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Has radiation or energy bending spacetime ever been observed?

If not, is it likely that it ever will, assuming current technology?

Note: This is not a question of space bending light, but of light bending space.

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Veedrac
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1 Answers1

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In short no. but...

While light has no mass, it does have non-zero energy. This means light contributes to the stress-energy tensor that in turn determines the gravitational field a.k.a. the shape of space. It takes a large energy to produce a measurable gravitational field, so we likely won't directly measure the bending of space due to light.

Compare the energy of a gamma-ray photon: $E\approx 10^6\, \mathrm{eV}$ to the energy of a typical terrestrial object $E=mc^2\approx 1 \mathrm{kg} \cdot c^2 \approx 5\times 10^{35} \mathrm{eV}$. We have trouble measuring the gravitational field of 1 kg objects, so a single photon is out of the question.

There is one place where the energy of photons matters for gravity: cosmology. It's possible to consider a universe filled with nothing but photons, we call it a 'photon gas'. The energy of the light changes the cosmological dynamics relative to an empty universe. This is called a 'Radiation dominated' cosmology. This model is actually a pretty good approximation for the early universe, which is filled with very fast, very small particles.

In that sense by learning about the dynamics of the early universe we can study the gravitational field of light.

Paul T.
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  • Thank you. It's worth considering that there are cases where large amounts of energy are expelled at once, such as in supernova. I believe the first observations of space bending were of gravitational lensing around a star, so it's not like we're constrained to worldly physics here! – Veedrac Jun 03 '14 at 21:47