# Bondi news in BMS supertranslation

What does "news" mean in "Bondi news"? Is it information? What about memory in gravitational memory, is it information as well?

• General tip: In order to receive useful answers, consider to add context and references. – Qmechanic Dec 14 '17 at 11:10

The Bondi news $$\mathcal{N}$$ does indeed describe the gravitational radiation in an asymptotically flat universe. And since gravitational waves contain information about the process that emitted them, you could certainly say that the news contains information. That was the original intent behind naming it the "news", because the research was directed toward finding the news it brings us of some phenomenon from the distant cosmos.

The name was first introduced in this paper by Bondi, van der Burg, and Metzner. They coined the term in section D.1 with this description:

Thus if anything happens at all at the source leading to changes in the field, it can only do so by affecting $$c_0$$, and vice versa. Thus all the news in the field is contained in $$c_0$$, which therefore merits the name news function.

[Emphasis theirs.] Though I haven't seen any real agreement on the topic, I think the most common notation for this quantity has changed over the years from $$c_0$$ to $$\mathcal{N}$$. (It's also important to note that this paper introduced much of the BMS group, though it was completed only by Sachs in this paper. Bondi, Metzner, and Sachs are the BMS in the group name. I don't know why van der Burg was kicked out. Maybe the BMvdBS group isn't as catchy.)

To be a little more clear, we generally approximate emission of gravitational waves as coming from some isolated source — like a binary black hole system — and model it as if we lived in an asymptotically flat universe. (We don't, if the standard ΛCDM model is anything close to correct; but asymptotic flatness is still a useful model.) Then, we pretend that our gravitational-wave detectors are basically "on" future null infinity. So LIGO, for example, will measure the strain $$h$$ of gravitational waves. Up to an irrelevant time derivative, the strain contains the same information as the Bondi news and the Newman-Penrose quantity $$\Psi_4$$. More specifically (up to certain sign conventions), we have $$$$\Psi_4 = \frac{d}{dt}\bar{\mathcal{N}} = -\frac{d^2}{dt^2}h$$$$ Any of these three will typically be produced by gravitational-wave models. The "integration constants" that are different between them are extremely low-frequency signals that can't actually be measured by realistic detectors, so we treat the equivalently.

The gravitational-wave memory is just a component of the gravitational waves given off. When there is memory, it changes the strain in the distant future relative to the distant past. But even then, because it is time-dependent, so it will show up in the news and $$\Psi_4$$.

I should point out a few issues with your question. First, contrary to the tags you put on this question originally, this doesn't really have much to do with quantum field theory. There are some extremely speculative quantum theories that involve the BMS group, but the news doesn't exactly go hand-in-hand with it. Also, I don't know of anything that this has to do with supersymmetry. The BMS group is a symmetry group, and does involve supertranslations, but not supersymmetry. And you wouldn't say that the news is "in" the BMS supertranslation. It is a gauge-dependent quantity that could be affected by the supertranslation, but that's about it.