Absence of gravitational-wave signal extends limit on knowable universe

April 10, 2015 by Diana Kwon, Fermi National Accelerator Laboratory
The Holometer is sensitive to high-frequency gravitational waves, allowing it to look for events such as cosmic strings. Credit: Reidar Hahn

Imagine an instrument that can measure motions a billion times smaller than an atom that last a millionth of a second. Fermilab's Holometer is currently the only machine with the ability to take these very precise measurements of space and time, and recently collected data has improved the limits on theories about exotic objects from the early universe.

Our universe is as mysterious as it is vast. According to Albert Einstein's theory of general relativity, anything that accelerates creates , which are disturbances in the fabric of space and time that travel at the speed of light and continue infinitely into space. Scientists are trying to measure these possible sources all the way to the beginning of the universe.

The Holometer experiment, based at the Department of Energy's Fermilab, is sensitive to gravitational waves at frequencies in the range of a million cycles per second. Thus it addresses a spectrum not covered by experiments such as the Laser Interferometer Gravitational-Wave Observatory, which searches for lower-frequency waves to detect massive cosmic events such as colliding and merging neutron stars.

"It's a huge advance in sensitivity compared to what anyone had done before," said Craig Hogan, director of the Center for Particle Astrophysics at Fermilab.

This unique sensitivity allows the Holometer to look for exotic sources that could not otherwise be found. These include tiny black holes and cosmic strings, both possible phenomena from the that scientists expect to produce high-frequency gravitational waves. Tiny black holes could be less than a meter across and orbit each other a million times per second; are loops in space-time that vibrate at the speed of light.

The Holometer is composed of two Michelson interferometers that each split a laser beam down two 40-meter arms. The beams reflect off the mirrors at the ends of the arms and travel back to reunite. Passing gravitational waves alter the lengths of the beams' paths, causing fluctuations in the laser light's brightness, which physicists can detect.

The Holometer team spent five years building the apparatus and minimizing noise sources to prepare for experimentation. Now the Holometer is taking data continuously, and with an hour's worth of data, physicists were able to confirm that there are no high-frequency gravitational waves at the magnitude where they were searching.

The absence of a signal provides valuable information about our universe. Although this result does not prove whether the exotic objects exist, it has eliminated the region of the universe where they could be present.

"It means that if there are primordial cosmic string loops or tiny black hole binaries, they have to be far away," Hogan said. "It puts a limit on how much of that stuff can be out there."

Detecting these high-frequency gravitational waves is a secondary goal of the Holometer. Its main purpose is to determine whether our universe acts like a 2-D hologram, where information is coded into two-dimensional bits at the Planck scale, a length around ten trillion trillion times smaller than an atom. That investigation is still in progress.

"For me, it's gratifying to be able to contribute something new to science," said researcher Bobby Lanza, who recently earned his Ph.D. conducting research on the Holometer. He is the lead author on an upcoming paper about the result. "It's part of chipping away at the whole picture of the ."

Explore further: Do we live in a 2-D hologram? New Fermilab experiment will test the nature of the universe

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12 comments

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docile
Apr 10, 2015
This comment has been removed by a moderator.
DonGateley
3.8 / 5 (4) Apr 10, 2015
Yes! There is no signal! Break out the Champagne!

That's not at all meant to be sarcastic.
Reg Mundy
2 / 5 (4) Apr 12, 2015
I'm still taking bets that no Gravitational Waves will be found in the next 5 years. That is, if they are not found within five years of your bet, I keep your money, if they are found I pay you. Personally, I believe that gravity does not exist as a force, and they will never be found. And thank you to all punters who took my bet before 2010!
Nik_2213
1 / 5 (2) Apr 12, 2015
Hope the SETI folk are taking an interest: such high-frequency gravity waves would be a real-neat transmission system...
viko_mx
1.7 / 5 (6) Apr 12, 2015
I am taking bet than gravitational waves exist only in imagination of some people and craftsmans whoose work is to searching such invented fictional phenomena because they are source for regular income.
derphys
3 / 5 (2) Apr 13, 2015
Everyone looking at a falling apple or in the sky to the moon or the sun sees that gravtitation force is real !!
This force must propagate using gravitational waves in the space at somme speed !!
In the future we will be abble to detect theses waves, extrememely difficult because gravitation is very small at human scale , but dissipation on orbiting binaries stars have shown gravitational waves, and not "in imagination of some people and craftsman" !!!
shavera
3 / 5 (4) Apr 13, 2015
Actually, in the strict sense of the word, the CURRENT mainstream view of gravity is that it's not a force. There is a *fictitious force* associated with being in a non-intertial reference frame, just like the *fictitious force* of "centrifugal force" pushing you towards the outside of a spinning object. There's not really any thing pushing you, you're just choosing a reference frame where inertia isn't perfectly conserved.

So too, if you are not free-falling toward a massive body (or floating freely sufficiently far away from any body), you are in a non-inertial reference frame. And in such a frame, a thing will feel very much like a force, even though it's your choice of frame that "creates" it, and there's no actual "force" in other frames where inertia is conserved.
docile
Apr 13, 2015
This comment has been removed by a moderator.
animah
5 / 5 (2) Apr 13, 2015
Why do you think that the gravitational scalar field should propagate like wave?

Because the speed of gravitational propagation (and its conformance with the inverse square law) has been measured. See for example:
http://relativity...-2001-4/
vidyunmaya
1 / 5 (1) Apr 14, 2015
Sub: Gravitational Waves has limits.
Many thanks for this information-it helps Cosmology studies in right perspective.
Your info: there are no high-frequency gravitational waves at the magnitude where they were searching.
The absence of a signal provides valuable information about our universe

docile
Apr 14, 2015
This comment has been removed by a moderator.
Reg Mundy
not rated yet Apr 15, 2015
Why do you think that the gravitational scalar field should propagate like wave? Because the speed of gravitational propagation
Has this speed http://www2.lbl.g...ty.html? Gravitational wave is the change of time dimension of space-time metric.. In which time dimension the time dimension is supposed to change and propagate? The space-time metric has no additional time available...

If TIME is dependent on the state of matter (and there is no other way of measuring it!) then if "gravity" is a property of matter it follows that what they are attempting to measure using time is a property of time, i.e. a meaningless exercise. In the final analysis, it comes down to what TIME actually is, as explained in my book. measured in TIME, "gravity" is not a force, i.e. does not exist except as an effect we perceive and interpret and model with mathematics.

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