# How light gravitates: a brief exploration

### Wednesday, 16 Sep. 2015, 11:00 - 12:00

**Presenter:** Dr. Dennis Rätzel

**Host:** Philip Walther

**Where:** Schrödinger room, 4th floor, Boltzmanngasse 5

As Einstein's equations tell us that all energy is a source of gravity, light must gravitate. However, because changes of the gravitational field propagate with the speed of light, the gravitational effect of light differs significantly from that of massive objects. In particular, the gravitational force induced by a light pulse is due only to its creation and annihilation and decays with the inverse of the distance to the pulse.

We can expect the gravitational field of light to be extremely weak. On the other hand, we have good control over the modulation of energy density, as well as the geometry and entanglement of light. Also, light interacts only at short ranges via other forces. Therefore, light could be a viable source of gravitational fields in the laboratory.

Furthermore, the properties of light are premises in the foundations of modern physics: they were used to derive special and general relativity and are the basis of the concept of time and causality in many alternative models. Studying the back-reaction of light on the gravitational field could give new fundamental insights to our understanding of space and time as well as classical and quantum gravity.

In this talk, a brief overview is given of the gravitational field of light pulses in the framework of general relativity. A glimpse is caught of the gravitational interaction of single photons.