Thought Experiment Notes

1. Is There More Light in a Room Lined with Mirrors?

I imagined lighting identical candles in two rooms: an ordinary room and one surrounded on all sides by mirrors. If the two candles were equally bright, they would emit the same amount of light energy over a given period. Yet a comparison of the total amount of light present in the two rooms at a particular moment could reveal a difference.

Ordinary walls reflect some light and absorb a substantial portion of it. The absorbed light energy is converted into the internal energy of the materials that make up the walls. Highly reflective mirrors return light repeatedly, increasing the average time photons remain inside the room.

Consequently, even when the same amount of light is emitted, more light energy may be present in the mirrored room at a given moment. This led me to distinguish between two quantities. One is the light energy produced by a candle over a certain period; the other is the light energy present inside the room at a particular instant. The latter depends on the rate at which light is produced and the length of time it remains in the room.

Once I understood this distinction, I became interested in a space capable of preserving light for long periods. By reducing the loss of light, it might be possible to accumulate a large amount of light within a space using the same energy source. I then wondered what would happen if light could be confined perfectly.

2. What Happens If Light Is Confined Perfectly?

Next, I imagined an ideal room whose walls absorbed no light at all and which had no opening through which light could escape. If a candle continued to emit light inside it, the newly introduced energy would accumulate. Under conditions of zero loss, the longer the candle remained lit, the greater the amount of accumulated light energy would become.

In reality, creating a perfectly reflecting wall is difficult. Actual mirrors absorb or transmit light at certain wavelengths. The air and the candle inside the room also interact with light. In real optical cavities, such losses are described using measures such as photon lifetime and the quality factor.

For the thought experiment, I assumed an extreme situation in which all these losses were eliminated. This allowed me to shift my attention from the problem of producing energy to that of preserving it. Initially, I imagined a perfectly mirrored room made of physical materials. Later, I considered whether space itself could serve as a boundary that confines energy. This idea became the starting point for the energy storage space that appears in the novel.

3. Can a Single Photon Exist Forever?

While considering ways to confine light, I also became curious about the fate of an individual photon. If a photon were reflected indefinitely inside an enclosed space, could we say that the same photon continued to exist? Here, it was necessary to distinguish between a situation in which light cannot pass through a wall and one in which the wall does not absorb it. Even if light cannot penetrate a wall, it may still be absorbed by the atoms or electrons composing it. Its energy can then heat the material or produce other internal states. The wall may subsequently emit light again, but that process is described as the emission of a new photon.

A photon is a quantized excitation of the electromagnetic field. Given this property, there are limits to imagining an individual photon as a small object that retains a fixed shape. In an ideal perfectly reflecting cavity, one can assume a situation in which a single-photon state is maintained indefinitely, provided that there are no interactions with the outside and the walls remain stationary. Such a thought experiment helped me distinguish between the conservation of energy and the physical state of a photon.

From this point on, my interest in the storage space also changed. Rather than preserving individual photons in their original form for long periods, the more important question became how to maintain the energy carried by light and the physical states it forms.

4. Does Time Not Pass for a Photon?

Thinking about the lifetime of a photon led me to examine the concept of time in relativity. It is often said that time does not pass for a photon. I needed to establish what this expression actually means.

An object with mass can have a reference frame moving along with it. A person aboard a spacecraft, for example, can measure elapsed time using a wristwatch. In relativity, this is called proper time. A photon travels at the speed of light in a vacuum, so a valid inertial rest frame for a photon cannot be defined. Along the lightlike path followed by a photon, the proper-time interval is zero, but this cannot be interpreted as meaning that the photon experiences its departure and arrival simultaneously.

This examination helped me distinguish between the travel time of light and the time measured by an observer watching it. It also raised the question of whether time flowing outside the storage space and time within that space would need to be considered separately. This question about time remained independent for some time. It became important again when I later considered a structure in which an action in the future could affect the galaxy of the past.

5. Can a Civilization Run Out of Energy Even Though Energy Is Conserved?

The idea of storing light for long periods led to questions about the nature of energy itself. Energy can be transformed into various forms. The chemical energy of fuel is converted into motion and heat, while nuclear fusion in stars converts some mass into radiant energy and other forms of energy. When light is absorbed by matter, its energy changes the internal state of that matter.

The important distinction here was between the total amount of energy and the energy that can actually be used. Heat flows between objects at different temperatures. This difference can be used to perform work. As the two objects approach thermal equilibrium, their capacity to perform work in the same way diminishes. Even when energy exists, making use of it requires suitable temperature differences, density differences, and arrangements of matter.

To understand this problem, I examined the concepts of entropy and available energy. At first, I thought the important question was how much energy could be secured, but as the investigation progressed, the form in which that energy could be preserved became more important.

This line of thought became connected to the decline of galactic civilization in the novel. I imagined that if a galactic civilization consumed enormous amounts of energy over a long period, its concentrated energy sources and the conditions required to exploit them might gradually diminish, rather than the total amount of energy itself.

6. What Does a Civilization That Uses an Entire Galaxy Consume?

I now expanded the scale of the thought experiment from a single room to an entire galaxy. If humanity had settled in countless star systems and built a civilization using a network of wormholes, its methods of energy consumption would differ greatly from those of the present day. Such a civilization might construct large-scale energy collection facilities around stars, extract matter from stars, draw rotational energy from black holes, and operate immense computing facilities.

What problems might arise if such a civilization continued to grow over a long period? Initially, I imagined the entire galaxy becoming hotter because of human energy consumption. But a galaxy is not an enclosed space surrounded by an atmosphere like Earth. The waste heat emitted by civilization can spread beyond the galaxy in the form of electromagnetic radiation.

I therefore changed the premise. The civilization would consume stellar fuel faster than it would be depleted through natural stellar evolution, collect interstellar gas that might otherwise form new stars, and exploit even the energy stored in black holes. Through these processes, the civilization would obtain the energy needed to sustain its way of life. At the same time, it would alter the long-term distribution of usable energy sources and release previously concentrated energy into the surrounding space in forms that would be difficult to recover.

In the novel, galactic civilization initially attributes these changes to conventional environmental problems and resource depletion. It might consider policies limiting stellar energy use, restricting energy consumption, and prohibiting the exploitation of black holes. This premise developed into a story about a civilization attempting to solve problems of its own making through technology and administrative systems. I also considered the possibility that some of the physical changes already underway might prove difficult to reverse, even if the civilization reduced its consumption.

7. Does Stored Energy Become More Valuable Over Time?

While examining the idea of storing energy for long periods, I once imagined a kind of cosmic compound-interest savings account. What benefit might there be in accumulating energy little by little for the distant future? In economics, compound interest refers to a process in which accumulated assets generate further returns, accelerating the growth of those assets. I wondered whether a similar principle could apply to energy storage.

Physically, energy does not increase merely because it has been stored. The idea of applying compound interest to energy itself therefore had to be revised. Instead, I considered the possibility that energy-collecting devices could be used to expand the capacity for further collection. If one collector used the energy it had acquired to build another collector, and those collectors subsequently built more, the collection network could expand rapidly. The relative value of stored energy could also change over time. Energy accumulated in an era when stars and gas were abundant would become a far more precious resource in the distant future, when most stars had disappeared.

This investigation also altered the function of the storage space in the novel. I began considering both the problem of increasing the amount stored and the problem of keeping that energy usable over long periods.

8. What Problems Arise When Light Is Concentrated in a Small Space?

At this point, I was reminded of a scene from the film Men in Black, which I had seen long ago. It showed a galaxy contained inside a small sphere attached to a cat's collar. When I first saw the scene, I was intrigued by the image itself: an immense world existing inside a tiny object.

After the mirrored-room thought experiment, I began examining that image as a physical problem. Although light differs from objects with rest mass, it possesses energy and momentum. Adding light to a sealed container increases the total mass-energy of the container and its contents. Light also transfers momentum to the walls, increasing the radiation pressure inside.

As the scale of the energy to be stored increases, the problems of gravity and pressure become more serious. Concentrating too much energy within a small volume also requires consideration of the conditions associated with black-hole formation. This revealed a problem with the idea of compressing the energy of an entire galaxy directly into a small sphere.

To resolve it, I changed the relationship between the sphere and the storage space. The small object visible from the outside would serve as an entrance connected to an immense storage space, while the energy itself would be kept in a separate, spacious spacetime. In general relativity, special geometrical models connecting a narrow entrance to a large interior region have been studied. Whether such structures could actually be constructed and maintained in a stable form remains unconfirmed. For the novel, I decided to take theoretically conceivable spacetime structures as a basis and add the technological assumptions needed for the story. In this way, the original sphere acquired a new role.

9. What Should a Separate Spacetime for Storing Energy Look Like?

The idea of using the sphere as an entrance led to questions about the internal structure of the storage space. First, the volume of the storage space would have to be sufficiently large. I also needed to consider how to deal with the gravity and pressure generated by the energy stored inside it.

Initially, I imagined a space in which time did not pass. If the energy inside remained unchanged no matter how much time elapsed in the outside universe, it seemed that such a space would make an ideal storage facility. But in a space where time does not exist at all, it is difficult to define processes through which energy enters or the internal state changes. The accumulation of energy requires a sequence of events.

I therefore changed the premise so that the storage space would have its own independent time. I imagined a structure in which time flowed differently from the outside universe, or one in which an independent cosmic history began after the connection to the outside was closed. During this process, I examined the concept of a baby universe: a new spacetime connected to the existing universe through a narrow passage, receiving energy and information until certain conditions were met, after which it would begin expanding independently. The formation of a baby universe and the maintenance of a stable connection to it are not established technologies in current physics. Several theoretical models explore related possibilities, and I extended some of these ideas for the novel.

I also examined the possibility that if time and the gravitational field inside the storage space differed from those outside, the measured energy of photons could change as they traveled. The hypothesis that some of the stored energy might contribute to the expansion of the interior space also emerged at this stage. As a result, the storage space developed from a simple warehouse for collecting light into a region possessing its own independent physical conditions.

10. What Must Be Stored to Start a New Universe?

As I considered using the storage space as the initial state of a new universe, questions arose about what should actually be stored. Would gathering light energy in one place be sufficient to ensure that stars, planets, and complex material structures could form in the new universe?

Even if the total amount of energy were sufficient, there might be too few usable differences if that energy were distributed in a state close to thermal equilibrium. The formation of stars and planets requires various conditions, including differences in matter density, motion, and gravitational instabilities.

I therefore conceived of a storage space that would contain not only energy but also particular compositions and distributions of matter, temperature differences, and initial density fluctuations. At this stage, the purpose of the storage space changed again. What had begun as a plan to preserve energy for future use developed into a plan to establish the initial conditions of a new universe.

From this perspective, the scientist of the future would have no reason to collect only starlight. The scientist might attempt to acquire energy remaining in stars, interstellar gas, elementary particles, and black holes. It was also necessary to consider how this process would affect the existing galaxy. As the scientist recovered the required energy and matter, the conditions necessary for forming stars and other celestial bodies in the existing universe could deteriorate. This was the point at which preparing the initial conditions of a new universe began to connect with the extinction of the existing galaxy.

11. Can Even Black Holes Be Harvested?

While examining the galaxy's energy reserves, I became interested in black holes. Black holes contain enormous amounts of mass-energy. In theory, some rotational energy can be extracted from a rotating black hole. Black holes are understood to lose energy through Hawking radiation over extremely long periods, but the natural disappearance of a massive black hole would take an extraordinarily long time. Black holes could therefore remain important energy resources in the distant future, even after most ordinary stars had been exhausted.

Initially, I imagined black holes collapsing into the storage space. But since black holes are themselves celestial objects formed through gravitational collapse, I needed to revise that description. I subsequently imagined a situation in which a black hole's rotation slowed, its mass and event horizon changed at an abnormal rate, and even the orbits of surrounding celestial bodies were affected. If such changes could be distinguished from ordinary Hawking radiation, scientists in the galaxy would have to search for another physical cause.

The first phenomenon observed in the novel is the photon deficit. If stars and black holes subsequently underwent abnormal changes, observers might begin investigating the possibility that the galaxy's various energy sources were being systematically depleted. This premise expanded the scale of the events while also changing how galactic civilization attempted to trace their cause.

12. When Does the Photon Deficit Begin Across the Galaxy?

I next needed to determine when the photon deficit would appear in different parts of the galaxy. Initially, it seemed possible to have observatories throughout the galaxy discover the deficit on the same day and at the same time. In relativity, however, defining the simultaneity of distant events requires consideration of reference frames and the time taken for signals to travel.

The galaxy's planets move at different velocities and exist in different gravitational environments. Even if they are connected by a wormhole network, the clocks and calendars used in different regions do not automatically agree. I therefore reconsidered how a single date could be assigned to all the observatories.

In cosmology, physical quantities associated with the expansion of the universe can be used to indicate its age. For example, the average temperature of the cosmic microwave background and the degree of cosmic expansion are used to study how the universe has evolved. On this basis, I examined the possibility of using a condition in which the universe reaches a particular physical state rather than specifying a date. In this scenario, devices in different parts of the galaxy would operate according to the same cosmological condition.

When comparing the observatories' records, it would be necessary to correct for differences in local motion, gravitational environments, and signal travel times. I developed the premise so that, through these corrections, the observers would discover that the onset of the deficit was associated with a single cosmological boundary. This idea also played an important role in conceiving the photon deficit and Galactic Standard Time in A Dictionary of Galactic Extinction. I needed to consider how galactic civilization would compare records based on different time systems and establish the sequence of events.

13. Can the Future Affect the Galaxy of the Past?

An even more difficult problem emerged when I considered the cause of the photon deficit. If a scientist in the distant future collected energy to create a new universe, how could that collection be connected to a photon deficit already being observed in the galaxy of the past?

Initially, I devised a scenario in which the scientist traveled backward through time and installed energy-collection devices in the past. Yet even if traversable wormholes were assumed to exist, traveling freely to any desired point in the past would be subject to various constraints. Methods of creating a time difference between the two mouths of a wormhole have been discussed theoretically, but this does not establish that a wormhole can be installed arbitrarily at a time before it existed.

I therefore reworked the premise of creating new passages in the past. This drew my attention to the wormhole network already used by galactic civilization. If a transportation network connecting countless star systems and settlements had existed for a very long time, the future scientist might be able to make use of those passages. This approach reduced the need to explain how the scientist had constructed every collection inlet and sent it into the past. Instead, the scientist became someone who developed a technology capable of changing the connections between existing passages or redirecting them toward a particular destination.

Of course, the assumption that an action in the future could affect the operation of wormholes existing in the past remained a fictional technology within the novel. To address this problem, I also examined other hypotheses concerning time and causality.

14. What If an Existing Wormhole Network Were Reconnected?

The idea of using the galactic wormhole network significantly changed the novel's temporal structure. In the past, humanity had established settlements in different parts of the galaxy and built a wormhole network connecting star systems. These passages became essential infrastructure for the movement of people and goods, communication, administration, and economic activity. After an immense period had passed, the scientist of the future prepared a new storage space. Using the wormhole network that had existed since the past, the scientist created a structure through which energy could move into that space.

I examined two possibilities. In the first, the mouths of existing wormholes would be redirected to connect with the new storage space. In the second, those connections would be incorporated into the boundary conditions of spacetime as a whole, encompassing both past and future. To understand the latter possibility, I examined the concept of the block universe. If spacetime in relativity is regarded as a single, complete structure, events in the past and future can be interpreted as belonging to one history.

This interpretation does not, by itself, give a scientist in the future the power to manipulate the past. For the novel, I introduced the additional assumption of a technology capable of establishing the connection between the existing wormhole network and the storage space within a complete spacetime structure. Under this premise, the photon deficit in the galaxy of the past occurs because the passages were already connected to the future storage space. Observers in the past do not know where those passages lead. The idea that the transportation network which brought prosperity to the galaxy would, after an immense passage of time, be used to collect energy also arose during this process.

15. How Did Noah's Role Change?

As I examined the structure of time and the wormhole network, the role of Noah Cain also changed. In the initial conception, Noah was the person directly responsible for the photon deficit observed in the galaxy of the past. Having reached the distant future, Noah would construct a device for collecting energy, and that device would cause light to disappear from the galaxy of the past.

Examining this premise raised several questions. How could Noah install devices that had not existed in the past? From what point in time could those devices begin operating? How could the era in which the photon deficit first appeared be connected to the era in which Noah lived? The emergence of the existing wormhole network as a possible solution allowed me to revise Noah's role. Instead of directly installing new collection inlets in the galactic past, Noah became someone who connected existing passages to the energy storage space. The central task shifted to preparing that space and completing the connections.

This change also affected the significance of Noah's actions. The galaxy of the past is already experiencing the photon deficit, and records and hypotheses concerning its cause have accumulated. In the distant future, Noah completes the structure associated with that phenomenon. This process led me to reconsider how the actions of one individual could be connected to the history of a civilization extending across an immense period. Noah's role did not become smaller; it changed into one that connected past and future within a single structure. This change also influenced the conception of the novel's ending. It is an example of an idea from the initial conception taking on a different role after its physical implications had been examined.

16. If an Action in the Future Affects the Past, What Happens to Causality?

A scenario in which Noah's actions are connected to phenomena in the galaxy's past gives rise to the paradoxes of time travel. If a scientist in the distant future collects energy from the galaxy of the past and destroys its civilization, the very history that allowed the scientist to be born and carry out the research might disappear. This would create a contradiction between the cause that led to the device's construction and the result produced by that device. I compared several possible ways of dealing with this problem.

The first was a scenario in which a new branch of the universe emerged whenever the past was changed. The future scientist would affect a different branch of the galaxy rather than the history the scientist had experienced. This could avoid the contradiction of time travel, but it would change the originally conceived relationship between the past and future of the same galaxy.

The second was a scenario in which the past and future were repeatedly altered. Under such a premise, the outcome of events would change each time history was rewritten. To trace the novel's chain of cause and effect to its conclusion, additional rules governing these changes would have to be devised.

The third was a scenario that preserved self-consistency. It would assume that the future scientist's construction of the device and the appearance of the photon deficit in the past had always belonged to the same history. In that case, despite the deficit occurring in the past, the history necessary for the scientist's birth would have to remain intact.

Of these possibilities, I judged the self-consistent model to be the most suitable for the novel's temporal structure. General relativity has provided a framework for theoretical investigations of unusual spacetime structures and the possibility of time travel. Concepts such as the Novikov self-consistency principle and Hawking's chronology protection conjecture were among those I examined to understand the problem.

For the novel, I settled on a hypothesis that allowed a self-consistent history. This made it possible to connect Noah's actions to events in the past without adopting a complicated temporal structure in which history was repeatedly rewritten.

17. How Can Traversable Wormholes Be Kept Open?

After adopting the premise of using an existing wormhole network, I also needed to examine the physical conditions of the wormholes themselves. General relativity permits special geometrical solutions connecting different regions of spacetime. Research has suggested that maintaining a wormhole stable enough for people to traverse may require energy conditions unlike those associated with ordinary matter.

In quantum field theory, effects related to negative energy density have been discussed under limited circumstances. Yet no method has been established for constructing and maintaining an immense wormhole network in practice. In the case of wormholes with a time difference between their mouths, quantum effects and gravitational backreaction have also been investigated as possible threats to their stability.

Considering these issues led me to conclude that several rules would be needed to explain how galactic civilization handled wormholes in the novel. I would have to determine whether wormholes had been discovered naturally or manufactured by humans, what kind of energy was needed to maintain them, and under what conditions a passage could close or change its destination. An especially important question was how the existing transportation network would be affected when Noah connected it to the storage space.

Through this examination, wormholes developed from a means of traveling over enormous distances into a device connecting the novel's temporal structure with the energy deficit.

18. If Energy Disappears from Our Universe, What Happens to Energy Conservation?

While examining the connection between the photon deficit and the storage space, I returned to the question of energy conservation. Suppose observers in the galaxy calculated the amount of light emitted by a particular star but found that less light was actually being detected than expected. If they could identify no intervening matter that had absorbed the light or celestial object that had scattered it, they would have to investigate where the missing energy had gone.

Initially, they might consider unknown forms of matter or new physical interactions. Instrumental errors, gravitational lensing, and problems with the propagation of electromagnetic signals would also have to be investigated. But if similar deficits were observed in different regions of the galaxy and their magnitude increased according to a consistent pattern, the phenomenon would become difficult to explain using existing explanations alone.

At this point, the scientists in the novel would begin questioning the scope within which the law of energy conservation should be applied. In general relativity, the local conservation of energy and momentum is an important principle. Defining the total energy of an expanding universe as a single quantity and discussing its conservation, however, is a far more complicated matter. Where the universe lacks a common time-translation symmetry, there are limitations on defining a globally conserved energy quantity.

For the novel, I assumed a situation in which energy no longer observable within the galaxy was moving into another spacetime. The movement of energy could be traced only by considering our universe, the storage space, and the structure connecting them together. This premise changed the way the photon deficit was explained. Initially, I had imagined energy actually disappearing. With the introduction of the storage space, the phenomenon developed into one in which energy moved beyond the observable region.

19. What Kind of Energy Does the Scientist of the Future Collect?

As I worked out the connection between the storage space and the galaxy, I designed the collection process in stages. Initially, the scientist collects photons emitted by stars. Observers discover that less starlight is arriving than expected and begin investigating the cause. At first, they might attribute the phenomenon to problems with their instruments or methods of observation.

I subsequently considered collecting energy from stars, interstellar matter, and other celestial bodies. As the energy recovery process expands, the distribution of matter within the galaxy and the motions of celestial bodies could also change. The abnormal depletion of energy stored in black holes could be placed in the later stages of these developments.

In the final stage, the scientist prepares the initial conditions necessary to start a new universe. What matters here is not only the total quantity of energy and matter but also how they are arranged.

If the scientist is assumed to be capable of expanding the storage space sufficiently and controlling its internal conditions, it becomes possible to imagine an independent universe beginning to expand within it. The Big Bang is better understood not as an explosion of matter within pre-existing space, but in relation to the history of a universe in which space itself expanded from an early hot, dense state. Accordingly, rather than depicting a new universe exploding within an existing space, I devised a structure in which a separate spacetime begins expanding according to its own initial conditions. By this stage, the purpose of the storage space had developed from the long-term preservation of energy into the formation of a new universe.

20. Can Life Reappear in a New Universe?

Having assumed that a new universe could form, I examined the possibility of life emerging within it. Even if a universe contained sufficient energy and matter, that would not necessarily guarantee the formation of complex chemical structures. Stable elementary particles and atoms, diverse chemical bonds, long-lasting energy sources, and environments capable of supporting material cycles and evolution might all be necessary.

Nor could it be known whether the physical constants of the new universe would be the same as those of the existing one. Stable atoms might never form, or stars might not survive long enough. There was therefore insufficient basis for assuming that life would necessarily emerge simply because the future scientist had created a new universe. This examination led me to reconsider the scientist's purpose. If the scientist wanted humanity to survive, preserving human DNA, memories, or the records of civilization would be possible alternatives. In an era of highly advanced galactic civilization, it would be reasonable to imagine technologies capable of preserving biological information and growing new bodies.

But the choice would be different if the scientist had no interest in preserving humanity as a species. Rather than restoring particular life forms, the scientist might attempt to establish the physical conditions under which life could arise again in a new universe. This premise raised an ethical question about the scientist's actions. The possibility of life emerging in the new universe was uncertain, while the existing galaxy, from which energy was being collected, was home to living beings who actually existed.

21. How Can Present Life Be Compared with Future Possibility?

The central question now shifted from physics to ethics. The scientist of the distant future concludes that the energy resources of the existing universe are steadily diminishing and that the chances of civilization's survival are also declining. Rather than restoring the existing civilization, the scientist chooses to establish conditions under which life might emerge in a new universe.

Here, I needed to consider what kind of person the scientist would be. If the character acted for personal survival or fame, the relationship between the scientist's ends and means would be relatively easy to explain. But if the scientist pursued the birth of a new universe while accepting the scientist's own death, the motivation would be different. The scientist compares the time that living beings in the present universe have left with the possibility of future life that does not yet exist. Believing that the present universe will eventually die, the scientist may consider it more valuable to secure the possibility of life emerging in a new universe.

Yet such a judgment contains assumptions that cannot be verified. Whether the new universe can actually be created, whether life will emerge there, and how long that life might endure are all unknown. The decision therefore gives rise to a profound ethical conflict. The scientist believes the action is being taken for the sake of future life, but the choice destroys the conditions necessary for the survival of countless beings who already exist.

Considering this problem led me to take an interest in how a person acting from benevolent intentions might come to determine the lives of others. This question also became connected to the political and social conflicts appearing throughout the novel.

22. How Does Galactic Civilization Understand Its Own Extinction?

After examining the physical premises, I turned my attention to how the inhabitants of galactic civilization might respond to these phenomena. Initially, they could conclude that the civilization's own energy consumption was responsible for the problem. If they believed that they had shortened the lifetimes of stars and extracted too much interstellar matter, they would introduce policies restricting energy use and conserving resources. Such policies would be based on the expectation that the civilization could solve the problem through its own actions.

Yet once the photon deficit was discovered and its cause proved difficult to explain, other interpretations might emerge. People could consider unknown natural phenomena, intervention by an extraterrestrial civilization, or changes that could not be explained by existing physics. Some would believe that they themselves were responsible for the problem, while others would regard it as a natural change for which they bore no responsibility. Disputes over scientific explanations could become entangled with questions of political responsibility.

If Noah's actions and the structure of the storage space were revealed in the distant future, these judgments would change again. The possibility would emerge that the decline of the existing civilization was connected to the creation of a new universe in the future. This premise led me to consider how a civilization's understanding of a crisis, and its response to that crisis, could be portrayed across different historical periods. The same phenomenon can be interpreted in entirely different ways depending on the era and the information available to those living in it.

23. In What Chronological Order Should the Events Be Arranged?

As I organized the scientific premises, I considered dividing the novel's temporal structure into three stages.

The first is the era in which humanity settles in different regions of the galaxy and develops the wormhole network. Civilization grows by connecting travel and communication between star systems and making use of enormous amounts of energy. The infrastructure that will later be connected to the storage space is established during this era.

The second is the era in which the photon deficit is discovered in different regions of the galaxy and civilization begins investigating its cause. What is initially regarded as an observational problem gradually develops into a crisis encompassing the entire galaxy. People propose various hypotheses, including natural phenomena, new physical laws, intervention by an extraterrestrial civilization, and environmental changes caused by energy consumption.

The third takes place in the far more distant future, when Noah appears. Most of the galaxy's energy resources have been exhausted and human civilization is approaching extinction. Noah prepares the initial conditions of a new universe and undertakes the task of connecting the storage space to the wormhole network that has existed since the distant past.

In connecting these three eras, I became interested in the possibility that the chronological order of events and the order in which their causes are understood might be different. Readers first encounter the photon deficit in the galaxy of the past. The era in which its cause can be understood lies much further in the future. People in the past act according to their own records and observations, while those in the future make judgments with more information about earlier events.

This structure also explains why it would be difficult for a single character to narrate the history of an entire civilization from beginning to end. Observers and records from each era interpret events within the limits of what they know. These differences also influenced my decision to construct the story through the forms of a dictionary and documentary records.

24. Which Parts Are Physics, and Which Are Assumptions Made for the Novel?

As the thought experiment became more complex, it was necessary to distinguish between established physical knowledge and the additional assumptions introduced for the novel. The fact that photons possess energy and momentum, that light energy contributes to gravity, and that the availability of energy depends on thermodynamic conditions are all grounded in established physics. The treatment of time and simultaneity in relativity, the mass and rotational energy of black holes, and observations relating to the expansion of the universe also provided a foundation.

Wormholes and baby universes presented a different problem. General relativity and quantum gravity research examine relevant mathematical models and theoretical possibilities. Yet traversable wormholes and devices capable of creating new universes for energy storage are not technologies whose existence has been confirmed by observation.

For the novel, several additional assumptions were necessary. These included the existence of a wormhole network connecting the entire galaxy, the ability of a scientist in the distant future to alter its connections, and the possibility of transferring energy and matter into a new spacetime to establish the initial conditions of an independent universe. In particular, the structure through which an action in the future affects the wormhole network of the past belongs to the technological possibilities invented for this novel. Even if a self-consistent temporal structure is adopted, whether such technology could actually exist remains a separate question.

Making these distinctions helped develop the premises. Where established physical laws could be applied, I examined the conditions they imposed. Where explanations proved difficult, I identified the assumptions needed to make the story work.

25. How Was the Novel's Form Determined?

As the scientific premises developed, I also began considering the form in which the story should be told. A Dictionary of Galactic Extinction encompasses immense spans of time, multiple civilizations, and characters living in different eras. A scientist in the era when the photon deficit is discovered cannot know what Noah will do in the distant future. Governments, the media, civic organizations, and religious groups interpret the same events from different perspectives. I thought it might be difficult to address events scattered across such vast stretches of time and space by following the experiences of a single individual.

Works by Borges and Milorad Pavić, which I had read long ago, influenced my thinking about this problem. I was interested in forms that allowed readers to construct a world through documents, records, and separate entries. I therefore decided to structure the novel as a dictionary. Its entries would present definitions of individual events, records of people, scientific concepts, administrative institutions, and the history of civilization. As readers move through these entries, they can discover relationships between events and compare interpretations that change over time.

This form was also connected to the development of the scientific thought experiments. Just as one question leads to another, one entry can provide information necessary to understand another. In this way, the structure of the thought experiments also influenced the novel's composition.

26. How Far Did the Question That Began with a Candle Lead?

Returning to the original question, it began with a comparison between a candle burning in an ordinary room and one burning in a room lined with mirrors. The thought experiment began with the realization that even when the same amount of light is emitted, the amount of energy present in a room can differ depending on how long that light remains inside it. I then considered whether light could be confined perfectly. I examined the properties of photons and the conservation of energy, along with the problems of gravity and pressure that arise when a large amount of energy is concentrated in a narrow space. The idea of containing enormous energy inside a small sphere developed into the premise of an entrance connected to another spacetime.

Thinking about the new storage space led me to questions about its internal time, its energy states, and the possibility of a separate universe forming within it. The proposed contents of the storage space also expanded, from light to matter and black holes, and eventually to the initial conditions required to start a new universe. I subsequently examined how a scientist in the future might collect energy from the galaxy of the past. The idea of installing new devices in the past was replaced by one that made use of an existing wormhole network, and Noah's role was revised accordingly. After examining the paradoxes of time travel, I organized the premise around a self-consistent history.

Finally, I considered whether life could emerge in a new universe, and how to understand the sacrifice of living beings in the present for the possibility of future life. A thought experiment that began with a question about physics had led to questions concerning human choices and the fate of civilization.

Of the various hypotheses examined during this process, some were incorporated into the novel, while others were revised or discarded. The remaining premises do not all represent established scientific facts. The world of A Dictionary of Galactic Extinction was constructed from these questions and assumptions. The finished novel did not need to contain every stage of the thought experiments. Its characters act according to the conditions of the world they inhabit, understanding the causes and consequences of events in their own ways. These notes preserve a record of the questions that arose as that world was being created, and of how its conception changed as those questions were examined.