APOD: Two Black Holes Merge (2016 Feb 12)

[RocketRon]

When you read the order of magnitude of the strength of the detected gravity wave signal,
that 'engineering problem' is awfully awfully awfully small....

"What we've measured in trying to record that signal is these two points in our interferometer, these two mirrors that are separated by four kilometres moved by an amount of an incredible 10-18 of a metre in a tenth of a second.
Even getting that concept of how small is 10-18 of a metre?
Well, it's 10,000 times smaller than a nucleus."

(Thats 10 to the -18th of a metre.)

A good sneeze at the wrong moment would blow this experiment into the next universe. !
For terrestrial based measurements, it would make a VERY sensitive earthquake measuring device.
Which presumably have to be filtered out of the 'results'.

[RocketRon]

A 10-19 "characteristic strain" for the 1km arm eLISA is a movement of 0.1 nanometers.

When the laser wavelength is 1064 nm

[alter-ego]

A 10-19 "characteristic strain" for the 1km arm eLISA is a movement of 0.1 nanometers.

When the laser wavelength is 1064 nm

I think you mean 1 million km arm, and the strain amplitude (length) is independent of wavelength. Since the LISA Pathfinder will use a 1064nm, and LIGO uses a 1064nm laser, most likely so will eLISA. The sensitivity is intended to be better than 10^-19:

eLISA can achieve 10-21 strain resolution by measuring displacements of the order of fractions of a picometer. Its observations in the quiet environment of space will not be disturbed by seismic and gravity-gradient noise. Thus eLISA's unparalleled sensitivity will allow studying sources within the Galaxy and up to the edge of the visible Universe.

Unlike LIGO, eLISA will be a highly accurate, range finder. A CW laser will make one pass to the test mirrors and back. The interferometer magic that determines the path length difference happens at the mother spacecraft:

eLISA uses precision laser interferometry across 10⁶ km of space to compare separations between test masses that are protected by the spacecraft from non-gravitational disturbances. The distance measuring system is a continuous interferometric laser ranging scheme, similar to systems used for radar-tracking of spacecraft.

[bls0326]

Very interesting topic to me (and obviously several others). thanks to all for the comments, clarifications, and details.

Brian

[RocketRon]

T'wasn't me that wrote that.
Look back through this thread ...

I think you mean

[alter-ego]

T'wasn't me that wrote that.
Look back through this thread ...

I think you mean

mmm... I guess it's guilt by association

[bmgriffi]

Three solar masses were carried away when the two black holes converged. I know this has something to do with the mass-energy equivalence. In nuclear fusion, for example, the lost mass is carried away by photons, if I understand correctly. What is carrying away the lost mass in this case?

[neufer]

Three solar masses were carried away when the two black holes converged. I know this has something to do with the mass-energy equivalence. In nuclear fusion, for example, the lost mass is carried away by photons, if I understand correctly. What is carrying away the lost mass in this case?

~10⁷⁹ gravitons of average energy ~1 pico-electronvolt.
(~10²⁴ gravitons passed through every square centimeter of your body.)

[50bmg]

So, gravitons are carrying away the energy/mass?
I'm not sure how to ask this question intelligently, but here is a try:
In a nuclear reaction where mass is lost in the form of radiated energy - are particles "turned into" photons as they radiate away, and in this way mass/energy is lost?
Then, in the case of the merging black holes losing mass/energy -
Where did this mass/energy come from that was lost (3 solar masses in this case)? Are there now fewer particles (three solar masses worth of particles) in the converged black hole than there were in the two black holes? And, if so, did these particles "turn into" gravitons and radiate away mass/energy?

[RocketRon]

~1079 gravitons of average energy ~1 pico-electronvolt.
(~1024 gravitons passed through every square centimeter of your body.)

And I didn't feel a thing....
Gravity from the Sun, moon, other planets and more adjacent stars would be somewhat stronger, wethinks ?
Not to mention planet Earth.

Define too what a graviton is, precisely ?
And how we identify it when we 'see' one.

And do gravitons meeting when travelling in opposite directions cancel each other out ??

[Chris Peterson]

So, gravitons are carrying away the energy/mass?

I wouldn't say that, because gravitons are purely hypothetical particles. An elegant quantum treatment of gravity largely requires them, but we don't have such a treatment yet. So the safe answer is that gravitational radiation is the form the energy is in.

In a nuclear reaction where mass is lost in the form of radiated energy - are particles "turned into" photons as they radiate away, and in this way mass/energy is lost?

Mass is converted to energy, and if that energy is electromagnetic, photons are its carriers.

Then, in the case of the merging black holes losing mass/energy -
Where did this mass/energy come from that was lost (3 solar masses in this case)? Are there now fewer particles (three solar masses worth of particles) in the converged black hole than there were in the two black holes?

The merged black hole has less mass than the sum of the progenitors. We don't know if black holes even have "particles" inside, so best not to go there. Somehow, mass escaped. Presumably that's something that happened right at the moment of merger, where the event horizon is very complex.

And, if so, did these particles "turn into" gravitons and radiate away mass/energy?

Possibly, assuming that gravitons exist. As noted above, what we can reliably say is that the mass (not "particles") was converted to energy (via mass-energy equivalence) in the form of gravitational radiation, the carrier of which is uncertain.

[neufer]

So, gravitons are carrying away the energy/mass?

I wouldn't say that, because gravitons are purely hypothetical particles. An elegant quantum treatment of gravity largely requires them, but we don't have such a treatment yet. So the safe answer is that gravitational radiation is the form the energy is in.

• Gravitons surely exist. The only reasonable question is whether gravitons are massless or not.

<<Gravitational waves may be viewed as the coherent states of many gravitons. If gravitational waves were observed to propagate slower than c, that would imply that the graviton has mass. Recent observations of gravitational waves have put an upper bound of 1.2 × 10^-22 eV/c² on the graviton's mass. It can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field must couple to (interact with) the stress–energy tensor in the same way that the gravitational field does. If a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.

Click to play embedded YouTube video.

String theory predicts the existence of gravitons and their well-defined interactions. As closed strings without endpoints, they would not be bound to branes and could move freely between them. If we live on a brane this "leakage" of gravitons from the brane into higher-dimensional space could explain why gravitation is such a weak force, and gravitons from other branes adjacent to our own could provide a potential explanation for dark matter. However, if gravitons were to move completely freely between branes this would dilute gravity too much, causing a violation of Newton's inverse square law. To combat this, Lisa Randall found that a three-brane (such as ours) would have a gravitational pull of its own, preventing gravitons from drifting freely, possibly resulting in the diluted gravity we observe while roughly maintaining Newton's inverse square law.>>

[RocketRon]

The only reasonable question is whether gravitons are massless or not.

To travel at the speed of light, they'd have to be mass-less ?

[neufer]

Then, in the case of the merging black holes losing mass/energy -
Where did this mass/energy come from that was lost (3 solar masses in this case)? Are there now fewer particles (three solar masses worth of particles) in the converged black hole than there were in the two black holes?

The merged black hole has less mass than the sum of the progenitors. We don't know if black holes even have "particles" inside, so best not to go there. Somehow, mass escaped. Presumably that's something that happened right at the moment of merger, where the event horizon is very complex.

That's something that happened at the moment of merger, where the gravitational forces are very strong and nonlinear. The source of the radiating gravitons are, in fact, virtual gravitons just outside the event horizon.

Lecture Notes on General Relativity
Sean M. Carroll, December 1997
Enrico Fermi Institute, University of Chicago

<<The nonlinearity of general relativity is worth remarking on. In Newtonian gravity the potential due to two point masses is simply the sum of the potentials for each mass, but clearly this does not carry over to general relativity (outside the weak-field limit). There is a physical reason for this, namely that in GR the gravitational field couples to itself. This can be thought of as a consequence of the equivalence principle - if gravitation did not couple to itself, a "gravitational atom" (two particles bound by their mutual gravitational attraction) would have a different inertial mass (due to the negative binding energy) than gravitational mass. From a particle physics point of view this can be expressed in terms of Feynman diagrams. The electromagnetic interaction between two electrons can be thought of as due to exchange of a virtual photon:

---

But there is no diagram in which two photons exchange another photon between themselves; electromagnetism is linear. The gravitational interaction, meanwhile, can be thought of as due to exchange of a virtual graviton (a quantized perturbation of the metric). The nonlinearity manifests itself as the fact that both electrons and gravitons (and anything else) can exchange virtual gravitons, and therefore exert a gravitational force:

---

There is nothing profound about this feature of gravity; it is shared by most gauge theories, such as quantum chromodynamics, the theory of the strong interactions. (Electromagnetism is actually the exception; the linearity can be traced to the fact that the relevant gauge group, U(1), is abelian.) But it does represent a departure from the Newtonian theory. (Of course this quantum mechanical language of Feynman diagrams is somewhat inappropriate for GR, which has not [yet] been successfully quantized, but the diagrams are just a convenient shorthand for remembering what interactions exist in the theory.)

[alter-ego]

So, gravitons are carrying away the energy/mass?

I wouldn't say that, because gravitons are purely hypothetical particles. An elegant quantum treatment of gravity largely requires them, but we don't have such a treatment yet. So the safe answer is that gravitational radiation is the form the energy is in.

Gravitons surely exist. The only reasonable question is whether gravitons are massless or not

Well, surely you may be right.
The day a GW is confirmed with electromagnetic radiation, the GW velocity = c could be bounded. Granted that correlation seems highly improbable today requiring visibility and good timing, but as detection technology evolves, precursor detection might gain enough time for to locate and swing a dedicated "optical" instrument to capture the GW event- analogous to the SWIFT gamma-ray satellite ensemble.

[neufer]

~10⁷⁹ gravitons of average energy ~1 pico-electronvolt.
(~10²⁴ gravitons passed through every square centimeter of your body.)

And I didn't feel a thing....

That's probably because your body is numb from the 65 billion neutrinos
passing through every square centimeter of it every second.

<<Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background of neutrinos, since the dimensions of the required neutrino shield would ensure collapse into a black hole.>>

Gravity from the Sun, moon, other planets and more adjacent stars would be somewhat stronger, wethinks ?
Not to mention planet Earth.

Astronauts aboard the ISS experience weak periodic ω²r Earth dynamic Weyl tensor tidal forces comparable in strength (though not frequency or duration) to the gravitational dynamic Weyl tensor tidal forces one would feel ~ 0.7 light years away from our merging black holes.

Define too what a graviton is, precisely ?
And how we identify it when we 'see' one.

If we 'hear' a massless spin-2 particle it will be a graviton.

Ned "Scotty" Scott: I bring you a warning: Tell the world, tell this to everybody wherever they are.

• Listen to the skies. Everywhere. Keep listening. Keep Listening to the skies.

And do gravitons meeting when travelling in opposite directions cancel each other out ??

Like photons, gravitons:

• 1) travelling in opposite directions generate standing waves... while those
  2) travelling in the same direction but 180º out of phase cancel each other out.

[Chris Peterson]

• Gravitons surely exist. The only reasonable question is whether gravitons are massless or not.

They certainly might exist. But all theory around them is speculative, poorly developed, and untested. So I find the best approach is simply to treat gravitational radiation as gravitational radiation, and not get too invested in any particular underlying mechanism.

[Markus Schwarz]

• Gravitons surely exist. The only reasonable question is whether gravitons are massless or not.

They certainly might exist. But all theory around them is speculative, poorly developed, and untested. So I find the best approach is simply to treat gravitational radiation as gravitational radiation, and not get too invested in any particular underlying mechanism.

I side with Chris. What has been detected was a classical gravitational wave, not a quantised graviton. Hertz discovered electromagnetic waves, not the photon. To detect the graviton would require some experiment analog to the photo electric effect. We don't have a consistent quantised theory of gravity that tells us the existence of gravitons.