Time loopcan you send a letter to the past?
Closed timelike curves are allowed by general relativity, but physics may forbid them. The real science behind the Cosmos Era ending “Time loop”.
Is it physically possible to send a letter to the past? In theory, closed timelike curves are solutions of the equations of general relativity (Morris, Thorne and Yurtsever, 1988). In 1991 David Deutsch showed that a quantum version can avoid the famous "grandfather paradox". But actually building a time machine and sending a message to before the universe began is pure fiction, and Stephen Hawking's chronology protection conjecture says physics would stop it.
This page first explains how to unlock the ending “Time loop” in the game. Then it covers the real science behind it: what a closed timelike curve is, why Deutsch's self-consistency condition sidesteps paradoxes, and which kinds of time travel really are allowed. It ends with a list of related real research and how each item connects to this ending.
How to unlock it
- Hint: At the end of the Spacetime constellation, there is a letter addressed to the past.
- Condition: Unlock “Message to the start”, then witness the end of the universe.
- Reward: A signature line is permanently added to the end of chapter 1 of the Observer's log.
This ending belongs to the Spacetime category. The node “Message to the start” sends a message to before the first second of this universe. Under Deutsch's self-consistency condition, it can only be a message that was already received.
Real science: loops and paradoxes
Closed timelike curves are solutions allowed by the equations of general relativity. This is a serious theoretical result, not science-fiction imagination.
The trouble is paradoxes, and the most famous is the grandfather paradox. Deutsch showed that the quantum version of a loop can avoid it. His self-consistency condition requires everything that happens inside the loop to be consistent, so a message sent to the past can only be one that was already received. The game's “Message to the start” follows this condition.
Time travel that is really allowed
H. G. Wells's The Time Machine (1895) is the best-known time-travel novel. Its scientific background is this: special relativity does allow travel to the future (time dilation), while no workable scheme for returning to the past has ever been found. Going forward is allowed by real physics. Going back is the problem.
Robert Heinlein's "—All You Zombies—" (1959) is the best-known literary example of a bootstrap paradox: a time agent moves between different decades, recruits new people, and finally discovers that everyone in the story is the same person. In physics, Deutsch's self-consistency condition permits loops like this.
Game lore, not real
The following is fiction: actually building a time machine and sending a message to before the universe began. Hawking's chronology protection conjecture says physics would prevent it. In the game, the “Message to the start” node asks the player one question: do you remember the first sentence you heard at the beginning?
Related real research
The studies below concern spacetime, relativity and the fate of the universe. They are real science related to this topic. They are not direct evidence for the Time loop ending, only background for the physics it touches.
- Maxwell unified electricity and magnetism and calculated that the speed of electromagnetic waves equals the speed of light: light is an electromagnetic wave (Maxwell, Philosophical Transactions of the Royal Society, 1865).
- Time and space are not absolute. A patent-office clerk published four papers in one year that changed physics (Einstein, Annalen der Physik, 1905).
- General relativity allows a "warp bubble" that compresses space ahead and stretches it behind, but it needs negative energy that may not exist in reality (Alcubierre, Classical and Quantum Gravity, 1994).
- Quantum effects make black holes radiate energy slowly. Black holes are not eternal and will eventually evaporate (Hawking, Nature, 1974).
- Two black holes merged 1.3 billion years ago, and humanity finally heard the ripples in spacetime, confirming Einstein's century-old prediction (Abbott et al., LIGO and Virgo, Physical Review Letters, 2016).
- The fate of the universe trillions of years from now and beyond: stars burn out, protons may decay, and black holes eventually evaporate (Adams and Laughlin, Reviews of Modern Physics, 1997). This last item is relevant because unlocking the ending requires witnessing the end of the universe.
Real basis
Closed timelike curves are solutions allowed by the equations of general relativity (Morris, Thorne, Yurtsever 1988); Deutsch showed in 1991 that the quantum version avoids the grandfather paradox.
Message to the start: Send a message to before the first second of this universe. By Deutsch's self-consistency condition, it can only be a message that has already been received.
—All You Zombies— (Robert A. Heinlein, 1959), the science: this is the most famous literary example of the bootstrap paradox; in physics, Deutsch’s self-consistency condition allows such closed loops to exist.
The Time Machine (H. G. Wells, 1895), the science: special relativity allows “travel to the future” (time dilation); going back to the past still has no workable proposal.
Related research
These studies relate to this topic but are not direct proof of this page.
Gravitational waves: Two black holes merged 1.3 billion years ago, and ripples in spacetime were finally heard by humans, confirming Einstein's prediction from a century earlier.
Source: Abbott et al. (LIGO & Virgo), Physical Review Letters 2016, "Observation of gravitational waves from a binary black hole merger"
Electromagnetic field theory: Maxwell unified electricity and magnetism and calculated that the speed of electromagnetic waves equals the speed of light: light is an electromagnetic wave.
Source: Maxwell, Philosophical Transactions of the Royal Society 1865, "A dynamical theory of the electromagnetic field"
Special relativity: Time and space are not absolute. A junior clerk at a patent office published four physics-changing papers in a single year.
Source: Einstein, Annalen der Physik 1905, "Zur Elektrodynamik bewegter Körper"
Black holes evaporate: Quantum effects make black holes radiate energy slowly. Black holes are not eternal and will eventually evaporate.
Source: Hawking, Nature 1974, "Black hole explosions?"
Warp drive: General relativity allows a “warp bubble” that compresses space ahead and stretches it behind, but it needs negative energy that may not exist in reality.
Source: Alcubierre, Classical and Quantum Gravity 1994, "The warp drive: hyper-fast travel within general relativity"
The last chapter of the universe: Worked out the fate of the universe over trillions of years and beyond: stars go out, protons perhaps decay, and black holes finally evaporate.
Source: Adams & Laughlin, Reviews of Modern Physics 1997, "A dying universe: the long-term fate and evolution of astrophysical objects"
Game lore (fictional)
- Actually building a time machine and sending a message to before the universe began are both fiction.
- Hawking’s chronology protection conjecture holds that physics would prevent it.
- Node “Message to the start”: Do you remember the first sentence you heard at the very beginning?
- Summary of the novel —All You Zombies—: A time agent travels between eras recruiting new people, and finally discovers that everyone in the story is the same person.
- Summary of the novel The Time Machine: An inventor rides a machine he built to the year 800,000 and beyond. Humanity has split into the frail Eloi on the surface and the Morlocks underground.