Observable Definition and 210 Threads

So the universe starts with an amount of matter, radiation, a fixed spatial curvature constant and a cosmic constant. Due to the expansion of the universe matter and radiation dillute and the spatial curtvature decreases whereas the cosmic constant remains fixed. Radiation dillutes faster...

Homework Statement I've been reading Leonard Susskind's Theoretical Minimum volume on QM, and enjoying it quite a bit - the book doesn't include exercise solutions at the end though, and if they exist online for this volume I haven't been able to find them. (Perhaps if such solutions...

Forgive the title if improper, the language of QM is not my native tongue. :) All scenarios where QM is invoked that I am used to involve parameters that do not commute, but I suppose I've never truly asked myself if QM is necessary to describe scenarios wherein you are not concerned with this...

It is thought that there are approximately 10^80 protons in the observable universe, but there are approximately 10^90 photons in the observable universe. If my googling is correct, there are also approximately 10^90 neutrinos in the observable universe, but their temperature is only 1.9 degrees...

Homework Statement I really do not understand why the expectation value of an observable such as position is <x> = \int\Psi*(x)\Psi Homework Equations If Q is an operator then <Q> = = \int\Psi*(Q)\Psi cn = <f,\Psi> The Attempt at a Solution What I understand this is saying is...

In Dr. Tyson's first episode of Cosmos he said "...there are parts of the Universe that are too far away. There hasn't been enough time in 13.8 billion year history of the Universe for their light to have reached us." Since the remnants of the Big Bang has a redshift over 1000 and large...

Why we define that average value of some observable ##\hat{A}## in state ##\psi## is ##(\psi,\hat{A}\psi)## Why this isnot perhaps ##26(\psi,\hat{A}\psi)##?

Homework Statement Given the following hamiltonian and the observable \widehat{B} find the possible energy levels (a is a real constant). If the state is in it's fundamental state what's the probability of measuring b_{1}, b_{2} and b_{3}? Homework Equations The Attempt at a...

Regarding scale. How big was the region of the universe we now understand to be the observable universe, immediately after early inflation? I'm tyring to understand the scale. I know our sphere of observation is very approximately 45 billion light years or something, I don't know for sure...

We know that observables correspond to hermitian operators on the Hilbert space of physical states of the system. We also know, via Wigner theorem, that for each symmetry there is a linear unitary operator (or anti-linear and anti-unitary). In the case of a continuous symmetry, that is in the...

Quantum mechanics has a well-known procedure for evaluating the expectation value of an observable quantity in a given quantum state. First one must obtain the quantum operator O that is associated with the observable quantity. Then the rule for computing the expectation value is: Apply O to the...

Hello everyone. As I was reading an article on wiki, I stumbled upon this by chance; If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of...

Homework Statement A two-level system is spanned by the orthonormal basis states |a_{1}> and |a_{2}> . The operators representing two particular observable quantities A and B are: \hat{A} = α(|a_{1}> <a_{1}| - |a_{2}> <a_{2}|) and \hat{B} = β(|a_{1}> <a_{2}| + |a_{2}> <a_{1}|) a) The state...

If the universe is a 3 dimensional (plus time) space expanding into something, what is that something? Am I correct in stating that the universe is an infinite eternal space in which the Big Bang happened 13.5 billion years ago, and that the observable cosmos is expanding into this space...

Hello, i'm reading through a book called "Discovering The Universe" and in the chapter about cosmology the familiar idea of the observable universe is discussed. It speaks of the radius of the cosmic particle horizon being ~15 billion light years, which makes perfect sense as light from...

We know that in Schrodinger equation, Ψ is called wave function, which is not observable, while Ψ·Ψ* is the probability, which is observable. can we rewirte the Schrodinger equation to a form without Ψ but only Ψ·Ψ*? because I think, in this way can I figure out all conservations in the...

Hi, I've been wondering about the charge balance in the known universe, factors that might alter it, and the consequences of any small imbalance that might exist. This is a sort of layman's theory. I don't expect it to be right but I'd love to understand how it can be refuted. I am surely...

From Wikipedia: http://en.wikipedia.org/wiki/Googolplex A Planck space has a volume of a Planck length cubed, which is the smallest measurable volume, at approximately 4.222×10^-105 cubic meters = 4.222×10^-99 cubic cm. Thus 2.5 cubic cm contain about a googol Planck spaces. There are only...

I started a new thread from a side discussion in https://www.physicsforums.com/showthread.php?t=681625&page=2, since it seems very off topic, but I still had questions. Is there a requirement for an operator that corresponds to an observable to be part of a complete set, ie. its...

Homework Statement This may be incredibly obvious, but I just need to check. Of course we all know that physical observables must yield real expectation values. What if you tried to calculate, say, <xV(d/dx)>, where x is the position, d/dx is a first derivative, and V is the potential? This...

Hi all. I relish hearing from our great cosmological explainers like Neil DeGrasse Tyson and Brian Greene and watch whenever I find something new on youtube, but one thing that I don't understand and haven't heard anyone specifically address is that, if the part of the universe we are able to...

Why do we define the uncertainty in some physical quantity ##A## as: $$\delta A = < \sqrt{<A^2> - <A>^2} > .$$ I know that it can be derived by computing the variance of ##A##, but what is the physical meaning? Thanks!

Hi Guys. I would like to ask what is the very nature of event in the same moment given a vast distances. Considering Point A (earth)-vast distance-Point B (galaxies). Would be possible that the event we're observing already happened long before it reaches us and may not exist at this very...

Hi, my first post here! Galaxies outside the observable universe (that we can't see their light) can affect us with their gravity? If the answer is no, we can say that gravity information travels at the speed of light. So, in a black hole how gravity information from an object inside...

My question is about both sides of the same coin. First, does a hermitian operator always represent a measurable quantity? Meaning, (or conversely) could you cook up an operator which was hermitian but had no physical significance? Second, are all observables always represented by a...

Hi, Does maybe someone know if there was ever the attempt to count the number of voids in the (observable) universe? I assume that is not the case, but would be very interested in any work that gives some estimation. Many thanks, Martin

The size of the observable universe usually gets put at 93 trillion light years, though some people believe it is infinite, though I'm not sure which camp is in the majority. Those who believe it is 93 trillion light years across are they just assuming that the size of the universe at the first...

are observables only those quantities which commute with system's Hamiltonian ?

Dark Flow: Tugs from Beyond the Observable Universe? Is this true ? If true then what lies Beyond the Observable Universe ?

Here is a chart showing the unobservable universe The green lines indicates the part we can see. My question is that figure that gets thrown around, there are 10^80 particles in the universe, is that for observable or the unobservable universe.

I was watching this lecture and the professor said that before inflation the universe's shape was spherical or curved and inflation blew it up to the point where the observable universe appears flat but the actual unobservable universe is still a curve or spherical. Did I understand correctly...

Here's a puzzle that I have been wondered about. I know the answer, but I do not understand the reasoning for it. Could someone explain it to me in a way that a layman like could understand in an intuitive manner? (The math involved in GR equations is too complex for me to grasp.) General...

In QFT of a real Klein-Gordon-Field, the field operator \phi(x) is an observable. Mathematically, this is the case because it is a sum (over all k) of a and a^\dagger and this yields a Hermitian operator. Physically, I can understand this because this equation would describe, for example, a...

is there any "gravitationally unbound' galactic clusters in observable universe my hunch is that everything in the universe is gravitationally tugged on each other ..we can say moon is connected to andromeda galaxy in a way moon->earth->sun->milkyway->andromeda even the slightest imbalance...

what i want to know is how much area of universe (in light years if possible) is surveyed by WMAP .when you say CMB permeates entire universe based on data from these satellites orbiting Earth how much of universe is actually surveyed by WMAP ? if you survey around where we are now in this...

I think the picture explains it better than I can with words. Is it possible that we can only see a small sphere around our planet (red circle around Earth in the picture), but there could be a 'planet x' on the real edge of the universe that can see light from 14 billion light years away in one...

Hey, How would you compute the probability of measuring a specific value of an observable (resp. range for continuous variables), given a wavefunction with an eigenbasis that is different from the one associated with the observable? An example: Let's consider my wave function is a...

Homework Statement a) Two observables A and B are represented by operators A(hat) and B(hat), which obey the following commutation relation: [A(hat), B(hat)] = iC, where C is the real number. Obtain an expression for the product of the uncertainties ΔAΔB. b) Hadrons, such as protons...

Homework Statement Find the allowed energies for a spin-3/2 particle with the given Hamiltonian: \hat{H}=\frac{\epsilon_0}{\hbar}(\hat{S_x^2}-\hat{S_y^2})-\frac{\epsilon_0}{\hbar}\hat{S_z} The Attempt at a Solution The final matrix I get is: \begin{pmatrix} \frac{3}{2} & 0 &...

Hi all, just like the atomic world is unobservable to our normal optical observation, can it not be that there can be out there some structure too large that we are deluded to believe that there are merely galaxies and all empty beyond.

Assuming a Hubble constant of 74.3 km/sec/Mpc, the critical density is about E-29 gm/cm3. To calculate observable matter based on 5% of this density (the other 95% is dark matter and dark energy), a volume has to be used. But which volume, the one with a 13.7 billion light year radius or the...

Here is a study in distances based on powers of ten of the meter. It gives easy-to-see graphic examples of relative size, from the Plank length out to the Observable Universe. Imho, a great teaching aid. http://www.newgrounds.com/portal/view/589217

According to my book: \frac{d}{dt} \langle Q \rangle = \frac{i}{\hbar} \langle [\hat{H}, \hat{Q}] \rangle + \langle \frac{\partial \hat{Q}}{\partial t} \rangle . No derivation for this is given. How can derive you this?

Please comment on my assumptions and results, this has confused me for some time. Thanks The following is an attempt to reconcile the critical density of the universe with the amount of observable matter as represented by number of stars. Assumptions for critical density: Hubble constant...

In quantum mechanics it seems that the operator for potential energy is U. Therefore, every state is an eigenstate of potential energy and there will never be any uncertainty in potential energy? It seems weird...

In another thread here, https://www.physicsforums.com/showthr...=548148&page=3 , post #36, Peter Donis posted this: "...But we *can* describe a generic spacetime in GR using a curved geometry (*which* curved geometry depends on the specific spacetime), and we *can* describe any given...

In classical mechanics, I can measure the inertial mass of a particle by measuring force and acceleration: m=F/a. In QM, this equation only holds for expectation values <F> and <a>. Does this lead to the fact that inertial mass is not an observable? Is there a deeper underlying principle which...

In an earlier thread--What is space?--a few people indicated that only a part of our universe is observable: Because space is expanding, photons from distant parts of the of the universe haven't had time to get here yet. So that's my next question. Is our entire universe observable? My...

Different "Slices of Now" Observable from Earth? Brian Greene's excellent series continued last week on PBS with "The Illusion of Time" in which he showed with graphics how different "nows" occur depending on the motion of an observer. Using the example of a space alien 10 billion light years...

I have heard it said that galaxies exist which are beyond the observable universe because the expansion causes them to be receding at super-luminal velocity. How can this be? We can see all the way back to the surface of last scattering, when the universe was just dense plasma. The limit to the...