FEEDBACK

Cosmos readers share their views

WRITE TO US

Letters to the editor must include the writer’s full name, address and daytime telephone number. They may be edited for clarity and length. Please do not send attachments.

AUSTRALIA’S ROLE IN LIGO

I enjoyed the articles in Cosmos 68 on the discovery of gravitational waves. But comprehensive as the coverage was, there was not one mention of the fact, that ALL the amazing optics for the LIGO instruments were made by the CSIRO in Australia.

I was one of the 16 people involved in the project, and we delivered 32 of the world’s smoothest optical surfaces ever fabricated. The waviness or low frequency spatial band ranged from 1.60 to 3.20 nanometres, and a roughness or high frequency spatial band from 0.20 to 0.40 of a nanometre! To put that in context, a human hair is 76,000 nanometres thick. — GLEN DAVIS Francis Lord Optics, Sydney, NSW

A NEW WAY TO DESCRIBE GRAVITY

With gravitational waves now in the public lexicon I think it is time for fresh analogies and images to describe gravity – the way mass warps space-time.

I always thought the analogy for gravity, of a bowling ball on a trampoline, was quite two-dimensional. Now we have gravitational waves being described as ripples on a flat pond. Couldn’t we start using analogies and graphics that reflect the way mass affects the three-dimensional space-time grid? Surely there’s a good comparison out there. Maybe something akin to a magnet placed in the centre of a box of filings to illustrate gravity. And gravitational waves shown like a rippling, layered sphere expanding ever outward from a central core. — MATTHEW HUNT Brisbane, Queensland

JUST WHAT IS SPACE-TIME?

What is space-time and why in your black hole article ( Cosmos 68, page 34) is it represented as a sheet with lines on it? — HARRISON YOUNG Melbourne, Victoria

PAUL DAVIES REPLIES:

Einstein taught us that gravitation is a warping or distortion in the geometry of space-time, which can involve both spacewarps and timewarps. That is, the geometry of space can differ from the geometry we learn at school which involves drawing on flat sheets of paper.

It can be more like the rules of geometry on a curved surface. For example, on the Earth’s surface you can draw parallel lines at the equator (lines of longitude) that nevertheless meet (at the poles). That can’t happen on a flat plane: parallel lines never meet.

Space is of course three-dimensional, but you can’t depict the curvature of three dimensions in a magazine that uses twodimensional sheets of paper, so to depict curved space one has to abandon one of the dimensions of space and represent it as a two-dimensional curve sheet.