7 ms·

I'm surprised this would need to be verified experimentally. Why couldn't a density gradient like this be derived from fundamental laws?

mLuby 8y ago
As I recall you can't even do this for Earth's atmosphere below the Karman line—it's experimentally derived. https://en.wikipedia.org/wiki/International_Standard_Atmosphere https://en.wikipedia.org/wiki/International_Standard_Atmosph...
- ummonk 8y ago
  You could just assume constant temperature (using e.g. blackbody temperature based on solar radiation) and get a first order approximation of the behavior of the atmosphere. Won't be as good as the standard atmosphere model which uses empirical temperature gradients, but it will provide a ballpark model of the atmosphere.
macawfish 8y ago
This is such a good question. Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders. And then another time my statistical mechanics prof informed me that "physics is just drawing cartoons of reality with math." That came as a huge existential relief by the way. Popular science holds that physics is the study of "fundamental laws", but really it's just a special practice of articulating these very stable, ubiquitous patterns. When you look at things holistically, you will notice that they are often irreducably dynamic, complex and full of potential surprises, along with fully actualized surprises. So I'd imagine that all these relationships of atmospheric gases with things like the earth's magnetic field, weather patterns, chemical reactions in the atmosphere, interactions with extraterrestrial particles, the dynamics of other magnetic fields, etc. would all make this very difficult to model accurately. First principles apply very well to our nice toy models, but the sheer vastness of existence usually out-scales our toy models very quickly.
- Rychard 8y ago
  > physics is just drawing cartoons of reality with math https://en.wikipedia.org/wiki/Spherical_cow https://en.wikipedia.org/wiki/Spherical_cow
- ithkuil 8y ago
  One thing that physics does well is to place upper and lower bounds around what that unpredictable thing that the actual reality can be or do
- bookmacpro 8y ago
  > Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders. This way of thinking is a bit flawed. Convenience is an important aspect, but it's not the entire reason. A physicist who wants to study how skiers might cause avalanches by modeling/simulation doesn't care about the details of the people they model because that's just not important. For what they want to study, it doesn't matter if they model the human as a sphere or as a real-life model with limbs and so on. It won't affect the grand picture. On the other hands, a physicist who wants to study how an avalanche would crush a person? Then yes, modeling the limbs and the person's physical properties become much more important. In modeling, you choose the model that best fits and represents what you aim to study. You start off with a complex system of equations and then you slowly go through and remove the parts that would be irrelevant. You don't need to account for turbulence if you know that you will never reach the high Reynolds numbers that require it. You don't need to model cellular biology if you're just trying to modeling how a piece of tissue stretches.
  - macawfish 8y ago
    All great points. But yeah it was a joke! And basically I remember his joke having a tangential quip about choosing the shape based on the coordinate system you wanted to stay in. And the point I got from it was that in modeling we often use crass approximations that are oversimplified in order to get a rough picture of what's going on with respect to certain dynamics, and that one shouldn't assume that the model resembles the full complexities of real life. By the way, he was a cosmologist, if that helps you understand a little better why he was so flippant with the shapes.
TeMPOraL 8y ago
I guess it's because of the interactions between gravity of Earth, Moon and other bodies in the Solar System, solar radiation pressure, and magnetic fields. It's hard to account for all the possible influences, and verifying things experimentally lets you both check your theoretical work, and possibly discover sources of influence you've missed.
abtinf 8y ago
Imagine how exciting it would be if you built a model, with every factor you could think of, and then it turned out to be even just a little bit wrong.
thorsday 8y ago
Fundamental laws are "just" models, and models are only as good as we can validate them. My day-to-day involves running simulations of physical systems and while things like the Navier-Stokes equations are well known and understood (to the best we can), the difficult part comes in the constants we use in the models to describe specific aspects. The Navier-Stokes equations themselves can describe and tell us how fluids behave, but we still need constitutive models to describe the fluid. We get those models by proposing ideas and testing them.
jules 8y ago
This is a typical exercise in thermodynamics courses, but simple models may not be very accurate.
erbium 8y ago
The magnetosphere and solar wind
BorRagnarok 8y ago
I'm surprised you're surprised. Why wouldn't you want to verify your theory? Look at all the missions to comets we did, they all returned totally unexpected results. Results we wouldn't have if we had never gone out there to verify our theories.

mLuby 8y ago

As I recall you can't even do this for Earth's atmosphere below the Karman line—it's experimentally derived. https://en.wikipedia.org/wiki/International_Standard_Atmosphere https://en.wikipedia.org/wiki/International_Standard_Atmosph...

ummonk 8y ago
You could just assume constant temperature (using e.g. blackbody temperature based on solar radiation) and get a first order approximation of the behavior of the atmosphere. Won't be as good as the standard atmosphere model which uses empirical temperature gradients, but it will provide a ballpark model of the atmosphere.

macawfish 8y ago

This is such a good question. Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders. And then another time my statistical mechanics prof informed me that "physics is just drawing cartoons of reality with math." That came as a huge existential relief by the way. Popular science holds that physics is the study of "fundamental laws", but really it's just a special practice of articulating these very stable, ubiquitous patterns. When you look at things holistically, you will notice that they are often irreducably dynamic, complex and full of potential surprises, along with fully actualized surprises. So I'd imagine that all these relationships of atmospheric gases with things like the earth's magnetic field, weather patterns, chemical reactions in the atmosphere, interactions with extraterrestrial particles, the dynamics of other magnetic fields, etc. would all make this very difficult to model accurately. First principles apply very well to our nice toy models, but the sheer vastness of existence usually out-scales our toy models very quickly.

Rychard 8y ago
> physics is just drawing cartoons of reality with math https://en.wikipedia.org/wiki/Spherical_cow https://en.wikipedia.org/wiki/Spherical_cow
ithkuil 8y ago
One thing that physics does well is to place upper and lower bounds around what that unpredictable thing that the actual reality can be or do
bookmacpro 8y ago
> Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders. This way of thinking is a bit flawed. Convenience is an important aspect, but it's not the entire reason. A physicist who wants to study how skiers might cause avalanches by modeling/simulation doesn't care about the details of the people they model because that's just not important. For what they want to study, it doesn't matter if they model the human as a sphere or as a real-life model with limbs and so on. It won't affect the grand picture. On the other hands, a physicist who wants to study how an avalanche would crush a person? Then yes, modeling the limbs and the person's physical properties become much more important. In modeling, you choose the model that best fits and represents what you aim to study. You start off with a complex system of equations and then you slowly go through and remove the parts that would be irrelevant. You don't need to account for turbulence if you know that you will never reach the high Reynolds numbers that require it. You don't need to model cellular biology if you're just trying to modeling how a piece of tissue stretches.
- macawfish 8y ago
  All great points. But yeah it was a joke! And basically I remember his joke having a tangential quip about choosing the shape based on the coordinate system you wanted to stay in. And the point I got from it was that in modeling we often use crass approximations that are oversimplified in order to get a rough picture of what's going on with respect to certain dynamics, and that one shouldn't assume that the model resembles the full complexities of real life. By the way, he was a cosmologist, if that helps you understand a little better why he was so flippant with the shapes.

TeMPOraL 8y ago

I guess it's because of the interactions between gravity of Earth, Moon and other bodies in the Solar System, solar radiation pressure, and magnetic fields. It's hard to account for all the possible influences, and verifying things experimentally lets you both check your theoretical work, and possibly discover sources of influence you've missed.

abtinf 8y ago

Imagine how exciting it would be if you built a model, with every factor you could think of, and then it turned out to be even just a little bit wrong.

thorsday 8y ago

Fundamental laws are "just" models, and models are only as good as we can validate them. My day-to-day involves running simulations of physical systems and while things like the Navier-Stokes equations are well known and understood (to the best we can), the difficult part comes in the constants we use in the models to describe specific aspects. The Navier-Stokes equations themselves can describe and tell us how fluids behave, but we still need constitutive models to describe the fluid. We get those models by proposing ideas and testing them.

jules 8y ago

This is a typical exercise in thermodynamics courses, but simple models may not be very accurate.

erbium 8y ago

The magnetosphere and solar wind

BorRagnarok 8y ago

I'm surprised you're surprised. Why wouldn't you want to verify your theory? Look at all the missions to comets we did, they all returned totally unexpected results. Results we wouldn't have if we had never gone out there to verify our theories.