# Can mass and space-time exist separately?

## Ten questions for Karsten DanzmannWaves, spacetime and all the rest

From this they reconstructed a rendezvous that lasted millions of years. The protagonists: two black holes. It should have been heavy guys, 29 and 32 solar masses - really?

Lots of questions lurk in the depths of space-time, and Karsten Danzmann, Director of the Albert Einstein Institute in Hanover and, as the top German gravitational wave hunter, significantly involved in the latest discovery, gives 10 answers.

** The giant LIGO antenna picked up a signal that was less than a second long. It is supposed to tell of an event that lasted millions of years. How does that fit together? **

**Karsten Danzmann: ** We detect the event, but of course not the entire million years, but only the last remainder. Here on earth we can only receive the last 0.2 seconds of this event, which lasts for millions of years, because only then are the two black holes so close come that the frequency has become so high that we can detect it with our earth-based detectors.

** How do you know it happened 1.3 billion away? **

**Karsten Danzmann: ** Merging black holes are "standard candles", which means that if we look at the event, we can immediately calculate how much energy it emitted. Then we just have to compare that with the energy that has arrived here and from that we can easily calculate how far it must have been.

** How do you know they were black holes - and not two stars? **

**Karsten Danzmann: ** Today we can solve Einstein's field equations very precisely with the help of computers and then you only have to compare the waveform that has arrived here with the one that you get from your solution of Einstein's field equations. Then you can see without a doubt that there must have been two black holes. No other object is able to produce such a waveform. In fact, it's so beautiful that it looks like something out of a textbook. Even Einstein couldn't have done it nicer.

** How do you know what mass the two black holes were? **

**Karsten Danzmann: ** The mass of the two black holes is reflected in the frequency and in the time dependence of the frequency. Depending on the masses the two black holes have, they radiate at different frequencies and also radiate different amounts of energy. And you can see that exactly in the waveform, that is, how the pitch changes and how quickly the pitch changes. That tells us exactly what masses the two must have had.

** What is spacetime? **

**Danzmann: ** Einstein taught us that space and time do not exist separately from one another, but rather have to be combined into one big, whole, new, the four-dimensional space-time. In the general theory of relativity, this goes so far that space and time are no longer separate from one another, but merge into one another and this is called spacetime.

*Can you imagine that? Does space-time have a substance, a filling? *

**Danzmann: ** It's hard for us to imagine it, because it's four-dimensional, and most of us already have problems with three dimensions. But you can make pictures of it by taking two-dimensional sub-spaces, projections. If you imagine that you are a surface being. And then you can imagine space-time quite well like a rubber blanket into which you can make dents. And that then represents the mass and the celestial bodies that lie in space-time and make dents in space-time.

**What is a black hole and how does the whole mass fit in there?**

**Karsten Danzmann: ** If the things that dent the space there move, then the dents in spacetime start to spread, just as if you throw a stone into a pond and the surface then carries waves that spread. These are gravitational waves, waves in the space-time curvature.

**What is a gravitational wave? **

**Danzmann: ** A black hole can very well be imagined as a hole in spacetime. Everything that is thrown into it disappears, and does not really disappear, the mass is retained, but it is compressed to one point, to infinite density. Okay, we can't explain that either, that is where the general theory of relativity ends: We know, infinite density, that will be difficult, that is also incompatible with quantum mechanics, that future generations will have to solve. We can't just fly over there and look at a black hole and come back, because there is a dangerous place near the black hole, the so-called event horizon, that is the place where gravity gets so strong that there isn't one Escape more there. If you get closer than this event horizon, then even light can no longer escape, and that is why it is called a black hole.

**Had I been in a spaceship near the collision - would I have noticed anything about the gravitational waves? Could they have destroyed my spaceship? **

**Danzmann: ** It depends on how close you would have been. At a reasonable distance you would not have noticed anything, because the gravitational waves are so weak that for all practical purposes it is impossible to notice anything. But if you are close enough, then you do not need a gravitational wave at all, then it is the curvature of space itself that would tear your spaceship apart or if you yourself fall into it as a body, then you will notice the gravity gradient, for example if you are there with your feet first jump in, then the gravity on your feet is much stronger than that on your head because the head is further away from the black hole, and if you are unlucky your body will simply be torn apart if you fall in. So don't do it.

!! Colorful images are circulating on the Internet, for example of colliding neutron stars that emit gravitational waves: yellow and orange tones waft away in a spiral.

Where do these pictures come from? !!

**Danzmann: ** These images are solutions to Einstein's field equations that were obtained by supercomputers, and the pretty colors, that is, false color representations of the curvature of space. The size of the curvature of the room is simply translated into these colors and then these slightly artistic images are created. But these are serious numerical solutions to Einstein's field equations.

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