According to special relativity, time travel cannot change history

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.

What Stephen Hawking said about time travel

When renowned theoretical physicist Stephen Hawking did not rule out the possibility of time travel in his posthumous book Brief Answers to the Big Questions, many hopefuls got excited. But, unfortunately for them, Hawking believed that yet-to-be discovered laws of physics will prove time travel impossible. This piece outlines some practicalities about the speed of light and wormholes that lend way to Hawking’s prediction.

Branching timelines are allowed and resolve grandfather paradoxes in physics

According to quantum physics, traveling back in time to kill one's grandfather would not cause a paradox because time travel would bring one to a new timeline, where the person killed is not one's grandfather. Alternatively, causality would not be violated because the universe would prevent the murder from occurring.

Photons do not experience distance or the passage of time

According to special relativity, the speed of light is the cosmic speed limit, and the faster an object moves through space, the less it moves through time. As a result, particles of light only move through space and have no velocity through time, perceiving all locations they travel through simultaneously.

Objects moving relative to one another will not agree on the simultaneity of events

Space and time shift when switching from a nonmoving to a moving perspective, causing events that seem to occur simultaneously for one person not to appear that way for someone else. This also means one person's present could be another's future, ensuring all of time already exists.

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.

Time dilation explains that movement through space affects the passage of time

According to special relativity, everything in the universe moves through space-time at the speed of light. Just as turning from north to northwest on a road reduces one's northward motion, moving through space leaves less of this speed to move through time, causing an object to age more slowly.

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.

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.

Visualizing Einstein's special relativity

Before Einstein developed his theory of general relativity, he worked out what is known as special relativity—or why time is experienced different for different observers. The breakthrough rests on his theory that light travels at a constant...

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.

The earliest mention of time travel

Time travel has fascinated humans for centuries. It appears in myths, literature, and science fiction. One of the earliest examples comes from the 400 BCE Hindu epic Mahabharata, describing time dilation long before Einstein’s Theory of Relativity. By the 19th century, stories introduced machines enabling time travel—setting the stage for modern sci-fi.

Time seems to stop for objects at the event horizon of black holes

At that location, the warping of space-time is so severe that the entirety of an object's space-time motion is solely through space, leaving it immobile in the time dimension. Faraway observers looking at the object would see it appear frozen before slowly dimming and reddening.

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.

Tips for writing time travel stories

Are you thinking of writing a story that involves time travel? You might want to read this guide first. It offers questions to consider, mistakes to avoid and strategies to employ to ensure you avoid paradoxes, plot holes and implausibilities.

Time disappears when encompassing all quantum configurations across the universe

The Wheeler-DeWitt equation, a modified version of the Schödinger equation—the formula that describes the evolution of the quantum configuration of a system—describes the quantum state of space-time across the universe. The transformation causes the variable of time to disappear, suggesting it is not a fundamental, physical parameter of the cosmos.

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.

What is space-time?

Space-time is a complex subject, but this Livescience article makes an effort to disentangle the topic. Somewhat akin to a sheet of rubber that can be warped and bent in parts, space-time is a bit more involved that that. However, the analogy makes this and other aspects of the concept simpler to grasp.

Philosophers have debated the nature of time since at least the 6th century B.C.E.

Greek philosopher Parmenides of Elea was skeptical of time and reasoned that if it were real, then the present would emerge from nothing because the future did not yet exist. Just as the color red is our perception of a specific wavelength of light, others have suggested time is a psychological projection based on how we interpret the world.

If the universe is infinite, it may be impossible to say what's beyond it

The universe is defined as all that exists, including space and time. If the universe is infinite, as observations currently suggest, there is no boundary from which we can ask what lies beyond.

Can music alter your memories?

You’re familiar with music’s ability to transport you to a past experience, but new research suggests that music can even alter how you remember your past experiences. A team of psychology researchers and music experts recently published a study that demonstrates how music can help shift our emotional recollection of a memory. Read to learn more about the study’s findings and how it could influence therapy practices.

Why atoms are used to keep track of time and define the meaning of a second

Small variations in the production of human-made devices, such as mechanical watches and pendulums, make them unreliable for the ultraprecise timekeeping required by modern technology, including financial transaction timestamping and GPS. Because atoms obey fundamental laws of nature and never wear out, their light emissions when energized provide a consistent timekeeping signal.

'I can’t go back to yesterday–because I was a different person then.'

- "Alice's Adventures in Wonderland" author Lewis Carroll (1832 - 1898)

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.

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.

A temperature of absolute zero is impossible to reach

Zero on the Kelvin scale—equivalent to -273.15 degrees Celsius—describes a system where all atomic motion completely stops, which is forbidden according to thermodynamics. As of 2025, the closest scientists have gotten to absolute zero is 0.00000000004 K.

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

Hurricanes, which rely on air consistently spinning in the same direction, cannot cross the equator because air curves in opposite directions north and south of it.

Visualize the path of every solar eclipse from 1900 to 2099 and see what each looks like when observed from different locations on Earth.

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.

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

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

How fusion—which releases energy when two lighter atoms combine to form a heavier atom—differs from fission

The first horror movie is believed to be George Méliès' 1986 short 'The Haunted Castle,' which pioneered the use of special effects

Explore fun facts from the USPS, including unique post office buildings, a stamp that traveled past Pluto, and more.

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Existentialist writer Jean-Paul Sartre coined the famous phrase 'Hell is—other people' in his play 'No Exit'

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.

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

Long before Major League Baseball began allowing instant replay for balls and strikes, an umpire reluctantly used the technology to determine whether a hit was a home run, a move that inadvertently ushered in a new era for the sport.

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.

The United States has had a tumultuous relationship with Cuba since its 1959 revolution, highlighted by a nuclear crisis and a decades-long trade embargo.

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Take a 3D tour of the Hajj, Islam's annual pilgrimage to Mecca, undertaken by over 1.5 million Muslims.

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.