# Why is Scalar Massless Wave Equation Conformally Invariant?

• I
• user1139
In summary, the scalar massless wave equation is conformally invariant, meaning that it is not affected by changes in scale. This can be seen mathematically through the solution of the equation in energy-momentum representation, which implies that momentum must be light-like. While the equation is also invariant under the Poincare group, the larger symmetry group that preserves light cones is the conformal group. This is because a massless field does not have a built-in scale, so the only important invariant is the light cone. In contrast, a massive field has a built-in energy/length scale, so its wave equation is only invariant under the restricted group that preserves both light cones and invariant mass.
user1139
TL;DR Summary
I wish to gain a physical intuitive understanding as to why the scalar massless wave equation is conformally invariant.
It can be shown mathematically that the scalar massless wave equation is conformally invariant. However, doing so is rather tedious and muted in terms of physical understanding. As such, is there a physically intuitive explanation as to why the scalar massless wave equation is conformally invariant?

Last edited:
It's easier seen in energy-momentum representation, i.e., with
$$\phi(x)=\int_{\mathbb{R}^4} \mathrm{d}^4 p \exp(-\mathrm{i} p_{\mu} x^{\mu}) \tilde{\phi}(p)$$
the solution of the massless wave equation implies that ##p_{\mu} p^{\mu}=0##, i.e., ##p^{\mu}## must be light-like.

Obviously the equation is invariant under the Poincare group, i.e., under Lorentz boosts, rotations and translations, but here you only need to preserve the light cone in momentum space not the Minkowski product between all vectors. So besides the Lorentz boosts and rotations (building together the proper orthochronous Lorentz). The corresponding symmetry group mapping light cones to themselves is larger, and that's the conformal group. Wikipedia gives a nice introduction

https://en.wikipedia.org/wiki/Conformal_symmetry

Thomas1 said:
is there a physically intuitive explanation as to why the scalar massless wave equation is conformally invariant?
One way to look at it is that a massless field does not have any built-in scale of energy or length; waves of the field of all possible frequencies/wavelengths are possible and "look the same" from the standpoint of the physics of how they propagate. The only invariant about the propagation of the waves is the light cones. So the invariance property you would expect the wave equation to have is the one that only preserves the light cones, i.e., conformal invariance.

A massive field, by contrast, has a built-in energy/length scale, given by its invariant mass. So you would expect its wave equation to only be invariant under the more restricted group of transformations that preserves, not just the light cones, but the invariant mass, i.e., the Poincare group.

vanhees71

## 1. Why is the scalar massless wave equation conformally invariant?

The scalar massless wave equation is conformally invariant because it is a special case of the more general conformal invariance principle in physics. This principle states that physical laws should remain unchanged under a conformal transformation, which is a type of transformation that preserves angles but not necessarily distances. Since the scalar massless wave equation is a fundamental physical law, it must also adhere to this principle.

## 2. What is the significance of conformal invariance in physics?

Conformal invariance is significant in physics because it allows for a deeper understanding of the underlying symmetries and structures of physical laws. It also allows for the simplification and unification of different theories and equations, making it a powerful tool for theoretical physicists.

## 3. How does conformal invariance relate to the concept of scale invariance?

Conformal invariance and scale invariance are closely related concepts. Both involve transformations that preserve certain properties, such as angles or distances. However, scale invariance specifically refers to transformations that preserve only the scale or size of an object, while conformal invariance includes a wider range of transformations that preserve angles as well.

## 4. Can the scalar massless wave equation be applied to all physical systems?

The scalar massless wave equation is a fundamental equation in physics and can be applied to a wide range of physical systems, including electromagnetic fields, quantum fields, and even gravitational fields. However, it may need to be modified or combined with other equations to accurately describe certain systems, such as those involving strong gravitational or quantum effects.

## 5. What are the implications of conformal invariance for the study of the universe?

Conformal invariance has important implications for the study of the universe, particularly in the fields of cosmology and string theory. It allows for the development of more comprehensive and elegant theories that can better explain the behavior of the universe at both the smallest and largest scales. It also provides a framework for understanding the underlying symmetries and structures of the universe, which can lead to new insights and discoveries.

• Special and General Relativity
Replies
4
Views
1K
• Special and General Relativity
Replies
7
Views
1K
• Special and General Relativity
Replies
144
Views
6K
• Special and General Relativity
Replies
1
Views
1K
• Special and General Relativity
Replies
10
Views
1K
• Special and General Relativity
Replies
7
Views
2K
• Special and General Relativity
Replies
6
Views
4K
• Special and General Relativity
Replies
9
Views
1K