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his
the
the
re commemorating
niversary of arguably
experiment of
We're going to commemorate its
importance 90 yearson by going out to
Principe for the anniversary.It's not
quite as challenging to get to as it was
in Eddington'stime. Back then it took
him a month and a half by ship. Still,
it's not the most convenientplace in the
world to get to even today; there's only
one flight a week, and you've got to stop
off on the African mainland. We've met
with the island'sgovernor,and he'skeen
to back the commemoration as a way to
bring overseasinvestmentand tourism
to the island.
Weie going to lay a plaque on 29
May at the place where Eddington
carried out his experiment,hold a
commemoration cergmonyand give a
seriesoftalks. I'm particularly keen to
seethe plaque unveiled becauseI
designedit. On the way out we'realso
going to be speakingin Lisbon, because
we've partnered with the Portuguese
ministry of science- Principe used to
be a Portuguese colony. We've also got
an exhibition planned in the island's
capital SdoTom6, explaining what
gravitationallensing is all about.
arriedout by
it was the
'stheoryof
experimentconfirmed that the way we
think about how the Universeoperates
had chaneed forever.
Eddington's experiment involved
observing a solar eclipse to test a
prediction of general relativity. This
statesthat light rays don't actually travel
in straightlines, but can be bent when
gravity'bends
spaceand lime, warping
the whole fabric of the Universe. We
can seethis effect as a star'slight
coming from a different direction than
it really is - the star appearsto move
slightly in the sky. It's an effect called
gravitationallensing.
Scientiststhought that they might be
able to look at starsjust behind Jupiter
to observewarped light rays,but Jupiter
turned out to not be massiveenough to
deflecl light. The only other optioi was
to look at starsjust behind the Sun and
seeif they moved slightly.Ordinarily,
the Sun is too bright to seestarsright
next to it, which meansthat you haveto
wait for a solareclipse.There wasa bit
ofa wait for such an event,because
Einstein publishedhis theory of general
relativity in the middle of the First
World War in 1915,and the first solar
eclipseafter the War was in 1919.
However, it was a particularly good one
becauseit occurred right in front ofthe
Hyadesstar cluster,a densegroup of
stars.Eddington neededa lot ofstars to
seewhether they moved or not.
He had to travel to where the eclipse
wasvisible,which meant going to the
island of Principe in West Africa. His
experiment had to be flawless.For a
responsiblefor Einstein'sfame. Before
start,he sent another expedition to
it, Einstein wasknown in scientific
Brazil, which was also in the path of the circlesbut not in the largerpublic
eclipse,and he also
took imagesof a
secondbunch ofstars theplaque
uweiledbecousearoundtheworldand
that didn't have the
comebackwith results
I des$nedit"
Sun in front ofthem to
verifying that Einstein was
show that any movement of the stars
right and that he was responsiblefor
wasn't causedby his telescope.
changing our view of the world. Many
Eddington'sexperimentreceiveda lot of the newspapersof the time ran big
of attention at the time and was largely
spreadson the experimenl.
"I'm
particularly
keen
tosee:fffiJiillfl'j]f
106SkyatNightMagazine
May2009
About
Richard
Massey
Richardis a STFC
advanced
fellowin
astronomy
at the
RoyalObservatory
Edinburgh.
h his
researchhehas
workedwith the
Herschel,
Subaru,
KeckandHubble
telescopes
to study
thedistribution
of
darkmatterin the
universe.
An astronomical legacy
I got involvedwith the commemoration
becauseI use gravitationallensing
every day to do my own research into
dark matter, and I was interestedin
finding out where the first detectionof
this effect happened.In the sarneway
that Eddington observedlight from the
Hyadesstarsbending as it travelledpast
the Sun, I look at the light from very
distantgalaxiesand observehow it's
bent as it traversesvastregionsof space,
which crucially contain dark matter.
Even though we can't seethe dark
matter, it doeshave mass,which bends
the light from thesedistant galaxiesso
that they appeardistorted.By
measuringthe shapeof thesegalaxies
we'vebeen able to pinpoint where this
dark matter is in the Universe.
Gravitationallensing was a novelty
lor Einstein
didn'tparti
beyondthe
v:
proveEin
bestwayoffinding
mostcommonand
in the Universe:
atter