PhysicsStop is one today!

That means I’m a year older than I was when I wrote the first entry, give or take a few nanoseconds as a result of special and general relativistic effects while on aircraft journeys. Eeek.

There are some lovely physics demonstrations that get repeatedly wheeled-out for things like Open Day and visits from school groups. Things like holding a spinning bike wheel on a rotating chair (flip it over and you start rotating – conservation of angular momentum) and levitating a piece of superconductor above a magnet at liquid nitrogen temperatures.  But one thing that I’ve yet to get my hands on to demonstrate is Cavorite.

I think it’s an excellent suggestion, but with two minor drawbacks. One, it comes with significant health and safety concerns, and two, it is fictional.

The material Cavorite makes an appearance in H.G. Wells’ novel "The First Men in the Moon". Its basic characteristic is that it is impervious to gravity – that is, if you put a sheet of Cavorite on the floor, anything you put on top of the Cavorite will be weightless.  Now, you can imagine that would be great fun, but it plays havoc with air pressure which is why one needs an appropriate risk management strategy before children can be allowed near it.

In the novel, Dr Cavor uses it to power a space ship to the moon – first of all he uses it to control air pressure to blast his ship off from the earth, then by opening and closing shutters of Cavorite he allows the moon’s gravitational attraction to pull  the ship towards its destination, but shuts off that of the earth.

A fun concept for a novel, but more seriously we can ask whether it is at all plausible that Cavorite could exist. For example, it is possible to shield the electrostatic force - simply putting a sheet of electrically conductive material between two charges will mean that neither can ’see’ each other (there are issues though – both charges will be attracted to the sheet)  – so why not gravity? Now, in some ways electrostatics is similar to gravity – both obey the inverse square law for example, but in other ways it is not. There are positive and negative charges, but there is not positive and negative mass. Gravity is always attractive. From what we understand from General Relativity, a mass distorts space-time, which will be felt by another mass. To create Cavorite, we have somehow to put a rip through space-time. Can it be done?

It is perhaps just possible that the Higgs Boson can shed some light on this. I don’t think anyone seriously expects its discovery  (if it happens) and subsequent analysis will help us create Cavorite (would we want to?)  but it will probably help us to understand just what mass really is. And that is of great interest to physics. Roll on Large Hadron Collider switch-on.

One of my undergraduate students has been researching gravitational waves this year. Last Friday, he gave a nice presentation on the subject.

Gravitational waves are one of the many examples of waves in physics. We are perhaps more used to waves on the surface of water, or waves along a guitar string, or electromagnetic waves (such as radio waves and light), and, in many ways, gravitational waves aren’t much different.

But they are a little strange. Whereas a radio wave travels through space and time, a gravitational wave (caused for example by a supernova)  travels ‘on’ space and time, rather like a water wave (caused by throwing a stone into a pond,) travels on the surface of water. This means that space-time distorts as the wave goes past. When a gravitational wave hits us front on, we will alternately squash in height and expand widthways, before squashing widthways and growing in height (the preferred option for most of us).

These changes in lengths are not some mathematical construction, they are real. At least, they are predicted to be real, but, to date, no-one has actually detected a gravitational wave. The problem is, that unless you are standing next to a supernova, the changes in length due to gravitational waves are very small indeed.  Imagine a rod the same length as the distance from the earth and the sun.  Now imagine it growing in length by about the width of an atom. That is the sort of distortion we are talking about. Not surprising that no-one has built a detector sensitive enough yet.

But that doesn’t stop people trying. Detectors on earth are limited by, for example, seismic vibrations, and the constraints on how large an object you can build. But space doesn’t have those problems. And so there is the LISA concept; three satellites in a large triangle 5 million kilometres apart, following the earth in its orbit around the sun, linked with laser beams. And you thought the Large Hadron Collider was ambitious.

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