2
votes
3answers
184 views

Has the universe we live in started as a black hole that is imploding?

As discussed in this question, as far back as the 1960s it was suggested that the Schwarzschild metric can be smoothly joined to a de Sitter metric. For example the idea has been used by Lee Smolin in ...
2
votes
0answers
50 views

Left (right) invariant vector fields on superspace

I read Freed'd book on "Five lectures on supersymmetry". For any vector space $V$ with metric of signature $(1,n-1)$ he constructs super Lie algebra $$ L=V \oplus S^*, $$ where $S$ is space of ...
1
vote
3answers
63 views

Difference between high-level and low-levels of electromagnetic radiation

can someone please explain me what we mean by 'high-level' or 'low-level' in electromagnetic radiation? for example, it is believed that high-level microwave radiation is harmful to human but not the ...
1
vote
2answers
84 views

DC Generator with magnet as rotor

DC generators convert the AC current in them by split ring commutators right and the graph of the current will be like this but the question is how would be the graph if the magnet is the rotor and ...
2
votes
2answers
133 views

How should I quote errors when measurements are asymetrically clustered?

Suppose five people measure the length of a stick and report the following values 4.90cm 4.92cm 4.93cm 4.94cm 4.94cm In high school science we are told that in ...
1
vote
0answers
77 views

Why is graphene the only (stable) 2D sheet structure? [duplicate]

I know that Carbon molecules can form different structures depending on how they bond with each other: graphite, diamond, graphene and fullerene. As far as I understand, graphene is just a "sheet" of ...
-2
votes
3answers
107 views

Why airports are generally near sea? [closed]

The generic way to measure the height is from the sea-level. If the airport is near the sea i.e. not at a very height from the sea, then the fuel required to make the air-plane above the sea level ...
0
votes
4answers
128 views

Can conservation of momentum and conservation of energy explain every possible event in the Universe?

I heard my friend, a researcher, say that we can, in theory, explain every event happening in the universe using the Conservation of momentum and energy. He added that we may not be able to do that ...
1
vote
2answers
190 views

Scattering theory textbooks

I am looking for a possibly extensive list of great textbooks on elastic and inelastic scattering of particles within quantum field theory. So far I am familiar with: Peskin and Schroeder: An ...
0
votes
1answer
64 views

Commutativity of Position Operators

Does the position operator $q_{i}$ of one harmonic oscillator commute with the position operator $q_{j}$ of another different harmonic oscillator? In other words, is $q_{i} q_{j} = q_{j} q_{i}$ true? ...
2
votes
0answers
47 views

Strength of Magnet in Magnetic Bearings

While reading about magnetic bearings, one reads about passive/active magnetic bearings. By passive, one means no electric current is put into the bearing for attaining magnetic levitation/flux. In ...
0
votes
1answer
26 views

Can electrons move back into the donor level

In a n-type semiconductor electrons from the donor level move up into the conduction band but can they move back down? Also in a p-type semiconductor electrons move to the acceptor level from the ...
1
vote
1answer
140 views

Time evolution of a quantum system

A quantum system has Hamiltonian $H$ with normalised eigenstates $\psi_n$ and corresponding energies $E_n$ ($n = 1,2,3...$). A linear operator $Q$ is defined by its action on these states: $$ ...
3
votes
2answers
920 views

Will a microwave heat sand?

I want to cook Turkish coffee on heated sand at school. I have difficulty accessing some easier method of heating, so I was going to try to heat sand in a microwave. It was then pointed out to me that ...
0
votes
1answer
69 views

Direction of EMF

I was reading electromagnetic induction and came across EMF.Now in my text book as the theory has been developed ,they have many a times talked about direction of EMF. But they have not clearly ...
3
votes
4answers
167 views

How are theories in physics validated against existing knowledge in practice?

Often you hear people mention how most attempts at Unified Theories haven't "accounted for all the experimental observations". The question is, how do you know you have accounted for all of the ...
2
votes
6answers
129 views

Thermodynamics only deals with homogenous systems?

In Thermodynamics quantities like pressure, temperature and entropy are associated with overall states of a macroscopic system. In that case, we do not talk about "the quantity $Q$ at the point $p$ of ...
1
vote
0answers
33 views

Momentum of electron problem [duplicate]

Recently, my friend bemused me with a question related to the momentum of an electron. The confusing logic is stated below: Since an electron is a particle and according to classical physics, we know ...
1
vote
3answers
381 views

Propellers and the Speed of sound

I was reading this website that described a novel wind turbine technology and it has this quote: In the mid-20th century, open propeller-driven planes with piston engines reached performance ...
0
votes
1answer
60 views

Broken Thin Lens Algorithm [closed]

We all know the thin lens equation. For $o$ being a horizontal object distance and $f$ being the focal length, the horizontal image distance $i$ is described by: $$\frac{1}{f} = \frac{1}{o} + ...
1
vote
2answers
61 views

Standard Usage of the word “per”

Math guy here. What is the usual meaning of "x per y per z?" Is this (ignoring details) (x/y)/z or x/(y/z)? Sorry to be mundane.
0
votes
1answer
118 views

Two-block system acceleration when applying a force in the $x$ direction

Two blocks are connected by a light-weight string that passes over a friction-less pulley (a.k.a. Atwood's machine). Block # 1 (called $m_1$) hangs from the edge of a friction-less table. Block # 2 ...
0
votes
0answers
21 views

Two Dimensional Kinematics and Projectile motion [duplicate]

I have a question. So a baseball is hit with a velocity of 30 m/s at 42.6 degrees to the ground. A fielder who is supposed to catch the ball is smart and knows that he is exactly 8 meters from where ...
4
votes
2answers
212 views

Why is the phase space a symplectic manifold rather than a manifold with a metric?

Why does phase space require a symplectic geometry rather than a metric? Is there some scenario where a metric is unable to describe the notion of length in phase space, specifically in relation to ...
2
votes
1answer
147 views

Does air conduct heat better than saturated steam?

This engineering toolbox table shows thermal conductivity of steam at 0.016. I understand that water is better in conducting heat than air, but if I read this correctly, steam is worse in conducting ...
1
vote
3answers
93 views

What is special about the ratio $Q/V$ that we give it the name Capacitance?

Why is the ratio Charge/Potential important? Also, usually when we add charge, the potential changes. Then why do we care how much charge we can put on a conductor for a given potential.
0
votes
1answer
60 views

Upcoming 'things to watch' in physics [closed]

What are the 'things to watch' in physics happening in the next year or so? E.g. A big astronomical event that happens only every n thousand years, or an experiment in particle physics which will ...
1
vote
2answers
88 views

Draw the charge configuration according to the electric field [closed]

I'm trying to draw the charge configuration outside the regions that lead to the given electric field patterns. Can someone show me how?
1
vote
0answers
58 views
1
vote
1answer
66 views

How does velocity relate to energy difference in Compton scattering?

I'm having trouble understanding what my professor is getting at asking in this question. I just visited her office and her explanation minutely helped. I'm hoping to get a bit more clarity on what is ...
3
votes
1answer
180 views

Why is the Earth's moon so big?

It could just be me, but it seems like our Moon is WAY bigger than it should be for a planet of our size. If you look at satellite-to-planet mass ratios for the largest moons of Jupiter, Saturn, ...
1
vote
0answers
26 views

Why isn't the sky violet? [duplicate]

Why isn't the sky violet? Is there any physical reason why the sky isn't perceived to be violet or is it all in our cones?
1
vote
0answers
23 views

What is “full clutching flow” in fluid-dynamics with regards venturi design?

As the title says, what is "full clutching flow" in fluid-dynamics with regards venturi design? I came across this reference a few times and can not find any further info on it, I'm currently ...
1
vote
1answer
535 views

Spring Powered Car - Which Design Is Best? [closed]

I'm in the preliminary stages of building a spring powered car to race over a distance of 10 metres. It is a yearly event held in my local area and so far we've only seen one spring powered car. I ...
2
votes
1answer
77 views

Can a lone black hole in a closed Universe evaporate?

If there is a closed Universe which only has a black hole in it, can that black hole evaporate? As the black hole evaporates, it gives off energy, which will eventually come back and be re-absorbed ...
1
vote
3answers
4k views

When is momentum not conserved?

What are some common examples where momentum is not conserved? This question arose in my mind when I read that a ball dropped from a height penetrates into a bed of sand and that momentum is ...
0
votes
1answer
31 views

N-type semiconductors and the number of holes?

Let us say we have Germanium which has been doped with antimony. Antinomy has a a fifth valence electron not involved in bonding. Is this electron in the valence band? I ask because all texts on the ...
2
votes
1answer
65 views

Is there a Fermi estimations toolbox?

I am a theoretical physics student and am a little ashamed at my inability to estimate any measurable quantity. I would like to develop my skills at Fermi estimations. Although it is hopeless to ...
0
votes
1answer
40 views

Why part of debroglie wave is negative?

I mean de Broglie wave for a particle represents the probability of the particle existing at a point, then, how can it be negative as well as positive?
0
votes
1answer
55 views

Why do roads appear wet on hot days? I do not understand [duplicate]

Why do roads appear wet on hot days? I don't understand the bending of light rays in relation to the density fluctuations. How are they related? Why do we see mirror images of cars on road in hot ...
0
votes
1answer
177 views

Is antimatter power theoretically possible?

There is a quote and citation on the "Antimatter" page of the obvious encyclopedic site (https://en.wikipedia.org/wiki/Antimatter), saying that positrons could be produced (in the future) at a rate of ...
4
votes
1answer
718 views

Why do we only talk about three options when it comes to the Shape of Spacetime?

The shape of spacetime is a hotly debated topic which can determine how we understand the ultimate fate of the universe and many more things. Despite this I only ever see factions supporting three ...
1
vote
1answer
105 views

Path of wheels of a bicycle

Why are the wheels of a bicycle moving in concentric circles with the center O? I know that the velocity of the back wheel is parallel to the frame of the bicycle and the velocity of the front wheel ...
2
votes
1answer
33 views

What theory unifies the electroweak and the strong forces? [closed]

As the title itself suggests, here's my question: What theory unifies the electroweak and the strong forces?
1
vote
1answer
287 views

Using Ampere's Law without Right-Hand-Rule to derive an expression for the magnetic field around a current

I'm a little confused over the textbook example of applying Amperians to get the magnetic field around a current. I understand we take a loop which shares the rotational symmetry of the wire (a ...
2
votes
3answers
235 views

Armageddon prevention

I was watching the movie Armageddon and it made me think of a few things. We would probably not need to send a crew to blow the asteroid in two, I think we could still send a warhead with enough power ...
1
vote
3answers
319 views

Can a bullet really fly through space forever?

Some people says that if it would be possible to shoot a bullet so high that it would get over the Earth gravitational pull, the bullet could fly through space forever, because of no deceleration of ...
3
votes
2answers
1k views

“Magic” Speed to Drive Over a Speed Bump: Myth or Reality?

A somewhat controversial aspect of speed bumps (sleeping policemen for those in the UK) is that they arguably cause more loss of life than they prevent by acting as a hindrance to emergency response ...
0
votes
0answers
45 views

Will charges redistribute on the surface of an insulator?

When I shot an electron beam onto an insulator, electrons will be knocked out of the material, leaving an immobile positive charge at their place. Some of the knocked out electrons might manage to ...
2
votes
2answers
169 views

How to define pseudovector mathematically?

The textbook describes pseudovector like this: Let $a,b$ be vectors and $c=a\times b$, $P$ be the parity operator. Then $P(a)=-a,P(b)=-b$ by definition. But $P(c)=c$ since both $a$ and $b$ reverse ...

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