# Linus Pauling, General Chemistry 1-2 - Does Light has mass?

Linus Pauling, General Chemistry 1-2:

"...it was pointed out by Einstein that energy also has a mass, and that light is accordingly attracted to matter by gravitation."

Isn't that wrong? It is curved by the fact that spacetime itself curves.

"1-2. Mass and Energy

Matter has mass, and any portion of matter on the earth is attracted toward the center of the earth by the force of gravity; this attraction is called the weight of the portion of matter. For many years scientists thought that matter and radiant energy could be distinguished through the possession of mass by matter and the lack of possession of mass by energy. Then, early in the present century (1905), it was pointed out by Albert Einstein (1879-1955) that energy also has mass, and that light is ac- cordingly attracted by matter through gravitation. This was verified by astronomers, who found that a ray of light traveling from a distant star to the earth and passing close by the sun is bent toward the sun by its gravi- tational attraction. The observation of this phenomenon was made during a solar eclipse, when the image of the star could be seen close to the sun. The amount of mass associated with a definite amount of energy given by an important equation, the Einstein equation, which is an essen- tial part of the theory of relativity: E= mc² (1-1) In this equation E is the amount of energy (J), m is the mass (kg), and c is the velocity of light (m s-1).* The velocity of light, c, is one of the funda- mental constants of nature;t its value is 2.9979 X 10 meters per second."

So one can rather say that spacetime curvature is not a causal explanation for gravity. It's just a geometric analogy? Does what Linus Pauling writes then make sense again? He has somehow a Newtonian way of looking.

• He's just using "massive" to mean "able to have its path affected by gravity". By that definition everything is massive. But by modern definitions, light is not. Nov 21 '21 at 20:16

As I understand the first quote is formulated really badly and a little bit wrong but not completely, as like you said the path light takes is due to the curvature of spacetime; Spacetime curvature isn't an analogy, in Einsteins formulation of general relativity spacetime curves due to energy being present, we can see this in the field equation: $$R_{\mu \nu}-\frac{1}{2} R g_{\mu \nu}+\Lambda g_{\mu \nu}=\kappa T_{\mu \nu}$$ here $$T_{\mu \nu}$$ is the energy-momentum tensor (to which all the present mass but $$\textbf{also light}$$ contributes as it has energy). This equation then locks up the metric $$g_{\mu\nu}$$ (The metric captures all the geometric and causal structure of spacetime, i.e the curvature) from which we can calculate the christoffel symbols $$\Gamma_{c a b}=\frac{1}{2}\left(\frac{\partial g_{c a}}{\partial x^{b}}+\frac{\partial g_{c b}}{\partial x^{a}}-\frac{\partial g_{a b}}{\partial x^{c}}\right)$$ ultimately giving us the possibility to determine the geodesic $$\frac{d^{2} x^{\mu}}{d s^{2}}+\Gamma_{\alpha \beta}^{\mu} \frac{d x^{\alpha}}{d s} \frac{d x^{\beta}}{d s}=0$$ of the particle (e.g light) we're investigating. This geodesic is essentially how the is particle moving in a straight line on curved spacetime.