Branching timelines are allowed and resolve grandfather paradoxes in physics

The grandfather paradox of time travel

If you went back in time and killed your grandfather when he was a child, what would happen to you? One of your parents would never be born, so you wouldn’t be born either. But, if you’re never born, how could you kill your grandfather in the first place? This so-called grandfather paradox is something physicists, philosophers and sci-fi writers constantly ponder. Read the piece to learn how they’ve been trying to reconcile it.

According to special relativity, time travel cannot change history

Although time travel is allowed—because all moments of the past, present, and future exist simultaneously and are locked in place—you will be unable to alter events beyond those that you changed because you were there before your travel to the past, including doing something to change your existence.

Within quantum physics, the order of events across time may not follow causality

Although time is considered a fixed, ordered mapping of events, where the past affects the future, quantum superposition breaks down causality, suggesting that future events can occur simultaneously with past ones. Within the quantum model, the notion of cause and effect is not a fundamental feature of our universe.

Is time travel really possible?

The laws of physics, as currently understood, don’t categorically reject the possibility of time travel. But, scientists are skeptical, especially when it comes to traveling to the past. Hopping between past, present and future would require the world to have some wacky yet-to-be-discovered physical properties like negative mass, negative energy, and infinite density. Time travel also introduces philosophical questions about free will, causality, and chronology.

A scientific analysis of famous time travel stories

This 8-minute video explores how time travel influences the narrative arc of several movies, books and video games, from Harry Potter to A Christmas Carol. (Warning: it contains spoilers!) J.K. Rowling and Charles Dickens set myriad parameters as they’re worldbuilding to avoid paradoxes and create quasi-logical timelines. By drawing diagrams as he’s speaking, the video breaks down how time works in some of popular culture’s most beloved fictional universes.

Five potential ways to achieve time travel

While the author discounts the possibility of traveling to the past, it lays out five hypothetical modes of catapulting into the future: speed, gravity, suspended animation, wormholes, and light. Each has some scientific basis, but none are realistic for humans. Read the article to learn where they offer promise and where they fall short.

In the 'block universe' model, the past, present, and future exist simultaneously

According to special relativity, observers do not experience the same events in the same order, but all perspectives are valid. This means what is considered one person's as yet unexperienced future may have already taken place in someone else's past.

Tachyons are theoretical particles that travel through space faster than light

The equations of special relativity require these objects to possess imaginary mass, and they would travel faster the more energy is taken away from them. The existence of tachyons would necessitate an unknown form of matter and lead to paradoxes involving information that can hypothetically travel backward in time.

Understanding how and when time began requires a theory of quantum gravity

Rewinding the universe to the big bang compresses all matter, energy, space, and time into a single point, necessitating a model that combines general relativity with quantum mechanics. Although physicists understand time to have begun when the universe did, a clear understanding of the mechanisms involved remains unclear.

Human teleportation through quantum principles raises questions about identity

Through quantum entanglement, particles have been "teleported" by measuring their state in one location and effectively transferring that state to a new location. Doing so for the particles that make up humans would destroy the state of the original particles, killing the person, and effectively creating a clone.

Within topology, time can be modeled as restarting after reaching its 'edge'

A manifold is any space that appears flat when zoomed in close enough, even if it is globally curved. Some of these structures can be curved to connect back on themselves, much like bending a line into a circle. This property can be mathematically established in a four-dimensional manifold by allowing time to loop back on itself.

The history of the gene: an interactive timeline

Humans didn’t need modern technology to figure out that kids tend to look like their parents. But what did humans believe about genetics before we knew about DNA or cells in general? Explore this interactive timeline to see how philosophers and scientists throughout the ages tried to explain how traits are passed from generation to generation.

The mathematics of general relativity does not rule out the existence of wormholes

Such objects, also known as Einstein-Rosen bridges, would resemble funnel-like tunnels connecting two regions of spacetime, allowing for instant travel across the universe. However, such structures would be incredibly unstable and require undiscovered materials with negative energy to keep them open.

J. Robert Oppenheimer’s grandson says his grandfather would back nuclear power

While the "father of the atomic bomb" referred to the bomb as "an evil thing," after the Manhattan Project, he also described nuclear energy’s potential as an environmentally friendly solution to address the world's energy problem.

Even in an infinite universe, its limited age prevents a sky full of starlight

Olbers' paradox asks why the sky would ever appear dark if the universe is full of stars in all directions emitting light everywhere. Light from the farthest stars requires more time to reach our skies than the universe's age.

Multiple new research paths may lead to slowing (or even reversing) aging

Aging has always been inevitable, but fasting, epigenetic reprogramming, and parabiosis seem to help people stay young. By studying centenarians, memory, and even the humble roundworm, scientists are looking for the genetic basis to live not just a long life, but a healthy one.

No one showed up to the party for time travelers organized by Stephen Hawking

In 2009, Hawking hosted a reception but only sent out invitations afterward, hoping that time travelers from the future would attend and prove time travel possible. He later joked it was "experimental evidence" that time travel doesn’t exist.

Delays in the decay of unstable particles provide evidence for time dilation

Physicists used the muon half-life measured in labs to predict how many should reach detectors on Earth's surface after their production in collisions between atmospheric molecules and cosmic rays. Higher detection rates were consistent with a longer half-life due to slower movement through time.

Boomerang earthquakes reverse direction and ripple back the way they came

Computer simulations show that rapid changes in friction along a fault where an earthquake has traveled for long distances can produce a smaller quake in the opposite direction. Data consistent with the phenomena has only been recorded a handful of times from 2011 to 2023.

How upheavals in accepted scientific theories helped popularize the term 'paradigm'

When Thomas Kuhn published "The Structure of Scientific Revolutions" in 1962, he described the history of science as comprising periods of "normal science" where these accepted frameworks—paradigms—gradually accumulate anomalies until they cannot describe nature. Revolutionary science, such as the development of a new theory, results in a paradigm shift, another term Kuhn helped popularize. (Some readers may experience a paywall.)

Friedrich Nietzsche theorized that we will live our lives over and over again: the myth of eternal recurrence

Rather than a theory of physics, the idea is designed as a moral argument. While Nietzsche famously eschewed the Christian afterlife, he wanted to avoid the nihilism that results from no longer having such an ultimate horizon. In a mystical moment, the philosopher came to believe that the fleeting moments of our lives would be repeated, presenting a test: would you want to relive this moment? If not, he would argue, you're not living well.

Transistors in traditional computers are physically limited to atomic scales

As computer parts shrink, transistors—tiny switches that control the path of electrical signals—become obsolete because electrons can quantum tunnel through switch barriers. This makes quantum computers, which take advantage of quantum effects, an enticing alternative.

The Year 2038 problem is the next potential computer time rollover bug

Many older systems store time as a signed 32-bit integer counting seconds since Jan. 1, 1970. That counter maxes out on Jan. 19, 2038—overflowing into negative time and sending clocks back to 1901, potentially crashing any older software that depends on accurate dates.

Paleontologists apply rules about rock layering, fossilization, and radioactivity to date geological samples

Concepts such as the understanding that older rock layers will form beneath newer ones allow scientists to compare the ages of samples from various layers. Although no geological site preserves evidence across all time, samples can be combined with fossils of known age, such as those dated by analyzing the decay of their radioactive isotopes, to reconstruct Earth's timeline.

Modified Newtonian dynamics revises gravity to eliminate the need for dark matter

Rather than imagining that an undiscovered particle is responsible for discrepancies in the orbits of stars and galaxies, MOND suggests that the mathematics underlying Newton's laws of gravity and force change beyond a certain scale. However, the suggested revisions struggle to explain relativistic effects involving gravity, including gravitational lensing and gravitational waves.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

According to the physics of black holes, our 4D reality may be a hologram—a projection of information coded on a 2D surface on the edge of the universe.

Apeirophobia is an overwhelming fear of living forever, perhaps stemming from the brain's inability to conceptualize an infinite life.

Salvador Dalí believed he was his own dead brother, who had died nine months before he was born

Learn more about the history of fireworks, which dates back to an accidental invention during an alchemist's quest for information about immortality.

Larger animals like giraffes, elephants, and blue whales have developed a unique defense against cancer, a phenomenon known as Peto's Paradox

Astronauts have repaired genetic damage in space using CRISPR—a molecular tool that can make precise changes to DNA and is used in single-celled organisms to attack invasive viruses.

See how each of the more than two million known species relates to each other—and to common ancestors—in this zoomable tree of life.

Recent research found that the brain doesn't seem to massively rewire following amputation—upending earlier beliefs about the cause of phantom limb pain.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

The first line of 'immortal' human cells, which can divide indefinitely and are used in scientific research, was nonconsensually taken from a Black woman.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

Abraham Lincoln was almost assassinated weeks before his inauguration as he traveled from Illinois to DC through Baltimore in 1861; it took two independent claims for Lincoln to be convinced the threat was real

Warren Buffett's former professor Benjamin Graham is known as the 'father of value investing,' a strategy in which investors seek to buy stocks trading below their intrinsic worth.

US housing costs have risen by roughly 175% between 1991 and 2026—but a growing number of Gen Zers are becoming homeowners anyway. In 2025, 4% of homebuyers were Gen Z, up from 3% the year before.

Volcanoes 'die' when their supply of magma runs out, if they are shifted away from their underlying magma reservoir, or if they get clogged by collapsing or their magma cooling back into rock.

Watch how algorithms can be designed to train themselves and self-improve without human input, to the point where the optimized code is too complex for programmers to understand.

In 1984, Operation Smash Hit intentionally crashed a 239-ton train into a nuclear waste canister at 100 miles per hour to prove the canister's safety, which remained completely intact and lost just 0.29 of its 100 pounds of pressure.

The Brothers Grimm were lawyers before they were storytellers, but became interested in fairy tales as they began investigating how rules and customs had been passed down through cultures.

'Grand Canyon of the Yellowstone,' a work by American painter Thomas Moran, inspired Congress to establish Yellowstone National Park.

Discover the catastrophic consequences of a failure of the US electrical grid—something that is more likely than many Americans think.

70 years ago today, city officials in Santa Cruz, California, banned rock 'n' roll to protect its youth from corruption after witnessing a 'provocative' concert the night before.

Explore a list of the 100 best novels of all time, compiled by some of the world's best living writers, including Salman Rushdie, Stephen King, and Jennifer Egan.