# Search results

1. ### I Are all symmetries in physics just approximations?

I came across this video of Leonard Susskind saying that all symmetries in physics are approximations. Unfortunately, I don't have the links on hand, but I have come across other sources of physicists claiming that all symmetries are approximations. My confusion though is that it was my...
2. ### I Observing black holes in a finite time (in Earth's frame)

My understanding from General Relativity is that if as distant observers we watch a probe or any test mass approach a black hole, time dilation goes to infinity as the probe gets closer to the event horizon. This would imply that we would never observe a black hole form, or the collision of two...
3. ### I Confused about some notation used by Griffiths

I worked out the expectation values of the components of a 1/2 spin particle. However, I'm confused about Griffiths notation for the x and y components. For the x component I got, ## \left< S_x \right> = \frac \hbar 2 (b^*a+a^*b)## which is correct, but Griffiths equates this to ##...
4. ### Particle Is Griffiths Introduction to Elementary Particles up to date?

I have a copy of Griffiths Introduction to Elementary Particles (1st Edition) and was thinking of beginning to work through it. I was curious if anyone knows if this text is sufficiently up to date or if there have been any major developments in particle physics that would make it worth getting...
5. ### Potential of a grounded conductor in the presence of an external charge

If we set the potential at infinity to be zero, we find that the potential of a grounded conductor is V=0. The conductor being grounded has no net charge and produces no external field, so I understand why in that situation we would say the potential of the conductor is zero. However, in...
6. ### I Bernard Schutz proof of the invariance of the interval

I've been going through Bernard Schutz's A First Course in General Relativity, and I'm hung up on his "proof" of the invariance of the interval. At the beginning of section 1.6, he claims that he will prove the invariance of the interval, and after a few lines shows that the universality of the...
7. ### I How to think about x/x, or (x-1)/(x-1) etc.

In solving physics problems, I have often done some type of simplifying where I eliminated an x in the numerator and denominator, or eliminated some other terms. For example, maybe I have something like ## \frac {x} {x^2 + x} ## and I simplify this to ## \frac {1} {x+1} ##. Or I have something...
8. ### Magnetic field of a cylinder with current down the center

I was just in a conversation with someone regarding the magnetic field resulting outside of a solid cylinder, with a current moving down the center of the cylinder, and then the resulting magnetic field after removing the current. Now I haven't thought about magnetic fields/magnets for a while...
9. ### I Conservative Forces

I actually have a few things I'm thinking about here. I'm curious as to whether a velocity dependent force field absolutely cannot be a conservative force field, in principle. I have at times come across statements in physics that I found out had mathematical exceptions for, but we don't...
10. ### Confusion about gravitational acceleration

I understand that gravitational acceleration is independent of mass. However, I've seen a common mathematical description of this that I can't help but find circular. I suspect that there's an error in my thinking that I'm hoping someone can point out for me. It goes like this; ##F=mg## but we...
11. ### Unnecessary step in deriving the Lorentz transformations?

I'm going through Ray D'Iverno's "Introducing Einstein's Relativity", and there is a step he makes in deriving the Lorentz transformations that doesn't seem necessary to me. So I'm not sure what I'm missing. He derives them from Einsteins postulates of relativity. From the postulate that the...
12. ### Gyroscope precession energy

I'm trying to think about gyroscope precession in terms of energy, and I'm a little confused. If you hold the spinning gyroscope at some tilt so that it doesn't precess, it will have energy associated with it's rotation. When you let it go, it will now have additional energy due to the...
13. ### Work Done on a System of Charges

Something I have read in Griffith's Electrodynamics is confusing me. In deriving the Poynting vector, he says to suppose that we have some charge and a current configuration which at some time t produces fields E and B. In the next instant, dt, the charges move around. He then asks the question...
14. ### Why the sky is blue and not violet?

I have some confusions about this that I'm not finding clear answers for. First, looking at the color violet, it looks like a sort of "purple". The answer as I've understood it to why the sky doesn't appear violet or this "purple" is that it has to do with a combination of the fact that there...
15. ### Commuting operators require simultaneous eigenfunctions?

Here is what I understand. The generalized uncertainty principle is: \sigma^{2}_{A} \sigma^{2}_{B} \geq ( \frac{1}{2i} \langle [ \hat{A}, \hat{B} ] \rangle )^2 So if \hat{A} and \hat{B} commute, then the commutator [ \hat{A}, \hat{B} ] = 0 and the operators are compatible. What I don't...
16. ### Time Dilation, Twin paradox etc.

I know there have been a lot of other threads on this topic but I wanted to get some feedback on my thoughts on this. Time dilation is a result of Einstein's second postulate of SR, although the first postulate is what gives rise to the apparent twin "paradox". The common solution to the paradox...
17. ### Experimental Quantum Mechanics Texts?

In studying QM, it's been very abstract. I'll read things such as, we get an eigenvalue when we measure an observable. Or we prepare an ensemble of particles in the state \Psi . I understand this in an abstract way, but I don't understand what it actually means to get an eigenvalue when a...
18. ### C and h are not fundamental constants?

I was looking through Zee's 'Quantum Field theory in a Nutshell" and he says that c and \hbar are "not so much fundamental constants as conversion factors." I've heard other physicists say this as well. I understand that these constants are used in some equations to give units of energy so...
19. ### Why is there no current in a reversed biased diode?

I understand there is some "leakage" current in a reversed biased diode but I don't understand why there is significantly less current in a reversed biased diode. I've seen the band diagrams and if all the electrons were only on the n-side of the diode, I would understand why the electrons can't...
20. ### When I look at the Milky Way, what am I looking at?

More specifically, when I look at the milky way am I seeing another arm of the galaxy or am I looking towards the center of the galaxy? I'm having a hard time finding a specific answer to this question. Thanks!
21. ### Infinite square well solution

In finding solutions to the time independent Schrodinger equation we have to normalize \psi to find the constant A. So we get \int_{0}^{a} |A|^{2} sin^{2}(kx) dx = |A|^2 \frac{a}{2}=1 For A we then get |A|^2 = \frac{2}{a} . Griffiths says that this only determines the magnitude of A but...
22. ### Electromagnetic waves with regard to photons

The electromagnetic spectrum contains wavelengths that are on the scale of macroscopic objects. What I'm not sure about then is, does this mean that photons corresponding to these "macroscopic" wavelengths are actually that size? My guess is that these photons have a corresponding electric field...
23. ### Electric Field of a Conducting Surface

I understand that taking the boundary condition that the electric field changes \frac{\sigma}{\epsilon_{0}} when you cross a charged surface and the fact that the electric field is zero inside a conductor that the electric field outside the conductor should be \mathbf{E} =...
24. ### Charge within a cavity inside a conducting material

Suppose I have a charge inside a conductor as shown in the image I've attached. For any charge distribution: \oint \mathbf{E} \cdot d \mathbf{a} = 0 I can see that if I took some path from the charge, through the conductor, and back to the charge, the integral would be zero still. Now, that...
25. ### Electrostatic Boundary Conditions

In Griffith's section about electrostatic boundary conditions, he says that given a surface with charge density \sigma , and take a wafer-thin Gaussian pillbox extending over the top and bottom of the surface, Gauss's law states that: \oint_{S} \mathbf{E} \cdot d \mathbf{a} =...
26. ### Larmor Precession Direction

For some reason, I don't understand the direction of larmor precession. Torque is: \mathbf{\Gamma} = \frac{d \mathbf{L}}{d t} = \mathbf{r} \times \mathbf{F} As an example, I understand with a gyroscope, using the right hand rule and with angular momentum in the direction as shown in this...
27. ### Is the higgs boson the mediator of the higgs field?

I'm a little confused about whether the higgs boson is the mediator of the higgs field. I haven't had a chance to study in depth the higgs field theory, but I have tried finding information from seemingly reliable sources and there are some apparent contradictions. I watched Leonard Susskinds...
28. ### Is chlorophyll green or green?

Is chlorophyll "green" or green? Something I can't find a precise answer on is if chlorophyll reflects only green light. Each color has an associated wavelength, but as shown in a CIE chromaticity diagram, you can combine different wavelengths of light to create the perception of a single...
29. ### Particle/dipole acceleration in non-uniform magnetic field

I've come across a few places that mention that dipoles and charged particles accelerate in non-uniform magnetic fields. Is this true? If the Magnetic force is always perpendicular to the velocity of a charge, I don't see why it would accelerate. I see it having centripetal acceleration with...
30. ### What happens to the electrons in a magnet

Suppose I'm holding a magnet and then lift a paperclip(or another magnet) with the magnet. Now the energy density of a magnetic field or the energy that went into making the magnetic field is W = \frac{1} {2 \mu_o} \int B^2 d \tau but since work was done in lifting the magnets, what happenes to...