What Happens When Black Holes Collide?

The sign of a truly great scientific theory is by the outcomes it predicts when you run experiments or perform observations. And one of the greatest theories ever proposed was the concept of Relativity, described by Albert Einstein in the beginning of the 20th century.

In addition to helping us understand that light is the ultimate speed limit of the Universe, Einstein described gravity itself as a warping of spacetime.

He did more than just provide a bunch of elaborate new explanations for the Universe, he proposed a series of tests that could be done to find out if his theories were correct.

One test, for example, completely explained why Mercury’s orbit didn’t match the predictions made by Newton. Other predictions could be tested with the scientific instruments of the day, like measuring time dilation with fast moving clocks.

Since gravity is actually a distortion of spacetime, Einstein predicted that massive objects moving through spacetime should generate ripples, like waves moving through the ocean.

The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle
The more massive the object, the more it distorts spacetime. Credit: LIGO/T. Pyle

Just by walking around, you leave a wake of gravitational waves that compress and expand space around you. However, these waves are incredibly tiny. Only the most energetic events in the entire Universe can produce waves we can detect.

It took over 100 years to finally be proven true, the direct detection of gravitational waves. In February, 2016, physicists with the Laser Interferometer Gravitational Wave Observatory, or LIGO announced the collision of two massive black holes more than a billion light-years away.

Any size of black hole can collide. Plain old stellar mass black holes or supermassive black holes. Same process, just on a completely different scale.

Colliding black holes. Credit: LIGO/A. Simonnet
Colliding black holes. Credit: LIGO/A. Simonnet

Let’s start with the stellar mass black holes. These, of course, form when a star with many times the mass of our Sun dies in a supernova. Just like regular stars, these massive stars can be in binary systems.

Imagine a stellar nebula where a pair of binary stars form. But unlike the Sun, each of these are monsters with many times the mass of the Sun, putting out thousands of times as much energy. The two stars will orbit one another for just a few million years, and then one will detonate as a supernova. Now you’ll have a massive star orbiting a black hole.  And then the second star explodes, and now you have two black holes orbiting around each other.

As the black holes zip around one another, they radiate gravitational waves which causes their orbit to decay. This is kind of mind-bending, actually. The black holes convert their momentum into gravitational waves.

As their angular momentum decreases, they spiral inward until they actually collide.  What should be one of the most energetic explosions in the known Universe is completely dark and silent, because nothing can escape a black hole. No radiation, no light, no particles, no screams, nothing. And if you mash two black holes together, you just get a more massive black hole.

The gravitational waves ripple out from this momentous collision like waves through the ocean, and it’s detectable across more than a billion light-years.

Arial view of LIGO Livingston. (Image credit: The LIGO Scientific Collaboration).
Arial view of LIGO Livingston. Credit: The LIGO Scientific Collaboration

This is exactly what happened earlier this year with the announcement from LIGO. This sensitive instrument detected the gravitational waves generated when two black holes with 30 solar masses collided about 1.3 billion light-years away.

This wasn’t a one-time event either, they detected another collision with two other stellar mass black holes.

Regular stellar mass black holes aren’t the only ones that can collide. Supermassive black holes can collide too.

From what we can tell, there’s a supermassive black hole at the heart of pretty much every galaxy in the Universe. The one in the Milky Way is more than 4.1 million times the mass of the Sun, and the one at the heart of Andromeda is thought to be 110 to 230 million times the mass of the Sun.

In a few billion years, the Milky Way and Andromeda are going to collide, and begin the process of merging together. Unless the Milky Way’s black hole gets kicked off into deep space, the two black holes are going to end up orbiting one another.

Just with the stellar mass black holes, they’re going to radiate away angular momentum in the form of gravitational waves, and spiral closer and closer together. Some point, in the distant future, the two black holes will merge into an even more supermassive black hole.

View of Milkdromeda from Earth "shortly" after the merger, around 3.85-3.9 billion years from now Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
View of Milkdromeda from Earth “shortly” after the merger, around 3.85-3.9 billion years from now. Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger

The Milky Way and Andromeda will merge into Milkdromeda, and over the future billions of years, will continue to gather up new galaxies, extract their black holes and mashing them into the collective.

Black holes can absolutely collide. Einstein predicted the gravitational waves this would generate, and now LIGO has observed them for the first time. As better tools are developed, we should learn more and more about these extreme events.

What is Time Dilation?

One of the most interesting topics in the field of science is the concept of General Relativity. You know, this idea that strange things happen as you near the speed of light. There are strange changes to the length of things, bizarre shifting of wavelengths. And most puzzling of all, there’s the concept of dilation: how you can literally experience more or less time based on how fast you’re traveling compared to someone else.

And even stranger than that? As we saw in the movie Interstellar, just spending time near a very massive object, like a black hole, can cause these same relativistic effects. Because mass and acceleration are sort of the same thing?

Honestly, it’s enough to give you a massive headache.

But just because I find the concept baffling, I’m still going to keep chipping away, trying to understand more about it and help you wrap your brain around it too. For my own benefit, for your benefit, but mostly for my benefit.

There’s a great anecdote in the history of physics – it’s probably not what actually happened, but I still love it.

One of the most famous astronomers of the 20th century was Sir Arthur Eddington, played by a dashing David Tennant in the 2008 movie, Einstein and Eddington. Which, you should really see, if you haven’t already.

So anyway, Doctor Who, I mean Eddington, had worked out how stars generate energy (through fusion) and personally confirmed that Einstein’s predictions of General Relativity were correct when he observed a total Solar Eclipse in 1919.

Arthur Eddington
Arthur Eddington

Apparently during a lecture by Sir Arthur Eddington, someone asked, “Professor Eddington, you must be one of the three people in the world who understands General Relativity.” He paused for a moment, and then said, “yes, but I’m trying to think of who the third person is.”

It’s definitely not me, but I know someone who does have a handle on General Relativity, and that’s Dr. Brian Koberlein, an astrophysics professor at the Rochester Institute of Technology. He covers this topic all the time on his blog, One Universe At A Time, which you should totally visit and read at briankoberlein.com.

In fact, just to demonstrate how this works, Brian has conveniently pushed his RIT office to nearly light speed, and is hurtling towards us right now.


Dr. Brian Koberlein:
Hi Fraser, thanks for having me. If you can hang on one second, I just have to slow down.

Fraser Cain:
What just happened there? Why were you all slowed down?

Brian:
It’s actually an interesting effect known as time dilation. One of the things about light is that no matter what frame of reference you’re in, no matter how you’re moving through the Universe, you’ll always measure the speed of light in a vacuum to be the same. About 300,000 kilometres per second.

And in order to do that, if you are moving relative to me, or if I’m moving relative to you, our references for time and space have to shift to keep the speed of light constant. As I move faster away from you, my time according to you has to appear to slow down. On the same hand, your time will appear to slow down relative to me.

And that time dilation effect is necessary to keep the speed of light constant.

Fraser:
Does this only happen when you’re moving?

A representation of the coordinate system of the warped space around Earth. Credit: NASA

Brian:
Time dilation doesn’t just occur because of relative motion, it can also occur because of gravity. Einstein’s theory of relativity says that gravity is a property of the warping of space and time. So when you have a mass like Earth, it actually warps space and time.

If you’re standing on the Earth, your time appears to move a little bit more slowly than someone up in space, because of the difference in gravity.

Now, for Earth, that doesn’t really matter that much, but for something like a black hole, it could matter a great deal. As you get closer and closer to a black hole, your time will appear to slow down more and more and more.

Fraser:
What would this mean for space travel?

Brian:
In many times in science fiction, you’ll see the idea of a rocket moving very close to the speed of light, and using time dilation to travel to distant stars.

But you could actually do the same thing with gravity. If you had a black hole that was going out to another star or another galaxy, you could actually take your spaceship and orbit it very close to the black hole. And your time would seem to slow down. While you’re orbiting the black hole, the black hole would take its time to get to another star or another galaxy, and for you it would seem really quick.

Orbiting near a moving black hole doesn't seem like the safest mode of transportation, but time dilation might make it worth the risk. Credit: NAOJ
Orbiting near a moving black hole doesn’t seem like the safest mode of transportation, but time dilation might make it worth the risk. Credit: NAOJ

So that’s another way that you could use time dilation to travel to the stars, at least in science fiction.

Fraser:
All right Brian, I’ve got one final question for you. If you get more massive as you get closer to the speed of light, could you get so much mass that you turn into a black hole? I’d like you to answer this question in the form of a blog post on briankoberlein.com and on the Google+ post we’re going to link right here.

Brian:
Thanks Fraser, I’ll have that answer up on my website.


Once again, we visited the baffling realm of time dilation, and returned relatively unscathed. It doesn’t mean that I understand it any better, but I hope you do, anyway. Once again, a big thanks to Dr. Koberlein for taking a few minutes out of his relativistic travel to answer our questions. Make sure you visit his blog and read his answer to my question.

How Far Can You Travel?

In a previous article, I talked about how you can generate artificial gravity by accelerating at 9.8 meters per second squared. Do that and you pretty much hit the speed of light, then you decelerate at 1G and you’ve completed an epic journey while enjoying comfortable gravity on board at the same time. It’s a total win win.

What I didn’t mention how this acceleration messes up time for you and people who aren’t traveling with you. Here’s the good news. If you accelerate at that pace for years, you can travel across billions of light years within a human lifetime.

Here’s the bad news, while you might experience a few decades of travel, the rest of the Universe will experience billions of years. The Sun you left will have died out billions of years ago when you arrive at your destination.

Welcome to the mind bending implications of constantly accelerating relativistic spaceflight.

With many things in physics, we owe our understanding of relativistic travel to Einstein. Say it with me, “thanks Einstein.”

The effect of time dilation is negligible for common speeds, such as that of a car or even a jet plane, but it increases dramatically when one gets close to the speed of light.
The effect of time dilation is negligible for common speeds, such as that of a car or even a jet plane, but it increases dramatically when one gets close to the speed of light.

It works like this. The speed of light is always constant, no matter how fast you’re going. If I’m standing still and shine a flashlight, I see light speed away from me at 300,000 km/s. And if you’re traveling at 99% the speed of light and shine a flashlight, you’ll see light moving away at 300,000 km/s.

But from my perspective, standing still, you look as if you’re moving incredibly slowly. And from your nearly light-speed perspective, I also appear to be moving incredibly slowly – it’s all relative. Whatever it takes to make sure that light is always moving at, well, the speed of light.

This is time dilation, and you’re actually experiencing it all the time, when you drive in cars or fly in an airplane. The amount of time that elapses for you is different for other people depending on your velocity. That amount is so minute that you’ll never notice it, but if you’re traveling at close to the speed of light, the differences add up pretty quickly.

But it gets even more interesting than this. If you could somehow build a rocket capable of accelerating at 9.8 meters/second squared, and just went faster and faster, you’d hit the speed of light in about a year or so, but from your perspective, you could just keep on accelerating. And the longer you accelerate, the further you get, and the more time that the rest of the Universe experiences.

The really strange consequence, though, is that from your perspective, thanks to relativity, flight times are compressed.

I’m using the relativistic star ship calculator at convertalot.com. You should give it a try too.

Proxima Centauri. Credit: ESA/Hubble & NASA
Proxima Centauri. Credit: ESA/Hubble & NASA

For starters, let’s fly to the nearest star, 4.3 light-years away. I accelerate halfway at a nice comfortable 1G, then turn around and decelerate at 1G. It only felt like 3.5 years for me, but back on Earth, everyone experienced almost 6 years. At the fastest point, I was going about 95% the speed of light.

Let’s scale this up and travel to the center of the Milky Way, located about 28,000 light-years away. From my perspective, only 20 years have passed by. But back on Earth, 28,000 years have gone by. At the fastest point, I was going 99.9999998 the speed of light.

Let’s go further, how about to the Andromeda Galaxy, located 2.5 million light-years away. The trip only takes me 33 years to accelerate and decelerate, while Earth experienced 2.5 million years. See how this works?

The Andromeda Galaxy. Credit: NASA/JPL-Caltech/WISE Team
The Andromeda Galaxy. Credit: NASA/JPL-Caltech/WISE Team

I promised I’d blow your mind, and here it is. If you wanted to travel at a constant 1G acceleration and then deceleration to the very edge of the observable Universe. That’s a distance of 13.8 billion light-years away; you would only experience a total of 45 years. Of course, once you got there, you’d have a very different observable Universe, and billions of years of expansion and dark energy would have pushed the galaxies much further away from you.

Some galaxies will have fallen over the cosmic horizon, where no amount of time would ever let you reach them.

If you wanted to travel 100 trillion light years away, you could make the journey in 62 years. By the time you arrived, the Universe would be vastly different. Most of the stars would have died a long time ago, the Universe would be out of usable hydrogen. You would have have left a living thriving Universe trillions of years in the past. And you could never get back.

Our good friends over at Kurzgesagt  covered a very similar topic, discussing the limits of humanity’s exploration of the Universe. It’s wonderful and you should watch it right now.

Of course, creating a spacecraft capable of constant 1G acceleration requires energies we can’t even imagine, and will probably never acquire. And even if you did it, the Universe you enjoy would be a distant memory. So don’t get too excited about fast forwarding yourself trillions of years into the future.

Time Dilation Confirmed in the Lab

It sounds like science fiction, but the time you experience between two events depends directly on the path you take through the universe. In other words, Einstein’s theory of special relativity postulates that a person traveling in a high-speed rocket would age more slowly than people back on Earth.

Although few physicists doubt Einstein was right, it’s crucial to verify time dilation to the best possible accuracy. Now, an international team of researchers, including Nobel laureate Theodor Hänsch, director of the Max Planck optics institute, has done just this.

Tests of special relativity date back to 1938. But once we started going to space regularly, we had to learn to deal with time dilation on a daily basis. GPS satellites, for example, are basically clocks in orbit. They travel at a whopping speed of 14,000 kilometers per hour well above the Earth’s surface at a distance of 20,000 kilometers. So relative to an atomic clock on the ground they lose about 7 microseconds per day, a number that has to be taken into account for them to work properly.

To test time dilation to a much higher precision, Benjamin Botermann of Johannes Gutenberg-University, Germany, and colleagues accelerated lithium ions to one-third the speed of light. Here the Doppler shift quickly comes into play. Any ions flying toward the observer will be blue shifted and any ions flying away from the observer will be red shifted.

The level at which the ions undergo a Doppler shift depends on their relative motion, with respect to the observer. But this also makes their clock run slow, which redshifts the light from the observer’s point of view — an effect that you should be able to measure in the lab.

So the team stimulated transitions in the ions using two lasers propagating in opposite directions. Then any shifts in the absorption frequency of the ions are dependent on the Doppler effect, which we can easily calculate, and the redshift due to time dilation.

The team verified their time dilation prediction to a few parts per billion, improving on previous limits. The findings were published on Sept. 16 in the journal Physical Review Letters.

Cosmonaut Sergei Krikalev, the World’s Most Prolific Time Traveler

Is time travel a fact or is it just science fiction? Thanks to time dilation and Einstein’s theory of relativity, we know that time travel can and actually does happen, albeit only in extremely tiny increments at the speeds and distances we can travel in space. If you add up the accumulated speed cosmonaut Sergei Krivalev has traveled in space – the most of any human with a total time spent in orbit of 803 days 9 hours and 39 minutes – he has actually time-traveled into his own future by 0.02 seconds.

Time dilation is caused by differences in either gravity or relative velocity — each of which affects time in different ways. When astronauts and satellites orbit the Earth, they are slightly further away from the center of the planet –compared to people on the ground – and so they actually experience less gravitational time dilation. This means the astronauts’ time would run slightly faster, and when they return to Earth, they’d have to “come back” to the past compared to when they were in space.

But time dilation due to velocity means that clocks for astronauts in space run slightly slower relative to people who are on the ground. When you come back to Earth, you’d be have to go into the future slightly to catch up with clocks on the ground.

The effect of time dilation due to gravity, however, “is quite small because Earth’s gravity is quite weak,” says educator Colin Stuart in this great instructional video from TedEd, “and so the time dilation due to their speed wins out and astronauts really do travel a tiny amount into their futures.”

But, as stated earlier, with our current technology limiting the velocities of astronauts, these differences are minuscule: after 6 months on the ISS, an astronaut has aged less than those on Earth, but only by about 0.007 seconds. The effects would be greater if we could get the ISS to orbit Earth at near the speed of light (approximately 300,000 km/s), instead of the actual speed of about 7.7 km/s.

This effect has been proven by GPS satellites, which orbit Earth at about 14,000 km/h (9,000 mph) which cuts several microseconds off their clocks daily, relative to clocks on Earth.

Watch the video for more information and see associated material from TedEd, or read these interesting articles from Huffington Post and DailyMail. Here are some calculations about time and the “Twin Paradox.”

And if you really want to know what time it is, check out the website for the primary atomic clock.

Astronomy Without A Telescope – Time Freeze

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There is a story told about traveling at the speed of light in which you are asked to imagine that you begin by standing in front of a big clock – like Big Ben. You realize that your current perception of time is being informed by light reflected off the face of the clock – which is telling you it’s 12:00. So if you then shoot away at the same speed as that light – all you will continue to see is that clock fixed at 12:00, since you are moving at the same speed that this information is moving. And so you discover that at the speed of light, time essentially stands still.

While there are a number of things wrong with this story – as it happens, one correct thing is that if you were able to travel at the speed of light you would experience no passage of time – although there are several reasons why this is probably an impossible situation to find yourself in.

But nonetheless, if you were able to travel at light speed and not experience the passage of time – then you would have no time available to reassess your situation – indeed there would be no time available for your neurons to fire. So, you might well leave Earth with the image of the clock fixed on your retina, but since your brain has stopped working, this has nothing to do with the information carried in the light beam you are moving along with. Your retina is never refreshed with a new image as long as you stay at the speed of light.

Some insight into special relativity is gained by considering the context of someone who stayed back on Earth. If your light speed trip was aimed at a mirror at Alpha Centauri (4.3 light years away) – then from their perspective, it takes you 8.6 years to go there and bounce back. This is true even though you leave and return with an image of 12:00 stuck on your retina and rightly announce that (from your perspective) no time has passed since your departure.

But moving at light speed and experiencing no passage of time is probably an impossible scenario for we mass-challenged beings. Relativity has it that you possess a proper mass, a proper length and a proper time – which persist regardless of your velocity. If you could survive the G forces to get up to such speeds, then you could happily coast at 99.95% of the speed of light and check your pulse against your watch to find your heart still beating at 72 beats per minute – just like it did back on Earth.

It’s only when you check back with Earth that you see that something remarkable is happening. Moving at 99.5% of the speed of light gives you a time dilation factor of around 10. So while someone back on Earth will still measure your trip duration at about 8.6 years – for you it will only be around 10 months. And with a remarkably good telescope you might look back to Earth and see a distorted Big Ben, red-shifted and running slow on the way there and then blue-shifted and running very fast on the way back.

At speeds of less than 10% of the speed of light (0.1c or 30,000 km/sec) time dilation is miniscule, but from 99% speed of light up it increases asymptotically towards infinite.

One of the reasons that probably makes the experience of light speed/time freeze unobtainable is that time dilation keeps increasing the faster you move. For example, at a speed of 99.99995% of the speed of light you get a time dilation factor of about 1,000. So even if you have a spacecraft with an infinite power source capable of seemingly infinite velocities – you will keep arriving at your destination before your speedometer makes it all the way from 99.99999(etc)% of the speed of light to c = 1.0.

This is perhaps how we will populate the universe – using difficult-to-imagine investments of energy, coupled with the principle of time dilation to cross vast distances. The trick is not to get homesick, because after covering such distances you can never go back – unless it is to meet your very, very, very great grandchildren.

(I have cheated a bit by ignoring any periods of acceleration and deceleration within the journeys described here).

Further reading: Relativity calculator.