Gravitational waves: Why the fuss?

October 16, 2017
A visualization of a supercomputer simulation of merging black holes sending out gravitational waves. Credit: NASA/C. Henze

Great excitement rippled through the physics world Monday at news of the first-ever detection of two ultra-dense neutron stars converging in a violent smashup.

The discovery, scientists say, would not have been possible without the detection of gravitational waves—a two-year-old feat awarded the 2017 Nobel Physics Prize.

A backgrounder:

Q: What are gravitational waves?

Albert Einstein predicted gravitational waves in his general theory of relativity a century ago. Under the theory, space and time are interwoven into seemless continuum—adding a fourth dimension to our concept of the Universe, in addition to our 3D perception of it.

Einstein postulated that mass warps space-time through its .

A common analogy is to view space-time as a trampoline, and mass as a bowling ball placed on it. Objects on the trampoline's surface will "fall" towards the centre—representing gravity.

When objects with mass accelerate—when two neutron stars or black holes spiral towards each other, for example—they send waves along the curved space-time around them at the speed of light, like ripples on a pond.

The more massive the object, the larger the wave and the easier it is to detect.

Gravitational waves do not interact with matter, travelling through the Universe almost unimpeded.

Q: Why are they so elusive?

Einstein himself doubted gravitational waves would ever be detected given how small they are.

Ripples emitted by a pair of merging , for example, would stretch a one-million-kilometre (621,000-mile) ruler on Earth by less than the size of an atom.

Q: How are they detected?

One technique involves detecting small changes in the distance between objects.

Gravitational waves passing through an distort its shape, stretching and squeezing it in the direction the wave is travelling, leaving a telltale, though miniscule, effect.

Detectors such as LIGO in the United States and Virgo in Italy, are designed to pick up such distortions in laser light beams.

At LIGO, scientists split the light into two perpendicular beams that travel over several kilometres to be reflected by mirrors back to the point where they started.

Any difference in length upon their return would point to the influence of .

Sources: European Space Agency, Institute of Physics, LIGO, Nature.

Explore further: Gravitational waves: Why the fuss? (Update)

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baudrunner
1.4 / 5 (9) Oct 16, 2017
they send waves along the curved space-time around them at the speed of light
The mainstream axiom that c is the upper limit to velocity in physics is still a presumption. Relating c to "gravitational waves" feeds the pseudo-science mill.

About faster than light speeds, the reader should be reminded that Einstein once said, "I could be wrong."

Apparently the LIGO instrument is so sensitive it would react to a person dropping a pen in an isolation booth at the site. It can be concluded that fracking in nearby states, or even an automobile accident near the site, can easily deliver the same data. Pointing to the sky and framing experimental results around an observed astronomical phenomenon is like randomly picking a "sure to win" Powerball lottery ticket. LIGO detects shock waves, and that is really all that it does, and shock waves have nothing to do with gravity. There is no such a thing as a gravity wave, because gravity doesn't work that way.
RealScience
5 / 5 (9) Oct 16, 2017
It can be concluded that fracking in nearby states, or even an automobile accident near the site, can easily deliver the same data.


That's why they have detectors separated by thousands of kilometers, and compare the results.
dirk_bruere
5 / 5 (3) Oct 17, 2017
"Relating c to "gravitational waves" feeds the pseudo-science mill."

We have just seen gravitational waves from colliding neutron stars at the same time as light from the collision.
antialias_physorg
5 / 5 (7) Oct 17, 2017
The mainstream axiom that c is the upper limit to velocity in physics is still a presumption.

c is the speed of causality (that light happens to have that speed is a consequence of it being massless...but the nature of light itself is not why c is the upper limit). If we posit that the universe does work via interchange of cause and effect then such an upper limit is a necessity.

LIGO detects shock waves

No. Shock waves make different signals. Read up on how LIGO works. It's not that complicated.

It can be concluded that fracking in nearby states, or even an automobile accident near the site, can easily deliver the same data.

No it can't, because:
a) the signature would be totally different
b) the speed at which the signal propagates to another such detector would be totally different

There's heavy filtering going on to avoid influence local perturbances (false positives).
antialias_physorg
5 / 5 (5) Oct 17, 2017
In answer to the headline of the article:
Gravitational waves: Why the fuss?

It's like we've been blind towards a large potion of what surrounds us, and are just now opening our -as yet very blurry- eyes. It's the first step into a much more complex universe.
billpress11
1 / 5 (1) Oct 17, 2017
Quote from article: "Q: Why are they so elusive?
Einstein himself doubted gravitational waves would ever be detected given how small they are."

I agree with Einstein here with one small change, change "waves" to a single gravitational wave. What they are detecting here is a sudden change in the strength of the gravitational force (field), or number of individual gravitational waves from these merging neutron stars.

shavera
5 / 5 (6) Oct 17, 2017
What they are detecting here is a sudden change in the strength of the gravitational force (field)

Actually, that's incorrect. We're detecting waves. We're detecting measures of space elongating along one axis and shrinking along another, and then alternating which axis is elongating and shrinking (If it's one kind of wave, I think it's a slightly different but same rough idea for a different mode of wave). We're not even sensing changes in "gravitational force" at all, since gravitational waves are waves in how we measure space and time, not in how we measure gravity.

or number of individual gravitational waves

What is an 'individual' wave? This is the question at the heart of quantum mechanics of course, are some waves broken down into smallest possible parts? The answer *may* be that, yes, there are 'individual' quanta of curvature changes arriving toward us, but we don't have the maths or data yet to know.
billpress11
1 / 5 (1) Oct 17, 2017
Shavera, I somewhat agree with what you are stating, but think an individual gravity wave is much to small of a unit to ever be detected. What you are stating is that they detected a single wave not a sudden change in the number of individual ways. Well I guess that is one way of looking at it but compare to a wave of water or nearly anything else, a single wave is usually composed of many smaller units.

Another way of looking at it is when these neutron stars collided they turn a lot of matter into a huge amout of energy. Are you trying to say that this change in mass into energy released a single gravitational wave and not a sudden reduction in the number of individual gravitational waves. With out having an understanding what the small units are, we still pretty much in the dark and unable to understand exactly how gravity works.
antialias_physorg
5 / 5 (2) Oct 17, 2017
I agree with Einstein here with one small change, change "waves" to a single gravitational wave.

A wave is a - generally - defined as a single change in amplitude.
What is detected here are many changes in amplitude (one for each revolution of the neutron star binary and then some after the crash).

a sudden change in the strength of the gravitational force (field)

Not really, since mass the integral over a volume the gravitational wave travels through is zero (because there is no mass contained there in). What you do get is, as shavera points out, *simultaneous* stretching and compression along different axes. But the sum of both cancels out.
billpress11
not rated yet Oct 17, 2017
AP, here is where we disagree, I say there is a sudden change in the gravity field between these now combined neutron stars and us by their combined lost of mass, mass turned into energy. That is carried away by radiation not by the gravitational field. This reduction in mass suddenly changes the total number of gravitons exchanged between us and these now combined neutron stars That is what they detected not individual gravitons, the fundamental unit of gravity. I don't think ever this article claims that they detected a single graviton.

Quote from article: "The more massive the object, the larger the wave and the easier it is to detect."

That is very nearly like saying the larger a wave of water the easier it is to be detected. They did not detect single gravity wave (graviton) they detected a sudden change in the gravity force or field.
shavera
5 / 5 (1) Oct 17, 2017
Maybe a better phrase might be that they receive a 'wave packet' from a collision. There's certainly the wave behaviour of oscillating amplitudes, but there's also a rise in amplitude up from "not detectable above noise" to a signal and up to some peak, and then a decay down back to a noise floor. Imagine hearing a half second "chirp" of an "A" note. The waves oscillate at ~540Hz, but the volume goes from "nothing" to "something" then back to "nothing" over a time period longer than the actual oscillation of the air pressure. Does that clear up the confusion?
IronhorseA
5 / 5 (1) Oct 17, 2017
"not individual gravitons"

Gravitons are a purely hypothetical particle in Quantum Mechanics. They have never been detected. What was detected here was a wave, whether it was a full sine wave or a soliton or some other type of wave will require reading the original papers.
antialias_physorg
5 / 5 (1) Oct 17, 2017
I say there is a sudden change in the gravity field between these now combined neutron stars and us by their combined lost of mass, mass turned into energy.

I just go by the math.
https://en.wikipe...ral_form
If you have no mass within an enclosing volume then you have no net gravitational field (Note that no *net* field is not the same as saying a *uniform* field. It just means that if you have a squeeze in one direction you have to have a stretch in another).

The suqeeze in one direction and the elongation in another that you get with a gravitational wave cancels out. That's what they measure at LIGO. After all, you need both. If it were a net squeeze for example the apparatus wouldn't produce a signal at all.
billpress11
not rated yet Oct 17, 2017
Quote AP: "The suqeeze in one direction and the elongation in another that you get with a gravitational wave cancels out."

How can that be correct? That implies there is not really any change in the strength of the gravity field when there has to be a change in the strength of the field caused by a sudden lost of mass.

Another way of looking at this to imagine the sun losing a lot of mass suddenly but to spare us from toast it is all radiated away as energy from the other side. What effect would this have on the earth? The earth's orbit would be changed forever with no corresponding return to its previous orbit. That effect is exactly what happen between the earth and the combined neutron stars which suddenly radiated a lot of energy and lost mass. We are forever slightly further apart. That is what the LIGO measured, this sudden change.

shavera
5 / 5 (2) Oct 17, 2017
sorry, again, this is nothing at all to do with a 'force' of gravity. It is only to do with how you measure lengths and times changing. It has nothing to do with 'strength of the gravity field' it only has to do with two things that are at rest with respect each other having the distance between them change, even with them being at rest.

The fact that how you measure lengths and times is variable is a pretty weird idea. But it turns out that because of this, when you look at how things move in this 'curved' spacetime, even when they don't have a force acting on them, they move in orbits as if there was a force being applied. This force that 'appears' out of the maths is what we used to call the "force" of gravity.

But what LIGO observes is a different solution to the same equations, it's the solution when to massive bodies are in orbit, and they stretch and compress space in a wave.
antialias_physorg
5 / 5 (1) Oct 18, 2017
That implies there is not really any change in the strength of the gravity field

No, as I said: that the integral over a volume is zero doesn't mean the value of a scalar at all points within the volume is zero. It just averages out to zero.

Note that for the integral it isn't important how large your volume is - it can be huge or miniscule - the zero must come out in each case as long as the volume in question contains no masses.

We are forever slightly further apart.

Erm...no. Where do you get this idea?
billpress11
not rated yet Oct 18, 2017
When a mass falls into a gravity field in gains velocity and kinetic energy. If it ends in a stable orbit its velocity and kinetic energy remain fairly stable. If the gravity field this mass entered later weakens it would have to lose some of the gained velocity and kinetic energy by climbing further out of the gravity well.

Benni
2.3 / 5 (3) Oct 18, 2017
"Einstein himself doubted gravitational waves would ever be detected given how small they are."

The author has it wrong, the below link is what Einstein actually wrote about gravitational waves:

In 1936, Einstein wrote to his close friend Max Born telling him that, together with Nathan Rosen, he had arrived at the interesting result that gravitational waves did not exist, though they had been assumed a certainty to the first approximation. He finally had found a mistake in his 1936 paper with Rosen and believed that gravitational waves do exist. However, in 1938, Einstein again obtained the result that there could be no gravitational waves.

Einstein and Gravitational Waves 1936-1938. Available from:

https://www.resea...936-1938

It's simple, you just need to read what Einstein actually wrote rather than what others say about what Einstein wrote.
shavera
5 / 5 (2) Oct 18, 2017
So you really can't just imagine that mass just "radiates away" along one axis and only solve for the one body that lost it. That mass that was lost is now momentum and energy, equal and opposite to the thing that lost it. And momentum and energy factor in to the stress-energy tensor, which determines how space is curved.

But more importantly, you seem to be thinking that we're measuring the gravitational strength of distant neutron stars attracting Earth. Like neutron stars in another galaxy. I don't know if we can measure the gravitational impact of our nearest star, let alone stars in distant galaxies. So, no, LIGO isn't measuring the change in strength of the gravitational force from these stars, because we are nowhere near being able to detect our gravitational attraction to them in the first place.
billpress11
not rated yet Oct 18, 2017


Shavera, you could be right but as noted below the LIGO is very sensitive. Also if there are collisions between matter and antimatter very large quantities mass could be turn into energy in short periods of time.

Quote from link: https://en.wikipe...iki/LIGO

" gravitational waves that originate tens of millions of light years from Earth are expected to distort the 4 kilometer mirror spacing by about 10−18 m, less than one-thousandth the charge diameter of a proton. Equivalently, this is a relative change in distance of approximately one part in 1021"
Da Schneib
1 / 5 (1) Oct 18, 2017
@shavy, perhaps to clarify a bit, it's possible to take a slight step in the direction @bill wants to go. While I agree that the Einstein GRT description does not work in detail as a force field, it is also striking that overall, gravity does behave very much like an inverse square force, and also striking that both LQG and string physics describe other phenomena we normally conceive of as force fields in the same way GRT describes gravity.

I wouldn't want to be misinterpreted as saying you're wrong; you're blatantly obviously correct in the GRT description of gravity. What I'm saying is it may not be the only possible description, just as the LQG/string description of "forces" is (pretty obviously) not the only possible description particularly of EM (as Kaluza demonstrated). And it is possible that the resolution of these two views may be one of the keys to quantum gravity, since it is one of the keys to quantum EM (Dirac's quantization).

[contd]
Da Schneib
1 / 5 (1) Oct 18, 2017
[contd]
On an instantaneous basis, as every mass that generates gravitational waves causes Earth to accelerate in one direction or another, in fact there is a very slight variation in the gravity field felt on Earth; however this variation is massively (pardon the pun) swamped by the local gravity fields of, for example, the Sun, the Moon, and for that matter the other planets, and in addition since it is very fast compared with those local fields and integrates over the timespans appropriate to those fields to zero, it has no discernible effect on the Earth's orbital mechanics.

To put this in very short terms, while @bill is notionally correct it's neither detectable above the noise floor nor does this affect what LIGO detects, which is minute variations that integrate to zero on the timespans orbital mechanics measures.

To put this in terms of the EM field, it's why radio waves from distant galaxies do not measurably increase radiation exposures in human bodies on Earth.
Da Schneib
1 / 5 (1) Oct 18, 2017
As further evidence, I will point out that current and voltage in the EM description do not instantaneously change together; remember ELI the ICEman rules for inductive and capacitive lags, which are duplicated in relativistic mechanics by lags in the response of masses in relativity. We thus see lags between the gravitational wave response by light masses in LIGO as opposed to response in terms of Earth orbital mechanics, due to Earth's far greater inertia swamping out such variations, which the lighter test masses in GWOs do not. This is why we can measure gravitational waves using instruments that are on Earth.

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