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INTRODUCTION
I have a google doc with the 17 equations that changed the world. See https://docs.google.com/document/d/1yi9X9_Zbrnpr3yKu3YsamTTCYyC6dqKEwZniZKKh3Y/edit#heading=h.ks5xj9eqv6la
I start this page with a simple equation with Walter Lewin at MIT May 16 2011 about the frequence of a pendulum.
INDEX
 introduction
 Letters in math and science (separate page)
 Partial derivative
 Chain rule
 Derivation with Leibnitz notation
 Newtons second law of rotation
 Trigonometry
 The period of the pendulum
 Maxwell equations
 Laplace equation
 L functions
 Euler’s Identity
 Eulers formula
 The Schrödinger equation derivation
 The wave function
 Einstein field equation (EFE) – presentation (separate page)
 EFE metric tensors derivation.
 Introduction to Information Theory
 String theory
 Conclusion
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Partial derivative
The term “Partial derivative” has confused me with its new symbol ∂(curly d).
Let say you have a function that depends on e.g. 2 variables or is a composite function h(x) like h(x) = g(f(x)) with a inner function f(x)(compare to a nested function in programming). If you want derivate this function, then you have to make 2 partial derivations with the so called chain rule. Every partial derivation is a partial derivative that is denoted with a curly d that has the character ∂ (Unicode: U+2202). It is a stylized cursive/curly d.
A example is
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Chain rule
To write a equation for a 2 frame system, we need the chain rule that tells how to derivate a external function with a inner function.
In calculus, the chain rule is a formula to compute the derivative of a composite function. That is, if f and g are differentiable functions, then the chain rule expresses the derivative of their compositef ∘ g — the function which maps x to — in terms of the derivatives of f and g and the product of functions as follows:
I found 3 applications of this rule: from NancyPi Youtube
1 example from Khan
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Derivation with Leibnitz notation
Derviation with the chain rule with Leibnitz notation is used in the derivation of the Metric tensor in EFE. I found a good teacher in the video below.
For a function
here isf(x) with a inner function g(x), one can write the chain rule in Lagrange’s notation, as follows:
The chain rule may also be rewritten in Leibniz’s notation in the following way. If a variable Φ depends on the variable x, which itself depends on the variable y (i.e., y and z are dependent variables), thenΦ, via the intermediate variable of x, depends on y as well. In which case, the chain rule states that
(Source: Wiki )
Here is the chain rule made using Leibnitz notation, explained:
I took this screenshot that describes the method used in the video. Now I can proceed with DrPhysics metric tensor derivation. 🙂
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Newtons second law of rotation
Khan Academy explains it well in this video
The rotational version of Newton’s law
is
where denotes the angular acceleration. As in the previous section, is torque (tangential force times a moment arm ), and is the mass moment of inertia. Thus, the net applied torque equals the time derivative of angular momentum , just as force equals the timederivative of linear momentum Read more at pccrma.stanford
Trigonometry
Early study of triangles can be traced to the 2nd millennium BC, in Egyptian mathematics (Rhind Mathematical Papyrus) and Babylonian mathematics. Systematic study of trigonometric functions began in Hellenistic mathematics, reaching India as part of Hellenistic astronomy.^{[1]} In Indian astronomy, the study of trigonometric functions flourished in the Gupta period, especially due to Aryabhata (sixth century CE), who discovered the sine function. During the Middle Ages, the study of trigonometry continued in Islamic mathematics, by mathematicians such as AlKhwarizmi and Abu alWafa. It became an independent discipline in the Islamic world, where all six trigonometric functions were known. (source: Wiki )
The sinus(x) functions can be calculated by a a computer program with the use of factorialss in the following formula:
sin(x) = x – x^3 / 3! + x^5 / 5! – x^7 / 7! + x^9 / 9!
Formula taken from https://www.mathworks.com where a program algoritm is discussed. I wonder who developed this solution.
it is called the Taylor series. Taylor’s series are named after Brook Taylor who introduced them in 1715.
For sin(x) it is as follows.
( Image source: Wiki )
For cos(x) it is as follows:
( Image source: Wiki )
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L functions
Studying anf trying to understand Riemann’s hypothesis, I came up looking at the L functions.
Lfunctions were introduced by Dirichlet in the early 19th century “to study prime numbers in arithmetic progressions. Twenty years later Riemann took a major step forward by demonstrating that how the study of these Lfunctions as functions of a complex variable, could hold the key to the deep mysteries about the distribution of prime numbers. In the same paper Riemann introduced his famous hypothesis which is now considered the ‘holy grail’ of mathematics.” (Read more at https://www.youtube.com/watch?v=A7Q3mLTq8M4 )
“In mathematics, an Lfunction is a meromorphic function on the complex plane, associated to one out of several categories of mathematical objects. An Lseries is a Dirichlet series, usually convergent on a halfplane, that may give rise to an Lfunction via analytic continuation. The Riemann zeta function is an example of an Lfunction, and one important result involving Lfunctions is the Riemann hypothesis and its generalization.”
In mathematics, a Dirichlet series is any series of the form
 where s is complex, and is a complex sequence. It is a special case of general Dirichlet series.
 In mathematics, a Dirichlet Lseries is a function of the form
Here χ is a Dirichlet character (In number theory, Dirichlet characters are certain arithmetic functions which arise from completely multiplicative characters on the units of . )
and s a complex variable with real part greater than 1. By analytic continuation, this function can be extended to a meromorphic function on the whole complex plane, and is then called a Dirichlet Lfunction and also denoted L(s, χ).
( Wiki )
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The Euler´s identity,
that the physicist Richard Feynman called “our jewl”, is derived from the Taylor series of sin (x) and cos(x) .
( Image source: www.livescience.com )
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The period of the pendulum
Walter Lewin demonstrates in this video , the validity of the pendulum Equation. Read more at www.acs.psu.edu/
derivation as described by www.acs.psu.edu
A simple pendulum consists of a ball (pointmass) m hanging from a (massless) string of length L and fixed at a pivot point P. When displaced to an initial angle and released, the pendulum will swing back and forth with periodic motion. By applying Newton’s secont law for rotational systems, the equation of motion for the pendulum may be obtained .
Equation with rearrangedment belows
If the amplitude of angular displacement is small enough, so the small angle approximation ($\sin\theta\approx\theta$) holds true, then the equation of motion reduces to the equation of simple harmonic motion
The simple harmonic solution is
where is the initial angular displacement, and the natural frequency of the motion. The period of this system (time for one oscillation) is
–
–
–
–
T=la
=
mg sin 0 L=ml
m2 do
–
d+2
–
and rearranged as
–
–
de 9
–
dt2 +
sinA=0
–
–
–
If the amplitude of angular displacement is small enough, so the small angle approximation ($\sin\thetalapprox\theta$) holds true, then the equation of motion reduces to the equation of simple harmonic motion
m
I
mg sino
d20
g
dt2 + 10=0
mg cose
mg
The simple harmonic solution is
e(t) = 0, cos(wt),
where an is the initial angular displacement, and w= 1 g/L the natural frequency of the motion. The period of this sytem (time for one oscillation) is
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Maxwell’s equations
Maxwell’s equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields. An important consequence of the equations is that they demonstrate how fluctuating electric and magnetic fields propagate at a constant speed (c) in a vacuum.
A good presentation is made in Youtube by 2Blue1Brown
Issues in the video:
 Quantum mechanics requires understanding of waves.
 The magnetic field is a vector fields
 Maxwell equations tells how electric and magnetic fields change each others
 Horizontal polarized wave: E=A_{x }cos(2π,f,t+Ø_{x}) Ι→〉+0Ι↑〉
where 0Ι↑〉 is the unit vector in vertical direction
 Vertically polarized wave: E=0 Ι→〉 + A_{y} cos(2π,f,t+Ø_{x})
where 0Ι→〉 is the vector in horizontal direction
where their videos are financed by course payments about €5.54 /month.
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Faraday’s law:
A introduction part 1 of electromagnetism is made here:
As Flux density = B = ϕ/A ( Wbm^{2 })
so Flux = field strength per unit area ϕ= BA (unit Wb)
B (unit tesla)
Flux linkage ( =BAn ) in a coil with N coils
 or
 where s the voltage across the device
 Maxwell came up with the right hand rule (or cork screw rule) to find out how magnetic field goes around a piece of wire with a current in it.. Khan academy presents this law here:

 Key notations:
 is a surface integral over the surface Σ,
 Symbols in bold represent vector quantities, and symbols in italics represent scalar quantities, unless otherwise indicated.
 the magnetic field, B, a pseudovector field, each generally having a time and location dependence.
The sources are:
 the total electric charge density (total charge per unit volume), ρ, and
 the total electric current density (total current per unit area), J.
The universal constants appearing in the equations (the first two ones explicitly only in the SI units formulation) are:
 the permittivity of free space, ε_{0}, and
 the permeability of free space, μ_{0}, and
In the differential equations,
 the nabla symbol, ∇, denotes the threedimensional gradient operator, del,
 the ∇⋅ symbol (pronounced “del dot”) denotes the divergence operator,
 the ∇× symbol (pronounced “del cross”) denotes the curl operator.
MaxwellFaraday equation
 is a surface integral over the boundary surface ∂Ω, with the loop indicating the surface is closed
 is a surface integral over the surface Σ,
 electric field, E, a vector field,ss
 the magnetic field, B, a pseudovector field
 Ω is any fixed volume with closed boundary surface ∂Ω, and
 Σ is any fixed surface with closed boundary curve ∂Σ,
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Laplace equation
Laplace’s equation is a secondorder partial differential equation named after PierreSimon Laplace who first studied its properties. This is often written as
 ∇ 2 f = 0 or Δ f = 0 , {\displaystyle \nabla ^{2}\!f=0\qquad {\mbox{or}}\qquad \Delta f=0,}
where Δ = ∇ ⋅ ∇ = ∇ 2 {\displaystyle \Delta =\nabla \cdot \nabla =\nabla ^{2}} is the Laplace operator,^{[note 1]} ∇ ⋅ {\displaystyle \nabla \cdot } is the divergence operator (also symbolized “div”), ∇ {\displaystyle \nabla } is the gradient operator (also symbolized “grad”), and f ( x , y , z ) {\displaystyle f(x,y,z)} is a twicedifferentiable realvalued function. The Laplace operator therefore maps a scalar function to another scalar function.
If the righthand side is specified as a given function, h ( x , y , z ) {\displaystyle h(x,y,z)} , we have
 Δ f = h . {\displaystyle \Delta f=h.}
This is called Poisson’s equation, a generalization of Laplace’s equation. Laplace’s equation and Poisson’s equation are the simplest examples of elliptic partial differential equations.
With Laplace equation we get:
Laplace’s tidal equations
the dynamic theory of tides, developed by PierreSimon Laplace in 1775,^{[9]} describes the ocean’s real reaction to tidal forces. Laplace’s theory of ocean tides took into account friction, resonance and natural periods of ocean basins. It predicted the large amphidromic systems in the world’s ocean basins and explains the oceanic tides that are actually observed. Laplace obtained these equations by simplifying the fluid dynamic equations, but they can also be derived from energy integrals via Lagrange’s equation. ( Wiki )
For a fluid sheet of average thickness D, the vertical tidal elevation ζ, as well as the horizontal velocity components u and v (in the latitude φ and longitude λ directions, respectively) satisfy Laplace’s tidal equations:^{[28]}
where Ω is the angular frequency of the planet’s rotation, g is the planet’s gravitational acceleration at the mean ocean surface, a is the planetary radius, and U is the external gravitational tidalforcing potential. ( Wiki )
Eulers formula
Eulers formula was used by Riemann to derivate his Zeta function described in a separate page.
( Image source: Wiki )
A nice presentation of Eulers formula is made by Khan Academy
The Taylor series for the exponential function e^{x} at a = 0 is
( Image source: Wiki )
( Image source: Khan video )
( Image source: Khan video )
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Euler’s identity
In this video Mark Newman describes beatifully the relation between sinusoids and Euler number in Eulers formula:
If I am not wrong….
If e^{iπ }= 1
then e^{iπ }= i^{2}
i sqr(e^{iπ})
Read more about it in
!EULER’S IDENTITY A MATHEMATICAL PROOF FOR THE EXISTENCE OF GOD! !in https://www.academia.edu/217151/Eulers_Identity
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The schrödinger equation derivation
In its simplest form, the Schroedinger equation looks like this
(Source: screenshots from Parth video )
where i is the imaginary number, thethe square root of 1
 ℏ is related to the Plank’s Constant as
 Ĥ is what we call Hamiltonian.
 ψ represent wave functions in quantum mechanics.
 the line and pointed bracket around theψ (PSI) is indicating that it is a a quantum state. The quantum state is a mathematical description of a quantum system, e.g. a electron near the nucleus.
The scroedinger equation Calculates the probability for a particle (e.g. electron) to be in a certain position. Iit s shared by Brian Green in this World Science Festival video.
presented in one dimension in this image fom Ul Islam from quora:
Unnikrishnan Menon, derived it for us in Quora.
I hope he don’t mind I share his loooong derivation here. He writes:
It is all about finding the different energies that a particle can have.
We’re looking at things that can have more than one answer! You might have studied in high school that atoms have energy levels. The Schrödinger’s Wave Equation lets us calculate what these energies are.
Let’s start with thinking what is Kinetic Energy…
It is defined as:
It turns out, talking about velocity isn’t very useful. So, we change this equation to make it depend on momentum p
We can think of particles as waves, at least at tiny scales where we need to use Quantum Mechanics!
To help us move between these 2 ways of thinking about matter, we can use De Broglie’s Equation
where is the wavelength, is the Planck constant, is the momentum, is the rest mass, is the velocity and is the speed of light in a vacuum.”
This theory set the basis of wave mechanics. It was supported by Einstein, confirmed by the electron diffraction experiments of G P Thomson and Davisson and Germer, and generalized by the work of Schrödinger. ( Wiki )
Now we don’t see matter around us in our everyday lives to be behaving like waves because Plank’s Constant h is absolutely tiny! ( h=6.62×10^{−34}m^{2}Kg/s )
But hold on! De Broglie’s Equation is useful when we are dealing with minute particles like protons and electrons 🙂
We also have something called ℏ
It’s is related to the Plank’s Constant as
Now, it is going to be useful to talk about ℏ and the wave number instead of h and λ…
Where
is the wave number. We did to avoid having 2π running around in our equations. It just turns out to be easier to work with!
We also know that k is change depending upon the energy. Since k depends only on λ , the factors that can affect λ can also affect k . In this case, it is Energy!
We use Wave Functions to encode information about waves. These are just equations with a few special properties. They look something like this…
This is just another way to write sines and cosines. (Waves are just combinations of sines and cosines!)
Here’s the reason for writing this in this weird format…
This type of problem is called an Eigen Value Problem.
is called an Eigen Function.
is an Operator.
Eigen Function of operators are functions which return themselves and a new Eigen Value after the operator is used on them.
As you might have guessed by now, on differentiating this we get the same thing back multiplied by some constants (differential of the argument)
Let’s do this again!
Here, −k^{2} is the new Eigen Value returned (that’s the extra bit)
Now let’s make a Kinetic Energy Operator (we want the energy EE and we are assuming a low potential)…
We know there are gonna be a bunch of answers so we are going to use our Wave Function to generate those answers and we also have a way of writing k^{2}
So on putting it all together we get…
Notice it is partial derivative instead of the usual one. This just means that there is more than one variable in the function we treat as a constant when differentiating.
What we just did is valid for a one dimensional particle that is time independent and is not in a potential.
Let’s generalize this equation a little…
For a photon,
We’re now gonna assume it to be true for particles. It turns out to be a good assumption…
We know that there is gonna be a set of energies so we want to know the Eigenvalue equation.
This time we’ll find an E by generating an ω
Let’s look at the wave function to do that…
As you might have guessed it by now, we’re gonna take the derivative with respect to time to pull it out front!
To account for the minus sign() we will simply use i.i=1
And that’s same as the equation we had before…
Please note that this equation is only for one spatial dimension! But we need to consider all 3 dimensions. The three dimensions in Cartesian Coordinate System are orthogonal to each other. That means, they will not interfere with each other.
Each of these contributes to the total energy.
So, in three dimensions, the time dependent wave equation becomes…
And the time independent version becomes…
This can more simplified as…
Where ∇is what we call Nabla.
To include Potential(V) we modify it a bit further like this…
where Ĥ is what we call Hamiltonian.
is an operator corresponding to the sum of the kinetic energies plus the potential energies for all the particles in the system (this addition is the total energy of the system in most of the cases under analysis ( Wiki )
So, the full Schrödinger’s Wave Equation becomes…
The classical wave function
I shared above The Schrödinger equation derivation.
The classical order wave function is a one dimension (linear) partial derivative function that has no relativity nor quantum mechanics included.
Parth shares a very good presentation of it at
Possible solution of the wave equation as explained by Parth:
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general relativity field equation (EFE)
First part copied from my page at www.kinberg.net/wordpress/stellan/einsteinsfieldequationsofgeneralrelativity/
As is correctly explained in Wikipedia
 R_{μν} is the Ricci curvature tensor,
the Ricci tensor is the part of the curvature of spacetime that determines the degree to which matter will tend to converge or diverge in time  R is the scalar curvature,
the scalar curvature (or the Ricci scalar) is the simplest curvature invariant of a Riemannian manifold.
 g_{μν} is the metric tensor,
 Λ is the cosmological constant,
 G is Newton’s gravitational constant,
 c is the speed of light in vacuum, and
 T_{μν} is the stress–energy tensor.
I found this Youtube that explains the general relativity field equation. I like it because of the good graphic representations.
As with the tensor youtube made by Dan Fleisch, this made by Eugene Khutoryansky show graphically how a tensor change depending on curvature of space. Very well done video.
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Einstein field equation (EFE) tensors
I copied this part to my tensor page.
This is Einsteins field equation (EFE) in simplified form. It is called Einstein Field equations as it is really 16 equations depending on the value of the indeces (μν) for g and T in the equation. 6 are duplicates. (read more below)
Source: Physicist page Facebook )
(Source: Dr Physics A Youtube )
This is a very good video that I recommend where . DrPhysicsA explains some basics and derives the equation.
 How you can feel gravitation and acceleration force
 How light bends in gravitation fields.
 5:33 Proven at a solar eclipse
 5:50 Why Newtons gravitation law does not apply to (0 mass) photons.
 6:42 All forms of movement are represented by movement in curved spacetime
 Trampoline metaphore
 9:00 What is spacetime?
 11:00 Not only one equation as it looks like.
The indeces μν represent the spacetime 4×4. 6 are duplicates = 10 EFE  13:15. “Everything on the left hand of EFE refers to curvature of spacetime. Everything on the right hand side is to do with mass and energy. What Einstein field equation basically say is that
‘mass tells spacetime how to curve
and curved spacetime tells mass how to move’“
Deriving EFE.  15:15 A Where I am on a bumpy field
 16;13 How does my height change as I move on the field
 17:40 1:10 gradient (dϕ/dx)
 14:50 metric tensor
 24:35 relableing the coordinates.
 19:17
 32:05 Tensor – relationship between two vectors.
 42:17 Contravariant and covariant transformation
 46:15 Kronecker delta
 49:45 Metric tensor – correct pythagoras equation on a curved space.
 53:14 Christophel symbol
Pythagoras and vectors
I were confused of his using ϕ in the same way as dh where h=height in the field. I understand that dϕ_{x}is a movement in the field in parallell to the x ax.
With Pythagoras we get a formula for 2 dimensional vectors like ds in the printscreen above.
The term “Partial derivative” has confused me with its new symbol . With this video I understand that a partial derivative is just a derivative that is not the only one to describe the movement. A derivative with respect to one of several variables. To denote a partial derivative character ∂ (Unicode: U+2202) is used. It is a stylized cursive/curly d.
relableing the coordinate system
This is necessary to be able to have more coordinates as in e.g. a 11 dimension system.If you are not used to it you get rather confused with the following calculations. So I keep this screenshot here as a memory backup. The ycoordinate will still be used but for another set of coordinates/frame of reference. x^{0} is used for time coordinate.
With the new coordinate system
can be rewritten as
If we have2 reference systems, then we have
Chain rule
In calculus, the chain rule is a formula to compute the derivative of a composite function. That is, if f and g are differentiable functions, then the chain rule expresses the derivative of their composite f ∘ g — the function which maps x to f ( g ( x ) ) {\displaystyle f(g(x))} — in terms of the derivatives of f and g and the product of functions as follows:
 Alternatively, by letting F = f ∘ g (equiv., F(x) = f(g(x)) for all x), one can also write the chain rule in Lagrange’s notation, as follows:
 F ′ ( x ) = f ′ ( g ( x ) ) g ′ ( x ) . F'(x)=f'(g(x))g'(x).}
 ( Wiki )
 The chain rule may also be rewritten in Leibniz’s notation in the following way. If a variable z depends on the variable y, which itself depends on the variable x (i.e., y and z are dependent variables), then z, via the intermediate variable of y, depends on x as well. In which case, the chain rule states that:
 d z d x = d z d y ⋅ d y d x . {\displaystyle {\frac {dz}{dx}}={\frac {dz}{dy}}\cdot {\frac {dy}{dx}}.}
More precisely, to indicate the point each derivative is evaluated at, d z d x  x = d z d y  y ( x ) ⋅ d y d x  x .
To write a equation for a 2 frame system we need the chain rule that tells how to derivate a function in a outer function.
I found 3 applications of this rule:
3 from NancyPi
1 example from Khan
So with the chain rule, we can describe the position in a 2 dimensional system as
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Introduction to Information Theory
I dont know why Edward Witten choose to start with a short introduction to Communication theory (the Shannon theory) at “Theoretical Physics 2018: From Qubits to Spacetime”
I presume it is important to know about this, to understand the theory of Quantum mechanics and aspects of General Relativity as he says to continue with at the conference.
I decided to take a look at C.E. Shannons book from 1948 ” A mathematical theory of Communication“
Shannon was an American mathematician, electrical engineer, and cryptographer known as “the father of information theory“. He wrote also “Theoretical Genetics.”^{[12]}
the video with English text is here.
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What Every Physicist Should Know About String Theory
I listened to Edward Witten in this Youtube:
The slide texts are very difficult to read and Witten is very fast and you get little time to think over it.
Here are the slide texts he presented so you can hopefully read these at your pace and maybe also get it satisfactory translated to your own language:
Slide 1.I am going to try today to explain the minimum that any physicist might want to know about string theory. I will try to explain answers to a couple of basic questions.
 How does string theory generalize standard quantum field theory?
 And why does string theory force us to unify General Relativity with the other forces of nature, while standard quantum field theory makes it so difficult to incorporate General Relativity?
 Why are there no ultraviolet divergences?
 And what happens to Einstein’s conception of spacetime?
I thought .that explaining these matters is possibly suitable for a session devoted to the centennial of General Relativity.
Slide 2. Anyone who has studied physics is familiar with the fact that while physics – like history – does not precisely repeat itself, it does rhyme, with similar structures at different scales of lengths and energies. We will begin today with one of those rhymes – an analogy between the problem of quantum gravity and the theory of a single particle.
Slide 3 04:32
Even though we do not really understand it, quantum gravity is supposed to be some sort of theory in which, at least from a macroscopic point of view, we average, in a quantum mechanical sense, over all possible spacetime geometries….
(We do not know to what extent this description is valid microscopically.) The averaging is done, in the simplest case, with a weight factor exp(1) (1 will write this in Euclidean signature) where I is the EinsteinHilbert action
with R being the curvature scalar and (lambda) the cosmological constant. We could add matter fields, but we don’t seem to have to.
Slide 4 05:20
Let us try to make a theory like this in spacetime dimension 1, rather than 4. There are not many options for a 1manifold
In contrast to the 4d case, there is no Riemann curvature tensor in 1 dimension so there is no close analog of the EinsteinHilbert action.
Slide 5.
Even there is no l Integral[(root(g)R] to add to the action, we can still make a nontrivial theory of “quantum gravity,” that is a fluctuating metric tensor, coupled to matter. Let us take the matter to consist of some scalar fields X_{i }j = 1…..D (matter fields). The most obvious action is
where g = (gut) is a 1×1 metric tensor and I have written ma^{2} / 2 instead of lambda
Slide 6 (repeating slide 5)
If we introduce the “canonical momentum P_{i}=dX_{i}/dt
then the “Einstein field equation” is just
In other words, the wavefunction ω(X) should obey the corresponding differential equation
Slide 7 (8:32)
This is a familiar equation – the relativistic KleinGordon equation in D dimensions – but in Euclidean signature. If we want to give this fact a sensible physical interpretation, we should reverse the sign of the action for one of the scalar fields X; so that the action becomes
Now the equation obeyed by the wavefunction is a KleinGordon equation in Lorentz signature:
Slide 8 9:07
So we have found an exactly soluble theory of quantum gravity in one dimension that describes a spin 0 particle of mass m propagating in Ddimensional Minkowski spacetime. Actually, we can replace Minkowski spacetime by any Ddimensional spacetime M with a Lorentz (or Euclidean) signature metric G_{IJ}. the action being then
The equation obeyed by the wavefunction is now a KleinGordon equation on our spacetime M:
This is the massive KleinGordon in curved spacetime
Slide 9 10:23
Just to make things more familiar, let us go back to the case of flat spacetime, and I will abbreviate G P_{I}P_{J}, as P^{2} (To avoid keeping track of some factors of i. I will also write formulas in Euclidean signature.) Let us calculate the amplitude for a particle to start at a point x in spacetime and end at another point y.
Part of the process of evaluating the path integral in a quantum gravity theory is to integrate over the metric on the onemanifold, modulo diffeomorphisms. But up to diffeomorphism, this onemanifold has only one invariant, the total length 1, which we will interpret as the elapsed proper time.
Slide 10 12:25 “Keeping To fixed”
For a given Tao we can take the 1metric to be just g_{tt} = 1 where
0 <t<T (As a minor shortcut, I will take Euclidean signature on the lmanifold. “as described by Feinman 50 years ago” Now on this lmanifold, we have to integrate over all paths X(t) that start at x at t = 0 and end at y at t = tao.
Slide 11 14:17
For a given tao we can take the lmetric to be just g_{tt} = 1 where 0<=t<=tao (As a minor shortcut. I will take Euclidean signature on the lmanifold.) Now on this lmanifold, we have to integrate over all paths X(t) that start at x att 0 and end at y at t = This is the basic Feynman integral of quantum mechanics with the Hamiltonian being H P2 + m and according to Feynman, the result is the matrix element of exp(taoxH) (H= Hamiltonian):
But we have to remember to do the “gravitational” part of the path integral, which in the present context means to integrate over
Slide 12 14:17
Thus the complete path integral for our problem – integrating over all metrics Sur(t) and all paths X(t) with the given endpoints, modulo diffeomorphisms – gives
This is the standard Feynman propagator in Euclidean signature, and an analogous derivation in Lorentz signature (for both the spacetime M and the particle worldline) gives the correct Lorentz signature Feynman propagator, with the iE
Slide 12 15:15
So we have interpreted a free particle in Ddimensional spacetime in terms of 1dimensional quantum gravity. How can we include interactions? There is actually a perfectly natural way to do this. There are not a lot of smooth lmanifolds, but there is a large supply of singular lmanifolds in the form of graphs.
Our “quantum gravity” action makes sense on such a graph. We just take the same action that we used before, summed over all of the line segments that make up the graph
Slide 13 16:02
Now to do the quantum gravity path integral, we have to integrate over all metrics on the graph, up to diffeomorphism. The only invariants are the total lengths or “proper times” of each of the segments:
(I did not label all of them.)
Slide 14 17:14
To integrate over the paths, we just observe that if we specify the positions y….. y4 at all the vertices (and therefore on each end of each line segment)
then the computation we have to do on each line segment is the same as before and gives the Feynman propagator. Integrating over the y; will just impose momentum conservation at vertices, and we arrive at Feynman’s recipe to compute the amplitude attached to a graph: a Feynman propagator for each line, and an integration over all momenta subject to momentum conservation.
Slide 15 19:07
We have arrived at one of nature’s rhymes: if we imitate in one dimension what we would expect to do in D = 4 dimensions to describe quantum gravity, we arrive at something that is certainly important in physics, namely ordinary quantum field theory in a possibly curved spacetime. In the example that I gave, the “ordinary quantum field theory’ is scalar 03 theory, because of the particular matter system we started with and assuming we take the graphs to have cubic vertices. Quartic vertices (for instance) would give 4 theory, and a different matter system would give fields of different spins. So many or maybe all QFT’s in D dimensions can be derived in this sense from quantum gravity in 1 dimension.
here is actually a much more perfect rhyme if we repeat this in two dimensions, that is for a string instead of a particle. One thing we immediately run into is that a twomanifold E can be curvedthe integral over 2d metrics promises to not be trivial at all
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conclusions
this is a tough issue to deal with. The last theory that mathematicians are working with are the Theory M. I decided to start a new issue “Introduction to Information theory“