3 ms·

How can you measure the strength of the gravitational constant if you don't know the masses of the objects in question?

greglindahl 10y ago
The Chandrasekhar mass depends on G and a bunch of fundamental constants, and that's the mass of a type Ia supernova. Unfortunately this Wikipedia article doesn't talk about astronomy much: https://en.wikipedia.org/wiki/Gravitational_constant#History_of_measurement https://en.wikipedia.org/wiki/Gravitational_constant#History... But it does point at one measurement of G on cosmological timescales and distances from 2006: https://arxiv.org/abs/1402.1534 https://arxiv.org/abs/1402.1534 There are other papers with similar measurements published in the past two decades.
- Armisael16 10y ago
  That's only true if all the other physics involved are the same wherever that supernova is; your own link mentions this. We can't prove that's the case since we can't test the individual parts of that theory - it's possible to construct a different set of physics that produces results identical to the physics we're familiar with. That isn't a particularly fruitful approach since it doesn't permit any real discovery - it's rather brain-in-a-jar - but it is still something you have to assume away.
dredmorbius 10y ago
There are some phenomena which allow good remote assessments of properties, based on various interactions. Cephid variable brightness, for example, serving to validate the expanding universe theory. The stellar Main Sequence gives a good sense of stellar masses. Spectroscopy works pretty well across a wide span of the Universe (several billions of light years). Etc. Since these operate in concert across the observable universe, and themselves involve various other interactions (elements, strong and week nuclear forces, gravity, speed of light, rates of hydrogen fusion, etc.), we can conclude that either there is no appreciable change in any of the underlying fundamental constants or that change occurs in a compensated fashion such that no net change is detectable. The second fails Occam's Razor. The conclusion that the laws of physics appear to be similar throughout observable space seems robust.

The Chandrasekhar mass depends on G and a bunch of fundamental constants, and that's the mass of a type Ia supernova. Unfortunately this Wikipedia article doesn't talk about astronomy much: https://en.wikipedia.org/wiki/Gravitational_constant#History_of_measurement https://en.wikipedia.org/wiki/Gravitational_constant#History... But it does point at one measurement of G on cosmological timescales and distances from 2006: https://arxiv.org/abs/1402.1534 https://arxiv.org/abs/1402.1534 There are other papers with similar measurements published in the past two decades.

Armisael16 10y ago
That's only true if all the other physics involved are the same wherever that supernova is; your own link mentions this. We can't prove that's the case since we can't test the individual parts of that theory - it's possible to construct a different set of physics that produces results identical to the physics we're familiar with. That isn't a particularly fruitful approach since it doesn't permit any real discovery - it's rather brain-in-a-jar - but it is still something you have to assume away.

dredmorbius 10y ago

There are some phenomena which allow good remote assessments of properties, based on various interactions. Cephid variable brightness, for example, serving to validate the expanding universe theory. The stellar Main Sequence gives a good sense of stellar masses. Spectroscopy works pretty well across a wide span of the Universe (several billions of light years). Etc. Since these operate in concert across the observable universe, and themselves involve various other interactions (elements, strong and week nuclear forces, gravity, speed of light, rates of hydrogen fusion, etc.), we can conclude that either there is no appreciable change in any of the underlying fundamental constants or that change occurs in a compensated fashion such that no net change is detectable. The second fails Occam's Razor. The conclusion that the laws of physics appear to be similar throughout observable space seems robust.