# Why is gravity related to time dilation?

## "Time is more than just a variable"

**What is time This question can not only be discussed among philosophers, physicists also have different ideas about the character of time: While time in quantum mechanics always advances at the same rate, according to Albert Einstein's theory of relativity, it can sometimes pass faster and sometimes slower. To combine the two theories, Martin Bojowald from Pennsylvania State University and his colleagues developed a theoretical model that describes time in a new way. In an interview with Welt der Physik, the theoretical physicist reports how they proceeded and whether their predictions can also be verified experimentally. **

**World of physics: what role does time play in physics? **

Martin Bojowald

Martin Bojowald: Time is one of the fundamental physical quantities. But what exactly we mean by time depends on the theory we are looking at: in Isaac Newton's classical mechanics, for example, time is a variable that always progresses at the same rate. This is how we usually imagine time in our everyday life. But Albert Einstein showed with his theory of relativity that the rhythm of time can change: the faster an object moves and the stronger the force of gravity it is exposed to, the slower time passes. However, the effects of so-called time dilation are so minor in our everyday lives that we do not notice them.

**Can you still observe the time dilation in experiments? **

Yes, with the help of very precise clocks - so-called atomic clocks - researchers can actually measure the effect. If you take an atomic clock with you on board an airplane and then compare it with an atomic clock that was on the ground, it becomes clear that the time has passed differently for the two clocks because they moved at different speeds and they also moved one were exposed to different degrees of gravity. The clocks behave exactly as Einstein's equations predict. This is why time dilation is also taken into account in our modern technology, such as when determining position with GPS.

**Does time dilation appear in other theories as well? **

In addition to the general theory of relativity, there is another fundamental theory in modern physics - quantum mechanics. But in this theory, which describes the world of the smallest particles, time takes on a completely different role: It advances steadily - similar to classical mechanics - and is not influenced by its environment. The two theories thus contradict each other in the character of time.

**What does this contradiction mean? **

Black hole

In most cases, the difference does not matter, because the two theories describe physical phenomena on different levels: For example, when physicists calculate the behavior of planets or entire galaxies, they use the general theory of relativity. In contrast, quantum mechanics predicts how the smallest particles - such as electrons - will behave. But there are areas in which strong gravitational fields occur at very small distances, such as inside black holes. In order to describe these areas correctly, theoretical physicists try to combine general relativity with quantum mechanics. But what role does time play in such a theory of so-called quantum gravity?

**And is that question you are dealing with? **

Exactly, with the help of a theoretical model, we have described time in a completely new way that takes into account aspects of both quantum physics and general relativity. In this theory, time is no longer described as a variable like in classical mechanics, but as a periodic oscillation. This time field pervades the entire universe and sets the pace of time at every location - we therefore speak of a fundamental time. The oscillation in our model can be imagined as a pendulum swinging anywhere in the universe: after the pendulum has swung back and forth once, a unit of time has passed.

**But how do you combine aspects of the two theories? **

On the one hand, the oscillation in our model - unlike a normal pendulum clock - is subject to the laws of quantum physics. This leads to small, random fluctuations - so-called quantum fluctuations - in the otherwise regular oscillation. On the other hand, like time in general relativity, fundamental time can interact with its environment. For example, the cycle of atomic clocks is influenced by the oscillation of fundamental time and the random fluctuations in fundamental time are also transferred to the cycle of atomic clocks. In theory, this effect could even be measured. The resolution of today's atomic clocks is not yet sufficient for this, but with ever more precise clocks this may one day be possible. Until then, we are looking for further predictions from our model that can be verified in experiments.

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