General Physics Thread

Fair coins tend to land on the same side they started: Evidence from 350,757 flips in a bit of 50.6-50.8% of cases.

Yeah, turns out the real world is just a little complexer than statisticians assumed. Turns out aerodynamics adds a slight bias.

I managed to correctly guess 2 things in Physics yesterday (subject was Heat and Thermodynamics). Why the old theory that heat was an invisible fluid was incorrect (it implies friction spontaneously generates heat) and how condensation works after the professor explained why some objects feel cold to the touch while others don't.

Edited by TheLivingDrawing on Nov 3rd 2023 at 10:50:44 AM

Aww Hell yes.

So, I've been interested in physics (among many things) for a long time...but my understanding of it has always been limited due to my weakness in mathematics.

But then boy oh boy. In my local used bookstore, I found a fairly recent copy of Conceptual Physics, a textbook by Paul G. Hewitt, which in my country is even translated as "Physics without Mathematics.''

Sweet. I know that I will never truly understand physics without mathematics, but eh, it's not like I'm gonna get a job that is any way related to STEM field anyways.

Watch sand defy gravity and flow uphill thanks to “negative friction”: Applying magnetic forces to single iron oxide-coated particles spurs strange collective motion.

Interesting. This should lead to some interesting applications.

A thought experiment: Is an universe where photons have a rest mass of about one nanoelectronvolt more habitable than our own?

Such a tiny mass would not noticeably change the speed of light or other natural constants, nor any electrical or magnetic fields with scales of less than a kilometre. Sure, we couldn't have a power grid in such an Earth, but that's hardly an obstacle to pre-19th century life.

The effects on stars, however, would be substantial. Without large magnetic fields, the stellar corona and the stellar wind shut down. That greatly increases the stability of planet atmospheres, leaving many more habitable even as more massive planets become gas planets.

Accretion disks would be substantially colder and more stable without magnetic braking. That would make 'em produce more in-disk bodies like planets, while supermassive black holes would be smothered in their crèches. This might be a double edged sword as you have more stars and more planets, but their masses might be higher.

The lack of magnetic winds and magnetic braking would make stars spin much more quickly, which combined with the disappearance of large scale magnetic phenomena would change the character of supernovae and neutron star mergers. What this means for nucleosynthesis is unclear, as the effects of more rotation and less magnetism may compensate each other to a degree.

What exactly would that imply for electromagnetism? The speed of light in a vacuum being constant is a consequence of Maxwell's equation. But here light has to be slightly slower than c, so its speed is frame-dependent, so Maxwell's equations must be wrong?

I think physics would break in numerous largely unpredictable ways with massive light. If massive light doesn't break causality itself however, it would make it impossible to determine the age of the universe accurately because light could slow down unpredictably.

Also it would probably make chemistry impossible, (changing most things about the electromagnetic force tend to break things in chemistry, and such a big change as making photons massive sounds like it almost certainly would do something like that) so there's that. I'd guess this would happen by breaking the fine structure constant entirely (because it relies on a constant speed of light)

I wonder if massive light would also affect star life development. Surely the amount of photons produced would churn up their insides something fierce.

Nah, the effects of massive light are actually quite predictable - quantum field theory on massive exchange particles is well-developed, and it posits that such an interaction decays both by inverse-square and exponentially, the "Yukawa potential".

But the exponent term becomes measurably different from 1 only for an interaction with a scale of 1km (for a rest mass of 1 nanoelectronvolt), considerably larger than any chemical process. Such a term would only minimally change orbitals, as the two body problem isn't changed by the exponential term unless it is very large - molecular chemistry isn't even discussed as a potential area for testing photon masses.

As demonstrated by neutrinos, photon speed would still be indistinguishable from c, which despite its name does not have to be the same as photon propagation speed. That said, one might expect photon speed to become wavelength dependent but that's more a "what does the sky look like" thing than a "can we live in it" aspect.

^1 nanoelectronvolt is considerably less than the energy carried by typical thermal photons. It wouldn't be even a rounding error.

I'm far to tired to be 100% on my math and recollection of that one general relativity course but I'm pretty sure you can't have life in a universe like this (at least without having photons decay to an infinite range force carrier) because an interaction length of one km means the habitable range for a planet would be inside it's stars roche limit. Photon's won't be able to carry energy the 93 million miles it takes to get to us.

Aye, you are misremembering. Relativity isn't the issue, the Yukawa potential is, and it pertains to the force exchanged via photons, not light. Otherwise, power lines wouldn't be able to carry electricity for distances exceeding a picometer.

Alright I am now slightly more conscious but now late for work. I am aware that general relativity is not the specific topic at hand but it was the course I took where we got into stellar mechanics. Astrophysics isn't my primary field, and I suspect that the evolution of a star to a neutron star as opposed to a black hole might be impacted.

That having been said Yukawa did give us a handy method for predicting the effective interaction length for a phenomenon based on the mass of its exchange particle. For a force with exchange particle of mass m the effective range of that force is approximately equal to hbar/(2mc). This handily reproduces the mass of the W and Z bosons for the weak force and the exchange of virtual pions for the residual strong force (i.e. nucleon to nucleon rather than the quark-quark strong force which is moderated by gluons).

This does not impact the range of a current in a wire because while the electron is a charge carrier the exchange particle for electromagnetic interaction is the exchange of virtual photons. Veritasium made a good video on the topic a year or so ago.

The Yukawa potential still only applies to the virtual photons, not to light itself. You may notice that a photon rest mass is constrained to about 10^-48 kg; were it to limit the range of light, we could infer a mass limit of 10^-68 kg from the visibility of objects billions of years away.

You are correct. I have got to stop doing napkin math after an owl shift, two hours of sleep is not enough. My primary concern in this case is going to be nucleogenesis. Magnetic interactions have a large impact on the evolution of massive stars and neutron stars collisions are thought to be the dominant source of many of the massive elements.