Interview with the German astrophysicist Martin Bojowald about spacetime atoms and the beginning of the world, which should not have existed
The 36-year-old assistant professor Martin Bojowald teaches and researches at Pennsylvania State University in the USA. For almost 10 years he has been the first of his genre to apply a comparatively young theory to the Big Bang. Based on loop quantum gravity (loop quantum theory or loop gravity for short), a model that was conceived as early as the late 1980s and has been continuously improved since then, Bojowald divides space and time into the smallest units: into so-called spacetime atoms (see Der Time arrow before the big bang). They prevent the universe from shrinking to size zero in the Big Bang. Once celebrated as the beginning of all being, the big bang, especially its singularity, comes off very badly with the German theorist.
During your diploma thesis you found the way to loop gravity, which was already considered and developed in the main several years before.
Martin Bojowald: Roger Penrose was the first to introduce spin networks back in 1971, a model that we continue to use in loop gravity. However, Abhay Vasant Ashtekar took the first real step towards loop gravity in 1986, when he first reformulated the theory of relativity mathematically in a purely classical way and introduced new variables. In my work, I focus on loop gravity, which was developed in several steps in the 1990s.
The loop gravity relativizes the big bang and at the same time devalues it. The initial singularity is to a certain extent dissolved, the big bang model is reduced to absurdity. Do you not see the danger that the theory of loop quantum gravity you have further developed could one day meet a similar fate?
Martin Bojowald: It's always like that in science. New questions and theories are raised, developed further and then discarded. However, it is not the case that with our theory we are competing with the Big Bang in a completely new way. With us, the big bang model remains largely intact. We only took a closer look at the initial singularity, which has always been a thorn in the side of many. It is by no means new that astrophysicists want to look beyond this point with mathematical models. From this point of view, we have expanded the old Big Bang model in some areas in such a way that the theory can no longer collapse at this point and the mathematical equations continue to work here. We can now extrapolate what could have happened before the Big Bang. To do this, we only need to question the equations. And they tell us that matter in the early universe had a very high but finite density. Around a trillion solar masses were concentrated in each region the size of a proton. We call this the Planck density. Quantum gravity now says that the balance of forces changes. Hence a singularity - a state of infinite density - can never arise.
In their model, the Big Bang does not mark the starting point, but a kind of limit.
Martin Bojowald: Yes. The general theory of relativity fails at the singularity, whereby the loop quantum gravity is able to control the extreme conditions prevailing there well. The Big Bang loses its meaning as a physical beginning and its status as a mathematical singularity. It puts a practical limit on our knowledge.
If loop gravity tries to define the “point in time” before the Big Bang, does it inevitably break through Planck time?
Martin Bojowald: But we just don't understand time as well as we do the distances in space. It is possible that the Planck period will play a bigger role again for us. The form of the time steps depends heavily on the spatial structures.
Obviously, our brains are not gifted with the ability to grasp the true nature of the Big Bang or other complex cosmological processes, let alone your theory. How can the loop gravity still be visualized?
Martin Bojowald: This is of course difficult, especially since we naturally use mathematics as a formalized language to set up and develop our thoughts and models. That way it is easier and at the same time harder to get an imaginary idea of the whole thing. But even as a physicist, I use a lot of images to find an explanation for my theory, or to better convey it to others. A single picture can never reproduce the complete picture, you have to generate a lot of pictures and pay attention to which of them fits and how you combine them. Only then can you begin to guess what the whole thing is about.
From an evolutionary point of view, our spatial thinking is conditioned on three dimensions. Don't you sometimes get in the way of the inadequacies of the human brain in your work?
Martin Bojowald:(smiles) Well, you get used to it. It used to be common in physics to view space as a non-physical structure. Its existence was simply assumed until Einstein came along. He was the first to recognize that space-time not only provided the stage on which the drama of the universe takes place, but rather actively participates in the events themselves.
There are also some alternative big bang theories that postulate cyclical universes or the existence of multiverses. What do you think of such metaphysical concepts?
Martin Bojowald: Yes, there is a lot of speculation here. Often in those fields that are beyond any observability or testability. Some colleagues rely on statistical properties such as the anthropic principle. In the end, such models are mostly still in the limit area that can just be called physics. But if it gets too hypothetical, I'll exercise caution. For example, I consider the multiverse theory to be pure speculation.
In view of the fact that the chain of supposed coincidences that determine our existence is so enormous, one wonders: Is there also a deterministic constant in our universe? This inevitably leads to the question of God. How do you feel about it?
Martin Bojowald: I find it very difficult to imagine - neither from a scientific perspective nor from any other. I also find it difficult to imagine that everything should have come and developed out of nowhere - also philosophically. I try to block out the question of God in my work. Most of the time she doesn't even pose at work.
With your theory you value space-time, which is neglected in many cosmological theories. Is that on purpose?
Martin Bojowald: Yes, but this also has to do with the fact that loop gravity is mainly postulated by researchers who have a relativistic background, while many string theorists who come from particle physics place more value on matter. We, on the other hand, neglect matter. Some see this as a disadvantage, but in this way we can concentrate better on the essentials, i.e. on space and time.
The spacetime atoms create space according to loop gravity. They do not exist in an existing space like "normal" atoms!
Martin Bojowald: Yeah right. Let's just imagine two points in space that, due to the lack of space-time atoms, have a distance of zero to one another. If we now add some spacetime atoms, the distance between the two points increases. The more space-time atoms are added, the greater the distance between the points.
Under normal conditions we do not even notice the existence of the spacetime atoms; the tissue is so dense that it looks like a continuum. How does a spacetime atom differ from a materially normal atom?
Martin Bojowald: A material atom is still somehow attached to space; you have different building blocks that are at different distances from each other. It's a completely different situation when you try to build the room yourself with building blocks. On the one hand, this is mathematically more complicated, on the other hand, this process is difficult to imagine and visualize. There are illustrations that show the whole thing as a grid. But even that is misleading because we imagine a grid can only exist in an existing space. There is simply space between the individual grid points.
How can potential spacetime atoms be detected? They write that evidence of their existence can be found with the help of neutrinos, gravitational waves and gamma-ray bursts.
Martin Bojowald: Yes, these are the possibilities. But these are ultimately only based on deliberation. Gravitational waves and neutrinos could indeed turn out to be very promising tools because they hardly interact with matter and therefore crossed the primordial plasma with almost no loss. They are messengers of the Big Bang and could convey messages from before that.
Do you believe that spacetime atoms can be detected directly, let alone photographed?
Martin Bojowald: No, definitely not. We will not prove them directly, any more than we will ever take photos of them. There is no space between atoms of spacetime in which light or other waves could propagate for imaging. If spacetime atoms exist, with luck we will be able to gather some indirect evidence of their existence within the next decade.
The big bang model is still the standard model of cosmology, which, thanks to indicators such as the microwave background radiation or the redshift (among others), still best describes the beginning of the universe. While it took most of its secrets with it into the inflationary phase, it is still mainstream. Do you still believe that it could one day tip over completely?
Martin Bojowald: The evidence you have given will not tip over anytime soon. However, there are some details, fluctuations and deviations in the background radiation that really irritate many researchers and pose major problems for them. Unfortunately, we only have this one big bang and consequently cannot repeat our observations arbitrarily. It could be that loop quantum gravity models explain these deviations better. But we have to do the math more precisely.
In their book Back Before the Big Bang, they present their thesis in detail. Will your readers fully understand loop gravity?
Martin Bojowald: Writing on such a subject is of course a risk. It can become critical when, as a scientist and author, one is forced to simplify without losing the common thread and trivializing it. But strictly speaking, I don't quite understand the theory of loop gravity itself. We still have to do a lot of math.
Do you read science fiction novels?
What is the source of inspiration for your creativity? Do you listen to classical music while you work?
Martin Bojowald: No, I don't listen to music at work either. I prefer to read classical literature, which sometimes also raises philosophical questions that certainly inspire me.
Who influenced them most of the famous astrophysicists, cosmologists!
Martin Bojowald: Hard to say. I wouldn't want to commit myself to any one here.
Do you define your work as basic research?
Martin Bojowald: Yes, in any case.
How do your colleagues react to the loop gravity they have developed? Was there any open criticism? After all, you are currently in the focus and are also likely to have some envious people in various ivory towers?
Martin Bojowald: Business is stimulated by criticism. I've had one response so far that has been positive, but interestingly, it came from a string theorist. I am still waiting for the first reactions in Germany.
How many sophisticated extraterrestrial civilizations do you think will engage in loop gravity as we do?
Martin Bojowald: If we keep in mind the numerous galaxies in the universe, one should at least strongly reckon with the existence of intelligent civilizations. We cannot even imagine where and how intelligence has spread everywhere in the cosmos. There are certainly forms of life that are very different from everything that animates our planet. I think it is rather unlikely that they will really develop the same theories as we do.
Suppose you had the opportunity to ask a single question to a scientist of a high-tech alien culture that is thousands of Earth years ahead of us in terms of knowledge. Which one would it be?
Martin Bojowald: I would probably ask him directly about the spatial structure, because it is not necessarily bound by theory. If my counterpart has grappled with the problem for centuries or millennia, I expect exciting answers from him.
Is loop quantum gravity a potential candidate for the world formula that should and can explain everything?
Martin Bojowald: Not at the moment. I see that rather pessimistically. (Harald fence)
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