Season 17, Episode 39: How Holmberg Simulated Colliding Galaxies Without a Computer

Hey StarTalkians! Episode 39 of season 17 saw Neil and Negin sit down with astrophysicist Mordecai-Mark Mac Low to discuss his work with simulations – and answer some cosmic queries. One part of the initial discussion stuck out to me:

Bug Splat Galaxies with Mordecai-Mark Mac Low - StarTalk Radio

(from 3:50)

It concerns Erik Holmberg, a Swedish astronomer and cosmologist who, in 1941, was able to simulate a galaxy collision without a computer. All it took was one key insight and some technical know-how.

Holmberg’s Key Insight

The crucial insight for Holmberg’s work was a similarity between gravity and light. The intensity of both falls off as you move away according to an inverse square law. This means that if r is the distance from the source, the measurement at that location is proportional to 1/r^2. If you’re twice as far away; the force is four times weaker.

The light given off by a bulb spreads out in all directions, like the surface of an expanding sphere. And the formula for the surface area of a sphere – 4πr^2 – has that same r^2 factor. The energy spreads out across this surface, so the bigger it gets, the weaker the force at any point on the surface.

Holmberg’s Experiment

Holmberg’s original paper describes the set up in meticulous detail, but the basics are all you need to get the picture.

He made two circular groups of 37 bulbs each, with every bulb representing a huge amount of stars in each host galaxy. He fine-tuned the current going to the bulbs so that the amount of light generated from different regions roughly corresponded to the mass distribution of the galaxies.

Starting as the two groups approach, he removed each bulb in turn and replaced it with a light meter. Measuring the light intensity – i.e. the gravitational force – in four directions at each point, he worked out the net gravitational force each “bulb” would experience. Then he moved them one step forwards in time according to this result.

Marking his results on a giant piece of paper – and working in two dimensions for simplicity – he showed how the weird galaxies Neil and Mordecai-Mark were talking about came to be.

Comparing his result to real-world antennae galaxies (see images) shows what a fantastic achievement this was using an ingenious analogue setup.

It wasn’t until 20 years later that actual computers were able to do galactic simulations.