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I've heard said by many physicists things along the lines of "Since light has no mass, we need general relativity to explain why light is affected by gravity."

But why is this necessary?

We know that light has energy $E=h\nu=\frac{hc}{\lambda}$, and we know that gravity is really based on total energy and not just rest mass, so therefore light has gravitational mass equivalent to $m=\frac{h\nu}{c^2}=\frac{h}{\lambda c}$.

So why can't we just say that it makes perfect sense that light is affected by gravity?

Matt
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1 Answers1

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we know that gravity is really based on total energy and not just rest mass

Before GR, it was thought that gravity acted on rest mass only. So even just to say gravity acts on total energy shows the need for a new theory.

Furthermore, the whole concept of rest mass doesn't exist without SR at the very least.

Señor O
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