## Top new questions this week:

### Does gravitation really exist at the particle level?

As I understand, we usually talk about gravity at a macro scale, with "objects" and their "centre(s) of mass". However, since gravity is a property of mass generally (at least ...

particle-physics newtonian-gravity experimental-physics atomic-physics
 asked by Hymns For Disco Score of 25
 answered by g s Score of 43

### Wouldn't Miller's planet be fried by blueshifted radiation?

In Interstellar, wouldn't Miller's planet be fried by blueshifted radiation? The 61,000x time dilation multiplier would make even cosmic background radiation photons into extreme UV photons. I was ...

black-holes time-dilation cosmic-microwave-background kerr-metric redshift
 answered by ProfRob Score of 32

### Why would black hole rip me apart?

According to my understanding of General Relativity, gravity is not a force and an observer which is falling freely under the influence of gravity should be considered inertial. Now, I have come ...

general-relativity gravity black-holes inertial-frames tidal-effect
 asked by Shivam Singh Aswal Score of 19
 answered by ProfRob Score of 43

### Where is the Lorentz signature enforced in general relativity?

I'm trying to understand general relativity. Where in the field equations is it enforced that the metric will take on the (+---) form in some basis at each point? Some thoughts I've had: It's baked ...

general-relativity spacetime differential-geometry metric-tensor curvature
 asked by Zinklestoff Score of 11
 answered by user1061016 Score of 9

### Does a volume stay constant when freely falling?

In general relativity, if a volume of particles moves unrestricted through spacetime, is their volume always conserved? Say we let a collection of particles at rest wrt each other, fall freely in a ...

general-relativity
 asked by Felicia Score of 11
 answered by Eric Smith Score of 11

Given the Einstein field equations, $$R_{\mu\nu} - \frac{1}{2}R g_{\mu\nu} = \kappa T_{\mu\nu}$$ that imply in particular that $\nabla_\mu T^{\mu\nu}=0$, one can show, using the explicit form of $T^{\... electromagnetism general-relativity maxwell-equations stress-energy-momentum-tensor geodesics  asked by Sjorszini Score of 7 ### Simulator for trajectories of charged particles in electric fields? I am looking for a numerical simulation program which computes the trajectory of a charged particle moving in an electric field. The electric field should be numerically determined by determining a ... electric-fields software  asked by Julia Score of 6  answered by cms Score of 5 ## Greatest hits from previous weeks: ### Given Newton's third law, why are things capable of moving? Given Newton's third law, why is there motion at all? Should not all forces even themselves out, so nothing moves at all? When I push a table using my finger, the table applies the same force onto my ... newtonian-mechanics forces everyday-life free-body-diagram faq  asked by user16458 Score of 172  answered by AndrewC Score of 190 ### Block on a block problem, with friction Consider two blocks, one on top of the other on a frictionless table, with masses$m_1$and$m_2$respectively. There is appreciable friction between the blocks, with coefficients$\mu_s$and$\mu_k$... newtonian-mechanics forces friction  asked by symplectomorphic Score of 17  answered by Chris Gerig Score of 8 ### Why is current a scalar quantity? Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the ... electromagnetism vectors electric-current  asked by user36159 Score of 35  answered by R004 Score of 29 ### Why does a glass rod when rubbed with silk cloth aquire positive charge and not negative charge? I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, and not negative charge? It ... electrostatics electricity  asked by Manisha Score of 26  answered by Lagerbaer Score of 16 ### How to determine the direction of induced current flow? There are three ways of inducing current in a loop/coil of wire as shown in my book. We can have a magnet approach a coil of wire, or a wire approaching a magnet. Both can be understood in the same ... magnetic-fields  asked by most venerable sir Score of 5  answered by gleedadswell Score of 7 ### Why does the comb attract the pieces of papers if they're neutral? When we rub our hairs with a comb, and then try to attract small pieces of paper, they're attracted by the comb. The pieces of the paper were not electrified before they were attracted. Then they ... electricity charge  asked by Samama Fahim Score of 17  answered by Tomáš Zato - Reinstate Monica Score of 10 ### Would you die if you put your hands on a powerline? You know how birds perch on powerlines without getting electrocuted? What if by some chance that I find myself falling and I grab on one of them? Let's say both of my hands are on the same line, would ... electricity potential  asked by Lemon Score of 14  answered by hdhondt Score of 15 ## Can you answer these questions? ### How much classical information is transmitted in continuous variable teleportation? In order to do quantum teleportation of$n$qubits, you need to send$2n\$ bits of classical information, in order to ensure that you get the original state and not a relative phase change of the ...

quantum-information quantum-computer quantum-teleportation
 asked by Keshav Srinivasan Score of 1

### Time taken to travel by light a minimum or maximum?

I'm stuck on the following. We know that the path taken by light to travel between 2 points A and B corresponds to the path which minimises time elapsed. However, from relativity we also know light ...

electromagnetism general-relativity geometric-optics geodesics
 asked by Vishal Jain Score of 1

### How to comprehend the fact that parity is an improper rotation in the odd dimension, but not in the even dimension, physically?

Some "clarification" To begin with, I'm not even talking about relativity so, in the following, rotations always act on the Euclidean space or only the space subpart of the Minkowski space. ...

group-theory rotation geometry spacetime-dimensions parity
 asked by Qi Tianluo Score of 1
 answered by Qmechanic Score of 0
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