There seems to be a lot of misunderstanding around the issue of the gravito-thermal effect as it appears in the work of scientists such as Hans Jelbring, and Nikolov & Zeller. Without trying to recapitulate their theories in detail, I thought it might be worth going through a few basics in order to dispel some of the fog some people seem to be surrounded by. I’ve thought about a few different ways of doing this, and settled on the style of a Platonic dialogue to give it some continuity, rather than a set of disconnected facts, like you might get in a Q&A, or FAQ. Some people might think I’ve got some stuff oversimplified or just plain wrong. Feel free to offer alternatives in comments below. H/T kdk33 for improved phrasing in the Glickstein section.
So these guys think most or all of the extra warmth there is at the surface of planets with atmospheres compared to those without is due to gravity? Are they serious?
Deadly serious. This is a real scientific theory.
But how can gravity cause heating of anything? It just pulls stuff together – right?
Right, but it’s what happens to the stuff that gets pulled due to other physical laws which come into play that causes the heating, not gravity itself.
But that means work has to be done by gravity to get anything else to happen doesn’t it? Otherwise it’s a perpetual motion machine.
In classical mechanics terms, gravity is not a type of energy, but a force. It is constantly applied by masses on other masses. It is an intrinsic property of mass, not an energy state which can ‘get used up’. In terms of relativity theory, it is a property that mass has which causes the warping of space-time around the mass, which causes other masses to fall towards its ‘gravity well’.
Ok, but how does ‘force’ make things like heating happen? Heating needs energy doesn’t it?
At the microscopic level, heat arises because all matter which is at a temperature above absolute zero vibrates and moves around, knocking into other bits of matter. The energy of collisions makes the atoms and molecules vibrate and the rate they vibrate at determines their temperature. The gravitational force can cause matter to fall, gather momentum and bash into something else. As the mass falls towards another mass it is gravitationally attracted to, the gravitational potential energy it has by virtue of its altitude from the other mass diminishes, and the momentum, which is a product of its mass and velocity increases. When it hits another mass on the way down, some of that energy of momentum gets turned into heat because the collision makes the molecules vibrate more.
But you said gravity isn’t energy. Now you are saying the mass turns gravitational potential energy into heat. What’s going on?
Although gravity itself isn’t an energy, by virtue of its action as a force, it causes mass which is not at rest at the centre of gravity to have the potential to accelerate towards that centre of mass. That’s why we talk about mass having ‘gravitational potential energy’. The higher above the centre of gravity a mass is, the more of its total energy is locked up as gravitational potential energy. This means less of the total energy is available to be thermalised as heat in collisions.
So is that why its cold at high altitude and warm near the surface? Ira Glickstein said it only works once, when the air is first pulled down and compresses, then the heat dissipates back to being the same temperature everywhere again.
That’s one way of looking at it, from the point of view of the classical mechanics of the microscopic scale. Ira is right in one sense, but wrong in another. Although initial heating caused by sudden compression dissipates, the ongoing action of gravity as a force keeps the air compressed more near the surface. This means air is denser at low altitudes, and that means more molecules are having collisions more often, thermalising energy.
But energy must be conserved to satisfy the first law of thermodynamics mustn’t it? Where does the extra energy come from?
There is no extra energy, it is equally spread through the troposphere. If the whole of the troposphere was the same temperature as the surface it wouldn’t make it warmer. But gravity causes there to be a temperature gradient from cold high up, because more of the total energy is locked away as gravitational potential energy compared to warm at the bottom where the near surface air is hotter than the average because less of the total energy is locked away. Again, total energy remains equally distributed throughout the troposphere, as the second law of thermodynamics demands, but because of the difference in gravitational potential energy between molecules at the bottom and top, there is a thermal gradient.
But that’s just the classical mechanics way of looking at it. What’s really happening physically? There’s convection to consider too.
Yes, the throughput of solar energy coming in, being absorbed and turned into other kinds of energy and causing processes like convection complicates the picture. All of our ways of looking at things are just our conceptions of reality, not reality itself. Whether our conceptions are right or not is tested by making predictions and seeing if reality does what we expect it to according to theory. A good start is to see if the ideas all fit together logically and without internal contradictions or paradoxes. If that test is passed, it’s experiment time.
OK, but how do we perform an experiment on the whole troposphere? It’s a messy place with all sorts of different energies and processes like convection going on in it.
Good point, that’s why the science isn’t settled. But we can perform gedanken experiments to see if they can shed any light on how stuff really works. That’s a kind of thought experiment where we simplify things and test our ideas in a framework which limits the complexity of the real world. A relevant example here is the ‘model planet’ used in the theory written by Hans Jelbring. That one is properly defined and conceived in such a way as the result can be accurately computed. Rather than looking at the microscopic level that theory deals with bigger ensembles of molecules of a billion or so. That way, it can consider other processes like convection which happen in the real troposphere.
But that theory doesn’t have any Sun and it doesn’t permit radiation to space. How can that be any use for understanding reality? And why do they talk about pressure?
It doesn’t need those in order to reach a conclusion regarding the way gravity affects the surface temperature of any planet, whether or not it’s close to a sun. The pressure in the troposphere varies being lower at the top and higher at the bottom because of all the extra weight of the rest of the atmosphere being piled on top of it, being pulled down by gravity. That means the air is denser at the bottom too, so there are more collisions happening and more energy is thermalised as heat. Stick around, and if we’re lucky, Hans himself will take up the challenge of explaining his theory and how it relates to the real world in terms anyone can understand.