Two people meet after a journey, and their functioning clocks have counted different intervals. One has aged less. Does that mean they could return and change the day of departure?
No: different accumulated times are a tested relativistic effect, whereas returning to one’s own past is a much stronger, unconfirmed possibility. Wormholes appear in theoretical models, but a solution to equations does not provide a ready way to create and maintain a traversable tunnel. The classic work on wormholes and time machines emphasizes this condition. Morris, Thorne and Yurtsever: a conditional theoretical conclusion.
In what sense can we reach the future?
Proper time is the duration a clock records along its path through spacetime. Between shared departure and reunion events, different paths can yield different durations. A traveller who ages less returns to other people’s future while spending fewer personal years. Their clock runs forward throughout. Einstein Online: travel and relative aging.
Differences in clock rates have been tested experimentally. In NIST experiments in 2010, changing an ion’s oscillatory motion in an atomic clock produced the predicted frequency change; raising one clock by about a third of a metre allowed a test of the gravitational contribution. These laboratory effects are tiny and do not mean that the ion or a person returned to the past. NIST: what was moved and measured, primary study abstract.
Ordinary video-signal delay is not the explanation. The clocks accumulate different proper times; bringing them back together and accounting for every part of the journey allows a comparison without arguing about image delivery times.
Branch · An idealized calculationOptional · you can skip this section
Ten years at the station, six aboard the ship
In flat spacetime, let a station be inertial and a ship travel away and back at speed magnitude v = 0.8c on both uniform-motion legs. We neglect the duration of brief accelerations and turnaround. If the station counts T = 10 years over both legs, the ship experiences
τ = T√(1 − ((v)^(2))/((c)^(2))) = 10√(1 − (0.8)^(2)) = 6 years.
Here c is the speed of light and τ the ship’s proper duration. The ratio v/c is dimensionless, leaving the unit as years. At v = 0, τ = T; for 0 ≤ v < c, duration remains positive. Approaching c does not turn it negative or allow a massive ship to reach c.
The traveller returns four years younger than an initially same-age person at the station, but reunion occurs after departure for both. These are teaching numbers, not the specifications of an existing ship. Turnaround breaks the symmetry of the paths; the entire age difference is not “stored” in an instantaneous jolt. A smooth manoeuvre requires including its proper time too. Einstein Online: the role of turnaround.
What does drawing a wormhole in equations mean?
In general relativity, spacetime geometry is linked to the distribution of energy, momentum and stresses. A wormhole is a model connecting different spacetime regions. Travel additionally requires traversability: a traveller must pass through and emerge, rather than encounter an impassable horizon or a destructive region.
We can specify a desired geometry and calculate the matter and stresses needed to support it. This does not prove that such a matter state can be obtained. The source, stability, initial conditions and geometry’s response to a traveller must be checked. A folded-sheet picture illustrates a shorter connection; it does not establish that our three-dimensional space is literally bent inside an external room.
Canonical static, spherically symmetric traversable models require unusual energy and stress properties: some observers must measure negative energy density. Quantum theory allows local negative values relative to a chosen vacuum state, but these cannot be accumulated arbitrarily over any duration and scale. Quantum-inequality studies impose restrictions under specific field and geometry assumptions. This is neither a universal ban on every imaginable model nor instructions for obtaining “wormhole fuel.” Ford and Roman: assumptions and constraints.
One phrase, different kinds of evidence
Different proper times
Measured effect
Clocks on different paths can accumulate different durations.
This is not a return to an event before departure.
Traversable wormhole
Conditional geometric model
Equations connect a specified geometry to the required energy and stresses.
A picture does not establish a real source, creation or stability.
A loop into the past
Additional hypothetical construction
A special time connection between mouths can produce a closed causal path in a model.
The existence of such a physical device is unconfirmed.
Quantum processor, 2022
Physical protocol, model interpretation
A quantum-information transfer protocol was implemented on a chip.
No traversable tunnel was created in our physical space.
What must be added for travel into the past?
An ordinary age difference does not connect a late event to an early one. A theoretical wormhole scenario assumes its two mouths can be maintained and given different time histories, for example by moving one. If the tunnel then connects external events with a sufficient time offset, the route through it together with an ordinary return route can form a causal loop. This adds the assumption of a special connection between events; an ordinary two-clock problem has none. The classic conditional proposal, Einstein Online: moving mouths and time shifts.
A closed timelike curve is a path that is locally future-directed for the traveller throughout, yet returns to their own earlier event. This raises a causality problem: how, for example, can an action remain consistent with the conditions of one’s departure? A paradoxical story alone is not physical proof that a device exists or cannot exist.
The chronology protection conjecture proposes that physics will prevent such loops from forming. Hawking investigated whether large quantum contributions and their backreaction on geometry would frustrate a time machine’s creation. The word “conjecture” matters: this argument should not become a proven theorem covering all possible theories of quantum gravity. Primary paper: assumptions and conclusion.
Was a wormhole not already created in a computer?
The 2022 quantum-processor experiment did not create a traversable wormhole in our physical space. The authors implemented a quantum protocol for a simplified model whose behaviour they compared with a theoretical gravitational picture. Transferring quantum information on a chip is not an object’s flight through a spatial tunnel. The authors’ explicit clarification.
Branch · Model and systemOptional · you can skip this section
What is physical in such an experiment?
The qubits, entanglement and measured protocol outcomes are physically real. This is more than drawing an animation. However, the gravitational interpretation relies on a correspondence between certain quantum systems and a different spacetime model. It has an applicable domain and approximations; these require checks separate from the processor’s operation.
The authors themselves note the small simplified model’s limitations and the absence of an explicit complete gravitational description for this particular system. Thus, “experimental dynamics of a wormhole model” does not mean “a new passage was observed in the laboratory room.” Authors’ explanation of assumptions, Nature primary paper abstract.
If a headline tomorrow promises a time machine, ask what was actually measured: clock readings, a quantum protocol, or a path through a created geometry? Has the matter required to support it been demonstrated? The answers determine the conclusion’s strength.
Different aging along different paths has therefore been tested. Traversable wormholes and journeys into one’s own past remain conditional theoretical constructions, and these quantum studies provide no way to build a macroscopic tunnel. For the tested side of the question, read motion and time and gravitational time dilation.