APOD: Advanced LIGO: Gravitational Wave... (2016 Feb 07)

[APOD Robot]

Advanced LIGO: Gravitational Wave Detectors Upgraded

Explanation: Accelerate a charge and you'll get electromagnetic radiation: light. But accelerate any mass and you'll get gravitational radiation. Light is seen all the time, but, so far, a confirmed direct detection of gravitational radiation has been elusive. When absorbed, gravitational waves create a tiny symmetric jiggle similar to squashing a rubber ball and letting go quickly. Separated detectors can be used to discern gravitational waves from everyday bumps. Powerful astronomical sources of gravitational radiation would coincidentally jiggle even detectors on opposite ends of the Earth. Pictured here are the four-kilometer-long arms of one such detector: the LIGO Hanford Observatory in Washington state, USA. Together with its sister interferometer in Louisiana, these gravitational wave detectors continue to be upgraded and are now more sensitive than ever.

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[narodnik]

Four-kilometer arms, not two!

[daddyo]

I wonder what's the latest theory as to why gravity waves have been so hard to detect.

[alter-ego]

I wonder what's the latest theory as to why gravity waves have been so hard to detect.

The predicted spatial distortions are exceedingly small, miniscule fractions of a proton size. Previous LIGO generations have been on the marginal end of picking up extreme mass events, e.g 10⁶ solar mass binary mergers. Combined with event size and required proximity for detection, these are rare events. aLIGO is believed to be sensitive enough to see smaller, more numerous (and closer) events like supernovae:

Click to view full size image

Over a 4-km length, the aLIGO can detect an arm-length change 1000 times smaller than a proton! This is an amazing achievement.

aLIGO represents a perceived turning point in gravitational wave detection. As long as it performs per design, I believe scientists will begin to question the physics if no detection is made over a reasonable (years) time frame.

[daddyo]

Thanks!

Given what looks like 5 prior generations, I'd be questioning those old theories myself.

[FLPhotoCatcher]

This sentence can be interpreted two ways: "Separated detectors can be used to discern gravitational waves from everyday bumps." I don't think I create gravity waves when I hit my head on something, though yesterday while helping a friend build his house, I did hit my head hard enough to see stars...

[RJN]

Four-kilometer arms, not two!

Yes, this error has been corrected on the main NASA APOD. The arms are four kilometers long. We apologize for the error.
- RJN

[heehaw]

All the good APOD folks should hear the rumor (from good sources): please read the New York Times this Thursday, February 11. No guarantees, but I hear ... this might be IT!

[Chris Peterson]

This sentence can be interpreted two ways: "Separated detectors can be used to discern gravitational waves from everyday bumps." I don't think I create gravity waves when I hit my head on something, though yesterday while helping a friend build his house, I did hit my head hard enough to see stars...

You generate gravitational waves every time you move. Every heartbeat produces gravitational waves. But even the ability to detect distortions in spacetime 1/1000 the size of proton isn't enough to catch those.

[RJN]

I have now added MIT to the credit list. I apologize (again) for the oversight.
- RJN

[bystander]

I have now added MIT to the credit list. I apologize (again) for the oversight.
- RJN

http://www.advancedligo.mit.edu/

[neufer]

This sentence can be interpreted two ways: "Separated detectors can be used to discern gravitational waves from everyday bumps." I don't think I create gravity waves when I hit my head on something, though yesterday while helping a friend build his house, I did hit my head hard enough to see stars...

You generate gravitational waves every time you move. Every heartbeat produces gravitational waves. But even the ability to detect distortions in spacetime 1/1000 the size of proton isn't enough to catch those.

• Neither you nor your heart is capable of generating a single graviton in your lifetime.

<<Suppose that [two orbiting masses] are m₁ and m₂, and they are separated by a distance r. The power given off (radiated) by this system is:



where G is the gravitational constant, c is the speed of light in vacuum and where the negative sign means that power is being given off by the system, rather than received. For a system like the Sun and Earth, r is about 1.5×10¹¹ m and m₁ and m₂ are about 2×10³⁰ and 6×10²⁴ kg respectively. In this case, the power is about 200 watts.>>

• So the Sun / Earth system radiates only about 200 watts or ~9.5x10⁴² gravitons per second.

But what if we substituted a full sized orrery model with a Sun & Earth of masses 2000 kg & 6 g respectively?
-----------------------------------------------------------------------------------------
The power depends upon the masses squared and the masses have dropped by a factor of 10²⁷; hence the radiation drops by a factor of 10⁵⁴ resulting in a radiation of ~9.5x10^-12 gravitons per second or 3x10^-4 gravitons per year.
-----------------------------------------------------------------------------------------
Things get even worse if we make the orrery a more feasible (1 AU => 150 m) size since gravitational radiation depends upon distances to the fourth power! Now our realistic yearly orrery puts out just 3x10^-32 gravitons per year. Clearly there is not enough time in the life of the universe for our realistic yearly orrery model to emit even a single graviton.
-----------------------------------------------------------------------------------------
Now what if we speed our realistic orrery model up to make an orbit in just one second? Now radiation depends upon frequency to the fourth power whereas graviton energy (= hv) only depends upon frequency to the first power such that graviton production increases as frequency cubed.

Our realistic very fast orrery now puts out ~10^-10 gravitons per year and just might put out a single graviton in the lifetime of the universe.

[Chris Peterson]

• Neither you nor your heart is capable of generating a single graviton in your lifetime.

Your analysis depends on the assumption that there's such a thing as a graviton.

I see little reason to believe that any moving mass doesn't produce gravitational radiation. However, even your analysis doesn't contradict my statement.

[neufer]

Neither you nor your heart is capable of generating a single graviton in your lifetime.

Your analysis depends on the assumption that there's such a thing as a graviton.

On a scale from 1 (weak belief) to 10 (strong belief)
how would you rate your belief in the existence of gravitons

I see little reason to believe that any moving mass doesn't produce gravitational radiation. However, even your analysis doesn't contradict my statement.

Graviton production goes as:

• 1) Time to the first power
  2) Mass squared
  3) Frequency cubed and
  4) Size to the fourth power.

Human sized objects are clearly too small, too slow and too light to generate a single graviton in a human lifetime.

[Chris Peterson]

Your analysis depends on the assumption that there's such a thing as a graviton.

On a scale from 1 (weak belief) to 10 (strong belief)
how would you rate your belief in the existence of gravitons :?:

Honestly, I don't feel like I understand the theory well enough to quantify my "belief" at all. But I do understand that the existence of gravitons- while providing some elegant theory- remains highly speculative.[/quote]

I see little reason to believe that any moving mass doesn't produce gravitational radiation. However, even your analysis doesn't contradict my statement.

Graviton production goes as:

• 1) Time to the first power
  2) Mass squared
  3) Frequency cubed and
  4) Size to the fourth power.

Human sized objects are clearly too small, too slow and too light to generate a single graviton in a human lifetime.

So like I said, your argument doesn't contradict my statement. A low intensity is not a zero intensity. I'm not aware of any interpretation of GR that doesn't conclude that a moving mass produces gravitational radiation, and that doesn't change if we assume that gravitation is quantum in nature.

[MarkBour]

. . .

• 1) Time to the first power
  2) Mass squared
  3) Frequency cubed and
  4) Size to the fourth power.

. . .

Not to quibble, but would it be better to say "size to the negative fourth power" ?
So, what kind of gravitational radiation would be occurring in a 1-billion sol mass black hole, with the mass presumably spiraling down into the most massive and tightest of spirals imaginable? I'm wondering about the masses interacting as has been suggested way down inside the event horizon.

[alter-ego]

I see little reason to believe that any moving mass doesn't produce gravitational radiation. However, even your analysis doesn't contradict my statement.

Graviton production goes as:

• 1) Time to the first power
  2) Mass squared
  3) Frequency cubed and
  4) Size to the fourth power.

Human sized objects are clearly too small, too slow and too light to generate a single graviton in a human lifetime.

So like I said, your argument doesn't contradict my statement. A low intensity is not a zero intensity. I'm not aware of any interpretation of GR that doesn't conclude that a moving mass produces gravitational radiation, and that doesn't change if we assume that gravitation is quantum in nature.

Therein lies the rub.
Within GR framework, gravitational radiation is a continuum; space-time ripples have no lower amplitude bound. Analogous to classical EM theory, we will likely explore the "wave" aspects of gravitational radiation within GR but first we have to discover them. Maybe some day long after the first detection gravitational waves and our monitoring capabilities are up to it, the bigger gravitational wave picture will reveal unique spectral distributions and "lines" that will then feed into the concept of gravity quanta on the cosmic scale and the theory needed to support those measurements.

It's fun to ponder on if/when/how GR will crack, but we are still experimentally basking in the middle-aged adult years of GR life. As many successes GR has had with predictions over the last century, we still haven't directly seen our first gravitational wave, let alone a gravitational "photon". So we can ponder and hypothesize as much as we like, yet new measurements will provide the needed foundation to build a theory on.

[Chris Peterson]

So like I said, your argument doesn't contradict my statement. A low intensity is not a zero intensity. I'm not aware of any interpretation of GR that doesn't conclude that a moving mass produces gravitational radiation, and that doesn't change if we assume that gravitation is quantum in nature.

Therein lies the rub.
Within GR framework, gravitational radiation is a continuum; space-time ripples have no lower amplitude bound.

A quantum theory of gravity doesn't entirely change that. In a sense, there's still no lower bound. There is a minimum amplitude defined by a single quantum, but the energy released is the same- just distributed discretely over some finite period of time. We trade the continuum for a probability.

[neufer]

. . .

• 1) Time to the first power
  2) Mass squared
  3) Frequency cubed and
  4) Size to the fourth power.

. . .

Not to quibble, but would it be better to say "size to the negative fourth power" ?

Actually, I have made an error

It should read:

• 1) Time to the first power
  2) Mass squared
  3) Size to the fourth power and
  4) Frequency to the FIFTH power

The strong sensitivity to frequency dominates in a binary gravitational situations such a merging black holes since Kepler tells us that Frequency = "size to the negative 3/2 power"

Allowing for this effect the graviton production = "size to the negative 7/2 power" [= 4 - 5x(3/2)]
and the power P = "size to the negative 5 power" as in the Wikipedia formula below.
(Note that each graviton has an increasing quantum energy = "size to the negative 3/2 power")

<<Suppose that [two orbiting masses] are m₁ and m₂, and they are separated by a distance r. The power given off (radiated) by this system is:



where G is the gravitational constant, c is the speed of light in vacuum and where the negative sign means that power is being given off by the system, rather than received. For a system like the Sun and Earth, r is about 1.5×10¹¹ m and m₁ and m₂ are about 2×10³⁰ and 6×10²⁴ kg respectively. In this case, the power is about 200 watts.>>

• So the Sun / Earth system radiates only about 200 watts or ~9.5x10⁴² gravitons per second.

But what if we substituted a full sized orrery model with a Sun & Earth of masses 2000 kg & 6 g respectively?
-----------------------------------------------------------------------------------------
The power depends upon the masses squared and the masses have dropped by a factor of 10²⁷; hence the radiation drops by a factor of 10⁵⁴ resulting in a radiation of ~9.5x10^-12 gravitons per second or 3x10^-4 gravitons per year.
-----------------------------------------------------------------------------------------
Things get even worse if we make the orrery a more feasible (1 AU => 150 m) size since gravitational radiation depends upon distances to the fourth power! Now our realistic yearly orrery puts out just 3x10^-32 gravitons per year. Clearly there is not enough time in the life of the universe for our realistic yearly orrery model to emit even a single graviton.
-----------------------------------------------------------------------------------------
Now what if we speed our realistic orrery model up to make an orbit in just one second? Now radiation depends upon frequency to the sixth power whereas graviton energy (= hv) only depends upon frequency to the first power such that graviton production increases as frequency to the fifth power.

Our realistic very fast orrery now puts out ~12 gravitons per hour.
-----------------------------------------------------------------------------------------

[b][color=#0000FF]generating a few gravitons per hour on a good day[/color][/b]

---

Hence...I concede that humans with masses 10⁴ larger than the orrery's 6 g "Earth" (the main generator) but sizes 10² smaller than the orrery are capable of generating a few gravitons per hour on a good day.

However, heart pumping with blood masses 10³ smaller and sizes 10¹ smaller will require 10¹⁰ hours to produce a single graviton.

[Boomer12k]

I don't think you generate gravitons, nor gravitational waves....just pushing air....making air waves....swish....
Drop a pebble in at one end of lake Superior...and try to detect that on the other side...an awful lot going on inbetween.

Most, if any, ripples in spacetime go back down very quicky... Like radio waves...they don't go out fifty light years intact...only around 1.5 ly...then they are so dispersed they are inditiquishable from the background radiation.... I think gravitational waves might be similar.

To borrow a concept.... If it hasn't been detected....it doesn't exist....

:---(===) *

[neufer]

I don't think you generate gravitons, nor gravitational waves....just pushing air....making air waves....swish....
Drop a pebble in at one end of lake Superior...and try to detect that on the other side...an awful lot going on inbetween.

Most, if any, ripples in spacetime go back down very quicky... Like radio waves...they don't go out fifty light years intact...only around 1.5 ly...then they are so dispersed they are indistinguishable from the background radiation.... I think gravitational waves might be similar.

To borrow a concept.... If it hasn't been detected....it doesn't exist....

Theoretically the ~ 1 Hz gravitons will never be intercepted in an infinite rapidly expanding universe.

If a graviton lives forever does it really exist at all

(Or does the energy loss and/or recoil from it's emission count.)

[RocketRon]

Natures' gravity detector - the oceans - readily detect the Moons gravity.

Regular as clockwork, twice a day, about a +/- 2 metre deflection hereabouts.
(And as much as 10m or more in some parts).

Can these fancy instruments do the same ?
What is the force calculated out to be - and in what units ??

[Beyond]

I don't think you generate gravitons, nor gravitational waves....just pushing air....making air waves....swish....
Drop a pebble in at one end of lake Superior...and try to detect that on the other side...an awful lot going on inbetween.

Most, if any, ripples in spacetime go back down very quicky... Like radio waves...they don't go out fifty light years intact...only around 1.5 ly...then they are so dispersed they are inditiquishable from the background radiation.... I think gravitational waves might be similar.

To borrow a concept.... If it hasn't been detected....it doesn't exist....

:---(===) *

I don't think they are inditiquishable from the background radiation, i think they are inditiquishabled by the background radiation.
Dropping a pebble into Lake Superior produces little waves that get lost in the wetness of the water. It's the wetness of the water that brings the ripples to a 'calm' state. In space, the background radiation is the 'calm' state.

[neufer]

Natures' gravity detector - the oceans - readily detect the Moons gravity.

Regular as clockwork, twice a day, about a +/- 2 metre deflection hereabouts.
(And as much as 10m or more in some parts).

Can these fancy instruments do the same ?
What is the force calculated out to be - and in what units ??

LIGO measures distant dynamic Weyl tensor tidal forces that drop off inversely with the distance.

The ocean measures nearby static Weyl tensor tidal forces that drop off inversely with the cube of the distance.

(It's similar to the difference between a radio receiver and a simple compass.)

<<The Weyl tensor expresses the tidal force that a body feels when moving along a geodesic. In general relativity, the Weyl curvature is the only part of the curvature that exists in free space—a solution of the vacuum Einstein equation—and it governs the propagation of gravitational radiation through regions of space devoid of matter. More generally, the Weyl curvature is the only component of curvature for Ricci-flat manifolds and always governs the characteristics of the field equations of an Einstein manifold.>>

[RocketRon]

So can LIGO measure the gravity changes from the moons orbit. ?
Never assume we can run before we can walk.

And the scale and units of those forces ?