Photons could uncover ‘massive gravity’,
brand new theory suggests
ravitationalwaves,ripplesinspace-
time,slipthroughEarthallthetime,
carryingsecretsabouttheuniverse.
Untila fewyearsagowecouldn’t
detecttheseatall,andevennowwehaveonlya
basicabilitytodetectthestretchingandsqueezing
ofthecosmos.Aproposednewgravitational-wave
hunter,whichwouldmeasurehowparticlesof
lightandgravityinteract,couldchangethat.Inthe
processitcouldanswerbigquestionsaboutdark
energyandtheuniverse’sexpansion.
ThedetectorsonEarthtoday,calledtheLaser
InterferometerGravitational-WaveObservatory
(LIGO)andVirgo,operateaccordingtothesame
principle:asa gravitationalwavemovesthroughthe
Earth,itfaintlystretchesandsqueezesspace-time.
Bymeasuringhowlonga laserlighttakestotravel
overlongdistances,thedetectorsnoticewhenthe
sizeofthatspace-timechanges.Butthechanges
areminute,requiringextraordinarilysensitive
equipmentandstatisticalmethodstodetect.
AteamledbySubhashishBanerjee,a physicist
attheIndianInstituteofTechnologyinJodhpur,G
India,proposeda radicalnewmethod:hunting
gravitationalwavesbylookingforeffectsof
directinteractionsbetweengravitons– theoretical
particlesthatcarrygravitationalforce– andphotons,
theparticlesthatmakeuplight.Bystudyingthose
photonsafterthey’veinteractedwithgravitons,
youshouldbeabletoreconstructpropertiesofa
gravitationalwave.Sucha detectorwouldbemuch
cheaperandeasiertobuildthanexistingdetectors,
Banerjeesaid.“Measuringphotonsissomething
whichpeopleknowverywell,”saidBanerjee.“It’s
extremelywell-studied,anddefinitelyitisless
challengingthana LIGOkindofset-up.”
Nooneknowsexactlyhowgravitonsand
photonswouldinteract,largelybecausegravitons
arestillentirelytheoretical.Buttheresearchers
madea seriesoftheoreticalpredictions.When
a streamofgravitonshitsa streamofphotons,
thosephotonsshouldscatter.Thatscattering
wouldproducea faint,predictablepattern,
whichphysicistscouldamplifyandstudyusing
techniquesdevelopedbyquantumphysicistswho
studylight.YOUR FIRST CONTACT WITH THE UNIVERSE
LAUNCH PAD
ININCOOPERATIONCOOPERATIONWITHWITH
Words by Rafi LetzterBy studying
how gravitons
and photons
might interact,
we could
detect these
waves better© Getty
Linking the physics of the tiny quantum world
with the large-scale physics of gravity and relativity
has been a goal of scientists since Albert Einstein’s
time. But even though the newly suggested
approach to studying gravitational waves would use
quantum methods, it wouldn’t fully bridge that tiny-
to-large-scale gap on its own, Banerjee said. Probing
the direct interactions of gravitons might solve
some other deep mysteries about the universe,
though, he added.
Banerjee and his team showed that the way the
light scatters would depend on the specific physical
properties of gravitons. According to Einstein’s
theory of general relativity, gravitons are massless
and travel at the speed of light. But according to a
collection of theories, together known as ‘massive
gravity’, gravitons have mass and move slower than
the speed of light. These ideas, some researchers
think, could resolve problems such as dark energy
and the expansion of the universe. Detecting
gravitational waves using photon scattering,
Banerjee said, could have the side-effect of telling
physicists whether massive gravity is correct.