digplanet beta 1: Athena
Share digplanet:


Applied sciences






















The Wheeler–DeWitt equation[1] is an attempt to combine mathematically the ideas of quantum mechanics and general relativity, a step toward a theory of quantum gravity. In this approach, time plays no role in the equation, leading to the problem of time.[2] More specifically, the equation describes the quantum version of the Hamiltonian constraint using metric variables. Its commutation relations with the diffeomorphism constraints generate the Bergmann-Komar "group" (which is the diffeomorphism group on-shell, but differs off-shell).

Because of its connections with the low-energy effective field theory, it inherits all the problems of the naively quantized GR, and thus it cannot be used at multi-loop level, etc, at least not according to the current knowledge.

The equation has not played a role in string theory thus far, since all properly defined and understood descriptions of string/M-theory deal with some fixed asymptotic conditions on the background. Thus, at infinity, the "right" choice of the time coordinate "t" is determined in every description, so there is a preferred definition of the Hamiltonian (with nonzero eigenvalues) to evolve states of the system forward in time. This avoids all the issues of the Wheeler-de Witt equation to dynamically generate a time dimension.

But at the end, there could exist a Wheeler-de Witt style manner to describe the bulk dynamics of quantum theory of gravity. Some experts believe that this equation still holds the potential for understanding quantum gravity; however, decades after the equation was first written down, it has not brought physicists as clear results about quantum gravity as some of the results building on completely different approaches, such as string theory.

Motivation and background[edit]

In canonical gravity, spacetime is foliated into spacelike submanifolds. The three-metric (i.e., metric on the hypersurface) is \gamma_{ij} and given by


In that equation the Roman indices run over the values 1, 2, 3 and the Greek indices run over the values 1, 2, 3, 4. The three-metric \gamma_{ij} is the field, and we denote its conjugate momenta as \pi^{kl}. The Hamiltonian is a constraint (characteristic of most relativistic systems)


where \gamma=\det(\gamma_{ij}) and G_{ijkl}=(\gamma_{ik}\gamma_{jl}+\gamma_{il}\gamma_{jk}-\gamma_{ij}\gamma_{kl}) is the Wheeler-DeWitt metric.

Quantization "puts hats" on the momenta and field variables; that is, the functions of numbers in the classical case become operators that modify the state function in the quantum case. Thus we obtain the operator


Working in "position space", these operators are

 \hat{\gamma}_{ij}(t,x^k) \to \gamma_{ij}(t,x^k)
 \hat{\pi}^{ij}(t,x^k) \to -i \frac{\delta}{\delta \gamma_{ij}(t,x^k)}.

One can apply the operator to a general wave functional of the metric \widehat{\mathcal{H}} \Psi[\gamma] =0 where:

 \Psi[\gamma] = a + \int \psi(x) \gamma(x) dx^3+ \int\int \psi(x,y)\gamma(x)\gamma(y) dx^3 dy^3 +...

Which would give a set of constraints amongst the coefficients \psi(x,y,...). Which means the amplitudes for N gravitons at certain positions is related to the amplitudes for a different number of gravitons at different positions. Or one could use the two field formalism treating \omega(g) as an independent field so the wave function is \Psi[\gamma,\omega]

Derivation from path integral[edit]

The Wheeler–DeWitt equation can be derived from a path integral using the gravitational action in the Euclidean quantum gravity paradigm:[3]

Z = \int_{C}\mathrm{e}^{-I[g_{\mu\nu},\phi]}\mathcal{D}\bold{g}\, \mathcal{D}\phi

where one integrates over a class of Riemannian four-metrics and matter fields matching certain boundary conditions. Because the concept of a universal time coordinate seems unphysical, and at odds with the principles of general relativity, the action is evaluated around a 3-metric which we take as the boundary of the classes of four-metrics and on which a certain configuration of matter fields exists. This latter might for example be the current configuration of matter in our universe as we observe it today. Evaluating the action so that it only depends on the 3-metric and the matter fields is sufficient to remove the need for a time coordinate as it effectively fixes a point in the evolution of the universe.

We obtain the Hamiltonian constraint from

\frac{\delta I_{EH}}{\delta N}=0

where I_{EH} is the Einstein-Hilbert action, and N is the lapse function (i.e., the Lagrange multiplier for the Hamiltonian constraint). This is purely classical so far. We can recover the Wheeler–DeWitt equation from

\frac{\delta Z}{\delta N}=0=\int \left.\frac{\delta I[g_{\mu\nu},\phi]}{\delta N}\right|_{\Sigma} \exp\left(-I[g_{\mu\nu},\phi]\right)\,\mathcal{D}\bold{g}\, \mathcal{D}\phi

where \Sigma is the three-dimensional boundary. Observe that this expression vanishes, implying that the functional derivative also vanishes, giving us the Wheeler–DeWitt equation. A similar statement may be made for the diffeomorphism constraint (take functional derivative with respect to the shift functions instead).

Mathematical formalism[edit]

The Wheeler–DeWitt equation[1] is a functional differential equation. It is ill defined in the general case, but very important in theoretical physics, especially in quantum gravity. It is a functional differential equation on the space of three dimensional spatial metrics. The Wheeler–DeWitt equation has the form of an operator acting on a wave functional, the functional reduces to a function in cosmology. Contrary to the general case, the Wheeler–DeWitt equation is well defined in mini-superspaces like the configuration space of cosmological theories. An example of such a wave function is the Hartle–Hawking state. Bryce DeWitt first published this equation in 1967 under the name "Einstein–Schrödinger equation"; it was later renamed the "Wheeler–DeWitt equation".[4]

Simply speaking, the Wheeler–DeWitt equation says

\hat{H}(x) |\psi\rangle = 0

where \hat{H}(x) is the Hamiltonian constraint in quantized general relativity and |\psi\rangle stands for the wave function of the universe. Unlike ordinary quantum field theory or quantum mechanics, the Hamiltonian is a first class constraint on physical states. We also have an independent constraint for each point in space.

Although the symbols \hat{H} and |\psi\rangle may appear familiar, their interpretation in the Wheeler–DeWitt equation is substantially different from non-relativistic quantum mechanics. |\psi\rangle is no longer a spatial wave function in the traditional sense of a complex-valued function that is defined on a 3-dimensional space-like surface and normalized to unity. Instead it is a functional of field configurations on all of spacetime. This wave function contains all of the information about the geometry and matter content of the universe. \hat{H} is still an operator that acts on the Hilbert space of wave functions, but it is not the same Hilbert space as in the nonrelativistic case, and the Hamiltonian no longer determines evolution of the system, so the Schrödinger equation \hat{H} |\psi\rangle = i \hbar \partial / \partial t |\psi\rangle no longer applies. This property is known as timelessness. The reemergence of time requires the tools of decoherence and clock operators[citation needed] (or the use of a scalar field).

We also need to augment the Hamiltonian constraint with momentum constraints

\vec{\mathcal{P}}(x) \left| \psi \right\rangle = 0

associated with spatial diffeomorphism invariance.

In minisuperspace approximations, we only have one Hamiltonian constraint (instead of infinitely many of them).

In fact, the principle of general covariance in general relativity implies that global evolution per se does not exist; the time t is just a label we assign to one of the coordinate axes. Thus, what we think about as time evolution of any physical system is just a gauge transformation, similar to that of QED induced by U(1) local gauge transformation  \psi \rightarrow e^{i\theta(\vec{r} )} \psi where \theta(\vec{r}) plays the role of local time. The role of a Hamiltonian is simply to restrict the space of the "kinematic" states of the Universe to that of "physical" states - the ones that follow gauge orbits. For this reason we call it a "Hamiltonian constraint." Upon quantization, physical states become wave functions that lie in the kernel of the Hamiltonian operator.

In general, the Hamiltonian vanishes for a theory with general covariance or time-scaling invariance.

See also[edit]


  1. ^ a b DeWitt, B. S. (1967). "Quantum Theory of Gravity. I. The Canonical Theory". Phys. Rev. 160 (5): 1113–1148. Bibcode:1967PhRv..160.1113D. doi:10.1103/PhysRev.160.1113. 
  2. ^ https://medium.com/the-physics-arxiv-blog/d5d3dc850933
  3. ^ See J. B. Hartle and S. W. Hawking, "Wave function of the Universe." Phys. Rev. D 28 (1983) 2960–2975, eprint.
  4. ^ Go to Arxiv.org to read "Notes for a Brief History of Quantum Gravity" by Carlo Rovelli

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Wheeler–DeWitt_equation — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
35 videos foundNext > 

Timeless Explanation: A New Kind of Causality, Julian Barbour

There are serious indications from attempts to create a quantum theory of gravity that time must disappear completely from the description of the quantum ...

Julian Barbour - What is Time?

For more on information and video interviews with Julian Barbour, please visit http://bit.ly/1aoc0oZ For more videos answering the question what is time click ...

Oliver Pooley – First-Class Constraints, Gauge, and the Wheeler–DeWitt Equation

Oliver Pooley (Oxford University) – First-Class Constraints, Gauge, and the Wheeler–DeWitt Equation Abstract: Recently, Pitts (2014) has argued that the claim ...

Interview with John Wheeler 1/3

Date- 2008? Source- http://www.webofstories.com/ Extracts from an interview with American theoretical physicist John Wheeler (who coined the phrase 'black ...

Joshua Norton – No time for problems

Joshua Norton (University of Illinois, Chicago) – No time for problems Abstract: The Wheeler–Dewitt equation is standardly interpreted using structures defined ...

How Spacetime Emerges

Response to Dhorpatan: Space-time does not exist without location, but location is the product of quantum probability. Locality (which defines space) breaks ...

Quantum God Episode Two: Properties of the Universal Orch-OR

In addition to being a literal mind (via Orch-OR), the wave-function of the universe is: Omnipresent: (Via the definition of wave-functions) Omnipotent (Via the ...

The Eccentric Gamer Re-Plays - System Shock 2 Part 3 - Engineering to Cargo Bay ½

In a world where time travel is only accessible by deloreans, phone boxes and the White Tulip from Fringe i must use the power of nostalgia and replay the ...

The Eccentric Gamer Re-Plays - System Shock 2 Part 1 - Training and the journey to Med

The Eccentric Gamer Re-Plays - System Shock 2 Part 2 - Med to Personnel and the Hunt for Key Cards

35 videos foundNext > 

18 news items

BBC Focus Magazine

BBC Focus Magazine
Tue, 29 Sep 2015 07:17:49 -0700

But the equation spawned a shocking insight. Of all the quantities it contained, one that everyone expected it to include had simply vanished: 't' for time. “According to the Wheeler-DeWitt equation, the quantum state of the Universe is just frozen ...

New Scientist

New Scientist
Fri, 25 Oct 2013 12:44:20 -0700

One early attempt in the 1960s was the Wheeler-DeWitt equation, which managed to quantise general relativity – by leaving out time altogether. “It means that the universe should not evolve. But of course we see evolution,” says Marco Genovese at the ...

Science News

Science News
Wed, 15 Jul 2015 07:33:07 -0700

... creating nucleons, atoms, etc. The Wheeler-DeWitt Equation shows mathematically how Time actually 'disappears' from the equation. The psychoiogical sense of time is either memories in the brain or projections of "future" events based upon physical ...


Mon, 24 Nov 2014 05:32:31 -0800

Sci-fi fans who hope humanity can one day zoom to distant corners of the universe via wormholes, as astronauts do in the recent film "Interstellar," shouldn't hold their breath. Wormholes are theoretical tunnels through the fabric of space-time that ...
Thu, 07 Aug 2014 11:33:45 -0700

In 1967, while in his mid-50s, he helped devise the 'Wheeler-DeWitt equation,' an important mathematical attempt to combine general relativity with quantum mechanics. In his 60s, Wheeler co-wrote one of the most influential textbooks on general relativity.

Scientific American (blog)

Scientific American (blog)
Thu, 22 May 2014 15:18:55 -0700

In March, a team of researchers based in Antarctica announced they'd detected gravitational waves, faint echoes from the first moments of the Big Bang. This discovery has enormous implications for cosmology, the world of physics and even our ...

Reason (blog)

Reason (blog)
Tue, 09 Sep 2014 09:45:37 -0700

Bill Nye the Science Guy ("He's not our Science Guy!" the Reason audience retorts) has waded into the Common Core debate. Per usual, he thinks those who disagree with him are—almost by definition—anti-science. After conceding one criticism of the ...

CNN (blog)

CNN (blog)
Fri, 28 Mar 2014 13:15:19 -0700

Touted as evidence for inflation (a faster-than-the-speed-of-light expansion of our universe), the new discovery of traces of gravity waves affirms scientific concepts in the fields of cosmology, general relativity, and particle physics. The new ...

Oops, we seem to be having trouble contacting Twitter

Support Wikipedia

A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia. Please add your support for Wikipedia!

Searchlight Group

Digplanet also receives support from Searchlight Group. Visit Searchlight