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Prof. Legolasov
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Renormalization group is basically a tool which shows us how our theory responds to the scale transformations. Since a classical theory is completely spawned by the action, this action should be invariant under the RG flow. It means that the classical RG flow is completely determined by the scale dimensions of physical quantities which can be derived from the action.

So yes, there is an analogue of the usual RG flow. Even more: the classical RG flow is used in the quantum theory when we try to determine whether the theory is renormalizable or not. We usually approximate the quantum RG flow by the classical RG flow (which gives its leading behaviour) and therefore classify couplings by their classical dimension.

Couplings with negative dimension ($m^{-d},\,d>0$) correspond to non-renormalizable interactions. This very situation occurs when determining the dimension of the gravitational constant $G$ (try this, it is actually extremely easy).

Couplings with positive dimension ($m^{d},\,d>0$) correspond to super-renormalizable interactions. They tend to vanish on short distances.

And finally, couplings with zero dimension (marginal couplings) are the most interesting ones. Since the leading (classical) behaviour of the RG flow vanishes, we are required to determine (at least to the first order of perturbation theory) the behaviour of the quantum corrections.

But there is a but: despite the fact that the gravitational coupling blows at short distances in the classical theory, it is not a problem since we don't use perturbative methods. But in perturbative QFT the disaster becomes when $G$ approaches one (~ the Planck scale), which indicates the meltdown of the perturbative approach.

UPD: note that gravity is only perturbatively non-renormalizable. It can still be that there is a complete quantum version of GR. For example, the 2+1 gravity was recently found to be exactly solvable despite that it had been considered non-renormalizable for a long time.

Renormalization group is basically a tool which shows us how our theory responds to the scale transformations. Since a classical theory is completely spawned by the action, this action should be invariant under the RG flow. It means that the classical RG flow is completely determined by the scale dimensions of physical quantities which can be derived from the action.

So yes, there is an analogue of the usual RG flow. Even more: the classical RG flow is used in the quantum theory when we try to determine whether the theory is renormalizable or not. We usually approximate the quantum RG flow by the classical RG flow (which gives its leading behaviour) and therefore classify couplings by their classical dimension.

Couplings with negative dimension ($m^{-d},\,d>0$) correspond to non-renormalizable interactions. This very situation occurs when determining the dimension of the gravitational constant $G$ (try this, it is actually extremely easy).

Couplings with positive dimension ($m^{d},\,d>0$) correspond to super-renormalizable interactions. They tend to vanish on short distances.

And finally, couplings with zero dimension (marginal couplings) are the most interesting ones. Since the leading (classical) behaviour of the RG flow vanishes, we are required to determine (at least to the first order of perturbation theory) the behaviour of the quantum corrections.

But there is a but: despite the fact that the gravitational coupling blows at short distances in the classical theory, it is not a problem since we don't use perturbative methods. But in perturbative QFT the disaster becomes when $G$ approaches one (~ the Planck scale), which indicates the meltdown of the perturbative approach.

Renormalization group is basically a tool which shows us how our theory responds to the scale transformations. Since a classical theory is completely spawned by the action, this action should be invariant under the RG flow. It means that the classical RG flow is completely determined by the scale dimensions of physical quantities which can be derived from the action.

So yes, there is an analogue of the usual RG flow. Even more: the classical RG flow is used in the quantum theory when we try to determine whether the theory is renormalizable or not. We usually approximate the quantum RG flow by the classical RG flow (which gives its leading behaviour) and therefore classify couplings by their classical dimension.

Couplings with negative dimension ($m^{-d},\,d>0$) correspond to non-renormalizable interactions. This very situation occurs when determining the dimension of the gravitational constant $G$ (try this, it is actually extremely easy).

Couplings with positive dimension ($m^{d},\,d>0$) correspond to super-renormalizable interactions. They tend to vanish on short distances.

And finally, couplings with zero dimension (marginal couplings) are the most interesting ones. Since the leading (classical) behaviour of the RG flow vanishes, we are required to determine (at least to the first order of perturbation theory) the behaviour of the quantum corrections.

But there is a but: despite the fact that the gravitational coupling blows at short distances in the classical theory, it is not a problem since we don't use perturbative methods. But in perturbative QFT the disaster becomes when $G$ approaches one (~ the Planck scale), which indicates the meltdown of the perturbative approach.

UPD: note that gravity is only perturbatively non-renormalizable. It can still be that there is a complete quantum version of GR. For example, the 2+1 gravity was recently found to be exactly solvable despite that it had been considered non-renormalizable for a long time.

Source Link
Prof. Legolasov
  • 16.3k
  • 2
  • 33
  • 70

Renormalization group is basically a tool which shows us how our theory responds to the scale transformations. Since a classical theory is completely spawned by the action, this action should be invariant under the RG flow. It means that the classical RG flow is completely determined by the scale dimensions of physical quantities which can be derived from the action.

So yes, there is an analogue of the usual RG flow. Even more: the classical RG flow is used in the quantum theory when we try to determine whether the theory is renormalizable or not. We usually approximate the quantum RG flow by the classical RG flow (which gives its leading behaviour) and therefore classify couplings by their classical dimension.

Couplings with negative dimension ($m^{-d},\,d>0$) correspond to non-renormalizable interactions. This very situation occurs when determining the dimension of the gravitational constant $G$ (try this, it is actually extremely easy).

Couplings with positive dimension ($m^{d},\,d>0$) correspond to super-renormalizable interactions. They tend to vanish on short distances.

And finally, couplings with zero dimension (marginal couplings) are the most interesting ones. Since the leading (classical) behaviour of the RG flow vanishes, we are required to determine (at least to the first order of perturbation theory) the behaviour of the quantum corrections.

But there is a but: despite the fact that the gravitational coupling blows at short distances in the classical theory, it is not a problem since we don't use perturbative methods. But in perturbative QFT the disaster becomes when $G$ approaches one (~ the Planck scale), which indicates the meltdown of the perturbative approach.