# Search results

1. ### B What is the relationship between mass and weight?

Weight is the force of gravity on a mass (typically, a mass that is not in freefall with respect to the source of the gravity). Because most human-related measurements and discussions occur near Earth's surface, we can take the proportionality between mass and weight to be g. g is not truly...
2. ### I How do we find/deduce Einstein's field equation?

John Baez, who has written a lot of articles for this site, has a nice formulation of this, which involves considering the deformation of a falling ball of liquid and applying various conservation laws. He arrives at the Einstein equations without the standard appeal to Riemannian geometry...
3. ### B How does a current "know" where to go

At equilibrium, the electric field inside a metal is zero. This is because inside a metal the electrons can move freely, and if an electric field is applied to a metal, the electrons will move in the opposite direction leading to a polarization that points in the opposite direction of the...
4. ### I 2-d equations of motion for a Slinky going down stairs?

Slinkies can actually get super complicated, math-wise. This paper has the most rigorous treatment that I know of, and includes discussions on the shapes formed by a slinky under various boundary conditions: https://arxiv.org/pdf/1403.6809.pdf This paper from the American Journal of physics...
5. ### Where could I find some basic problems in physics?

I have a PhD in physics and I'm not sure what the seven basic quantities in physics are. Is that supposed to be asking about the SI base units?
6. ### A Eric Weinstein's Geometric Unity theory

George Jones, the only way the situation changed is that he (or someone) put his talk onto youtube, so now people can watch. The situation had evolved from "we can't really evaluate his theory based on a one paragraph abstract" to "we can't really evaluate his theory just based on his talk."...
7. ### B Can we use relativity to speed up computer calculations?

This would work if you kept the computer in an inertial frame and then sent the Earth off on an out-and-back relativistic journey. Do not recommend.
8. ### Physics A post doc in an area that differs from my PhD?

I did a postdoc that was quite similar to my PhD (same sub-field, maybe even same sub-sub-field?) and I feel like there would have been some better opportunities to expand my knowledge and skillset. I don't regret it, but if you're applying for faculty positions it might be better to be an...
9. ### Schools Laurentian vs. Queens for Physics

Hi, I did my undergrad at Queen's in physics. I took the first year astrophysics course there with Stephen Courteau, which was honestly perhaps the best and most informative course I took my entire time there. I also took general relativity with Laurence Widrow in my senior year, and did an...
10. ### A Effective Spring Constant of a Chain Polymer

The model you are looking for is the Ideal Chain. It is based on a chain of monomers that can rotate freely (or in one dimension, be aligned or anti-aligned with their neighbors) and ignores interactions between them. You can essentially treat the location of the Nth monomer like a Brownian...
11. ### A How is the thermal conductivity affected by magnetic field?

If such an effect exists, it is certainly not a standard part of materials physics. You could perhaps handwave an argument for it that electrical and thermal conductivity are related through phonon mechanisms, and electrical magnetoresistance is a thing, so perhaps thermal magnetoconductance is...
12. ### I The Second Law of Thermodynamics and the Stefan-Boltzmann Law

The Stefan-Boltzmann law was initially empirical, but it can be derived by integrating Planck's blackbody spectrum over all wavelengths. The derivation of Planck's law explicitly uses entropy-maximization to figure out how energy is distributed between different electromagnetic wave modes, so...
13. ### Job Skills Future Mechanical Engineering Domains

Complex fluids is (are?) an area of physics research that is slowly morphing into the engineering domain. Sometimes this is known as rheology, but the study of complex fluids is a bit broader. There are a lot of nontrivial fluid mechanics that go into figuring out how thing like polymer...
14. ### Which school is stronger in condensed matter theory?

Probably University of Chicago, but if there's someone at Brown you're excited to work with, don't let the school's rankings dissuade you.
15. ### Programs What is the best major or double major for Nuclear Fusion research?

The main domain of physics involved in fusion research is plasma physics, so you want a school with a good plasma physics program. The other things mentioned would also be useful, but plasma is number one. Nuclear engineering would be good for understanding the radiation safety aspects...
16. ### B Electromagnetic fluid acceleration as effective rocket propulsion?

Not exactly leaving the atmosphere, but this is something that was done for the first time in small aircraft just a few years ago http://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121 I imagine there are some "engineering" problems associated with just pointing the thing in the...
17. ### I Photon induced magnetic effects

I mean, we don't know what it's referring to because we don't know the details.
18. ### I Biological solid state physics

Or in terms of energy scales, the energy difference between different conformational states in a protein is typically of order kT (25 meV), whereas phonons are associated with modes in the interatomic bonds, which are of order single eV, like 40 times greater in energy. There are important...
19. ### I Photon induced magnetic effects

Can't be sure of the specifics, but I suspect it may be referring to the transverse magnetic field that makes up the electromagnetic wave that is the laser. A beam of light consists of a tranverse electric (E) and magnetic (B) field, and although the magnitude is typically described in terms of...
20. ### Is Mathematica more incomplete than the other math solver software packages?

I am more familiar with Maple, but have used Mathematica a bit. I am more used to Maple because I learned it first, and did my undergrad honors thesis in general relativity with it. The main things I use Maple for are simplifying gross expressions, finding the minima of gross functions, and...
21. ### Physics Career path forward for a physics autodidact?

I think your best is to get into a physicsy field as a programmer or a computer engineer, rather than as a physicist. All areas of physics research require computer programming, and a lot of bigger operations (startup companies, national labs, research collaborations, etc) tend to hire computer...
22. ### Physics Do Physics (Research) Related jobs exists for Bachelor's holders?

There are jobs for physics majors to do physics research, but they are often not labelled as such. I had such a job after I graduated, where I was doing fluid dynamics simulations in a hospital research lab. These jobs are often not treated like potentially permanent positions, but rather as...

Overall I think your letter is good, although I'd move the PS into the main text, in case people stop reading after the signature. Professors get a looooot of emails and sometimes have to get through them quickly. Similar to what Vanadium 50 said, the letter is much more likely to get...
24. ### Insights Either the Sun Is Getting Smaller or Gravity Is Getting Weaker

Greg Bernhardt submitted a new PF Insights post Either the Sun Is Getting Smaller or Gravity Is Getting Weaker Continue reading the Original PF Insights Post.
25. ### Insights New Research on Untying Knots in Polymers - Comments

They do. You can try tying a cooked spaghetti noodle into a knot and pulling on the ends. The noodle will break at the knot. All this stuff about thermodynamic stability though basically assumes that the length is constant (no breaking or growing). There are lots of things that knots are useful...
26. ### Insights New Research on Untying Knots in Polymers - Comments

Good question. The lowest energy state of a polymer is a random coil, that is the one that maximizes entropy. When you stretch a polymer and let it relax, it returns to the coiled state as entropy increases (this is also why rubber is elastic). However, if there's no knot it is unlikely (but not...
27. ### Insights New Research on Untying Knots in Polymers - Comments

Greg Bernhardt submitted a new PF Insights post New Research on Untying Knots in Polymers Continue reading the Original PF Insights Post.
28. ### Insights Using the Spaghetti-Twist to Align DNA - Comments

Greg Bernhardt submitted a new PF Insights post Using the Spaghetti-Twist to Align DNA Continue reading the Original PF Insights Post.
29. ### Insights Assessing the Physics in Edmond Hamilton's "The Second Satellite" - Comments

klotza submitted a new PF Insights post Assessing the Physics in Edmond Hamilton's "The Second Satellite" Continue reading the Original PF Insights Post.
30. ### Insights History's First Science Experiments - Comments

klotza submitted a new PF Insights post History's First Science Experiments Continue reading the Original PF Insights Post.