Observable Definition and 215 Threads

In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum physics, it is an operator, or gauge, where the property of the quantum state can be determined by some sequence of operations. For example, these operations might involve submitting the system to various electromagnetic fields and eventually reading a value.
Physically meaningful observables must also satisfy transformation laws which relate observations performed by different observers in different frames of reference. These transformation laws are automorphisms of the state space, that is bijective transformations which preserve certain mathematical properties of the space in question.

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  1. V

    Expectation value of an operator (not its corresponding observable value)

    1. Problem statement This isn't a homework question itself, but is related to one. More specifically, I'm computing the time-derivative of \langle x \rangle using the correspondence principle. One side simplifies to \left\langle \frac{\hat{p}}{m} \right\rangle, but what is the physical meaning...
  2. B

    Capturing Light with a Theoretical Mirror Sphere: Observable? Possible?

    What would happen if: I had a sphere produced from a theoretical perfectly reflective one-way-mirror(reflective on the inside) and a shone a light into it? Would the light accumulate inside the sphere for a noticeable period of time? Would it be observable? Could light be captured this way...
  3. B

    Calculating the Volume of the Observable Universe: A Scientific Approach

    This question seems simple but I cannot recall the formula to solve it. I hope someone can help me: If the age of the universe is 1.4E10 years, estimate the volume of the observable universe?:confused: Urgent help required.
  4. H

    Could Gravitational Lensing Confirm the Existence of Dark Matter?

    Hi. Couldn't dark matter be confirmed with the gravitational lense effect? Shouldn't we see a difference in the lensing effect that is not accountable to the traditionally considered visual mass?
  5. Loren Booda

    The cross section of a photon traversing the observable universe

    What is the cross section of a photon traversing the observable universe? That of a neutrino? Dark matter in general?
  6. L

    Observable and interaction

    Let us consider the http://en.wikipedia.org/wiki/Stern-Gerlach_experiment" experiment. The inhomogenuous magnetic field produces a drift force that can separate particles according to their spin and offer a possibility for a "measurement". This (very) small magnetic interaction can be...
  7. C

    Observable Universe vs. HyperUniverse

    I've noticed recently that many posts are veering towards the question of just how much of our total Universe can we observe. So I thought a new thread could address this question. 1) What is the radius of the observable Universe? 2) What are our best guesses as to the radius of the...
  8. Chronos

    Evolution of the Observable Universe

    Note from SpaceTiger: This discussion originated in the "what existed before the big bang?" thread. Well put marcus. I think we are on the same page [albeit you are more eloquent] - and I tend toward being brutally blunt. We reside in an OBSERVABLE universe. And, by definition, it includes...
  9. S

    Randall-Sundrum; Observable particles at LHC?

    The model that Lisa Randall and Raman Sundrum propose consists of two four dimensional (Minkowskian) branes, bounding a five dimensional bulk between them. In their initial model the branes are very close together; this is not a "large extra dimensions" model. The motivation of the model is...
  10. S

    What Is Beyond The Observable Universe?

    Is it just black space extending forever? Or perhaps black space for a finite distance until another universe? I find it hard to believe our universe is just the only universe. I don't see how it wouldn't extend for eternity instead. What is so special about our universe and the space we are...
  11. M

    Poynting's vector - Observable - Force?

    Certainly an horrible way of thinking and that's why I should like to get a fundamental critic about this: As I can read in some threads, the Poynting’s vector is of a great importance (e.g.: to know if a radiation is present or not) in a lot of works and theories; in mine too. One of the...
  12. M

    Is Time an Observable in Quantum Mechanics?

    In the standard formulation of QM, time is a scalar parameter. I have seen time being treated as the 0th dimension of spacetime in a covariant Dirac equation, but is there any way of having time as an observable (i.e. associated with a Hermitian operator etc) which has a spectrum and so on?
  13. S

    The mind's 'projection' & the position observable:

    The Law of Projection: "No matter where a particular sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the receptor." ('Review of Medical Physiology', 6th Edition, W.F. Ganong, p. 63) OK, the term 'projection' might...
  14. marcus

    What is the current radius of the observable universe?

    How far away are the most distance objects we can see? What is currently the volume of space that we can study with the light that is reaching us? You might want to look at this brief Cosmology FAQ paragraph: http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN *******footnote...
  15. Loren Booda

    Is all physics logically expressible and observable?

    Say I observe a physical phenomenon; does that mean I can necessarily mathematicize it? Does possible observation reach only a fraction of physical reality? Will mathematics ever be able to explain a unification between general relativity and quantum mechanics? If such a TOE is ever...
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