Tuesday, March 31, 2015

New type of gravitational wave detector proposed

The existence of gravitational waves is one of the central predictions of Einstein’s theory of general relativity. Gravitational waves have never been directly observed, though the indirect evidence is so good that Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland, recently said
“We are so confident that gravitational waves exist that we don’t actually need to see one.”
But most scientists for good reasons prefer evidence over confidence, and so the hunt for a direct detection of gravitational waves has been going on for decades. Existing gravitational wave detectors search for the periodic stretching in distances caused by a the gravitational wave’s distortion of space and time itself. For this one has to very precisely measure and compare distances in different directions. Such tiny relative distortion can be detected very precisely by an interferometer.

In the interferometer, a laser beam is sent into each of the directions that are being compared. The signal is reflected and then brought to interfere with itself. This interference pattern is very sensitive to the tiniest changes in distance; the longer the arms of the interferometer, the better. One can increase the sensitivity by reflecting the laser light back and forth several times.

The most ambitious gravitational wave interferometer in planning is the eLISA space observatory, which would ping back and forth laser signals between one mother space-station and two “daughter” stations. These stations would have distances of about 1 mio kilometer. The interferometer would be sensitive to gravitational waves in the mHz to Hz range, a range in which one expects signals from binary systems, probably one of the most reliable sources of gravitational waves. eLisa might or might not be launched in 2028.

In a recent paper now two physicists from Tel Aviv University, Israel, have proposed a new method to measure gravitational waves. They propose not to look for periodic distortions of space, but periodic distortions of time instead. If Einstein taught us one thing, it’s that space and time belong together, and so all gravitational waves distort both together. The idea then is to measure the local passing of time in different locations with atomic clocks to very high precision, and then compare it.

If you recall, time passes differently depending on the position in a gravitational field. Close by a massive body, time goes by slower than farther away from it. And so, when a gravitational wave passes by, the tick rate of clocks, or of atoms respectively, depends on the location in the field of the wave.

The fineprint is that to reach an interesting regime in which gravitational waves are likely to be found, similar to that of eLISA, the atomic clocks have to be brought into distances far exceeding the diameter of the Earth and more like the distance of the Earth to the Sun. So the researchers propose that we could leave behind a trail of atomic clocks on our path around the Sun. The clocks then would form a network of local time-keepers from which the presence of gravitational waves could be read off; the more clocks, the better the precision of the measurement.

Figure from arXiv:1501.00996

It is a very ambitious proposal of course, but I love the vision!


Chaitanya Dev said...

Correction- '1 million km' in fourth paragraph.

Uncle Al said...

"prefer evidence over confidence" Religion never offers overflowing collection plates, then take what you want.

1) Build an astronomical interferometer with aperture synthesis of a segmented radio telescope. Resolution is effective antenna diameter/wavelength, amplitude is summed area of collectors. Elements in the South Pole, Australia, South Africa, South America; North Pole, Canada, Svalbard, Siberia.

2) Observe gravitational wave emitters: 2.01 solar mass 25.58 Hz PSR J0348+0432 plus 0.172 solar-mass white dwarf in a 2.46-hour binary system. Gravitation radiation orbital decay is GR-predicted -8.6 μsec/yr.

3) Ripples in spacetime periodically distort the pulsar signal. Add phase-lock detection.

L. Edgar Otto said...

I like this idea also. I especially like the Fibonacci ratios of frequency powers. But it brings to my mind the early experiments of Michelson-Morley (on steroids!) and all the old books that questioned how sound the experiment and theory was. If GW were evidence by this way, not the least of it building on Newton for a reference frame of calculations as the authors noted, would it not give a better status to a dynamic superderministic idea of time which may override our quantum edifice into a more general physics?

It is like at some point in Einstein's theory, in GR what seemed a general grounding he said "we may as well call it aether." to use the old term in a different way.

I get the feeling that I need black chalk for the blackboard or maybe white chalk for the white board like when I first opened a book on this subject. GW proves a much deeper mystery... but even if not found by such methods at that stage - If we garbled the systems of general relativity from radically different approaches in a wider fundamental cosmic theory, it would still be general relativity.

But we cannot see too deeply into the sun if the Hydrogen is ionized so without electrons how would we see the penetrating (through the dust) X-rays?

nemo said...

It is one of the most fascinating argument of GR... the most difficult to accept for me...

Randall Smith said...

There's good and bad news on eLisa. It wasn't selected for ESA's L2 slot (the 2028 launch), which went to the Athena X-ray Observatory mission. However, it was selected for the L3 slot, with a 2032 launch, so the plan is for something to happen. That said, I've heard strong suggestions from the advanced LIGO team that they have good reason to expect a direct detection sometime in the next two years; if so, I suspect there will be greatly renewed interest in a LISA-type mission.

hush said...

Dear Alice,

What do you think?

What do I think about what?

What do you think about your beautiful and attractive afterglow?


Our afterglow makes us attractive to everyone, not just physicists.
Afterglow is timeless. You can see our contours there too. Imagine a 'contour' (as in gravitational waves) being there as part of our afterglow in the form of information making us everyone's darling, not just the darling of all physicists.

Bob: That's our (lure of) mutual gravitational attraction for you.

Bonkers Bob

Sabine Hossenfelder said...

Randall: Thanks for the info. I thought I had read this somewhere, but then I couldn't find it on the website. Best,


L. Edgar Otto said...

There is still a little romance in the idea that time as the fourth dimension is much a stance as philosophy. I recall, which seems children have a natural sense of time and space, relatives over the coffee table debating simple things like is the moon bigger than the earth About the time that Einstein died 1955 and it was common household news.

So the question I found interesting, like the photo here of 7 years in one place equal to 1 year in another, that would an ant on Neptune live 300 or so years longer? It set off some hours of debate and I think I had a brief time of sleep walking traveling some landscape in my dreams by such discussions at a young age.

My grandmother would meet her lady friends by the Hague in Norfolk which I felt a magical place and say to each other: "Why Thelma, I haven't seen you in a dog's age." To which I said "Aren't we leaving Neal home by himself too long? He must be bored spending his time like this?" or Why will Neal live for so short a time now that he is in ripe old age?"

There is more than afterglow (nice post Hush) between the subjective and objective sense of time - something like the color of Love, or maybe gravity.

CapitalistImperialistPig said...

Perhaps the most important astrophysical implication of gravitational wave detection would be clarification of the nature of gamma ray bursts, especially the short duration ones thought to be caused by neutron star merger.

William Wallace said...

Fascinating, and what I like best, cheap. Hope they can pull this off.

Zephir said...

/* not to look for periodic distortions of space, but periodic distortions of time instead */

Best luck with it..;-) Gravitational waves cannot be periodic by their definition.

Unknown said...

As a non-theoretical physicist, I've never understood why gravitational wave stretching of space wouldn't also stretch the light wave in exactly the same proportion as the physical interferometer, thereby producing a null interferometer signal.
Is there some straightforward explanation?

Zephir said...

/* Is there some straightforward explanation? */

It does generate jobs for physicists and profit for companies involved..

L. Edgar Otto said...

To unknown:

If they are seen Strait then they can go Forward. But if seen forward then they cannot be strictly proportional, they are asymmetric to some ideal limit of being cyclic. So we do not know if in this experiment some deeper measure will not show up. Nor as in the Bicepts data that we can rule out a cyclic universe either (along with early days as inflation).

There could be a unique path thru the jittery maze of it all (Say a god who was a non-theoretician but more of an engineer physicist) in which we may think a gods influence is at best the universe as mechanical. God waves beyond the scope of the physics thus such influence unknowable, invisible, not even a chance disaster as an act of said god would be theoretical evidence.

This is sort of a religious level of debate, the constrained collection plate as Uncle AI points out. Or Deism, loading the dice and walking away. So we are still debating what amounts to the deep connection between QM and GR in a Bigger Picture.

NOVA had a program that concerned the place of One God in Jewish history and four eras that established the sacred text. The Greeks complained of an unknown god, theirs at least having material idol representations and polytheistic. But the Kingdom of David and Solomon had a place where this one god lived on earth forever. Containing the ark. Those who followed the Biblical play and made one of gold and such found it to built up a very large charge as a capacitor.

But the kingdom was destroyed and the temple burnt down...Thus the expression in the New Testament "My God, why hast thou forsaken me?" So in exile in Babylon the idea of monotheism (repenting of idolatry despite the feminine element found in the tribal household as fertility worship) became a more general but abstract unifying of the diverse people. I suspect too the bias against god as women still prevalent in the world today.

But we do not need to appeal to philosophy to answer some of the questions you raise on a straightforward level as these could be explained a little deeper by existing problems in our mathematics.

Do you understand that two photons moving in opposite direction is not double the speed of light? (the time slowed twin is the one undergoing acceleration). Is there an analog to this concept in the vague so called non-linear for this proposed experiment? Nature seems to give us an example of a straightforward answer, some things have direction but no magnitude when we condense them according to our first blush models.

Uncle AI, we are trying to express the aperture of thermodynamic equilibrium that in a closed container (such as Planck et al considered for quantum theory) as depending on the square root of 2 when in 4D the invariant graph of lenght requires an aperture of the golden ratio (the slowest growing irrational number). Phase lock detection would require equal consideration for the finite or Klein inverse groups as well as the massive Lie group formulations. A flattened shadow of a polytope in an odd dimension into an odd dimension has a center not as 0 but equals 2. Euler.

Can we get any simpler or straightforward than that, a new beginning really, or for that matter very much more complicated if we want to make sense of the universe?

Zephir, would your rather flat aether ideas have an even or odd center or perhaps the question of both - even for our advanced concept of a general brane dynamics?

L. Edgar Otto said...

Pardon, should read even dimension into an odd dimension (Euler). Our conception of odd and even centers are too strict as our separate formulas apply them. Logic and consistency or the scope of uniform laws over the universe is somewhat tentative to the level of our current experiments and understanding.

hush said...

Abandon the label 'stretch'.
The label is everything except straightforward.

Onto the topic. There are tools for every scale.

Nothing is revealed. Now what?

What is wrong? The scale? Or the tools searching at this scale?

What is the tool for invariance?

Sabine Hossenfelder said...


You're not measuring and comparing wavelengths, you're measuring path lengths, but the speed of light isn't affected. Best,


Albert Zotkin said...
This comment has been removed by the author.
Sabine Hossenfelder said...

I deleted your comment because our comment policies, which you apparently didn't read, don't allow for links. A nice day to you too,


PS: It's called mainstream

Albert Zotkin said...

I meant mainstream, It was a typo, I know very well the name of that religious, sectarian and ENDOGAMIC cult of yours dubbed mainstream. I challenge you to recover my short and inoffensive comment from your trash and post it without its links.

Regards, A nice Easter Egg for you too.

Sabine Hossenfelder said...

Recovery of deleted comments not possible. Besides this, I reserve the right to delete whatever I want. If you want to insult other people, please do so elsewhere, it is not welcome here.

kashyap vasavada said...

Hi Bee: Interesting proposal, but may be unrealistic.How many atomic clocks they plan to set up in the orbit? Who will maintain the clocks?!!

Sabine Hossenfelder said...


I think at least 3 (see image). Of course the central question is: who will pay. Unfortunately, I am afraid the timescale for this proposal would be even longer than eLISA :/ Otoh, by the time it could be launched the precision of atomic clocks would likely be even higher. Best,


Uncle Al said...

A modern GPS satellite has at least two miniature cesium plus up to two rubidium atomic clocks. Atomic clocks occasionally hiccup.
Fig. 2.2, p. 14; Fig 3.1, p. 16

Albert Zotkin said...

I wonder how many thousand of years will physicists keep looking for gravitational waves.

Henning said...

The improvement in timekeeping precision over the last decade has been nothing short of astounding, and the NIST record holders have a well defined program to keep pushing the envelope.

This is a very smart proposal and I hope it will be funded.