Your Questions About Gravitational Waves, Answered

Your Questions About Gravitational Waves, Answered

Gizmodo readers asked a lot of great questions about yesterday’s big announcement on the discovery of gravitational waves. And Dr Amber Stuver of the LIGO Livingston Observatory in Louisiana is here today with some answers.

Gizmodo Readers: There has been a huge amount of work put into the detection of a single gravitational wave to this point and it is a huge breakthrough. It sure seems this could open up a lot of new exciting possibilities in astronomy – but is this first detection “merely” a proof that the detection in itself is possible or will you already be able to gain further scientific advancements from this? What do you hope to do with this in the future? Will there be easier methods of detecting these waves in the future?

Stuver: This is indeed the first detection, which is a breakthrough, but the goal has always been to use gravitational waves to do new astronomy. Instead of looking into the universe seeing light, now we are able to feel the very small changes in gravity caused by some of the largest, most violent, and (in my opinion) most interesting things in the universe — including things that light will never be able to bring us information about.

We have been able to do this new kind of astronomy using the waves of this first detection. Using what we already know about general relativity, we can predict what gravitational waves from objects such as black holes or neutron stars look like. The signal we found matches what is predicted for a pair of black holes, one 36 times as massive as our Sun and the other 29 times, orbiting each other faster and faster as they get close together. Finally, they merge into one black hole. So, not only was this the first detection of gravitational waves, it was also the first direct observation of black holes since light cannot be used to observe them (only how the matter around them moves).

Credit: LIGO collaboration.

How can you be certain that outside effects (such as vibration) aren’t impacting the results?

Stuver: At LIGO, we record much more data related to our environment and equipment than data that can contain a gravitational wave signal. The reason for this is that we want to be as sure as possible that outside effects don’t trick us into thinking we’ve discovered a gravitational wave. If the ground was moving an abnormal amount at the time we think we see a gravitational wave signal, we will likely dismiss it as a detection candidate.

Another measure we take to not see something accidentally is that both LIGO detectors must see the same signal within the amount of time it would take a gravitational wave to travel between the two facilities. The maximum amount of time for this trip is about 10 milliseconds. To be considered a potential detection, we must see a signal with the same shape and almost the same time and the extra data we collect from our environment must be clean of abnormalities.

There are also many other tests a detection candidate must pass before we consider it to be a valid detection, but these are the basics.

Is there any practical way for us to generate gravity waves that could be detected by a device such as this? So that we could build a gravity radio or laser?

Stuver: What you suggest is exactly what Heinrich Hertz did in the late 1880s to detect electromagnetic waves in the form of radio waves. However, gravity is the weakest of the fundamental forces holding the universe together. Because of that, moving masses around in a lab or other facility will create gravitational waves but they will be too weak for even sensitive detectors like LIGO. To make the waves strong enough, we would need to spin a dumbbell at speeds so high that it would rip any known material apart in the process. The next place to look for large amounts of mass moving around extremely fast is the universe and that is why we build detectors that target these far away sources.

Will this confirmation change our future at all? Could we harness the power of these waves for space exploration for example? Would it be possible to communicate via these waves?

Stuver: Because of the amount of mass that needs to be moving with extreme acceleration to produce gravitational waves detectors like LIGO can detect, the only known mechanism for this is pairs of neutron stars or black holes orbiting just before they merge into one (there are other sources too, but this works for our discussion). The chances of there being an advanced civilisation with the means to manipulate matter like this are incredibly small. Even if these civilisations did exist, there are much more efficient ways to communicate with us. Personally, I don’t think it would end well for us if we encountered a civilisation that had the ability to use gravitational waves as a means of communication since they would also be able to destroy us handily.

Credit: Randall Munroe/xkcd

Credit: Randall Munroe/xkcd

Are gravity waves coherent? Can they be made to be coherent? Can they be focused? What would be the effect upon a massive object of being subjected to a focused beam of gravity? Could this effect be employed to improve particle accelerators?

Stuver: Certain kinds of gravitational waves can be coherent. Consider a neutron star that is nearly perfectly spherical. If it is spinning quickly, small deformities of less than an inch will produce gravitational waves of a very consistent frequency making them coherent. But it is very difficult to focus gravitational waves because the universe is transparent to them; that is, gravitational waves pass through matter and come out unchanged. You would need to change the path of at least part of the gravitational waves in order to focus them. There may be an exotic form of gravitational lensing that could at least partially focus gravitational waves, but these would be difficult if not impossible to use for a purpose. If they could be focused, they are still so weak that I don’t know of a practical application they could have. But that is also what they said about lasers, which are just focused coherent light, so who knows?

What is the speed of a gravitational wave? Does it have mass? If it doesn’t have mass, is it possible for it to move faster than the speed of light?

Stuver: Gravitational waves are expected to travel at the speed of light. This is the speed that is implied by general relativity. However, experiments like LIGO will get to test this. It is possible that they could travel slower but very near the speed of light. If that is the case, then the theoretical particle associated with gravity (and what gravitational waves are made up of) called the graviton would have a mass. Since gravity acts between masses, this would add complications into the theory. The complications don’t make it impossible, just improbable. This is a great example of the use of Occam’s razor: the simplest explanation is usually the correct one.

How far away do you have to be from this kind of black hole merger to live to tell the tale?

Stuver: For the black hole binary we detected with gravitational waves, they produced a maximum change in the length of our 4km long arms [of] 1×10-18 metres (that is 1/1000 the diameter of a proton). We also estimate that these black holes were 1.3 billion light-years away.

Now assume that we are 2m tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright). This is more than survivable.

Using this new sense to listen to the cosmos, what are some big areas on which scientists are focusing to find out more about the universe?

Stuver: The potential is really unknown, meaning that there will be many more areas than we have even though up so far. The more we learn about the universe, the better the questions we will be able to answer with gravitational waves. Just a few are below:

· What is the cause of gamma-ray bursts?

· How does matter behave in the extreme environment of a collapsing star?

· What were the first instants after the Big Bang like?

· How does matter in neutron stars behave?

But what I am really interested in is discovering gravitational waves we didn’t anticipate. Every time humans have observed the universe in a new way, we’ve always discovered something unexpected that revolutionised our understanding of the universe. I want to find those gravitational waves and find something we couldn’t have even imagined before.

Will this have any impact on the possibility of ever making a real warp drive?

Stuver: Since gravitational waves don’t have a significant interaction with matter, there really isn’t a way to use them to propel matter. Even if you could, a gravitational wave could only travel up to the speed of light. Using them as a means to power a warp drive to go faster than the speed of light isn’t possible. I wish it was though!

What are the implications now about anti-gravity devices?

Stuver: For an anti-gravity device we would need to turn the attractive gravitational force into a repulsive one. While a gravitational wave is a propagating change in gravity, this change never becomes repulsive (i.e. negative).

Why gravity is always attractive is because negative mass doesn’t seem to exist. After all, there are positive and negative electric charges, north and south magnetic poles, but only positive mass. Why? If there was negative mass, a ball made out of the stuff would fall up instead of down. It would be repulsed from the positive mass of the Earth.

What does this mean for the possibility of time-travel and teleportation? Could we conceivably find practical applications for this phenomenon beyond learning about the universe?

Stuver: Right now, the best ways to time travel (and only into the future) are to travel round-trip at nearly the speed of light (this is the twin-paradox of special relativity) or to move into an area with much higher gravity (this is like the general relativity time travel that was featured on Interstellar). Since a gravitational wave is a propagating change in gravity, there will be very small fluctuations in the speed of time, but since gravitational waves are inherently weak, the time fluctuations are as well. While I can’t think of a practical application towards time travel (or teleportation), I’ve learned to never say never (but don’t hold your breath either).

Image courtesy of Amber Stuver

Do you anticipate a day when we stop confirming Einstein and start finding unexpected weirdness again? At least in cosmological physics terms it sometimes feels like we live in a world where Nostradamus wrote clearly and in English.

Stuver: Absolutely! Since gravity is the weakest of the forces, it is also the hardest to test. So far, every time Einstein’s relativity has been put to the test, it has accurately predicted the results of those experiments. Even the tests of general relativity we were able to do with the gravitational waves we have detected have confirmed general relativity. But I expect that we are going to be able to start testing such fine details of the theory (maybe with gravitational waves or other ways) that we will start seeing “funny” things, like having the results of an experiment be very close to what was expected, but not exactly. That won’t mean that relativity is wrong, it just may need to have some of its details refined.

Every time we answer one question about nature, it leads to more. Eventually we will have questions that will be “more” than general relativity can completely explain. That’s what makes being a scientist exciting.

Can you explain how this discovery relates to/affects the Unified Field Theory? Are we closer to confirming that or closer to debunking it?

Stuver: Right now, the results of the discovery we’ve made focus mainly on testing and confirming general relativity. Unified field theory seeks to develop a theory that can explain the physics of the very small (quantum mechanics) and the very large (general relativity). Right now, these two theories can be generalised that explain the scale of the world we live in, but not past that. Since this is focusing on the physics of the very large, there isn’t much that this discovery alone can do to advance us towards a unified theory. But as we learn more is it not out of the question. Right now, the field of gravitational wave physics is newly born. As we learn more, we will be able to possibly extend it to work toward a unified theory. But we must walk before we can run.

Now that we’re listening to gravity waves, what’s the most outrageous thing we might hear that could cause scientists to lose their collective shit? (1) Unnatural patterns/structure? (2) Gravity wave source from a region we were certain was empty? (3) Rick Astley – Never gonna give you up?

Stuver: As soon as I read your question, I immediately thought of the scene in Contact where the radio telescope picks up patterns of prime numbers. This is not anything that would be naturally occurring (at least that anyone has thought of yet). So the unnatural pattern/structure you suggest is something I think would be most likely.

I’m not sure that we are ever sure that a certain part of space is empty. After all, the black hole system we found was isolated and no light would ever come from this region, but we found gravitational waves there anyway.

Now music… I specialise in separating gravitational wave signals from the static-like noise we constantly measure from environmental influences. If I found music as a gravitational wave, especially music I’ve heard before, I would know that I was on the receiving end of a practical joke. But music that has never been heard on Earth before… That would rank up there with the prime number sequence from Contact.

Since the experiment detects the wave by a change in distance between two locations, is the amplitude greater in one direction than the other? IOW, do the readings imply that the universe is changing in size? And if so, does it confirm expansion, or something unexpected?

Stuver: We would need to observe many gravitational waves coming from many different directions in the universe before we could begin to answer that. In astronomy, this is creating a population model. How many different kinds of things are where? That’s the main question. Once we have many observations and start seeing uneven patterns, like there are many more of this kind of gravitational wave coming from a certain part of the universe and almost never anywhere else, that would be an extremely interesting result. Some patterns could confirm expansion (which we are already very sure of) or other phenomena that we haven’t thought of yet. But we need to see many more gravitational waves first.

I don’t have time to read the article, but I read the earlier post today, and it didn’t explain how, exactly, they knew the waves that were measured were due to these two supermassive blackholes. How can they know precisely what caused any of the waves they measure?

Stuver: The data analysis methods used a catalogue of predicted gravitational wave signals to compare to our data. If there is a strong correlation to one of these predications, or templates, then we not only know this is a gravitational wave candidate, but we also know what system made it from what system was used to create the template.

Each different way of making a gravitational wave, black holes merging (like this discovery), stars orbiting each other, stars dying in explosions or creating black holes, all of these will have different shapes. When we detect a gravitational wave, we use these shapes as predicted by general relativity to determine what caused it.

How do we know that these waves originated from the collision of two black holes and not some other event? is it possible to have any idea where/when the event happened to any degree of accuracy?

Stuver: Once we know what system made the gravitational wave, we can predict how strong the gravitational wave was near where it was produced. Measuring its strength once it gets to Earth and comparing our measurement to the predicted strength at the source, we can calculate how far away the source is. Since gravitational waves travel at the speed of light, we also can calculate how long the gravitational waves have been travelling to Earth.

For the black hole system we discovered, we measured a maximum change in the length of LIGO’s arms of 1/1000 the diameter of a proton. For this system, that places it about 1.3 billion light-years away. So, the gravitational wave discovered in September and announced yesterday have been ontheir way to us for 1.3 billion years. This was before animal life formed on Earth but after multicellular life formed.

During the announcement mention was made of other detectors to look at longer wave periods — some of the graphics shown imply that these would be space-based detectors. Can you give more details about these larger detectors — where we are in implementation, what additional challenges we face to create them, how long it will take to get them on line, what types of information they could yield versus LIGO?

Stuver: There is indeed a space-based detector in the works. It is called LISA (Laser InterferometerSpace Antenna). Since it would be in space, it would be sensitive to low frequency gravitational waves that ground-based detectors won’t be due to the Earth’s natural vibrations. This is a huge technological challenge since these satellites need to be farther away from Earth than humans have ever gone. That means if something malfunctions, we can’t send astronauts up to fix it like we did the Hubble in the 1990’s. To test the needed technologies, a mission called LISA Pathfinder was launched in December 2015. So far, it has completed all of its benchmarks successfully but this mission isn’t over yet.

Can gravitational waves be translated into sound waves? And if so, what would it sound like?

Stuver: Absolutely. Of course, you can’t hear just a gravitational wave. But if you take the signal and put it through speakers, you can hear it.

What can we do with this information? Do other astronomical objects of sizable mass emit these waves? Can this be used to find planets or just black holes?

Stuver: It isn’t just the mass that matters when looking for gravitational waves (although more is better). It is also the acceleration that the object is undergoing. The black holes we discovered were orbiting around each other at about 60 per cent the speed of light when they merged. That is why we could detect them as they were merging. But there are no more gravitational waves coming from them now that they have settled down into one mass with little motion.

So anything very massive moving around very quickly can possibly make gravitational waves we can detect.

Exoplanets are much less likely to have the mass or the acceleration to make detectable gravitational waves. (I’m not saying they aren’t making gravitational waves, just that they aren’t strong enough or the right frequency for us to detect). Even if an exoplanet were massive enough to possibly make detectable gravitational waves, the accelerations that it would need to undergo would likely tear the planet apart. This is especially an issue since the most massive planets tend to be gas giants.

Credit: LIGO collaboration.

How valid is the wave-like-in-water analogy? Can we “surf” these waves? Are there gravity “peaks” like there are “wells”?

Stuver: Because gravitational waves can travel through matter unchanged, there isn’t a way to surf them or use them for another kind of propulsion. So no gravitational-wave surfing.

The”peaks” and “wells” is an excellent point. Gravity is always attractive because there is no negative mass. We don’t know why but it has never been observed in a lab or any evidence found elsewhere in the universe. So gravity is usually represented on spacetime graphics as being a downward curvature, or your “well.” A mass travelling by the “well” will tend to bend inward toward it; this is gravitational attraction. If you had negative mass, you would have repulsion, which would be represented by a “peak.” A mass moving by a “peak” would tend to bend away from it. So there are”wells” but no “peaks.”

The water analogy is very good at talking about how the strength of the wave decreases as it travels away from its source. A water wave will get smaller and smaller just like a gravitational wave will get weaker and weaker.

How does this discovery effect the narrative of the inflation period of the big bang?

Stuver: So far, this discovery really doesn’t touch on anything having to do with inflation. To make statements about that, you would need to observe the relic gravitational waves from the Big Bang. The BICEP2 project believed that they indirectly observed these gravitational waves — meaning they thought they saw the effects these gravitational waves had on the relic light from the Big Bang. If this stood up to greater scrutiny, it would have virtually proven that there was a period of inflation shortly after the Big Bang. Unfortunately, their accounting for the effects that the dust in our galaxy had on their data wasn’t sufficient.

LIGO may be able to directly see these gravitational waves eventually (they are the weakest kind of gravitational wave we hope to detect). If we see those gravitational waves, we will be seeing farther back into the history of the universe than we ever have before and we will be able to make statements about inflation then.

What can the Public do so we can ensure to keep Funding Science like this. Over the years the Science budget has been cut in so many places I’m afraid our next generation will not have enough funds and Equipment to make such Amazing Discovery’s like this one?

Stuver: Contact your representatives and senators and tell them you support science funding. And I don’t mean just for LIGO. Many of the technologies that we take for granted today, many of which make our smartphones work, were originally developed by government-funded research whose results found practical applications. And research like we do at LIGO will not only help us learn more about our universe we are a part of, but it will allow us to turn the universe into our own lab to observe things we can never replicate on Earth. We can learn more about nuclear physics from how matter behaves in a collapsing star. These refinements can have far reaching applications.

Projects like LIGO also need to develop new technology to make our discoveries. These technologies often work their way into industry and everyday use. The laser was once called the “physicist’s playtoy” because it wasn’t believed to have any practical applications. Today we use it to scan our groceries, play CDs, perform medical procedures, and look for gravitational waves.

Top image: LIGO Livingston facility in Louisiana. Credit: LIGO Collaboration/NSF.