What is Time

What is time? While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative — it can vary for different observers depending on your speed through space. To Einstein, time is the "fourth dimension." Space is described as a three-dimensional arena, which provides a traveler with coordinates — such as length, width and height —showing location. Time provides another coordinate — direction — although conventionally, it only moves forward.

Einstein's theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein's theory of general relativity, gravity can bend time.

Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board. The effects of gravity, as well as the satellites' increased speed above the Earth relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day.

In a sense, this effect, called time dilation, means astronauts are time travelers, as they return to Earth very, very slightly younger than their identical twins that remain on the planet.

Through the wormhole

One possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometers per second) in a vacuum. Einstein's equations, though, show that an object at the speed of light would have both infinite mass and a length of 0. This appears to be physically impossible, although some scientists have extended his equations and said it might be done.

A linked possibility, NASA stated, would be to create "wormholes" between points in space-time. While Einstein's equations provide for them, they would collapse very quickly and would only be suitable for very small particles. Also, scientists haven't actually observed these wormholes yet. Also, the technology needed to create a wormhole is far beyond anything we have today.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some groups have proposed alternate solutions to jump back and forth in time.

Black holes

Another possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge, rotating structure. "Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," physicist Stephen Hawking wrote in the Daily Mail in 2010.

"Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had."

However, he added, the crew would need to travel around the speed of light for this to work. Physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel pointed out another limitation if one used a machine: it might fall apart before being able to rotate that quickly.

Cosmic strings

Another theory for potential time travelers involves something called cosmic strings — narrow tubes of energy stretched across the entire length of the ever-expanding universe. These thin regions, left over from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them.

Cosmic strings are either infinite or they’re in loops, with no ends, scientists say. The approach of two such strings parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.

Time machines

It is generally understood that traveling forward or back in time would require a device — a time machine — to take you there. Time machine research often involves bending space-time so far that time lines turn back on themselves to form a loop, technically known as a "closed time-like curve."

To accomplish this, time machines often are thought to need an exotic form of matter with so-called "negative energy density." Such exotic matter has bizarre properties, including moving in the opposite direction of normal matter when pushed. Such matter could theoretically exist, but if it did, it might be present only in quantities too small for the construction of a time machine.

However, time-travel research suggests time machines are possible without exotic matter. The work begins with a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields to form a closed time-like curve.

To go back in time, a traveler would race around inside the doughnut, going further back into the past with each lap. This theory has a number of obstacles, however. The gravitational fields required to make such a closed time-like curve would have to be very strong, and manipulating them would have to be very precise.

Time travel in fiction

Two 2015 articles by described different ways in which time travel works in fiction, and the best time-travel machines ever. Some methods used in fiction include:

One-way travel to the future: The traveler leaves home, but the people he or she left behind might age or be dead by the time the traveler returns. Examples: "Interstellar" (2014), "Ikarie XB-1" (1963)

Time travel by moving through higher dimensions: In "Interstellar" (2014), there are "tesseracts" available in which astronauts can travel because the vessel represents time as a dimension of space. A similar concept is expressed in Madeleine L'Engle's "A Wrinkle In Time" (2018, based on the book series that started in 1963), where time is folded by means of a tesseract. The book, however, uses supernatural beings to make the travel possible.

Travelling the space-time vortex: The famous "Doctor Who" (1963-present) TARDIS ("Time And Relative Dimension In Space") uses an extra-dimensional vortex to go through time, while the travelers inside feel time passing normally.

Time travelling while standing still: Both the "Time Machine" (1895 book) and Hermione Granger's Time-Turner from "Harry Potter" keep the traveler still while they move through time.

Traveling faster than light: In "Superman: The Movie" (1979), Superman flies faster than light to go back in time and rescue Lois Lane before she is killed. The concept was also used in the 1980 novel "Timescape" by Gregory Benford, in which the protagonist sends (hypothetical) faster-than-light tachyon particles back to Earth in 1962 to warn of disaster. In several "Star Trek" episodes and movies, the Enterprise travels through time by going faster than light. In the comic book and TV series "The Flash," the super-speedster uses a cosmic treadmill to travel through time.

Difficult methods to categorize: There's a rocket sled in "Timecop" (1994) that pops in and out of view when it's being used, which has led to much speculation about what's going on. There's also the Time Displacement Equipment in "The Terminator" movie series, which shows off how to fight a war in four dimensions (including time).

So is time travel possible?

While time travel does not appear possible — at least, possible in the sense that the humans would survive it — with the physics that we use today, the field is constantly changing. Advances in quantum theories could perhaps provide some understanding of how to overcome time travel paradoxes.

One possibility, although it would not necessarily lead to time travel, is solving the mystery of how certain particles can communicate instantaneously with each other faster than the speed of light.

In the meantime, however, interested time travelers can at least experience it vicariously through movies, television and books.

"The Large Hadron Collider...routinely sends subatomic particles into the future.”

Hadron Collider

The particle accelerator has the ability to propel protons at 99.999999 percent the speed of light, a speed at which their relative time is moving about 6,900 times slower compared to their stationary human observers. So, yes, we’ve been sending atoms into the future and we’ve been doing it for the last decade, but humans are another matter.

Gott says given that we propel particles nearly the speed of light on a regular basis, conceptually, it’s rather simple for humans to time travel into the future. “If you want to visit Earth in the year 3000,” Gott says, “all you have to do is to get on a spaceship and go 99.995 percent the speed of light.”

Let’s say a human is put on such a ship and sent to a planet that’s a little less than 500 light years away (for example, Kepler 186f), meaning if they traveled at 99.995 percent of the speed of light, it would take them about 500 years to get there since they are going at nearly the speed of light.

After a quick snack and a bathroom break, they would then turn around and head back to Earth, which would take another 500 years. So in total, it would take about a thousand years for them to arrive safely back home. And, on Earth, it would be the year 3018.

However, since they were moving so fast, the resulting time dilation wouldn’t seem like a thousand years for them since their internal clock has slowed. “[Their] clock will be ticking at 1/100th of the rate of the clocks on Earth. [They] are only going to age about 10 years,” says Gott. While a millennium would pass for us, for them it would be a decade.

“If we [on Earth] were watching through the window, they would be eating breakfast veeeerrry slooooowly,” says Gott, “But to [them], everything would be normal.” But there is a massive gulf between what is theoretical and what is real. So how do we overcome the immense technological challenges of building a time machine?

The Not-So-Distant Future of Human Time Travel

Building a time-traveling spaceship may be the best place to start, but the engineering obstacles, at least for now, are enormous. For one, we are not even close to having a spaceship that can travel the speed of light. The fastest spacecraft ever created will soon be the Parker Solar Probe, which will launch this summer and travel only .00067 percent the speed of light.

There’s also the enormous amount of energy that would be needed to propel a ship to go that fast. Gott suggests that highly efficient antimatter fuel could be the key and other world agencies and scientists also think such a fuel could be a potentially invaluable piece to interstellar travel.

But ensuring the safety of the human cargo on such a futuristic mission would also be tricky. First of all, the ship would need to carry enough supplies, like food, water, and medicine, and be self-sufficient for the entire journey.

Then there’s the whole acceleration thing. To make sure our hypothetical traveler wouldn’t be obliterated by overwhelming g-forces, the ship would need to gradually and steadily accelerate. While steady 1g acceleration (like what we feel on Earth) for a long period of time would eventually get the ship to approach near speed of light, it would add to the length of the trip and minimize how far in the future one could go.

Using our 500-light-year planet example, Gott predicts that the steady acceleration of 1g up to near light speed would increase the aging of the time traveler to 24 years, “but you would still get to visit Earth in the year 3000,” says Gott.

"The technology isn’t far off...we could do it in the next twenty years."

Since the speed of light is the absolute maximum, physicists are concentrating on finding phenomena like wormholes, which could provide tunnel-like shortcuts that jump across curved spacetime and, in theory, beat a light beam to a particular point in spacetime.

While wormholes do work within the confines of Einstein’s theories of relativity, they have yet to be observed in space, and scientists have no concrete evidence that these galactic shortcuts would even work.

So while time traveling to the past may be the more exciting concept, scientists are much more likely to fling someone into the unknown future rather than the well-trodden past. But despite overwhelming odds—fiscal and scientific—Mallet believes the future of a time-traveling society is possible.

“What happened with going to the moon...we wanted to go there, Kennedy asked for it, and there was proper funding so we got there within a decade,” Mallet says. “The technology isn’t far off. If the government and taxpayers wanted to pay for it, we could do it in the next twenty years.”

For now, wannabe time travelers will still have to look to science fiction for its time travel fix, with some movies being much more accurate than others. “A good movie... was the original Planet of the Apes,” says Mallett. "The astronauts thought they had landed on another planet that was ruled by apes, but what they found out...was that they had traveled so fast, that they had arrived into Earth’s future. That movie accurately depicts Einstein’s special theory of relativity.”

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