# Quantum gravity increases with increasing speed

## Gravitation - What Curves Space?

The nice thing about gravity is that everyone is familiar with it. It is extremely practical because it holds the earth together to some extent and also prevents tools and keys that have been released from drifting uncontrollably through space. Thanks to gravity, we know that we should look for lost objects near our feet.

But how do we explain gravity today? For this we have the general theory of relativity. Published in 1915 by Albert Einstein and since then further developed and recalculated by numerous scientists, the general theory of relativity explains gravity by means of a curvature of space. No, that is wrong! At least incomplete. You can only fully understand gravity if you understand space and time as a unit. Time is assigned as a further dimension to space with its three dimensions, which have now even found their way into the cinema. This is by no means a new idea from Einstein. Almost a hundred years ago, the only new thing was the idea of bending space and time together to explain gravity.

A popular model for understanding space-time curvature is the rubber blanket model. Let us imagine that the earth is a ball that lies on a stretchy rubber cloth. The weight of the ball presses in the rubber blanket in the middle and creates a hollow of curved two-dimensional space. XKCD recently published what happens when you try to explain gravity like this:

*You can find a translation of the comic in the language log next door.*

One will immediately hear the objection that the rubber blanket model explains gravity with gravity. Because it is gravitation that presses the ball into the cloth. Is this objection justified? Do we now have to give up the popular representation of general relativity? Should we encourage all interested laypeople to learn and understand a set of equations? Not quite. The rubber blanket model does not explain how the mass curves the space. It shouldn't either. With that we would actually explain gravity by gravity. The blanket model only explains how the curvature of a surface can lead to a deflection of flying particles and waves.

This deflection takes place in the stretched rubber blanket without the influence of external gravity. If an ant tries to walk straight on the blanket, it will inevitably make a curve due to the curvature. She experiences a deflection in the field of curvature of the blanket. Such are the deflections of objects in the curved 3 + 1-dimensional space.

But the blanket can only give a first impression of what Einstein's theory of gravity really says. In this theory it is not only space that is curved. The curvature also affects time. In fact, the curvature of space can often be neglected compared to the curvature of time.

**What should time warp mean?**

I blogged about the phenomenon of time three times last October. Time is essentially what the watch measures. When we speak of the curvature of time, this means that clocks that are independent of the strength of the gravitational field itself run at different speeds in different depths of the gravitational field. Time passes more slowly inside a gravitational field than far away from the heavy object. This effect is measured very precisely every day in the GPS navigation satellites.

In order to understand why a stretching of time leads to gravity, one has to switch from the idea of three space and one time dimensions to a four-dimensional idea. In this imagination everything moves through time. Even an object at rest rushes through the dimension of time at a whirlwind. And this movement through evenly passing time is a straight line. If an object tries to run on a straight line in the curved time, it becomes a curve. A curve in space-time is no longer a state of rest but an accelerated movement. This is how an object accelerates in a curved space-time. And not only for heavy objects, but also for light waves and everything that moves through time and space.

But the blanket model doesn't explain one thing at all: **Why does the room actually bend?**

On my website Relativity Principle.info I started a long time ago to write a popular science explanation on the special and general theory of relativity. Like all my projects, the site is constantly being expanded and I have already received some suggestions for improvement that I will incorporate. I get important impulses, for example, from the discussions in my forum.

One question relates to what actually bends the room. Is it the rest mass of the elementary particles that are in an object? That cannot be because then light could not be involved in gravity. The elementary particles of light, the photons, are known to be massless, but the influence of gravity on light can easily be demonstrated. So is it the total mass including kinetic energy that bends the room? That would not do justice to the general theory of relativity, which must be formulated independently of observers. Kinetic energy is a relative quantity.

In fact, it's a slightly more complicated quantity that curves space: the **Energy-momentum tensor**. Probably the most popular book on *Gravity* of *Misner, Thorne and Wheeler* describes the energy-momentum tensor as a machine into which two velocity vectors are inserted to get a value. If you know the field of energy-momentum tensors from a matter distribution, you know everything about the mass, energy, momentum and pressure distribution in this mass. And because a tensor has very specific mathematical properties, one also knows how to get these quantities in any coordinate system for any observer.

The tensor algebra in general relativity is a powerful tool to describe symmetries of the world around us. But it is not as clear as a rubber blanket. In my spare time I work to find a better popular science description. I would be grateful for any relevant information.

Joachim Schulz is group leader for sample environment at the European XFEL GmbH in Schenefeld near Hamburg. His scientific career began in quantum optics, where he studied the interaction of individual atoms with laser fields. Among other things, she introduced him to atomic physics with synchrotron radiation and cluster physics with free-electron lasers. For four years he planned, set up and carried out experiments on coherent X-ray diffraction on biomolecules at the Center for Free-Electron Laser Science (CFEL) in Hamburg. In his free time, for example, he writes "Joachim's Quantenwelt" on the blog or on his homepage.

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