Tagged Questions
2
votes
1answer
59 views
Anti-symmetric 2 particle wave function
Suppose we want to construct a wave function for 2 free (relativistic) fermions. As we are dealing with fermions the total wave function has to be antisymmetric under interchange of the coordinates,
...
0
votes
0answers
42 views
How does special relativity affect this photon/mirror experiment?
Here is a simple experiment showing how amplitudes add and cancel in a simple photon-mirror experiment.
In the comments, someone pointed out
What confuses me about the actual verification of ...
1
vote
3answers
68 views
Measurements and simultaneity [duplicate]
Picture a situation where we have two observers, $A$ and $B$, and a system in a certain quantum state. If $B$ makes a measurement of some observable, say energy for example, the state will collapse to ...
2
votes
1answer
62 views
Quantum Mechanical Effects of an object accelerating near speed of light $c$?
Consider a space ship, undergoing constant acceleration (which for our purposes means that the same amount of energy is being used per second to increase its speed). According to special relativity ...
6
votes
3answers
259 views
Why is the Dirac equation not used for calculations?
From what I understand the Dirac equation is supposed to be an improvement on the Schrödinger equation in that it is consistent with relativity theory. Yet all methods I have encountered for doing ...
3
votes
3answers
105 views
Reaching the speed of light via quantum mechanical uncertainty?
Suppose you accelerate a body to very near the speed of light $c$ where $v = c - \epsilon$. Although this would take an enormous energy, is it possible the last arbitrarily small velocity needed -- ...
5
votes
3answers
414 views
Does entanglement not immediately contradict the theory of special relativity?
Does entanglement not immediately contradict the theory of special relativity? Why are people still so convinced nothing can travel faster than light when we are perfectly aware of something that ...
1
vote
1answer
92 views
The Klein–Gordon equation
As we know that the Schrödinger equation presents basis of Quantum Mechanics and analogy with Newton second law in Classical Mechanics, I thought that relativistic interpretation of Schrödinger ...
0
votes
2answers
83 views
If inherent randomness exist in quantum mechanics, what then of eternalism implied by relativity?
I am nothing but a curious layman so don't go too technical on me.
First of all, I am well aware that a lot of people consider the question of determinism vs indeterminism to be unsolved and others ...
5
votes
1answer
146 views
Meaning of spin
I'm pretty astounded that I did not hear about this sooner, but in my course on QFT our professor told us that the concept of spin can be used to mean three things:
Mechanical spin (apparently a ...
4
votes
4answers
343 views
Can a photon get emitted without a receiver?
It is generally agreed upon that electromagnetic waves from an emitter does not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non ...
1
vote
1answer
94 views
The status / acceptance of block time?
What is the current status or acceptance of block time as it relates to Einstein's theory of relativity? Has quantum mechanics ruled it out or is it still the favored view of the world? Perhaps there ...
5
votes
2answers
322 views
Does quantum mechanics allow faster than light (FTL) travel?
Let's suppose I initially have a particle with a nice and narrow wave function[1] (I will leave these unnormed):
$$e^{-\frac{x^2}{a}}$$
where $a$ is some small number (to make it narrow). Let's also ...
5
votes
1answer
227 views
Is there any uncertainty between mass and proper length or time?
I was trying to naively draw a parallel between special relativity and the Heisenberg uncertainty principle. I try to understand uncertainty principle as a consequence of 4-position and 4-momentum ...
1
vote
0answers
164 views
Matrix manipulation for Dirac matrices
From the Dirac equation in gamma matrices, we know that $$\gamma^i=\begin{pmatrix}
0 & \sigma^i \\
-\sigma^i & 0
\end{pmatrix}$$ and $$\gamma^0=\begin{pmatrix}
I & 0 \\
0 & -I
...
3
votes
2answers
207 views
Matrix operation in dirac matrices
If we define $\alpha_i$ and $\beta$ as Dirac matrices which satisfy all of the conditions of spin 1/2 particles , p defines the momentum of the particle, then how can we get the matrix form ?
...
9
votes
1answer
310 views
How is quantum mechanics compatible with the speed of light limit?
Consider a free electron in space. Let us suppose we measure its position to be at point A with a high degree of accuracy at time 0. If I recall my QM correctly, as time passes the wave function ...
3
votes
1answer
71 views
Empirical meaning of relativity in the context of QM
In special relativity an event E is mapped to coordinates (x,t) in one inertial frame, and to coordinates (x',t') in another, and SR provides the relation between (x,t) and (x',t').
What is the ...
7
votes
1answer
286 views
Classical vs. Quantum use of the spin 4-vector
I have a few basic questions about the Pauli-Lubanski spin 4-vector S.
I've used it in quantum mechanical calculations as an operator, that is to say each of the components of S is a matrix operator ...
2
votes
2answers
227 views
Two photons of different frequencies collide to create electron and positron
A photon of frequency f, and another of frequency f' (take f' as given) collide to create an electron-positron pair. The frequency f is such that when the collision is head on, there is exactly enough ...
4
votes
1answer
113 views
Temporal part of Quantum Wavefunction
I was hoping that someone could give me the more fundamental reason that we take as the temporal part of a quantum wavefunction the function $e^{-i\omega t}$ and not $e^{+i\omega t}$? Clearly ...
-3
votes
1answer
59 views
Assuming collision , are there fundamental forces associated with absorbtion?
I just learned that strong and weak nuclear forces relate to decay/emission.
I know absorbtion depends on Energy levels(QM) and heat(thermodynamics , kinetic energy , entropy) and E = gamma mc^2 ( ...
0
votes
0answers
18 views
Is there a way to compute or explain if a decay prefers decaying into mainly mass or mainly energy?
Is there a way to compute or explain if a decay prefers decaying into mainly mass or mainly energy ?
I know quarks prefer to decay into the most massfull quarks : ...
4
votes
1answer
80 views
What are relativistic and radiative effects (in quantum simulation)?
I'm reading about Quantum Monte Carlo, and I see that some people are trying to calculate hydrogen and helium energies as accurately as possible.
QMC with Green's function or Diffusion QMC seem to be ...
0
votes
0answers
143 views
Newton Gravitational constant $G$, Plank constant $\hbar$ , Speed of Light $c$ : The Dream Team of moderators?
The 3 great constants of Nature are well known :
The Speed of light $c$ (special relativity)
The Plank constant $\hbar$ (quantum mechanics)
The Newton ...
2
votes
1answer
233 views
Explanation of equation that shows a failed approach to relativize Schrodinger equation
I'm reading the Wikipedia page for the Dirac equation:
$\rho=\phi^*\phi\,$
......
$J = -\frac{i\hbar}{2m}(\phi^*\nabla\phi - \phi\nabla\phi^*)$
with the conservation of probability ...
1
vote
3answers
255 views
What is the 'quantum-developed' or 'relativistic-developed' equation of the electrostatic force?
Quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics that is the first theory where full agreement between quantum mechanics, special relativity and ...
2
votes
3answers
199 views
Spooky action appears to contradict Relativity of time order of multiple events
It is well known that in special relativity observers can disagree on the time ordering of two events.
It is also well known that entangled particles exhibit so called spooky action at a distance.
...
5
votes
1answer
201 views
Relativistic contraction for a wave packet and uncertainty on momentum
Consider an electron described by a wave packet of extension $\Delta x$ for experimentalist A in the lab. Now assume experimentalist B is flying at a very high speed with regard to A and observes the ...
3
votes
1answer
570 views
Darwin term and Zitterbewegung
I've noticed that in the discussion of the fine structure of Hydrogen atom standard QM texts claim that the Darwin term, which corrects energy of $\ell=0$ (or $s$-) states only, is related to the ...
7
votes
8answers
489 views
Is “Causality” the equivalent of a claim that the future is predictable based on the present and the past?
In classical (Newtonian) mechanics, every observer had the same past and the same future and if you had perfect knowledge about the current state of all particles in the universe, you could ...
1
vote
4answers
925 views
Is it possible to transfer classical bits of information faster than light speed?
Is there any known, verifiable way to transfer classical information faster than light, using quantum entanglement or other phenomenon?
Does quantum teleportation, or other known phenomenon, allow ...
14
votes
2answers
701 views
Definitions: 'locality' vs 'causality'
I'm having trouble unambiguously interpreting many answers here due to the fact that the terms locality and causality are sometimes used interchangeably, while other times seem to mean very different ...
3
votes
1answer
121 views
quantum curvature
If a state can be a superposition of energy states, and mass equals energy (special relativity), and mass curves space-time (general relativity), then could we say that space-time around a quantum ...
1
vote
2answers
334 views
Spin matrices in Dirac equation
Why in every textbook when deriving Dirac's equation the smallest possible matrices ($2 \times 2$) are used?
I wonder why one couldn't use spin 1 matrices ($3 \times 3$) and get relativistic equation ...
0
votes
0answers
19 views
Is the universe a quantum computer - is light speed barrier a computational constraint [duplicate]
Possible Duplicate:
Is the universe a quantum computer - is light speed barrier a computational constraint
Cross-posting this question, since physics.stackexchange has not provided any ...
12
votes
3answers
969 views
Is the universe a quantum computer - is light speed barrier a computational constraint
There is currently a debate ongoing on leading maths blog Gödel’s Lost Letter, between Gil Kalai and Aram Harrow, with the former arguing that building a quantum computer may not be possible due to ...
11
votes
3answers
322 views
Do particles have different spins in different frames of reference?
Let's say we have two photons, whose momentum vectors point to opposite directions. Also spin angular momentum vectors of the photons point to opposite directions. (Sum of spins is zero)
Now we ...
3
votes
0answers
43 views
Spectrum of a quantum relativistic “distance squared” operator
This question disusses the same concepts as that question (this time in quantum context). Consider a relativistic system in spacetime dimension $D$. Poincare symmetry yields the conserved charges $M$ ...
0
votes
1answer
261 views
Relativity and Entanglement
Say we have two particles which are entangled so that they have opposite spins. If one is up, the other is down. They are sent off to two spatially separated observers A and B. Both observers can ...
1
vote
0answers
2k views
Kinetic energy of Electron [closed]
I had to find the kinetic energy of electron with wavelength $2$ pm. I used the formula
$$ KE = \frac{p^2}{2m} = \frac{h^2}{\lambda^2 2m}$$
which gave me result, $KE = 376.9$ KeV. But the answer ...
14
votes
10answers
7k views
Does the Pauli exclusion principle instantaneously affect distant electrons?
According to Brian Cox in his A night with the Stars lecture$^1$, the Pauli exclusion principle means that no electron in the universe can have the same energy state as any other electron in the ...
0
votes
2answers
272 views
Momentum Energy and Higgs
So, as an object accelerates it gains energy. And energy is mass. So an object becomes more massive as it approaches the speed of light.
But, if mass is ONLY due to an object's interaction with the ...
7
votes
5answers
884 views
Why Negative Energy States are Bad
The argument is often given that the early attempts of constructing a relativistic theory of quantum mechanics must not have gotten everything right because they led to the necessity of negative ...
2
votes
1answer
261 views
Naïve relativistic schrodinger equation [duplicate]
Possible Duplicate:
Why are higher order Lagrangians called 'non-local'?
Bjorken and Drell presents the equation:
$$i\hbar\frac{d\psi}{dt}=H\psi=\sqrt{p^2 c^2+m^2 ...
9
votes
2answers
356 views
Essential background for QFT study
The preface to Mark Srednicki's "Quantum Field Theory" says that to be prepared for the book, one must recognize and understand the following equations:
$$\frac{d\sigma}{d\Omega} = ...
3
votes
1answer
174 views
Does decoherence single out a preferred frame?
Environmentally induced decoherence makes wave function collapse unnecessary. But the environment, usually taken to be some heat bath, introduces a preferred frame. (That in which the total (spatial) ...
8
votes
2answers
904 views
Poincare group vs Galilean group
One can define the Poincare group as the group of isometries of the Minkowski space. Is its Lie algebra given either by the equations 2.4.12 to 2.4.14 (..as also given in this page - ...
4
votes
1answer
324 views
Commutation of operators in quantum theory
I have always written the commutation rules of quantum theory as ,
$[q,p] = i\hbar\delta _{ij}$
But seems that some people write this as,
$[q^i,v_j]= \frac{i\hbar}{M}\delta^i _{j}$
(..this is ...
3
votes
3answers
567 views
Time in special relativity and quantum mechanics
The time is treated differently in special relativity and quantum mechanics. What is the exact difference and why relativistic quantum mechanics (Dirac equation etc.) works?
