What Is Time Travel? Paradoxes, Physics, and the Philosophy

Direct Answer

Time travel is the possibility that a person or object could arrive at a time other than the time it would normally reach — traveling to the future faster than ordinary processes allow, or traveling to the past to arrive before one's departure. Time travel to the future is physically uncontroversial: special relativity shows that fast-moving or strongly accelerated travelers age more slowly, so an astronaut can return to find more time has passed on Earth.

Time travel to the past is the hard case. It seems to generate paradoxes: if you travel back and kill your grandfather, you are never born, so you never travel back, so your grandfather lives, so you are born... The question of whether such scenarios are impossible in principle, or merely impossible to realize, is one of the most intriguing in the philosophy of time.

Historical Context

Serious reflection on time travel began with the discovery of non-Euclidean geometries and their application to physics. In 1895, H. G. Wells published The Time Machine, imagining a fourth dimension through which a machine could move. In 1949, the mathematician Kurt Godel found a solution to Einstein's field equations — a rotating universe — containing closed timelike curves: paths through spacetime that loop back to their starting point in time. Godel noted with delight that in such a universe, time travel is physically possible, and he took this as evidence that time is not what it seems.

David Lewis's 1976 paper "The Paradoxes of Time Travel" remains the philosophical benchmark. Lewis argued that the paradoxes rest on a confusion: the time traveler cannot both kill his grandfather and not kill him, but that is a fact about the actual history, not about his freedom. The traveler, in the relevant sense, "can" kill his grandfather in the same way any person can do what they in fact will not do — compatibilism extended to the time traveler.

Key Concepts

Closed timelike curve (CTC). A path through spacetime that is everywhere future-directed yet returns to its starting event; in the presence of CTCs, time loops.

The grandfather paradox. The scenario in which a time traveler prevents her own birth by killing her grandfather; the paradox is that the traveler both exists and does not exist.

The autofanticide paradox. Lewis's name for killing one's younger self; he argues it is impossible because no consistent history contains it — the traveler "can" in one sense, "cannot" in another.

Consistent histories. The constraint that any time travel story must yield a single, self-consistent history; physicists building CTC models impose "self-consistency" conditions.

Time dilation. The relativistic effect by which moving observers age more slowly — the basis of mundane time travel to the future.

Philosophical Perspectives

Lewis's compatibilist resolution is the classic move. The time traveler who sets out to kill his grandfather cannot succeed — because if he did, he would not exist — but this is not a violation of his freedom in any sense we ordinarily care about. "Can" is ambiguous: the traveler can kill his grandfather relative to his beliefs and abilities, but cannot relative to the actual history. The paradox dissolves once we stop conflating the two.

Physicists approach the question through general relativity. CTCs appear in several solutions to Einstein's equations, but it is unclear whether they can exist in realistic spacetimes: attempts to construct time machines require exotic matter with negative energy, and quantum effects may destroy the machine before it operates. Even if CTCs are possible, the self-consistency principle suggests that whatever happens on a CTC will form a single, coherent history — which means the traveler can visit the past but cannot change it.

Bergson's philosophy of time suggests a different moral. If real time is duration — irreversible becoming — then time travel is not merely physically unlikely but conceptually confused: the past is not a place to visit because it is not a location; it is the accumulated past of a moving present. On this view, the very picture of time as a landscape through which one can move is the error that makes time travel look like a meaningful possibility.

Modern Reflection

Time travel remains a laboratory for philosophical and physical ideas. Quantum gravity research considers whether CTCs could arise at the Planck scale; some formulations of quantum mechanics allow "retrocausation" — effects influencing their past causes — which would be a kind of time influence, if not time travel of people. The philosophical tools developed for time travel — consistency constraints, "can" semantics, the relation between time and causation — now feed directly into debates about quantum foundations.

The cultural grip of time travel is undiminished, and philosophy has followed: the grandfather paradox appears in fiction, ethics, and law (if you could change the past, would you?). What the debate teaches is not whether time machines will be built — almost certainly not — but that our ordinary concepts of time, causation, and possibility are deeper and stranger than we thought.

Related Thinkers

• Henri Bergson — duration excludes travel in time
• Bertrand Russell — the logic of time and change
• Karl Popper — the open future and scientific prediction

Related Quotes

• "Time travel is possible, but not the changing of the past: whatever happened, happened." — paraphrase of David Lewis, The Paradoxes of Time Travel
• "Duration is irreversible; the past is not a place." — Henri Bergson
• "In a universe where time loops, the distinction between past and future breaks down." — paraphrase of Kurt Godel, 1949

Sources

• Time Machines — Stanford Encyclopedia of Philosophy
• Time — Stanford Encyclopedia of Philosophy
• Lewis, The Paradoxes of Time Travel — American Philosophical Quarterly (JSTOR)
• Henri Bergson — Stanford Encyclopedia of Philosophy

Further Learning

• Continue with What Is Time? and What Is the Block Universe?
• Explore the Understanding Reality collection
• Read about the philosophy of time