European plan for gigantic new gravitational wave detector passes milestone | Hacker News Reader

European plan for gigantic new gravitational wave detector passes milestone

sciencemag.org

107 pointsrbanffy87 comments

What are the practical applications of gravitational wave research? Or is it purely basic science for now?

It's another type of telescope. In history we first observed waves in the visible spectrum, then perhaps infrared, radio, microwave, ultraviolet, X-ray, etc.

* https://en.wikipedia.org/wiki/Telescope#By_spectrum

This will allow measurements of another type of wave.

* https://en.wikipedia.org/wiki/Gravitational-wave_astronomy

No one expects the science to be of practical value, but the development of the technology and engineering practices needed to perform the science may have many applications elsewhere.

It has some merit sometimes, but I prefer not to rely on this justification. These are incidental benefits at the margins at best.

My preferred argument is, if we can spend billions on trivialities like carbonated sugar water and computer games, as a civilisation we can afford basic science research. It’s not actually all that expensive in the grand scheme of things, and cutting science would not necessarily have any impact on funding for the alternative things (world peace, feeding the poor, etc) people who complain about such funding want. If they want to complain about waste they are picking a particularly poor target. A bit of a tangent, as the head comment wasn’t actually making this complaint.

I don't know of any field that necessitates better noise reduction than gravitational wave detection. Seems to me it has the potential to spin off tons of researchers/techniques able to handle noisy data.

Better measuring macro effects of gravity might help us build better model of physics which can enable better quantum theory which can provide us with better technologies.

The several already existing gravitational wave detectors have already detected signals believed to come from black hole-black hole, black hole-neutron star and neutron star-neutron star mergers.

Using the time delta between detecting the same events at different detectors around the globe, we have inferred the directions the signals are coming from.

The intensity curves of the events tell us details about the masses and other characteristics of the colliding object. Also over time the distribution of their masses and other characteristics will give us information about the populations and distribution of these objects, which can help validate our theories of star, neutron star, black hole and galaxy formation.

> What are the practical applications of gravitational wave research?

Currently, not much. We’re Galileo grinding lenses at home.

Theoretically? Everything from a whole new mode of active sensing, e.g. deep into the Earth and/or Sun, all the way to communications. Being able to directly measure gravity also lets us do new classes of experiments (not yet, but if we can measure more sensitively). We don’t know what we don’t know when it comes to gravity, so that’s another pot of potential.

Less fantastically, these are immensely precise instruments. The manufacturing methods and know how will almost certainly translate to other domains.

If and when ET goes whipping by earth, these things might detect it. Setting all the science and engineering breakthroughs aside, every scientist wants a warp drive detector network.

Get them working well enough and we might be able to see the cosmic gravity wave background. That might let us see how/if the big bang actually happened, pre-inflation.

If we build enough of these things, can we combine the data and use them to take gravitational photographs?

This is more like a microphone than a camera. I don't think you could really build a camera out of a large number of microphones, but you can detect events and infer their direction and distance with good precision.

You CAN build a camera out of a large number of microphones

Citation?

Ocean floor mapping [1] and ultrasound are two examples.

[1] https://www.acousticimaging.com

That requires bouncing sounds off of things though. With gravity waves I guess you would have to measure how objects deflect gravity waves produced by other sources. May be useful to try to measure properties about black holes or other invisible objects.

You use the detector as a camera and wait for a black hole merger to use it as a flash.

Seems pretty doable with a number of detectors. The more the better the “photo”

So much science happening these days. For some reason seeing smart people doing things I couldn't possibly understand makes me hopeful.

When I was a kid, a laser was a fantastical machine the size of a small house that cost a million dollars. Now they're everywhere and cost pennies.

I wonder if the children of today will someday have gravitational wave detectors on their keychains and use them to entertain their cats the way we do with lasers, and think it's nothing special.

> the edge of the observable universe, 45 billion light-years away

If the universe is 13.x billion years old, how is the edge 45 billion light years away? Assume we're on an edge (probably not, but ...); the radius between us and the center should be <13.x billion light years, and then the radius to the far edge would be another 13.x billion light years; that seems to total <28 billion light years max.

Looking out my window ... definitely doesn't look like 45 billion ...

Space expands faster than light. Don't forget that C is the max speed of stuff in space, not of space itself.

And space without stuff creates gravitational waves? How? Thanks for the impromptu physics lesson, BTW.

Space expands uniformly across all its volume, so the parts far away from us also have stuff in it. Some stuff already moves away from us faster than light.

When space expands, it carries matter with itself just like our dog ran away with my sandwich so many years ago.

Sorry to be pedantic or if I'm missing the obvious:

The article says: Using gravitational waves we can sense what is 45 billion light years (bly, for purposes of this post) away. That implies that something(s) at 45 bly once did generate gravity waves.

If matter can't move faster than C, and if it has only been on the move for 13.x billion years, it must be less than 28 bly distant.

I am given to understand that the universe can expand faster than C; I presume that the universe must lack matter beyond 28 bly. I confess that I'm not sure what exactly is expanding.

Therefore, what could be located between 28 and 45 bly distant that is generating gravity waves?

One possibility: the matter moves no faster than C relative to points in the universe as it stretches, but is carried faster than that relative to other parts of the universe, like a sandwich.

If we receive light that is 13b years old the light has travelled that distance, but the source of that light is now 45bly away from us because the universe expands.

Imagine it like that: you are in a train and per the rules of the railway company, nobody can run in the corridors aboard the trains. Your train passes by another train where the same no-run rule applies. If you can't run and a person in the other train cannot run, how the two of you can see each other for only a brief moment before you are carried kilometers from each other? Well, the rules do not apply to the trains both of you are occupying. So, though the two of you are relatively static, the underlying infrastructure is moving in different directions with speeds which are unfeasible for the passengers.

In my previous comment, me and the sandwich were sharing the brief moment of unity that was broken apart by the runaway trajectory of the dog which made reunification impossible.

Thank you. I recognize the trains analogy, BTW, and perhaps frames of reference, but my knowledge of the topic is very spotty. And I've also had a sandwich snatched from my fingertips, seemingly faster than light.

I'm not an expert, but as far as I understand it the universe expanding means space itself is stretching at every point. Meaning if something was 1ly away in the past, it might be farther away due to that effect. This also applies to gravitational waves and anything else inside the universe.

You need something with mass to generate gravitational waves, but those waves themselves can pass through empty space without any medium, or rather the empty space itself is the medium.

Wait, what, so atoms, neutrons, quarks etc. are all growing and moving apart at the same rate as the Universe?

The expansion at a local level is too slow and is overwhelmed by the local forces holding these structures together. Only at the cosmic scale, where the gravitational forces between galaxies and clusters or super-clusters of galaxies are very tenuous, the expansion takes over.

The forces that keep atoms, neutrons, quarks etc bound to each other is stronger than the force caused by space expanding. Therefore, space expanding is mostly observed when measuring distance between galaxies.

Expansion of space

What do you think 45 billion ly deep space would look like, to the naked eye?

Like the absence, in the brain, of the grey matter that detects humor. :)

Something something lots of Olympic-size swimming pools.

Black.

Yep. If space didn't expand there would be a star or galaxy at every point we looked.

So not black then.

Only if stars and galaxies were eternal and hadn’t first formed about 13 billion years ago.

We apparently don't know why space looks black. See Olbers' Paradox.

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/que...

Oh, but we do know. It is because the age of the universe and the speed of light are both finite. (We knew that back in 2002 as well; the page you linked seems to be written the way it is mostly for pedagogical reasons.)

Well I have a citation from NASA for my claim. What do you have? :)

I recently read another article saying something similar but I personally know nothing about it. It definitely looks black to me.

Edit: Looking around, some other credible websites agree with you. That would seem to imply that the night sky is very slowly getting brighter.

But nowhere does the page make the claim that we don't know! It only says that it's not a trivial question, and that there was a time that we didn't have a good explanation for that. Like we used to not have a good explanation for gravity or illnesses or earthquakes or what have you. It gives the best explanation that we have which is also overwhelmingly likely to be true given the evidence but of course there's never anything that we can actually prove in science, so in the interest of pedagogy it doesn't outright say that we know.

Anyway, as I said, the page is from 2002 and many things have happened in cosmology since then. Indeed there are things that we didn't know back then but are now pretty sure about.

For someone who knows more than me: what is space? If it's able to "move" in the sense of expanding into existence faster than light can travel, what is the composition of it? Is this what dark matter is meant to explain? For something to expand faster than C means it has to be pushed at least that quicky by some other thing or some other force, right?

I have no science background at all, just a few decades of sysadmin work. Is there a book I should start with?

No need to push anything. Distances between objects (that are sufficiently far away from each other) just increase as if there’s "more space" inserted between them in some sense. There’s the concept of spacetime metric, a function that’s sort of a universal measuring stick for distances in the universe, and it is "shrinking" (as in there’s a parameter that varies as a function of time) so that a distance that used to be three measuring sticks long is now three point one.

“A Brief History of Time” by Stephen Hawking is written for the average reader and wonderful.

The acceleration of the expansion of the universe is sometimes attributed to “dark energy”, but that’s a placeholder term. We don’t actually know what is doing it exactly. We can just see that it seems to be happening.

The ‘faster than the speed of light’ thing is only if you measure the rate of divergence of objects at very great distances from each other, tens of billions of light years. At more local scales, such as between our neighbouring galaxies and clusters, the effect is relatively small and negligible compared to gravity.

No one knows what space is.

We know there's a thing we call "spacetime", because space and time are closely related. And we know some things about its properties.

But we know nothing at all about what it's made of, or why it has the properties it does.

You can think of this space as something that tells for every point a path passes through, how long the distance of the path in that point is. So when I connect A and B with a path, and I take the integral of space on that path, I get the length of the path I would travel if I go from A to B along this path. One step further, the distance from A to B is the shortest length of all paths from A to B.

Now if for every point in the universe, we have those numbers for each path through that point, we call that a "metric". Einsteins laws tell us how those numbers all relate to each other and how changing the numbers at one point affects the numbers at other points over time. And from those laws, you can see that you can have waves within those numbers.

So it is not "something" that is waving, it is space itself, the thing that defines distances, the metric, that is waving.

Now the annoying thing is that that means that two points at rest with each other, can be at a changing distances from each other. What happened is that the space between them was waving, changing which path between the points was the shortest path, and how long that path was.

Noob here so forgive my ignorance but I thought a light-year was a unit for measuring distance, not time.

It is. Light travels one lightyear per year.

Although, when you have to account for anything in relativity, the expansion of the universe included, you have to be pedantic about what you mean by both distance and time because they’re both observer-dependent, albeit in a way that keeps the speed of light itself a constant.

Do we know which of the two sites was selected yet?

Don't these gravitational wave detectors usually come in pairs?

That would be the European thing to do, true.

Not just a European thing, but also an important science thing.

LIGO, which made the first gravitational wave detections, is located in the US and also has a pair of detectors (one in Hanford Washington; the other in Livingston, Louisiana).

The reason to build in pairs is because the detectors need to be _extremely_ sensitive in order to detect gravitation waves. Such sensitive detectors can easily pick up all kinds of noise from local phenomena (such as a truck driving nearby), so having two sites separated by a good distance helps rule out local noise. Only signals that are replicated across both sites need to be evaluated.

There are other benefits to have multiple detectors, too. Having two detectors gives us some ability to determine the direction the wave is coming from. More detectors will allow us to determine the direction even more precisely.

Wouldn't 3 better than? For quorum?

They would. But it is harder to get funding for three than it is to get funding for two...

If you had infinite resources, sure!

It’s a cost benefit analysis (and one I’m not remotely equipped to make). My guess is they went with two sites because that was the smallest number they could get away with while still being able to make detections.

Great explanation, thanks. I'd like to add that the cost for two would be much less than twice the cost for one, due to the design reuse of all the custom equipment.

It should be noted that while LIGO has one detector per site in an L shape, the Einsten Telescope (ET) will have three, each one in the corner of an equilateral triangle[1]. The idea is that this provides enough of a cross-check to rule out local sources and glitches.

[1]: https://en.wikipedia.org/wiki/Einstein_Telescope#Current_des...

This decision will be made next year at the earliest.

Source: I work in one of the physics labs that is pushing ET.

Hi Lewis, keep me posted ;)

Is there a gravitational background wave of the universe? Could we receive G-signals from outside of our universe visual horizon? Also, if our universe is inside a 4-dimensional blackhole like some theory suggests, could we receive G-signals from other surrounding 4-D bodies?

There’s probably a cosmic gravitational wave background “behind” the cosmic microwave background, but given that the gravitational wave events we can currently only just detect are multiple stellar masses of radiated energy, I doubt we’ll be able to see that background until after we can build a Dyson swarm.

If the US and Europe are both working on this, why not combine resources and build a bigger and better one?

If I had to guess: scientists want different sites to be able to cross-validate their findings. Ie there’s value in redundancy when it comes to fundamental research.

Indeed, but also the time delta between detecting the same event at different detectors around the globe gives us directional information.

Because it makes sense to have multiple independent detectors for several reasons.

For this stuff, more detectors, as far apart as possible, improve observations ,ore.

Only €1.9B?

If I was a centi-billionaire, I'd just pay for one.

That should save at least 5 years of bureaucratic process to advance this scientific frontier!

What is the benefit of being able to observe these so far away? The article does not provide the justification for the project.

It's basic research and has no immediate application for normal life if this is what you are wondering about.

For astronomy, it is giving us a completely new way of observing the universe. Most other ways were based on different ways of observing electromagnetic radiation.

https://en.wikipedia.org/wiki/Basic_research

Primarily it means there are more potential events to observe, which again leads to better statistics and hopefully more of the interesting events.

Better statistics can help guide us towards a viable theory of quantum gravity, as well as improve our understand of what happens around black holes and neutron stars.

I'm not a scientist but: I believe it provides access to the only real "data" on events that actually occurred very large distances away. This data is valuable in aiding our understanding of the early universe e.g. the big bang theory.

The justification is knowledge.

Just creating these machines moves science and engineering forward.

Why isn’t America doing more to be on the forefront?

From the article,

“U.S. gravitational wave physicists welcomed the announcement, too, as they think it may bolster their plans to build a pair of detectors even bigger than the Einstein Telescope in a project called Cosmic Explorer.”

Americans in fact have kept investing in gravitational wave detectors when everyone else didn't because they thought you'd need way more sensitivity to detect anything. This led to the first direct observations of gravitational waves happening with US built detectors, together with the Nobel prize being awarded to three americans.

The LIGO observatory, in the United States (Washington and Louisiana) was the first to make gravitational wave detection.

I agree that the US often lets itself fall behind in some areas (Superconducting Supercollider!), but gravitational wave astronomy is an area where the US is significantly participating at the forefront of research.

I hope we continue to invest further!

Why does it matter, as long as gravitational wave detectors are available and the research is performed? In a purely selfish sense, isn't it better if someone else spends their money on it?

What? And let them have all that delicious gravitational knowledge before us?

What's the point even. This experiment doesn't test any important hypotheses so it's nothing to do with advancing science

More detections gives us information about the population characteristics of various sized black holes and neutron stars. We have models for star and galaxy formation that these events can validate, which in turn can also tell us about conditions in the early universe.

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