Having some issue with notation in a Hilbert spaceInterference, 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
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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
$begingroup$
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?
quantum-mechanics hilbert-space notation
$endgroup$
add a comment |
$begingroup$
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?
quantum-mechanics hilbert-space notation
$endgroup$
add a comment |
$begingroup$
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?
quantum-mechanics hilbert-space notation
$endgroup$
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
quantum-mechanics hilbert-space notation
edited 7 hours ago
PM 2Ring
3,72721227
3,72721227
asked 8 hours ago
kaykay
162
162
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
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 $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
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 $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$
$endgroup$
add a comment |
$begingroup$
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 $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$
$endgroup$
add a comment |
$begingroup$
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 $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$
$endgroup$
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 $$langle x | xrangle = beginbmatrix1&2&3endbmatrix beginbmatrix1\2\3endbmatrix = 1 + 4 + 9 = 14.$$
answered 8 hours ago
CR DrostCR Drost
24.2k22169
24.2k22169
add a comment |
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
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.
edited 7 hours ago
answered 8 hours ago
gabegabe
545315
545315
add a comment |
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
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.
answered 7 hours ago
ZeroTheHeroZeroTheHero
21.7k53465
21.7k53465
add a comment |
add a comment |
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