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Posts posted by geordief

  1. 33 minutes ago, studiot said:

    Yes of course there is, but direction of travel has quantum implications only sometimes.


    By this I mean that there is a quantum solution for a particle freely travelling in space in any direction the solution is the same unless of course the travel medium  is not homogeneous and istropic.

    Normally we do not employ this solution because the quantum levels are so closely spaced that it is effectively a continuum.

    The direction of travel becomes important in the band theory of metals and semiconductors.


    As regards your question in time, Genady is correct that some solutions are time independent, such as the particle in the box or the potential well models of an electron in an atom or a crystal


    33 minutes ago, studiot said:
    35 minutes ago, studiot said:

    But I don't see this has any relevence to travelling

    Thanks.(I hope nobody took the use of the word literally ,as in time travel.I just used it as a turn of phrase)

    56 minutes ago, Genady said:

    I just try to clarify the question because I don't know what they were referring to in the thing you've read. 

    Let's consider an example, a particle in a Hamiltonian eigenstate ψ(x) with energy E . It evolves in time as eiEtψ(x) . The probability density for it to be in position x is ψ¯(x)ψ(x) . This outcome does not depend on time and thus doesn't change if the sign of time is flipped.

    Thanks.It doesn't  seem that extraordinary to my untrained ears. 

    Would probability  equations in QM  normally have a time component?(it comes as a surprise to me that they would but I only aspire to a layman status )

  2. 1 hour ago, swansont said:

    Frames of reference are important in relativity, where they are tied to a velocity, not necessarily a physical point. All points are at rest with respect to each other in that frame. 

    To rephrase your statement, can you have a valid frame that does not apply to some potential scenario? I don’t know. I can’t think of one at the moment.


    Can a scenario involving only  quantum objects be modeled using spacetime diagrams and their frames of reference?

    Is it ever done? Would there be a need?

    I understand that special relativity is used in such scenarios.


  3. 14 minutes ago, swansont said:

    I can’t parse “Does a frame of reference have to be applicable to a potential physical  scenario to be physically valid?”

    There’s an adage in particle physics that goes “that which is not forbidden is mandatory” so I can’t imagine a frame of reference that’s valid that would not somehow correspond to a physical scenario, but I don’t know if one exists, or what you might have in mind.

    Well a frame of reference can have its origin tied to a  physical point in space (even though ,I think it applies generally and not to one in particular)

    If ,though the frame of reference with a spatiotemporal  origin   coinciding with a quantum object  is populated with other physical quantum objects then it seems to me that it is not simple to map their positions  onto the frame of reference

    I did have a look at @MigL video and perhaps I begin to see how some circles can be squared. 


    So perhaps frames of reference  can apply to physical scenarios where objects' position and momentum are not separately defined?

    Might one map objects' combined states  into a Minkowski like frame of reference chart? (Eg position  and momentum combined)


    As to the  parsing of my phrase

    ,"Does a frame of reference have to be applicable to a potential physical  scenario to be  physically valid?” ....

    I had thought first of writing

    "Does a frame of reference have to be applicable to a potential physical  scenario to be  valid?” 


    Maybe the second "physically" made it less comprehensible? 

  4. 11 minutes ago, swansont said:

    Not exactly sure what you mean here. 

    Physicists have no problem approximating things, so something can be treated as being at rest despite all of the caveats we’ve mentioned

    Ok.I am a bit of a literalist myself.If someone greets me in the street  with a "How are you?" my reflex is to wonder how I am and to communicate my state of being with the person :(

  5. 2 minutes ago, swansont said:

    You don’t know what the momentum is, so saying it’s zero isn’t strictly possible, though this might be unimportant for certain problems.

    There could never be two physical objects at rest wrt each other could there?

    Does a frame of reference have to be applicable to a potential physical  scenario to be physically valid?

  6. The thing I find confusing is that on the one hand it must be clear that Israel has such a back history of truly despicable behaviour against the Jews down the years culminating in the most odious collective stain on our common civilisation  that it is incredible to imagine that it can be forced out of its one  last refuge. 

    On the other hand its  behaviour has more or less united (if they dare speak it) all the countries in the world against them.

    It seems like the irresistible object meeting the immovable force.

    Noone has any idea how this car crash develops

    I hope Netanjahu gets his comeuppance but it would be more than naive to suppose that his removal (and being held to account) will move anything forward.

    It feels like this drama has its own life force  and is pulling us all along with it.

    Against that Israel has only been in existence for 75 years -a blink in the eye of nothing at at and so there is all the time in the world for things to evolve (at their own pace) 

  7. 9 minutes ago, sethoflagos said:

    The cat's clear overreaction could well suggest some flight response to a predator. Perhaps the sound of the engine stirs some ancient memory of a leopard's roar. I'll have to investigate their response to a car being started.

    Maybe put on Stray Cat Blues on the car stereo as well:-)

  8. 46 minutes ago, sethoflagos said:

    I was outside having a smoke recently and whiled away the time watching a cat cautiously cross the pavement a couple of metres ahead of me seemingly intent on crossing the road.

    The view down the road was obstructed by a parked van but from my vantage point I could see a pedestrian approaching on the far side pavement.

    The cat  froze in its tracks. At first I thought that this was due to seeing the pedestrian. However, a car suddenly came into view and the cat bolted to hide underneath another vehicle parked off the road beside me.

    After it had passed, the cat emerged once more and approached the road only to see a vehicle coming from the opposite direction whereupon it bolted yet again to its place of safe refuge.

    Its third attempt at crossing the road passed uneventfully.

    The cat clearly was a little cautious of nearby humans. It seemed to have no fear at all of stationary vehicles, indeed seeing them as places of safety, Yet it fled in absolute terror at the sight of a moving vehicle. 

    In my experience, cats tend not to survive any degree of impact with the latter, so I was left wondering what the learning process might be. 

    Motion  itself is something that would occupy  the attention of a cat.

    Does it see like a big dangerous  animal?

    11 minutes ago, studiot said:

    Yes, but I think you will find that the original statement referred to changes in the observed system, not the observer


    This original statement?

    6 hours ago, Luc Turpin said:

    my contention here is that the act of measurement or observing changes something in the world, or is this again too far fetched

    Perhaps you are right.

  9. 6 hours ago, Markus Hanke said:

    While this is of course true, I think it’s very important to remember that such a transformation changes the physical meaning of the time coordinate - it will no longer correspond to a clock co-moving with the cosmological fluid.

    Which is which?

    The space time expansion is the one where the clock does co-move with the cosmological fluid?

    But if we are talking only if space expanding  this is not so?

    First time I have heard mention of "cosmological fluid"...


  10. Since the Big Bang I understand that the distances between galaxies and ,generally regions not bound together gravitationally have been  increasing in an accelerating way. 

    I have seen this process described as either space expanding or space-time expanding.

    Which is the more correct way of describing it?

    To my mind it should be the former.

    I see space time as a mathematical model (and don't see how a model can expand)

    I see space as the distances between objects and can understand how these measurements can be continuously  increasing.

    On the other hand  I think I can also see that the space time intervals between the galaxies might also be increasing.....

  11. 1 hour ago, Genady said:

    Yes, if a particle has a definite momentum, measuring its momentum does not change its state. The same holds for its position

    Coming back to you earlier post (hopefully with a better understanding)

    So the particle is in a superposition of eigenstates  and you measure its momentum....

    Are you saying that

    none of the superimposed eigenstates are changed  by the measurement, 

    or that jst the momentum eigenstate is unchanged 

    or that all the other eigenstates (not including the momentum) are unchanged?

  12. 11 minutes ago, swansont said:

    It would have to be in a momentum eigenstate

    What might it be if it was not in a momentum eigenstate?

    Any one of the other eigenstates it could be in?

    Can it be in more than one eigenstate at a time ?

  13. 5 minutes ago, Genady said:

    (We can call it, "particle".)

    Yes, if a particle has a definite momentum, measuring its momentum does not change its state. The same holds for its position.

    So a particle whose momentum  is measured does not undergo a change in momentum as a consequence of the measurement?

    Or is it rather that a particle whose momentum  is measured does not undergo a change in position as a consequence of the measurement.?

    I feel it should be the latter as the position is unknown both before and after the measurement.

    And ,if that is right ,the same would apply to a measurement of the position of the particle.That should not change its momentum ,for the same reasons.

  14. 28 minutes ago, Genady said:

    Sure. If the state of a system is an eigenstate of the observable in question, then it does not change. 

    Would that apply to the position/momentum of a quantum object?

    (I hope "quantum object "is a meaningful description  of  something)

  15. 37 minutes ago, Genady said:

    A measurement usually changes the state. Non-commuting implies that the measurement of one observable affects the measurement of the other. The two cannot be both measured independently on the same state.

    You say "usually".

    Does that mean or imply that some measurements do not change the state of a system?

  16. 20 minutes ago, KJW said:

    Just in case you don't, an operator L() is linear if and only if it satisfies:


    Linearity is essential to QM because quantum superposition demands it.



    Are there  other  features of QM that could be described as essential  in the way that it differs from Classical Physics?

  17. 4 hours ago, AIkonoklazt said:

    Not sure if any group of people would "support" those who directly lords over them through a sort of "terror governance..."



    Clearly some support if not enough to  govern democratically. 

    Ally that to fear of coercion and ostracization  then  they have  for now all the "support" they need.

    These are the other polls I alluded to


    I expect many here will already be aware of them


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