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Thought Experiments – All in Your Mind

Challenging perceptions of the world, overcoming the limitations of thought, and uniting what seemingly doesn’t go together – thought experiments are an often undervalued tool in the world of science. They are also useful for research at Jülich.

Zeichnung: Eine Frau im Vordergrund und ein Mann im Hintergrund mit Darstellungen, was in ihrem Kopf vorgeht. / Drawing of a woman and a men and their thinking processes.Copyright: Forschungszentrum Jülich / Constanza Rojas-Molina

You take a pair of twins and a space ship that travels almost at the speed of light. One of the twins travels in his space-ship at rapid speed through the galaxy; the other stays at home. Once the galaxy-exploring twin returns after a long time, he is astounded to see that his twin brother on Earth is now years older than he is. This is obviously a fictitious scenario. "The 'twin paradox' is a thought experiment," explains Prof. Bert Heinrichs from the Institute of Neuroscience and Medicine (INM). It was conceived around 100 years ago by Albert Einstein to vividly demonstrate the very real consequences of his theory of relativity. The dilation of time described in the twin paradox, for example, is the reason why certain particles from cosmic rays are able to reach Earth at all. And satellite navigation systems only work so reliably because they take such effects into account.

Bert Heinrichs is always contemplating thought experiments. He is a philosopher – one of seven currently tackling ethical issues in science at the Ethics in the Neurosciences subinstitute (INM-8). "Thought experiments are a tool of imagination that help us to consider the nature of things from new angles," Heinrichs explains. "They are mostly performed whenever it is not possible to carry out an actual experiment – whether it be for physical, technological, financial, or ethical reasons. Thought experiments can sometimes illustrate very abstract concepts and thus contribute to the process of understanding," he adds. "They have been around since ancient times and in all kinds of fields including philosophy, economics, history, mathematics, and science – especially physics."

A better understanding of quantum physics

The fact that thought experiments help to give us a better understanding of processes can be seen in the field of quantum physics, for instance. In this area of science, objects do not behave as we are accustomed to in everyday life. They do not assume a solid state. "An atom in the process of collapsing, for example, finds itself in a superposition of states. It is both collapsed and not collapsed at the same time – as long as we don’t observe it," explains Prof. Kristel Michielsen from the Jülich Supercomputing Centre (JSC). "It is a similar idea to Schrödinger’s infamous thought experiment in which his cat can be simultaneously dead and alive."

Portrait von Prof. Kristel Michielsen.Prof. Kristel Michielsen: "I use computer simulations to test one of Einstein’s thought experiments."
Copyright: Forschungszentrum Jülich / Ralf-Uwe Limbach

"I use thought experiments as hypotheses that I can then test using scientific tools," says Michielsen, who heads the Quantum Information Processing group at JSC. She and her colleagues use computer simulations to better understand a special phenomenon of quantum theory: quantum entanglement. The latter was described in 1935 as part of a thought experiment, the Einstein–Podolsky–Rosen paradox (EPR paradox).

Quantum entanglement is a phenomenon in which two particles are connected to each other, even when situated far apart. But how does this connection work over a large distance? How do the particles know about their respective states? Do they exchange information or "communicate" with each another? Einstein doubted this possibility right to the end and famously described quantum entanglement as "spooky action at a distance". It would otherwise have meant that information is exchanged faster than at the speed of light. Not all of Einstein's colleagues agreed with him and still today there are a range of different interpretations as to how quantum entanglement works. For 80 years now, discussions on the topic have been the subject of numerous books, journal articles, and internet sites. With the aid of their computer simulations, Kristel Michielsen and her international team have tested the thought experiment and discovered that Einstein may have been right. There must be reasons other than "communication" that explain why the states of entangled particles influence each other.

Using computer simulations to put thought experiments to the test is a – relatively recent – major advantage for the scientific community. "Computer simulations allow us to play with the particles and test hypotheses in a kind of protected setting. We can take a look at everything without influencing – or even destroying – our object of investigation," says Michielsen. This is not always the case in the laboratory, she adds. "In the lab, the required apparatus also needs to be technically implemented in the correct way, for example particle sources and detectors. This is not always possible," Michielsen says. Laboratory experiments on the EPR paradox have also been undertaken since the 1970s. “The difficultly is always in clearly assigning two particles to an entangled pair. For example, particles can be falsely registered that are not even part of the experiment, such as from cosmic radiation. The laboratory also requires additional procedures that Einstein hadn’t even envisaged in his thought experiment. Both problems lead to incorrect conclusions, as we have been able to prove,” the physicist explains. “With simulations, however, we can control all these framework conditions precisely.”

Developing approaches for research

But thought experiments can also have an indirect influence on research. Take the silicon brain’ as an example, a thought experiment in which each neuron of a human brain is successively replaced by an electric circuit. The replacement is always made one after another. The end result would see a biological brain replaced by a technical one. But would the human still be the same and have the same consciousness?

This is just one of a whole range of thought experiments related to the human brain. They often tackle complex and – in part – highly philosophical subjects such as consciousness, artificial intelligence, and the role of sensory stimuli. "But at the core is always one fundamental question: How does the human brain work?" says Prof. Markus Diesmann, head of Computational and Systems Neuroscience at INM-6. And this is precisely what he aims to find out.

"Thought experiments show where assumptions and hypotheses ultimately lead, above all when thinking goes beyond what can actually be achieved or determined in practice," Diesmann says. A thought experiment such as the silicon brain, for instance, simply cannot be tested in the lab. Diesmann and his team of physicists also use computer simulations for their research approach. The Jülich researchers are working on a simulation of the human brain. "We are able to describe a neuron in just a few equations and can thus assemble networks that should be similar to the human brain. We believe that the collaboration of neurons within a network is the really interesting aspect."

Over years of work, Diesmann and his colleagues have simulated a cubic millimetre of a human brain – roughly 100,000 cells, each one of which has roughly 10,000 contacts that lead to other cells. “As natural scientists, we have our own restricted toolbox, our scientific methods, which we use to try and understand the brain.” The philosophical question regarding consciousness naturally plays more of a secondary role initially. But Diesmann’s research may one day also contribute to solving this question.

Ethical issues

"The silicon brain is all about identity and self-consciousness. As with many thought experiments, however, it also includes an ethical component," explains Bert Heinrichs, whose work particularly focuses on the field of applied ethics, especially in neuroscience and medicine. The Institute of Neuroscience and Medicine (INM) invited the philosopher to one of its annual retreats during which he spoke about the world of thought experiments. “Jülich neuroscience is concerned with many ethical issues. This is why it is important that our doctoral researchers are introduced to helpful tools such as thought experiments,” explains Prof. Johannes Ermert from INM-5, coordinator of the retreat.

When asked what thought experiment Heinrichs would present to Jülich colleagues for contemplation, he describes the "Chinese room" argument made by philosopher John Searle: "A person who speaks no Chinese is sitting in a closed room. In front of him is a text written in Chinese characters to which he receives question cards – also in Chinese – through a slot in the wall that he has to respond to. The person also has a set of instructions in his native language. Although he does not understand the answers he gives in Chinese, he writes them down again and again on the cards and returns them through the slot. And the people on the other side? They assume that the person sitting inside the room understands Chinese."

Thought experiments

Schrödinger's Cat

A cat is placed in a box together with a small amount of a radioactive substance, a detector, and a flask of poison. The box is then sealed. If just a single atom decays, the detector kicks in and automatically releases the poison. The cat then dies. However, it is equally possible that no atom decays, in which case the cat stays alive. So as long as you don’t look in the box, the cat’s condition is undetermined – it is both alive and dead at the same time. This world-famous thought experiment was proposed by the Austrian physicist Erwin Schrödinger in 1935. It is, of course, impossible for a cat to be simultaneously alive and dead. However, it is possible for an (unobserved) atom to be in two states at once according to the rules of quantum mechanics. In setting up the experiment – the interconnection of all experimental elements from the atom to the cat – we can directly apply these rules in our head to our everyday lives.

Achilles and the Tortoise

gezeichnete Umsetzung vom Gedankenexperiment 'Achilles und die Schildkröte': Achilles läuft auf einer geschlängelten Laufbahn einer Schildkröte nach. Achilles und die Schildkröte
Copyright: Forschungszentrum Jülich / Constanza Rojas-Molina

Achilles, the fastest runner in ancient times, is challenged to a race by a tortoise. Confident of his victory, Achilles gives the tortoise a head start. The race commences and both start running. But once Achilles
reaches the tortoise’s starting point, the latter has moved a bit further down the track. Achilles thus has to reach this next point before he can overtake the tortoise. But once he gets there, the tortoise has again moved forward – this time an even shorter distance. The tortoise’s advantage therefore keeps getting smaller, but he always remains ahead. Achilles is thus unable to catch up to the tortoise and overtake him. This
thought experiment from ancient times was proposed by the ancient Greek philosopher Zeno of Elea (5th century BCE). It has puzzled many thinkers for over 2,000 years. Of course, any runner would easily be able to overtake the tortoise. But Zeno’s logical arguments are not so easy to dismiss. Where is the error in the logic?

Regine Panknin