On Friday, November 9, 2018 at 12:26:52 PM UTC, Bruno Marchal wrote: > > > On 8 Nov 2018, at 18:25, [email protected] <javascript:> wrote: > > > > On Thursday, November 8, 2018 at 11:04:20 AM UTC, Bruno Marchal wrote: >> >> >> On 6 Nov 2018, at 12:22, [email protected] wrote: >> >> >> >> On Tuesday, November 6, 2018 at 9:27:31 AM UTC, Bruno Marchal wrote: >>> >>> >>> On 4 Nov 2018, at 22:02, [email protected] wrote: >>> >>> >>> >>> On Sunday, November 4, 2018 at 8:33:10 PM UTC, jessem wrote: >>>> >>>> >>>> >>>> On Wed, Oct 31, 2018 at 7:30 AM Bruno Marchal <[email protected]> wrote: >>>> >>>>> >>>>> On 30 Oct 2018, at 14:21, [email protected] wrote: >>>>> >>>>> >>>>> >>>>> On Tuesday, October 30, 2018 at 8:58:30 AM UTC, Bruno Marchal wrote: >>>>>> >>>>>> >>>>>> On 29 Oct 2018, at 13:55, [email protected] wrote: >>>>>> >>>>>> >>>>>> >>>>>> On Monday, October 29, 2018 at 10:22:02 AM UTC, Bruno Marchal wrote: >>>>>>> >>>>>>> >>>>>>> On 28 Oct 2018, at 13:21, [email protected] wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>> On Sunday, October 28, 2018 at 9:27:56 AM UTC, Bruno Marchal wrote: >>>>>>>> >>>>>>>> >>>>>>>> On 25 Oct 2018, at 17:12, [email protected] wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> On Tuesday, October 23, 2018 at 10:39:11 PM UTC, agrays...@ >>>>>>>> gmail.com wrote: >>>>>>>>> >>>>>>>>> If a system is in a superposition of states, whatever value >>>>>>>>> measured, will be repeated if the same system is repeatedly measured. >>>>>>>>> But >>>>>>>>> what happens if the system is in a mixed state? TIA, AG >>>>>>>>> >>>>>>>> >>>>>>>> If you think about it, whatever value you get on a single trial for >>>>>>>> a mixed state, repeated on the same system, will result in the same >>>>>>>> value >>>>>>>> measured repeatedly. If this is true, how does measurement distinguish >>>>>>>> superposition of states, with mixed states? AG >>>>>>>> >>>>>>>> >>>>>>>> That is not correct. You can distinguish a mixture of particles in >>>>>>>> the up or down states with a set of 1/sqrt(2)(up+down) by measuring >>>>>>>> them >>>>>>>> with the {1/sqrt(2)(up+down), 1/sqrt(2)(up-down}) discriminating >>>>>>>> apparatus. >>>>>>>> With the mixture, half the particles will be defected in one >>>>>>>> direction, >>>>>>>> with the pure state, they will all pass in the same direction. >>>>>>>> Superposition would not have been discovered if that was not the case. >>>>>>>> >>>>>>> >>>>>>> >>>>>>> *And someone will supply the apparatus measuring (up + down), and >>>>>>> (up - down)? No such apparatuses are possible since those states are >>>>>>> inherently contradictory. We can only measure up / down. AG* >>>>>>> >>>>>>> >>>>>>> You can do the experience by yourself using a simple crystal of >>>>>>> calcium (CaCO3, Island Spath), or with polarising glass. Or with >>>>>>> Stern-Gerlach devices and electron spin. Just rotating (90° or 180°) an >>>>>>> app/down apparatus, gives you an (up + down)/(up - down) apparatus. >>>>>>> >>>>>> >>>>>> *I don't understand. With SG one can change the up/down axis by >>>>>> rotation, but that doesn't result in an (up + down), or (up - down) >>>>>> measurement. If that were the case, what is the operator for which those >>>>>> states are eigenstates? Which book by Albert? AG * >>>>>> >>>>>> >>>>>> David Z Albert, Quantum Mechanics and Experience, Harvard University >>>>>> Press, 1992. >>>>>> >>>>>> https://www.amazon.com/Quantum-Mechanics-Experience-David-Albert/dp/0674741137 >>>>>> >>>>>> Another very good books is >>>>>> >>>>>> D’Espagnat B. Conceptual foundations of Quantum mechanics, I see >>>>>> there is a new edition here: >>>>>> >>>>>> https://www.amazon.com/Conceptual-Foundations-Quantum-Mechanics-Advanced/dp/0738201049/ref=sr_1_1?s=books&ie=UTF8&qid=1540889778&sr=1-1&keywords=d%27espagnat+conceptual+foundation+of+quantum+mechanics&dpID=41NcluHD6fL&preST=_SY291_BO1,204,203,200_QL40_&dpSrc=srch >>>>>> >>>>>> It explains very well the difference between mixtures and pure states. >>>>>> >>>>>> Bruno >>>>>> >>>>> >>>>> *Thanks for the references. I think I have a reasonable decent >>>>> understanding of mixed states. Say a system is in a mixed state of phi1 >>>>> and >>>>> phi2 with some probability for each. IIUC, a measurement will always >>>>> result >>>>> in an eigenstate of either phi1 or phi2 (with relative probabilities >>>>> applying). * >>>>> >>>>> >>>>> If the measurement is done with a phi1/phi2 discriminating apparatus. >>>>> Keep in mind that any state can be seen as a superposition of other >>>>> oblique >>>>> or orthogonal states. >>>>> >>>> >>>> I don't know if you're restricting the definition of phi1 and phi2 to >>>> some particular type of eigenstate or not, but in general aren't there >>>> pure >>>> states that are not eigenstates of any physically possible measurement >>>> apparatus, so there is no way to directly measure that a system is in such >>>> a state? >>>> >>> >>> *Yes, such states exist IIUC. That's why I don't understand Bruno's >>> claim that Up + Dn and Up - Dn can be measured with any apparatus, * >>> >>> >>> Not *any*¨apparatus, but a precise one, which in this case is the same >>> apparatus than for up and down, except that it has been rotated. >>> >>> >>> >>> >>> *since they're not eigenstates of the spin operator, or any operator. * >>> >>> >>> This is were you are wrong. That are eigenstates of the spin operator >>> when measured in some direction. >>> >> >> *If what you claim is true, then write down the operator for which up + >> dn (or up - dn) is an eigenstate? AG * >> >> >> >> It is the operator corresponding to the same device, just rotated from >> pi/2, or pi (it is different for spin and photon). When I have more time, I >> might do the calculation, but this is rather elementary quantum mechanics. >> (I am ultra-busy up to the 15 November, sorry). It will have the same shape >> as the one for up and down, in the base up’ and down’, so if you know a bit >> of linear algebra, you should be able to do it by yourself. >> >> Bruno >> > > *You don't have to do any calculation. Just write down the operator which, > you allege, has up + dn or up - dn as an eigenstate. I don't think you can > do it, because IMO it doesn't exist. AG * > > > > If up and down are represented by the column (1 0) and (0 1) the > corresponding observable is given by the diagonal matrix > > 1 0 > 0 -1 > > Then the up’ = 1/sqrt(2) (1 1), and down’ = 1/sqrt(2) (1 -1), > > So the operator, written in the base up down, will be > > 0 1 > 1 0 > > Here the eigenvalue +1 and -1 correspond to up (up’) or down (down’). > > I have no clue why you think that such operator would not exist. >
*Because the measured spin state is Up or Dn along some axis, never anything in between. Up + Dn or Up - Dn is not physically realizable in unprimed basis. AG* All pure state can be seen as a superposition, in the rotated base, and you > can always build an operator having them as eigenvalues. > > Bruno > > > > > > > > > >> >> >> >> >> >> >>> Julian Swinger (and Townsend) showed that the formalism of (discrete, >>> spin, qubit) quantum mechanics is derivable from 4 Stern-Gerlach >>> experiments, using only real numbers, but for a last fifth one, you need >>> the complex amplitudes, and you get the whole core of the formalism. >>> >>> Bruno >>> >>> >>> >>> >>> *Do you understand Bruno's argument in a previous post on this topic? AG >>> * >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Everything List" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To post to this group, send email to [email protected]. >>> Visit this group at https://groups.google.com/group/everything-list. >>> For more options, visit https://groups.google.com/d/optout. >>> >>> >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "Everything List" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> Visit this group at https://groups.google.com/group/everything-list. >> For more options, visit https://groups.google.com/d/optout. >> >> >> > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > To post to this group, send email to [email protected] > <javascript:>. > Visit this group at https://groups.google.com/group/everything-list. > For more options, visit https://groups.google.com/d/optout. > > > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
