Time Definition and 1000 Threads

Dear PF, As an excercise I am to find out how the expectation value of the spin operator evolves over time. There was a hint, stating that it is enough to show that $$ e^{i \frac{\phi ( \hat{n} \cdot \sigma )}{2}} \sigma_i e^{- i \frac{\phi ( \hat{n} \cdot \sigma )}{2}} = [R_{ \hat{n} }]_{ij}...

Can two particles ever be said to occupy exactly the same space at the same time while remaining distinct objects?

Hi there, So as far as I know, after long time, current doesn't flow through the resistor That means the voltage drop across the capacitor is equal to the EMF of the cell = 9V So will the charge be ##U = 0.5CV^2## = ##0.5(10)(81)##? Answer appears to be wrong, not sure what I am doing wrong here.

So I just wanted to see if anyone could offer some suggestions. So in my mind this seems impossible, in the case of electric field a jump in time derivative of that field would indicated in my mind that electric charge was either introduced or removed from the system instantaneously which would...

Hi, I'm struggling to solve this problem. Can anyone please solve it and explain it to me? Thanks.

Please could I ask for help with the following question: Part (a) is no problem. Acceleration is the gradient of the graph in regions OA and AB which gives 3 and 0.5 Part (b), I believe, requires me to calculate the greatest and least value of the gradient of the curve in region BC Part...

Recently I have come into Special Relativity and specifically Lorentz transformation. Let's assume two frames A and B moving relative with speed ##v##. The position of a particle moving with respect to B is given by ##x′=f(t′)=3t′##. What is the function of position ##x=f(t)## of the particle...

So I can across this question in class and there were a multitude of answers, such as 0 work (J), my answer was 8100J based on the fact it doesn't tell you any distance moved, just that its 45N for 3 minutes, so my logic was 45N per second so 45*3(60)=8100J

I received a bachelors in mathematics fairly recently and I am thinking about a career in teaching. I know that you must posses a license to be a full time teacher in most schools, but what requirements are there for being a substitute teacher? I hear teaching can be too overwhelming for some...

Homework Statement: spin dephasing time Homework Equations: spin relaxation time Hello, i have seen the next equation that relates the spin dephasing time of an ensemble of électrons T2* with the spin relaxation of a single electron T2, and I would like to know how it was deduced/proposed...

While deriving continuity equation in Fluid mechanics, our professor switched the order of taking total time derivative and then applying delta operator to the function without stating any condition to do so(Of course I know it is Physics which alows you to do so) . So,I began to think...

I want to know the total energy contained in a magnetic field due to a long wire (just consider a 1m segment) as a 1amp current is turned on starting at time zero. I'm assuming zero turn-on time for convenience. At t=0 the cylindrical field is formed and I wish to know the total energy as a...

I know that for normalized quaternion, $$\hat{q}$$, the derivative is given by $$\frac{d\hat{q}}{dt}=\frac{1}{2}\hat{q}\cdot \omega$$ where $$\cdot$$ denotes the quaternion multiplication. I want to calculate the time derivative of a non-normalized quaternion q. I tried to calculate the...

Edit: I'm leaving the original post as is, but after discussion I'm not confused over coordinate time having a physical meaning. I was confused over a particular use of a coordinate time difference to solve a problem, in which a particular coordinate time interval for a particular choice of...

I've bumped into a few interesting papers talking about time-reversal symmetry in QM (eg: https://arxiv.org/abs/1507.07745) but I can't seem to wrap my head around the concept. 1) What does it mean for one to say that standard QM isn't time-reversal symmetric? Does this have to do with the...

Why can't absolute time be used to describe events? Previously I tried to describe entanglement collapse on this forum in terms of absolute time, but I was told more or less this was not valid. I don't understand why. If the proper time that we use is based on the fact that the speed of light...

I don't know how to make that question better, to not seem as if I'm asking why is time relative (that's not what I'm looking, I think). When we say that time is relative, we look at how many time takes light to go from one point to another, in order to be seen from another perspective. So...

With the Liouville's theorem $$\frac{{d\rho }}{{dt}} = \frac{{\partial \rho }}{{\partial t}} + \sum\limits_{a = 1}^{3N} {(\frac{{\partial \rho }}{{\partial {p_a}}}\frac{{d{p_a}}}{{dt}} + \frac{{\partial \rho }}{{\partial {q_a}}}\frac{{d{q_a}}}{{dt}})} = 0$$ when we calculate the time evolution...

Time travel teleportation can be achieved in small scale experiment in millisecond. Using a computer byte in radio signals. I look forward for comments.

Sorry if the question has been already answered, but I didn't manage to find it. Let's go back to ligo detection of gravitational waves, my question is the following: if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so...

on the conquering the physics gre book it says e.g. for time invariant "if you can see someones eyes in a mirror, they can see yours as well" so what the hell does that mean? isnt person A sending photons to person B and person B sending different sets of photons to person A? how does that...

A long horizontal rod has a bead which can slide along its length and is initially placed at a distance ## L ## from one end A of the rod.The rod starts from rest in angular motion about A with a constant angular acceleration ##\alpha## .If the coefficient of friction between the rod and the...

Problem Statement: I am having trouble deriving the expression from the initial equations. (Calculate the emptying time considering Volume conservation) Relevant Equations: Q=A*sqrt(b(H-h(t)) And we have dh/dt =Q/S (conservation equation) and we have to obtain h/H = 1-(1-t/te)^2 with te=...

In another forum, some people argue that time and space are discrete, due to Planck time and Planck length. However, I disagree with this idea. I think, the Planck time and Planck length are just some scales that we can measure, but they do not forbid continuous time and space shorter than...

Suppose someone randomly went to Starbucks Monday thru Saturday between 8:00 AM and 10:AM. Suppose one Tuesday in May they purchased a drink and the credit card transaction occurred at 9:14:12 AM. What are the odds that person could randomly return on a Wednesday in July and purchase a drink...

I am interested in climate change and thereby interested in tipping points. So for the last few months, I have been investigating the dynamics of a toppling brick. I derived a differential equation of for the motion and wrote a computer program to solve this from initial conditions. I have more...

Hello! The time ordered product for Dirac spinors is defined as: $$<0|\psi(x)\bar{\psi}(y)|0>-<0|\bar{\psi}(y)\psi(x)|0>$$ Can someone explain to me how should I think of the dimensionality of this. For a Dirac spinor, ##\psi(x)## is a 4 dimensional column vector, so the first term in that...

Apologies if there are already an abundance of threads on related questions. I have tried searching for threads on here and have read quite a few, as well as reading other sources. I've kind of reached a point where I need help to parse some of the information that I have been reading and to get...

Hey! I wanted to ask and learn more about what time is, and whether it even exists. Can relativity be understood solely in terms of rates of motion and subjective experiences, or do we really need to say this 4th dimension is a real thing. And if it is real, is it material? Thank you

Aristotle's absolute space and time can be represented as ordered pairs (s, t) but not as fibers π(s) = t of time as is the case of Galileo and Newton's space time. That is to say that the space of Galileo and Newton is the projection π(s) = t on the time axis. The time space of Galileo and...

Consider the example of flashing light in spaceship. The observer in the spaceshipe and the observer in the Earth measure different times. Both observers could argue that the clock of the other tick slower(motion is relative). But is anyone right ? I mean the one will measure proper time...

Suppose I have Gaussian white noise, with the usual dirac-delta autocorrelation function, <F1(t1)F2(t2)> = s2*d(t1-t2)*D12 Where s is the standard deviation of the Gaussian, little d is the delta function, and big D is the kronecker delta. For concreteness and to keep track of units, say F...

I’m just imagining spacetime that is spatially infinite but has a beginning in time. Naively, there would be only one (class of) reference frame(s with the same velocity) in which time starts simultaneously everywhere. (This is a naive application of “relativity of simultaneity” from SR.) But...

This is my wrong attempt, the bolt's equation for ##t\geq 2## would be ##y_b(t)=-\frac{1}{2}g(t-2)^2+e(2)+l## where ##e(t)=\frac{1}{2}wt^2## the position of the elevator's floor in the absolute frame of reference. ##d(t)=-\frac{1}{2}(w+g)t^2+2gt+2(w-g)+l## the distance between the bolt and the...

Could one come to think that time is irrelevant in quantum mechanics? we know that the QM equations are written with the time variable, (schrodinger equation). Yet everything suggests that time is irrelevant, as the search for loop quantum gravity seems to indicate

I m a noob, I wanted to figure out the time dilation for an observer on Earth who passed 50,000 years, and the Traveler at light speed(99.9%) passed ONE DAY. I mean we pass 50,000 years on Earth and the Traveler spends One day.Problem is I tried many online calculator, but each one is giving me...

How do you calculate the time it take a high pressured gas to fill a chamber? I have a gas in a chamber that is at 600psi and then another chamber at atmospheric pressure. When the valve between the two chambers is opened, how do you calculate the time it would take to fill the chamber? Are...

I am 99.99% sure I am correct but would like some clarification from the other members here. I occasionally read that "if you could travel faster than light you go travel backards in time." This statement does not make sense to me and seems to be a misinterpritation of reality. I will provide...

OK I just had time to post and hopefully ok but still typos maybe the graph was done in Deimos wanted to try tikx but not sure about the polynomial trying to as many physics homework before classes start on Aug 26 Mahalo

So I was reading a book on special relativity and it was explaining how, if we were to go very near the speed of light, time (relative to us traveling) would slow and space itself would condense. It used the example that if we were to try and travel to a galaxy 1 million light years away, Yet we...

Time travel, or more precisely time dilation, has been a well known proven result of Relativity. It has been demonstrated most notably in satellites traveling thousands of mph around the world with GPS. You can only go forward with our current understanding of physics. Have there been any...

my paper was in "awaiting referee report" during 1 month. After this, the asociate editor says "We have now received both primary referee reports. However, as they do not agree we have to send your manuscript and the referee reports to an adjudicator. " The page of the journal says : "If the...

##V_{oy} = V_{0}.sinθ = (20 m/sec)(sin 30) = 10 m/sec## ##V_{ox} = V_{0}.cosθ = (20 m/sec)(cos 30) = 17.3 m/sec## ##y = y_{0} + V_{oy}t - \large\frac {1}{2}\normalsize gt^{2}## ##0 = 50 + 10t - 4.9t^{2}## ##0 = 4.9t/6{2} - 10t - 50## ##t = \large\frac {-b±\sqrt {b^{2} - 4ac}}{2a}## ##t =...

Let us suppose a spaceship moving from Earth to another star that is 10ly away with a speed of 0.99c. Calculate the of years spaceship takes to reach its destination (a) in the rest frame of an observer on Earth and (b) as perceived by a passenger on board the ship For (a) I find that ##t_0 =...

I'm having trouble processing math atm, I'm a perfectionist so each time I delve into a topic on my "self-study" I try to understand all of it. but that generally takes too long a time to focus on other "important" math stuff that actually matters right then like studying some trig stuff and...

Figure given in question-: I first found out the time when force will be equal ro friction.After force becomes equal to friction there will be no acceleartion further,so velocity after that time will be same.I calculated the velocity by getting a relation of it with time.I got the velocity as...

Hello! In Griffiths chapter on Time independent perturbation theory, he has a problem (9.20) in which he asks us to calculate the first order contribution to the electron Hamiltonian in an atom if one takes into account the magnetic dipole/electric quadrupole excitations, beside the electric...

In relativity, time is a dimension like space, a co-ordinate system. Depending on your motion, your co-ordinates shift and this leads to time dilation, simultaneity and length contraction. So, does time flow at all in relativity? Does time 'move forward'? If not, what would be different if time...

For the past few weeks, I have been trying to review the math courses I've taken over past two years. All I can really do, though, is just read over the chapters. I acknowledge that the only way to actually (re)learn the material is to do the problems. I always tell myself that I can do it in my...

A free photon can have any wavelength and energy; no discreteness there. Just because something is quantized, or fundamentally quantum in nature, doesn’t mean everything about it must be discrete. The idea that space (or space and time, since they’re inextricably linked by Einstein’s theories...