What Do We Know about Spacetime?

This is an edited version of an interview between timeanddate’s Graham Jones, and Professor Astrid Eichhorn of the University of Southern Denmark.

Graham Jones (timeanddate): What is your answer to the question “What is time?”

Astrid Eichhorn: I would say time is a dimension, just like space, that we exist in. But the difference to space is that we can’t step right and left, we can only go in one direction. And this is why we perceive time very differently to space.

Does time have a beginning and an end?

That is a question we would love to have the answer to. We currently don’t know because our best theories of space and time break down if we follow the universe to its very beginning.

So, as we look back at the history of the universe, the further into the past we go, the smaller it becomes, and the more energy is packed into a very small region of space.

If we go back more than 13 billion years, we end up at this point where we expect the quantum effects in space and time become really important, so we need to understand the quantum structure of space and time. We simply don’t know what happens when we get to this very beginning. We don’t know what happened before the big bang.

For example, was there an earlier, very big universe that sort of contracted and went through a very, very hot, small phase and then expanded? Is that the real origin of time?

What makes this question about the nature of time such a difficult one to answer?

I think if we had a better grasp of pinning down the problem more clearly we would be better at finding an answer. We have made some progress in understanding how time behaves. For instance, we now know that one shouldn’t think about space and time in isolation, but there really is this thing called spacetime—space and time together forming one entity.

But there are still very deep, unanswered questions. Why is there time in our universe at all? Why do we live in a universe that has time, and not just space? If we have three dimensions of space, why is there only one dimension of time?

Can you explain a bit more about the concept of spacetime?

Anything that has energy or mass, curves space and time. And that means that time stretches or contracts depending on whether we are close to something very massive or far away. So for instance if we go very, very close to something like a black hole, then time will stretch and pass much more slowly.

In fact, there is a tiny effect of something like that even on Earth. If you’re closer to Earth, time passes a little bit slower than if you’re farther away. GPS satellites need to take that into account to be accurate.

In practical terms, that would mean if you want to age as slowly as possible, you should really lie flat on the floor. Of course, it’s an extremely tiny effect that accumulates over a lifetime. But that’s the way I think about it. If you’re climbing Mount Everest, you will age a bit faster.

Tell us a little about your work.

I’m a researcher at the University of Southern Denmark trying to understand the fundamental building blocks of all matter, how space and time interact with these elements, and how the world we know emerges from whatever is going on at this fundamental, microscopic level.

Why is it challenging to explain these ideas to the general public? Is it because there is so much math involved?

Yes. Mathematics is a language of its own. With other languages, not everything is translatable. For instance, you have words in German that there is no English version of, and vice versa.

We have concepts that we can write down in math, but if we try to translate those ideas into everyday language, it doesn’t really work. So we come up with analogies that can capture part of it, but overall it’s something that can’t really be translated.
So this is what makes it difficult to explain sometimes, because it really means that you need to learn this language of mathematics to get the precise meaning.

Professor Eichhorn, thank you for this interview.