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Having some issue with notation in a Hilbert space


Interference, photon's phase, and the Hilbert spaceDifficulties with bra-ket notationSome Dirac notation explanationsDealing with dirac notation with regards to different basis'Equivalence classes in a Hilbert spaceAre these two spin states the same?Equivalence between wavefunction and Dirac ket notationPartial inner product on Hilbert spaceAmbiguity with Dirac NotationSpin states in hilbert space













3












$begingroup$


What is the difference between these two?



$langle x|xrangle$ and $|xranglelangle x|$



Are they the same? If they're the same, why are they used in these two different forms?










share|cite|improve this question











$endgroup$
















    3












    $begingroup$


    What is the difference between these two?



    $langle x|xrangle$ and $|xranglelangle x|$



    Are they the same? If they're the same, why are they used in these two different forms?










    share|cite|improve this question











    $endgroup$














      3












      3








      3


      1



      $begingroup$


      What is the difference between these two?



      $langle x|xrangle$ and $|xranglelangle x|$



      Are they the same? If they're the same, why are they used in these two different forms?










      share|cite|improve this question











      $endgroup$




      What is the difference between these two?



      $langle x|xrangle$ and $|xranglelangle x|$



      Are they the same? If they're the same, why are they used in these two different forms?







      quantum-mechanics hilbert-space notation






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited 7 hours ago









      PM 2Ring

      3,72721227




      3,72721227










      asked 8 hours ago









      kaykay

      162




      162




















          3 Answers
          3






          active

          oldest

          votes


















          9












          $begingroup$

          They are not the same. One is the outer product and one is the inner product.



          In the finite-dimensional real-number case for example, if $$|xrangle = beginbmatrix1\2\3endbmatrix,$$ then $$|xranglelangle x| = beginbmatrix1\2\3endbmatrix beginbmatrix1&2&3endbmatrix = beginbmatrix1&2&3\2&4&6\3&6&9endbmatrix,$$ while $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$






          share|cite|improve this answer









          $endgroup$




















            3












            $begingroup$

            In Bra-Ket notation $| x rangle$ is a vector and $langle x |$ is a covector. Formally, a covector is a map that goes from $V rightarrow mathbbR$, i.e. it “eats a vector” and gives you a number. When we write
            $$
            langle x | x rangle
            $$

            we are saying that the covector $langle x |$ is acting on the vector $|xrangle$, so $langle x | xrangle$ is a real number.



            On the other hand,
            $$
            | x rangle langle x|
            $$

            isn’t acting on anything. If we toss some vector at it $|arangle$, we would have
            $$
            |xrangle langle x | a rangle
            $$

            which is just a real number times $|x rangle$. We have given the above a vector and we got a vector back out so it is a map between vectors. To summarize then, $|xrangle langle x|$ and $langle x | x rangle $ represent different types of objects: the first is a map from $V rightarrow V$, wheras the second is an element $b in mathbbR$.



            To be concrete, let
            $$
            |xrangle =
            beginpmatrix
            1\
            0
            endpmatrix
            $$

            . Then we would write
            $$
            langle x | = beginpmatrix 1&0endpmatrix
            $$

            which is the Hermitian conjugate of $|xrangle$, in this case just the transpose. So
            $$
            langle x | x rangle = beginpmatrix 1&0 endpmatrix
            beginpmatrix
            1\
            0
            endpmatrix = 1
            $$

            And
            $$
            | x rangle langle x | =
            beginpmatrix
            1\
            0
            endpmatrix beginpmatrix1&0endpmatrix =
            beginpmatrix
            1&0\
            0&0
            endpmatrix
            $$

            A number and a map, as expected.






            share|cite|improve this answer











            $endgroup$




















              0












              $begingroup$

              In general $langle Avert Brangle$ is a scalar product whereas $vert Aranglelangle Bvert$ is an operator. To this this last suppose
              $vert psirangle$ is an arbitrary vector in your space. Then
              $$
              vert Aranglelangle Bvert psirangle
              $$

              is a vector proportional to $vert Arangle$ since $langle Bvert psirangle in mathbbC$ is a (complex) number, so that $vert Aranglelangle Bvert$ maps a vector to another vector.






              share|cite|improve this answer









              $endgroup$













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                3 Answers
                3






                active

                oldest

                votes








                3 Answers
                3






                active

                oldest

                votes









                active

                oldest

                votes






                active

                oldest

                votes









                9












                $begingroup$

                They are not the same. One is the outer product and one is the inner product.



                In the finite-dimensional real-number case for example, if $$|xrangle = beginbmatrix1\2\3endbmatrix,$$ then $$|xranglelangle x| = beginbmatrix1\2\3endbmatrix beginbmatrix1&2&3endbmatrix = beginbmatrix1&2&3\2&4&6\3&6&9endbmatrix,$$ while $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$






                share|cite|improve this answer









                $endgroup$

















                  9












                  $begingroup$

                  They are not the same. One is the outer product and one is the inner product.



                  In the finite-dimensional real-number case for example, if $$|xrangle = beginbmatrix1\2\3endbmatrix,$$ then $$|xranglelangle x| = beginbmatrix1\2\3endbmatrix beginbmatrix1&2&3endbmatrix = beginbmatrix1&2&3\2&4&6\3&6&9endbmatrix,$$ while $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$






                  share|cite|improve this answer









                  $endgroup$















                    9












                    9








                    9





                    $begingroup$

                    They are not the same. One is the outer product and one is the inner product.



                    In the finite-dimensional real-number case for example, if $$|xrangle = beginbmatrix1\2\3endbmatrix,$$ then $$|xranglelangle x| = beginbmatrix1\2\3endbmatrix beginbmatrix1&2&3endbmatrix = beginbmatrix1&2&3\2&4&6\3&6&9endbmatrix,$$ while $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$






                    share|cite|improve this answer









                    $endgroup$



                    They are not the same. One is the outer product and one is the inner product.



                    In the finite-dimensional real-number case for example, if $$|xrangle = beginbmatrix1\2\3endbmatrix,$$ then $$|xranglelangle x| = beginbmatrix1\2\3endbmatrix beginbmatrix1&2&3endbmatrix = beginbmatrix1&2&3\2&4&6\3&6&9endbmatrix,$$ while $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered 8 hours ago









                    CR DrostCR Drost

                    24.2k22169




                    24.2k22169





















                        3












                        $begingroup$

                        In Bra-Ket notation $| x rangle$ is a vector and $langle x |$ is a covector. Formally, a covector is a map that goes from $V rightarrow mathbbR$, i.e. it “eats a vector” and gives you a number. When we write
                        $$
                        langle x | x rangle
                        $$

                        we are saying that the covector $langle x |$ is acting on the vector $|xrangle$, so $langle x | xrangle$ is a real number.



                        On the other hand,
                        $$
                        | x rangle langle x|
                        $$

                        isn’t acting on anything. If we toss some vector at it $|arangle$, we would have
                        $$
                        |xrangle langle x | a rangle
                        $$

                        which is just a real number times $|x rangle$. We have given the above a vector and we got a vector back out so it is a map between vectors. To summarize then, $|xrangle langle x|$ and $langle x | x rangle $ represent different types of objects: the first is a map from $V rightarrow V$, wheras the second is an element $b in mathbbR$.



                        To be concrete, let
                        $$
                        |xrangle =
                        beginpmatrix
                        1\
                        0
                        endpmatrix
                        $$

                        . Then we would write
                        $$
                        langle x | = beginpmatrix 1&0endpmatrix
                        $$

                        which is the Hermitian conjugate of $|xrangle$, in this case just the transpose. So
                        $$
                        langle x | x rangle = beginpmatrix 1&0 endpmatrix
                        beginpmatrix
                        1\
                        0
                        endpmatrix = 1
                        $$

                        And
                        $$
                        | x rangle langle x | =
                        beginpmatrix
                        1\
                        0
                        endpmatrix beginpmatrix1&0endpmatrix =
                        beginpmatrix
                        1&0\
                        0&0
                        endpmatrix
                        $$

                        A number and a map, as expected.






                        share|cite|improve this answer











                        $endgroup$

















                          3












                          $begingroup$

                          In Bra-Ket notation $| x rangle$ is a vector and $langle x |$ is a covector. Formally, a covector is a map that goes from $V rightarrow mathbbR$, i.e. it “eats a vector” and gives you a number. When we write
                          $$
                          langle x | x rangle
                          $$

                          we are saying that the covector $langle x |$ is acting on the vector $|xrangle$, so $langle x | xrangle$ is a real number.



                          On the other hand,
                          $$
                          | x rangle langle x|
                          $$

                          isn’t acting on anything. If we toss some vector at it $|arangle$, we would have
                          $$
                          |xrangle langle x | a rangle
                          $$

                          which is just a real number times $|x rangle$. We have given the above a vector and we got a vector back out so it is a map between vectors. To summarize then, $|xrangle langle x|$ and $langle x | x rangle $ represent different types of objects: the first is a map from $V rightarrow V$, wheras the second is an element $b in mathbbR$.



                          To be concrete, let
                          $$
                          |xrangle =
                          beginpmatrix
                          1\
                          0
                          endpmatrix
                          $$

                          . Then we would write
                          $$
                          langle x | = beginpmatrix 1&0endpmatrix
                          $$

                          which is the Hermitian conjugate of $|xrangle$, in this case just the transpose. So
                          $$
                          langle x | x rangle = beginpmatrix 1&0 endpmatrix
                          beginpmatrix
                          1\
                          0
                          endpmatrix = 1
                          $$

                          And
                          $$
                          | x rangle langle x | =
                          beginpmatrix
                          1\
                          0
                          endpmatrix beginpmatrix1&0endpmatrix =
                          beginpmatrix
                          1&0\
                          0&0
                          endpmatrix
                          $$

                          A number and a map, as expected.






                          share|cite|improve this answer











                          $endgroup$















                            3












                            3








                            3





                            $begingroup$

                            In Bra-Ket notation $| x rangle$ is a vector and $langle x |$ is a covector. Formally, a covector is a map that goes from $V rightarrow mathbbR$, i.e. it “eats a vector” and gives you a number. When we write
                            $$
                            langle x | x rangle
                            $$

                            we are saying that the covector $langle x |$ is acting on the vector $|xrangle$, so $langle x | xrangle$ is a real number.



                            On the other hand,
                            $$
                            | x rangle langle x|
                            $$

                            isn’t acting on anything. If we toss some vector at it $|arangle$, we would have
                            $$
                            |xrangle langle x | a rangle
                            $$

                            which is just a real number times $|x rangle$. We have given the above a vector and we got a vector back out so it is a map between vectors. To summarize then, $|xrangle langle x|$ and $langle x | x rangle $ represent different types of objects: the first is a map from $V rightarrow V$, wheras the second is an element $b in mathbbR$.



                            To be concrete, let
                            $$
                            |xrangle =
                            beginpmatrix
                            1\
                            0
                            endpmatrix
                            $$

                            . Then we would write
                            $$
                            langle x | = beginpmatrix 1&0endpmatrix
                            $$

                            which is the Hermitian conjugate of $|xrangle$, in this case just the transpose. So
                            $$
                            langle x | x rangle = beginpmatrix 1&0 endpmatrix
                            beginpmatrix
                            1\
                            0
                            endpmatrix = 1
                            $$

                            And
                            $$
                            | x rangle langle x | =
                            beginpmatrix
                            1\
                            0
                            endpmatrix beginpmatrix1&0endpmatrix =
                            beginpmatrix
                            1&0\
                            0&0
                            endpmatrix
                            $$

                            A number and a map, as expected.






                            share|cite|improve this answer











                            $endgroup$



                            In Bra-Ket notation $| x rangle$ is a vector and $langle x |$ is a covector. Formally, a covector is a map that goes from $V rightarrow mathbbR$, i.e. it “eats a vector” and gives you a number. When we write
                            $$
                            langle x | x rangle
                            $$

                            we are saying that the covector $langle x |$ is acting on the vector $|xrangle$, so $langle x | xrangle$ is a real number.



                            On the other hand,
                            $$
                            | x rangle langle x|
                            $$

                            isn’t acting on anything. If we toss some vector at it $|arangle$, we would have
                            $$
                            |xrangle langle x | a rangle
                            $$

                            which is just a real number times $|x rangle$. We have given the above a vector and we got a vector back out so it is a map between vectors. To summarize then, $|xrangle langle x|$ and $langle x | x rangle $ represent different types of objects: the first is a map from $V rightarrow V$, wheras the second is an element $b in mathbbR$.



                            To be concrete, let
                            $$
                            |xrangle =
                            beginpmatrix
                            1\
                            0
                            endpmatrix
                            $$

                            . Then we would write
                            $$
                            langle x | = beginpmatrix 1&0endpmatrix
                            $$

                            which is the Hermitian conjugate of $|xrangle$, in this case just the transpose. So
                            $$
                            langle x | x rangle = beginpmatrix 1&0 endpmatrix
                            beginpmatrix
                            1\
                            0
                            endpmatrix = 1
                            $$

                            And
                            $$
                            | x rangle langle x | =
                            beginpmatrix
                            1\
                            0
                            endpmatrix beginpmatrix1&0endpmatrix =
                            beginpmatrix
                            1&0\
                            0&0
                            endpmatrix
                            $$

                            A number and a map, as expected.







                            share|cite|improve this answer














                            share|cite|improve this answer



                            share|cite|improve this answer








                            edited 7 hours ago

























                            answered 8 hours ago









                            gabegabe

                            545315




                            545315





















                                0












                                $begingroup$

                                In general $langle Avert Brangle$ is a scalar product whereas $vert Aranglelangle Bvert$ is an operator. To this this last suppose
                                $vert psirangle$ is an arbitrary vector in your space. Then
                                $$
                                vert Aranglelangle Bvert psirangle
                                $$

                                is a vector proportional to $vert Arangle$ since $langle Bvert psirangle in mathbbC$ is a (complex) number, so that $vert Aranglelangle Bvert$ maps a vector to another vector.






                                share|cite|improve this answer









                                $endgroup$

















                                  0












                                  $begingroup$

                                  In general $langle Avert Brangle$ is a scalar product whereas $vert Aranglelangle Bvert$ is an operator. To this this last suppose
                                  $vert psirangle$ is an arbitrary vector in your space. Then
                                  $$
                                  vert Aranglelangle Bvert psirangle
                                  $$

                                  is a vector proportional to $vert Arangle$ since $langle Bvert psirangle in mathbbC$ is a (complex) number, so that $vert Aranglelangle Bvert$ maps a vector to another vector.






                                  share|cite|improve this answer









                                  $endgroup$















                                    0












                                    0








                                    0





                                    $begingroup$

                                    In general $langle Avert Brangle$ is a scalar product whereas $vert Aranglelangle Bvert$ is an operator. To this this last suppose
                                    $vert psirangle$ is an arbitrary vector in your space. Then
                                    $$
                                    vert Aranglelangle Bvert psirangle
                                    $$

                                    is a vector proportional to $vert Arangle$ since $langle Bvert psirangle in mathbbC$ is a (complex) number, so that $vert Aranglelangle Bvert$ maps a vector to another vector.






                                    share|cite|improve this answer









                                    $endgroup$



                                    In general $langle Avert Brangle$ is a scalar product whereas $vert Aranglelangle Bvert$ is an operator. To this this last suppose
                                    $vert psirangle$ is an arbitrary vector in your space. Then
                                    $$
                                    vert Aranglelangle Bvert psirangle
                                    $$

                                    is a vector proportional to $vert Arangle$ since $langle Bvert psirangle in mathbbC$ is a (complex) number, so that $vert Aranglelangle Bvert$ maps a vector to another vector.







                                    share|cite|improve this answer












                                    share|cite|improve this answer



                                    share|cite|improve this answer










                                    answered 7 hours ago









                                    ZeroTheHeroZeroTheHero

                                    21.7k53465




                                    21.7k53465



























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