BBC Knowledge Asia Edition 3

Can gravitational waves teach

us about the inside of a black hole?

The recent detection of gravitational waves from the

merger of two black holes was a success for astrophysics.

But from the outside it is impossible to tell anything

about the inside of a black hole. The shape of the signal

detected by LIGO revealed the masses of the two merging

black holes, which led to an estimate of how powerful the

event was at its source. But no observations, including

those of gravitational waves, can probe beyond the event

horizon of a black hole. AG

WHAT’S IN...

...TOOTHPASTE

Most of the ingredients in toothpaste are there to make it taste, smell and feel nice,

act as preservatives, or bind everything together. The rest of the ingredients that are

actually included for the good of your mouth include:

SODIUM FLUORIDE

This helps maintain the strength

of tooth enamel and, despite

controversies, is perfectly safe at the

amounts found in toothpaste.

HYDRATED SILICA

This mild abrasive used to

rub away stains and debris.

Alternatively, your toothpaste

of choice might contain

calcium carbonate (chalk),

magnesium carbonate or

aluminium oxides, all of

which do the same job.

SODIUM LAURYL SULPHATE

This detergent crops up in all sorts of products

including shampoos, washing powder and

washing-up liquids. It helps remove fats, but

mostly it’s just included to make a nice foam

(which doesn’t actually help with the cleaning

but we expect bubbles in cleaning products). It’s

this stuff that makes orange juice taste vile after

you’ve cleaned your teeth.

TRICLOSAN

This antibacterial agent appears in many

toothpastes. It helps prevent gum disease, but

it is a cause for concern after you spit. Triclosan

is difficult to remove from sewage and when

it enters the water systems it is toxic to some

aquatic organisms.

&

To keep 1,000-tonne, 150m long Tunnel

Boring Machines (TBMs) like those used

for London’s Crossrail project on track,

engineers rely on a laser-based system.

Precise reference points are set up below

ground behind the TBM, and laser beams

are sent out from them into receivers

in the machine. This keeps the TBMs

heading in the right direction to within a

millimetre or so over distances of up to

100 metres. RM

How do tunneling

machines know

where they are?