# I Why is the gravitational constant used in GR?

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1. May 11, 2016

### Zak

(precursor: I have not formally studied GR)

I have noticed that the gravitational constant found in classical gravitation is also used in GR. Why is this the case? Am I correct in thinking that the constant was determined by Cavendish and was for the classical theory of gravitation? So, my question: how do we know that the same gravitational constant is suitable for GR and why did we not have to, say, measure a new constant?

2. May 11, 2016

### JorisL

We get $G$ into the Einstein equations by requiring that they reduce to the classical gravitation law in the limit of slow (test-)particles in low gravitational fields.

Essentially the reason is that we want GR to supersede Newtonian gravity.

3. May 11, 2016

### Staff: Mentor

As JorisL mentioned, you can look at is as a requirement so that GR can reduce to Newtonian gravity in the appropriate limit.

Another way to look at it is that G is just a universal conversion factor between different units, so any time that you will convert between the same units you will get a the same conversion factor.

4. May 11, 2016

### Staff: Mentor

You can think of it in terms of reducing to Newtonian gravitation in the appropriate limit. You can also look at it as a conversion factor between different units. So then it is the same in both theories as long as you use the same units.