Problem of time in quantum field theory?

[mitchell porter]

@PrashantGokaraju says a number of intriguing intuitive things in this thread. They deserve a broader response, but I will just focus on two things.

First, on de Sitter and Chern-Simons, this comes from Smolin 2002 and drew a response in Witten 2003.

Second, @Demystifier is correct that "space dimensions in the bulk could be equivalent to time dimensions on the boundary" is a wrong guess. In AdS/CFT, there are two kinds of extra bulk dimension that need to be accounted for: the extra "radial" space dimension of AdS per se, which gives the bulk one more dimension than the boundary, and then enough compact space dimensions to reach the 10 dimensions of string theory or the 11 dimensions of M-theory.

It's clear that the radial dimension has something to do with scale, and renormalization group flow, in the boundary theory. The similarity between AdS geometry, and the tensor networks of MERA (multiscale entanglement renormalization ansatz), is now well-known, although I believe work remains to be done in rigorously relating the two.

As for the compact extra dimensions, what they represent, in terms of the boundary theory, is a lot harder to generalize about. Five years ago, David Berenstein, who must be one of the top people to have studied this topic, gave three "sketches" of how the geometric description of objects in the bulk, corresponds to internal degrees of freedom of objects (e.g. "droplets") on the boundary.

I suggest that the way to think about this, is to recall that points in the space-time of the CFT, correspond to asymptotic "points" on the boundary of the AdS theory; and that correlation functions between points in the CFT space-time, equal scattering amplitudes for objects that asymptotically enter and exit AdS at the corresponding "points" on the boundary. That may sound a little abstract, but look up what a Witten diagram is (it's the AdS/CFT counterpart of a Feynman diagram) and it may become a little clearer.

The creation of an inbound or outbound object, at the boundary of AdS, corresponds to a source or sink in the CFT. The objects in the AdS theory should all be strings or branes, and the sources in the CFT are gauge-invariant combinations of CFT operators, localized at a point. A particular operator combination, creates a particular AdS object, moving into the holographic radial dimension. So the compact extra dimensions, must somehow have to do with parameters of these CFT operator combinations.

I am sorry for being technical and vague at the same time, and for going on at such length about AdS/CFT, in a thread which started out as being about time in QFT.

Actually, I would like to comment on a third thing that Prashant said: "The open strings and closed strings are in some sense equivalent because of the equivalence between gauge theory and gravity." Well, there are dualities between open and closed strings, the simplest of which is that a cylinder can be regarded as a closed string evolving over a time interval, or as an open string evolving in periodic time. And there may even be a way to "apply" this to gauge/gravity duality, in the context of black D-brane stacks, where the gauge theory described open strings attached to the D-branes, and the gravity theory describes closed strings within the event horizon of the stack. But I do not remember how this works.

 

[PGaccount]

The current j is conjugate to A in the same way x is conjugate to p. This means that in a sense

j = d/dA

The addition of the term jA to the Lagrangian is a Legendre transformation. The Legendre transformation allows us to consider L as a function of p instead of v. This is related to the fact that the kinetic energy

T = 1/2 (dφ)²

is a function of v = dφ instead of p.

H = pv - L

is the hamiltonian. This is the sense in which the Lagrangian can be considered to be a Hamiltonian.

 

[Auto-Didact]

The current j is conjugate to A in the same way x is conjugate to p. This means that in a sense

j = d/dA

The addition of the term jA to the Lagrangian is a Legendre transformation. The Legendre transformation allows us to consider L as a function of p instead of v. This is related to the fact that the kinetic energy

T = 1/2 (dφ)²

is a function of v = dφ instead of p.

H = pv - L

is the hamiltonian. This is the sense in which the Lagrangian can be considered to be a Hamiltonian.

That the direct application of the Legendre transform to a function gives another function is only applicable contingently and not completely necessarily, i.e. only given that certain mathematical conditions are met w.r.t. both explicit functions.

Often when presenting a function implicitly - as functions are generally presented in the context of physics - most physicists just tend to outright assume without any justification that these conditions are met, purely for reasons of mathematical tractability, when in actuality whether or not such conditions are met is unknown and constitutes an open problem in mathematics.

Therefore, while your argument may be somewhat convincing at the level of rigour for many - if not most - physicists, your premise is actually de facto logically false and worse, mathematically flawed.