APOD: A Message from the Gravitational... (2023 Jun 29)

[APOD Robot]

A Message from the Gravitational Universe

Explanation: Monitoring 68 pulsars with very large radio telescopes, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has uncovered evidence for the gravitational wave (GW) background by carefully measuring slight shifts in the arrival times of pulses. These shifts are correlated between different pulsars in a way that indicates that they are caused by GWs. This GW background is likely due to hundreds of thousands or even millions of supermassive black hole binaries. Teams in Europe, Asia and Australia have also independently reported their results today. Previously, the LIGO and Virgo detectors have detected higher-frequency GWs from the merging of individual pairs of massive orbiting objects, such as stellar-mass black holes. The featured illustration highlights this spacetime-shaking result by depicting two orbiting supermassive black holes and several of the pulsars that would appear to have slight timing shifts. The imprint these GWs make on spacetime itself is illustrated by a distorted grid.

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[alter-ego]

Nanoherz GW detection is "not trivial". 200 collaborators, 70 institutions, very small variations in timing due to several factors, directional-dependent "synchronization" errors and 15 years of observations yields a 3-σ confidence error (0.001 random chance). Optimistic to reach 5-σ confidence error in a couple years. Evidence currently indicates first confirmation that supermassive blackholes merge. Despite physical loss of the Arecibo dish, and redirection of the 14-dish Westerbork Synthesis Radio Telescope, new / improved systems should be online by the end of this decade that will fill the gap and be better suited for nanohertz GW detection. Really cool stuff!

I'm always impressed and excited when new technology applications significantly (and suddenly!) expand our scientific prowess. I've wondered over the years where we'd be with not understanding the cosmos, and I'm enjoying the push to answer the current questions. I'm finally settling in on what I'd like to see within my remaining lifetime. Believe me, I realize things change quickly, so I'm attempting to have reasonable expectations.

[Christian G.]

Apologies for the tangent but I am still stuck with some basic questions regarding gravity as bending of spacetime, replacing the old notion of a "force" that pulls:
We are sometimes told that spacetime is not a "thing" like some physical fabric, that this is an analogy; it is not a fabric but a metric. In this case, what is it that mass bends? If we say that it bends the "path" of objects, why should objects follow that path? If there is no "force" pulling objects down that path, why do objects not go past it and ignore it so to speak? Moreover if there is no pulling "force", what makes objects SPEED UP as they follow the path in the case of strong gravitation?
In short if relativity predicts WHERE objects will go, WHAT makes them go there?

[Chris Peterson]

Apologies for the tangent but I am still stuck with some basic questions regarding gravity as bending of spacetime, replacing the old notion of a "force" that pulls:
We are sometimes told that spacetime is not a "thing" like some physical fabric, that this is an analogy; it is not a fabric but a metric. In this case, what is it that mass bends? If we say that it bends the "path" of objects, why should objects follow that path? If there is no "force" pulling objects down that path, why do objects not go past it and ignore it so to speak? Moreover if there is no pulling "force", what makes objects SPEED UP as they follow the path in the case of strong gravitation?
In short if relativity predicts WHERE objects will go, WHAT makes them go there?

You could ask the same question about "forces".

[Christian G.]

You could ask the same question about "forces".

Indeed. I guess what I'm asking is, er, what's the nature of gravity…

[Chris Peterson]

You could ask the same question about "forces".

Indeed. I guess what I'm asking is, er, what's the nature of gravity…

I would say its nature is described by GR. The problem is trying to fit that into something limited by how our senses and sensibilities have evolved. It's like QM. These bother us because they are not intuitive in the way older, simpler physics often is. But the Universe cares nothing for our intuition...

[njudell]

Are the data available anywhere. Would love to take a stab at the inverse problem.

[VictorBorun]

those who learned that the CMB means the cosmic microwave background now face memorizing that the GWB means the gravitational wave background.

No detailed map of distinct sources so far, just the total buzz amplitude of 2,4×10⁻¹⁵ at the frequencies about 1/year (that translates to 76 nanoseconds in pulsars' ticking; a lot of trouble to account for Earth's orbit elements and the pulsar's orbit elements, including minor planets around the pulsar)

[VictorBorun]

by the way we are lucky that gravity waves are polarized.

If you try the same trick with sound waves, which are not polarized, you fail. Say you pick the acoustic noise in a room by subtracting signals at microphones A and B, then subtracting signals at microphones A and C, and hope to see a correlation between the BAC angle and the similarity between A—B signal amplitude and and A—C signal amplitude. There will be no effect of the angle at all.

However there is a correlation for the gravitational wave background. Because the noise of polarized waves is another tale

[Chris Peterson]

The edge of the visible universe as we generally understand the term is at z=1089, which corresponds to the Universe at an age of 379,000 years. We cannot see beyond this to the earliest times because the Universe was opaque to electromagnetic radiation before that. But gravitational waves are subject to no such transparency limitation. One day we may be able to use gravitational waves to "see" right back to the Big Bang itself.

[orin stepanek]

I was hoping to get the annotations with the photo!

Kitty peeking at something!

[bystander]

Gwb_Nanograv_960.jpg
I was hoping to get the annotations with the photo!
cat-looking-surprised-peering-over-the-edge-of-the-picture-john-daniels.jpg
Kitty peeking at something!

Click to view full size image 1 or image 2

[johnnydeep]

So, it's Thursday stupid question time again:

As I understand it, the GWB is apparently the total combined effect of millions or billions of events like black hole and neutron start mergers and rotating neutron stars, all emitting gravitational waves at various frequencies.

1. Why are there different frequencies in the first place?
2. Are these GWs directional, like the EM waves emitted by rotation or "lighthouse-like" pulsars that emit radiation along their spin axes?
3. I would think GWs, in a manner naively similar to waves on water, have the potential to cancel each other out when they overlap, which is something that EM waves don't do (or do they?). Is the GWB simply what's left over after such cancellation effects? (Hmm, I can also imagine that overlapping waves might result in low frequency waves with high frequency waves embedded in them resulting in smaller ripples on top of longer ripples.)
4. Aside: why are GWs limited by the speed of light, and why are they obliged to travel at light speed anyway? After all, they are ripples IN space-time that are propagating, not actual matter or energy travelling THROUGH space-time.

[Chris Peterson]

So, it's Thursday stupid question time again:

Not stupid questions at all!

As I understand it, the GWB is apparently the total combined effect of millions or billions of events like black hole and neutron start mergers and rotating neutron stars, all emitting gravitational waves at various frequencies.

1. Why are there different frequencies in the first place?

Because most are produced by cyclical phenomena, like binary masses orbiting one another.

2. Are these GWs directional, like the EM waves emitted by rotation or "lighthouse-like" pulsars that emit radiation along their spin axes?

I don't know if there are any directional sources. But most are likely produced by two bodies, and the propagation is substantially on the plane of their orbit.

3. I would think GWs, in a manner naively similar to waves on water, have the potential to cancel each other out when they overlap, which is something that EM waves don't do (or do they?). Is the GWB simply what's left over after such cancellation effects? (Hmm, I can also imagine that overlapping waves might result in low frequency waves with high frequency waves embedded in them resulting in smaller ripples on top of longer ripples.)

I agree that interference effects seem inevitable. Don't know any of the theory on this, though.

4. Aside: why are GWs limited by the speed of light, and why are they obliged to travel at light speed anyway? After all, they are ripples IN space-time that are propagating, not actual matter or energy travelling THROUGH space-time.

The Universe seems to have a problem with allowing information to propagate at faster than c. And gravitational waves carry information. You'll have to take that question up with the Universe.

[markb212]

... have the potential to cancel each other out when they overlap, which is something that EM waves don't do (or do they?). Is the GWB simply what's left over after such cancellation effects?

Depends on what you mean by "cancel". EM waves, like all waves, form interference patterns where two or more waves meet. Nulls or minimums occur where the individual wave peaks are out of phase and will tend to cancel. Peaks or maximums occur where the individual wave peaks are in phase and reinforce each other.

You may notice this while driving and listening to an FM station on the radio. Some of the fades you hear are caused by the radio waves (EM waves) taking two or more paths to reach your antenna and canceling out in spots. Move a few feet and the signal returns. This is called multi path fading and is EM wave interference in action.

Not an expert on gravity waves, but I would be surprised if they don't behave the same.

mb

[VictorBorun]

2. Are these GWs directional, like the EM waves emitted by rotation or "lighthouse-like" pulsars that emit radiation along their spin axes?

I don't know if there are any directional sources. But most are likely produced by two bodies, and the propagation is substantially on the plane of their orbit.

no jets of gravitational waves from events in mainstream astrophysics.
But a merger of two BHs if they happen to have spins of the same orientation does fire a gravitational wave packet carrying a momentum; not jet-narrow, but narrow enough to cause a recoil.

The mergers are matter of a second for stellar mass BHs and must be matter of an hour or a month for million suns or billion suns mass BHs.
NANOGrav is dealing with year-long events like a binary supermassive BH making one orbit.
If there were exotic things early on, before the CMB, they could invest in the GWB too… or could they?
After all deuterium leftovers tell us that the first minutes after Big Bang saw the same baryonic/dark matter ratio as we see currently. Maybe there were nothing exotic early on.

[johnnydeep]

Thanks guys. So, is it only changing gravitational fields that produce GWs? Does a single mass moving alone in space produce GWs? How about one mass orbiting another: does that always produce GWs? Is the Earth's orbit around the Sun creating GWs?

[Chris Peterson]

Thanks guys. So, is it only changing gravitational fields that produce GWs? Does a single mass moving alone in space produce GWs? How about one mass orbiting another: does that always produce GWs? Is the Earth's orbit around the Sun creating GWs?

It is accelerating masses that produce GWs. And extended masses that change size non-spherically. A single mass moving at a constant velocity does not radiate GWs. Masses orbiting each other always do so. Yes, the Earth radiates GWs as it orbits the Sun.

[alter-ego]

Thanks guys. So, is it only changing gravitational fields that produce GWs? Does a single mass moving alone in space produce GWs? How about one mass orbiting another: does that always produce GWs? Is the Earth's orbit around the Sun creating GWs?

It is accelerating masses that produce GWs. And extended masses that change size non-spherically. A single mass moving at a constant velocity does not radiate GWs. Masses orbiting each other always do so. Yes, the Earth radiates GWs as it orbits the Sun.

GW power ≈ 200 Watts
Inspiral time ≈ 10²³ years

[johnnydeep]

Thanks guys. So, is it only changing gravitational fields that produce GWs? Does a single mass moving alone in space produce GWs? How about one mass orbiting another: does that always produce GWs? Is the Earth's orbit around the Sun creating GWs?

It is accelerating masses that produce GWs. And extended masses that change size non-spherically. A single mass moving at a constant velocity does not radiate GWs. Masses orbiting each other always do so. Yes, the Earth radiates GWs as it orbits the Sun.

GW power ≈ 200 Watts
Inspiral time ≈ 10²³ years

Nice! So I could power a few dozen LED light bulbs with this power!

As for mechanisms that cause GW "radiation", what about an object that changes in mass due to accreting matter? I'm thinking mainly about the BHs in the centers of galaxies that can accrete huge amounts of mass over time.

Also, I presume that "inspiraling" of orbiting objects is not required for GW emission, and even the "outspiraling" Moon orbiting the Earth is radiating GWs sine there is still acceleration involved.

[Chris Peterson]

It is accelerating masses that produce GWs. And extended masses that change size non-spherically. A single mass moving at a constant velocity does not radiate GWs. Masses orbiting each other always do so. Yes, the Earth radiates GWs as it orbits the Sun.

GW power ≈ 200 Watts
Inspiral time ≈ 10²³ years

Nice! So I could power a few dozen LED light bulbs with this power! :-)

As for mechanisms that cause GW "radiation", what about an object that changes in mass due to accreting matter? I'm thinking mainly about the BHs in the centers of galaxies that can accrete huge amounts of mass over time.

Also, I presume that "inspiraling" of orbiting objects is not required for GW emission, and even the "outspiraling" Moon orbiting the Earth is radiating GWs sine there is still acceleration involved.

The process of accretion involves accelerated mass. So it produces gravitational radiation. (Accretion always involves inspiraling matter.) It is the acceleration that matters. Spin a barbell and you produce gravitational radiation, despite the fact that the two masses remain at a fixed distance from each other.

[Fred the Cat]

So, we live in a universe full of older and newer gravitational waves. Let’s call the idea – POND. There are little, old splashes and big, new splashes that we can measure.

What happens when they run into each other? Constructive and destructive interference? Sounds like a gravity-type mechanism to begin to work with.

[johnnydeep]

So, we live in a universe full of older and newer gravitational waves. Let’s call the idea – POND. There are little, old splashes and big, new splashes that we can measure.

What happens when they run into each other? Constructive and destructive interference? Sounds like a gravity-type mechanism to begin to work with.

Hmm - old and new GW based solely on the newness of the source phenomena, just like there are old and new photons.

Hmm: POND - Pulsar Observations for Nanogravity wave Detection?

Also, I presume that "one off" events like a BH merger will cause a big splash of GW ripples that has a finite duration, and there would be many billions that have already passed us by never to be detected (by us) at all. Same as one off emissions of photons I suppose.

[johnnydeep]

GW power ≈ 200 Watts
Inspiral time ≈ 10²³ years

Nice! So I could power a few dozen LED light bulbs with this power!

As for mechanisms that cause GW "radiation", what about an object that changes in mass due to accreting matter? I'm thinking mainly about the BHs in the centers of galaxies that can accrete huge amounts of mass over time.

Also, I presume that "inspiraling" of orbiting objects is not required for GW emission, and even the "outspiraling" Moon orbiting the Earth is radiating GWs sine there is still acceleration involved.

The process of accretion involves accelerated mass. So it produces gravitational radiation. (Accretion always involves inspiraling matter.) It is the acceleration that matters. Spin a barbell and you produce gravitational radiation, despite the fact that the two masses remain at a fixed distance from each other.

Just to be clear, the outspiraling Moon is also giving rise to GWs, right? That is, there's acceleration happening there too.

And I presume that "decretion" of matter such as what occurs when stars blow off their layers for various reasons will also cause GW?

[Chris Peterson]

Nice! So I could power a few dozen LED light bulbs with this power! :-)

As for mechanisms that cause GW "radiation", what about an object that changes in mass due to accreting matter? I'm thinking mainly about the BHs in the centers of galaxies that can accrete huge amounts of mass over time.

Also, I presume that "inspiraling" of orbiting objects is not required for GW emission, and even the "outspiraling" Moon orbiting the Earth is radiating GWs sine there is still acceleration involved.

The process of accretion involves accelerated mass. So it produces gravitational radiation. (Accretion always involves inspiraling matter.) It is the acceleration that matters. Spin a barbell and you produce gravitational radiation, despite the fact that the two masses remain at a fixed distance from each other.

Just to be clear, the outspiraling Moon is also giving rise to GWs, right? That is, there's acceleration happening there too.

And I presume that "decretion" of matter such as what occurs when stars blow off their layers for various reasons will also cause GW?

There is acceleration happening anytime a body isn't moving in a straight line.

If a body blows off mass in a spherically symmetric way, it won't create gravitational radiation.