We know lots about gravity in a vacuum but very little about gravity inside extremely massive objects. But astrophysicists say the Sun could reveal all
Einstein's theory of general relativity is one of the cornerstones of modern physics. As such, it is unquestionably a towering achievement. And yet it also raises uncomfortable questions for physicists. The most widely discussed is the conflict between relativity and quantum mechanics.
But there are other less well known problems. Einstein's equations tell us about gravity in a vacuum at some distance from a massive body. This distortion of spacetime by a massive body is famously analogous to a heavy ball on a rubber sheet.
But what of gravity inside a massive body such the Sun or a neutron star? In a weak gravitational field, like that inside the Earth, Einstein's equations reduce to their Newtonian equivalent and are well understood.
But in a much stronger field, the answer is not so clear cut because nobody knows how the distortion of spacetime occurs inside matter.
This coupling is a mystery and various theorists have proposed modifications to the theory that make little or no difference to gravity in a vacuum but have important implications for gravitational fields inside big, massive things.
That's important for the way astrophysicists think about things like neutron stars, since the strength of gravity inside the star determines its internal structure. But since these effects only come into play in extreme fields, nobody has thought of a way to distinguish them from plain vanilla gravity.
Until now. Today, Jordi Casanellas at the Technical University of Lisbon in Portugal and a few pals say that these small modifications to gravity should influence the internal structure of the Sun and that we ought to be able to spot them with measurements we can make today (or at last constrain how big they can be).
The workings of the Sun are reasonably well understood. Astrophysicists assume that our star is in hydrostatic equilibrium--the force of gravity exactly balances the hydrostatic pressure generated by the fusion of hydrogen to form helium.
This assumes an entirely Newtonian gravitational field inside the Sun. So any small modifications should have a significant effect. "Any corrections would affect the thermal balance and, in turn, the temperature profile inside the star, leaving potentially observable signatures," say Casanellas and co.
Perhaps the best window into the solar interior comes from the flux of neutrinos it emits. That's because the rate of neutrino production is highly sensitive to temperature. In particular, the neutrinos produced during the decay of boron-8 are very sensitive to temperature and so a good litmus test of the internal temperature profile.
As it turns out, neutrino telescopes on Earth have accurately measured the flux of neutrinos from this reaction. So this has the potential to place strong constraints on the size of any modification to gravity.
Another way of probing the Sun's interior is to look at the way it vibrates. Again, solar physicists can see the Sun ringing like a bell and this tells them about the internal density and temperature profile.
So what do the current observations tell us about potential modifications to gravity? Casanellas and buddies say current observations place powerful constraints on the size of any new coupling between matter and gravity inside a mass.
But interestingly, the observations do not rule out the main modified theories.
Casanellas and pals take a positive view: "Our results show that the Sun is a very good testing ground to constrain generic modified theories of gravity."
So there may yet be new gravitational physics to find by studying the Sun in more detail.
Ref: arxiv.org/abs/1109.0249: Testing Alternative Theories of Gravity Using the Sun
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