six years in North America have softened the edges. As I keep telling myself, at least that means my post-doccing was good for something ;)

Phillip Helbig said...

If they're not interested, it's probably too late. Or you need a lot of discipline on your part. One really has to start shortly after birth, to make it effortless. (My two youngest children are trilingual. We actually started out with five languages, but cut it back to three when the older one didn't talk at all even when he was three years old. We later learned that this had absolutely nothing to do with so many languages, but by that time the second one was about the same age, so we didn't want to confuse him and wanted to go back to five after he started talking. But he didn't start until he was almost five years old. So NOW we could go back to five, but it will take a while before the children accept it, and some discipline on our part. It turns out that both children have a form of autism, which explains the late talking and many other things. Interestingly, the older one taught himself to read without us noticing. We never tried to teach him to read; we didn't even read things to him, because he wasn't interested. We noticed it by chance in a hospital waiting room where we had never been before when he read physicians' and departments' names from a sign, making it obvious that he really was reading by knowing what letters make what sounds.)

Yes, they'll learn English eventually, but not as well as if they learn it now. Also, learning more than one language at this age makes it much easier to learn still more later on. Even Martin Schulz, famously with no Abitur, speaks six languages (Dutch, German, English, French, Spanish, Italian). :-)

Phillip Helbig said...

"If they're not interested, it's probably too late. Or you need a lot of discipline on your part."

That will make you a busy Bee. :-D

akidbelle said...

Hi Sabine,

I noticed my girls were very interested in English when it came to watching cartoons and funny TV shows... we lived in the US at that time.


Sabine Hossenfelder said...


They do watch English cartoons, preferably Peppa Pig and Shaun the Sheep. It's kinda interesting - they watch the full thing and I'm perfectly sure they understand parts of it, but they claim they don't.

I have seen this also the other way 'round - we have a relative whose son insists he doesn't speak German, even though his mother does. I am sure he understands a big part of it, and could probably speak it too, he just doesn't want to.

However, I recall that I also more or less refused to speak English (or French, or Latin) unless I really, really had to. So that's why I'm saying, I don't want to push them. Sooner or later they'll figure out by themselves that it's useful. Best,


Phillip Helbig said...

I agree that pushing is not good. That's the advantage of starting at birth; all languages are on an equal footing. At some point they realize that some people speak only one language. But that is no different than discovering that other people differ from their parents in other ways as well. :-)

akidbelle said...

Hi Sabine,

I agree pushing would be very wrong. If they are in the age of reading (even a little), I suggest you try to put captions on in English. That's the way my daughters did for maybe 2 months or so; after that it was just for me :)


Rob van Son (Not a physicist, just an amateur) said...

"I've tried to teach the girls English, but they're not interested. And I'm not interested enough in teaching to push hard. They'll learn it soon enough. "

The best, and simplest, way to learn a new language is to watch TV or movies in that language. If you want your kids to learn a foreign language, put them in front of the screen. There is a very deep divide in language proficiency between countries that dub foreign programs on TV and in the theater and those that use subtitles.

And you are right, they will learn it all by themselves.