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Spacetime: A smoother brew than we knew?

Aug. 23, 2012
Courtesy of Marcia Goodrich/Michigan Tech
and World Science staff

The fabric of space and time may be less like beer than like smooth whis­key. Or so an in­ter­ga­lac­tic pho­to fin­ish would seem to sug­gest.

Phys­i­cist Rob­ert Ne­miroff of Mich­i­gan Tech­no­log­i­cal Uni­vers­ity reached this heady con­clu­sion af­ter stu­dy­ing traces of three dif­fer­ent pho­tons, or par­t­i­cles of light, recorded by NASA’s Fer­mi Gamma-ray Space Tel­e­scope in May 2009.

The par­t­i­cles orig­i­nat­ed about sev­en bil­lion light years away from Earth in one of three pulses from an enor­mous ex­plo­sion known as a gamma-ray burst. A light year is the dis­tance that light trav­els in a year, so sev­en bil­lion of these is a mind-boggling dis­tance and it rep­re­sents more than half the size of the known uni­verse. De­spite that dis­tance, the pho­tons ar­rived at the or­bit­ing tel­e­scope just one thou­sandth of a sec­ond apart, in a vir­tu­al tie.

Gamma-ray bursts are short-lived bursts of gamma-ray pho­tons, the most en­er­get­ic form of light. They can orig­i­nate far across the uni­verse, and as­tro­no­mers be­lieve many are caused by gi­ant stars col­laps­ing, of­ten bil­lions of years be­fore the Earth was formed.

“Gamma-ray bursts can tell us some very in­ter­est­ing things about the uni­verse,” Ne­miroff said. In this case, those three pho­tons recorded by the Fer­mi tel­e­scope sug­gest that space­time may not be not as bubbly as some sci­en­tists think, he ex­plained.

Cer­tain the­o­ries of phys­ics, known as quan­tum gra­vity the­o­ries, say the uni­verse is not smooth but foam­y—made of fun­da­men­tal un­its called Planck lengths. These are less than a tril­lionth of a tril­lionth the width of a hy­dro­gen at­om. 

Planck lengths are so small that there’s no way to de­tect them—ex­cept via pho­tons like those that make up gamma-ray bursts. This is due to the wave­lengths of the pho­tons. “Wave­length” lit­er­ally means the length of a wave from front to back. Pho­tons, in ad­di­tion to be­ing par­t­i­cles, are al­so dis­cern­i­ble as a type of wave, with a meas­ur­a­ble wave­length. For these par­tic­u­lar pho­tons, the wave­lengths are are some of the short­est dis­tances known to sci­ence. This should al­low the waves to in­ter­act with even the un­fath­omably small Planck length. And if they do so, the pho­tons should be scat­tered during their trek through Planck length–pix­e­lated space­time.

In par­tic­u­lar, they should dis­perse in dif­fer­ent ways if their wave­lengths dif­fer, just as a ping pong ball and a soft­ball might take al­ter­nate paths down a gravely hill­side. You would­n’t no­tice the scat­ter­ing over short dis­tances, but across bil­lions of light years, the Planck lengths should dis­perse the light. And three pho­tons from the same gamma-ray burst should not have crashed through the Fer­mi tel­e­scope at the same mo­ment.

But they did, and that calls in­to ques­tion just how foamy space­time really is. “We have shown that the uni­verse is smooth across the Planck mass,” Ne­miroff said. “That means that there’s no chop­pi­ness that’s de­tectable. It’s a really cool disco­very. We’re very ex­cit­ed.”

A team of re­search­ers at the Uni­vers­ity of Mel­bourne in Aus­tral­ia and else­where re­cently pro­posed an al­ter­na­tive test of wheth­er space is “pix­e­lat­ed,” based on “cracks” that would form in the struc­ture of the uni­verse if it were.

The work by Ne­miroff and col­leagues was pub­lished June 8 in the jour­nal in Phys­i­cal Re­view Let­ters.


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Spacetime may be less like beer than like smooth whiskey. Or so an intergalactic photo finish seems to suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying traces of three different photons, or particles of light, recorded by NASA’s Fermi Gamma-ray Space Telescope in May 2009. The particles originated about seven billion light years away from Earth in one of three pulses from an enormous explosion known as a gamma-ray burst. A light year is the distance that light travels in a year, so seven billion of these is a mind-boggling distance and it represents more than half the size of the known universe. Despite that distance, the photons arrived at the orbiting telescope just one thousandth of a second apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. “Gamma-ray bursts can tell us some very interesting things about the universe,” Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think, he explained. Certain theories of physics, known as quantum gravity theories, say the universe is not smooth but foamy—made of fundamental units called Planck lengths. These are less than a trillionth of a trillionth the width of a hydrogen atom. Planck lengths are so small that there’s no way to detect them—except via photons like those that make up gamma-ray bursts. This is due to the wavelengths of the photons. “Wavelength” literally means the length of a wave from front to back. Photons, in addition to being particles, are also discernible as a type of wave, with a measurable wavelength. For these particular photons, the wavelengths are are some of the shortest distances known to science. This should allow the waves to interact with even the unfathomably small Planck length. And if they do so, the photons should be dispersed—scattered—on their trek through Planck length–pixelated spacetime. In particular, they should disperse in different ways if their wavelengths differ, just as a ping pong ball and a softball might take alternate paths down a gravely hillside. You wouldn’t notice the scattering over short distances, but across billions of light years, the Planck lengths should disperse the light. And three photons from the same gamma-ray burst should not have crashed through the Fermi telescope at the same moment. But they did, and that calls into question just how foamy spacetime really is. “We have shown that the universe is smooth across the Planck mass,” Nemiroff said. “That means that there’s no choppiness that’s detectable. It’s a really cool discovery. We’re very excited.” A team of researchers at the University of Melbourne in Australia and elsewhere recently proposed an alternative test of whether space is “pixelated,” based on “cracks” that would form in the structure of the universe if it were. The work by Nemiroff and colleagues was published June 8 in the journal in Physical Review Letters.