What if I tell you time travel is easier than it looks?
Don't have to take my word for it. But in Stephen Hawking's last book for the general public, the late cosmologist and physicist raised a few tantalizing possibilities about time travel, which might provide you a long-awaited answer.
A decade ago, Hawking famously threw a party for time travelers. Unlike any ordinary gathering, the invitations were only sent out after the party. If time travel were to be invented in the future, one would expect that a space-time portal should have opened up mid-air, from where a person of the future would walk right into the room.
As you have probably guessed the outcome: no one showed up. Is this because time travel is simply impossible? Or as Hawking suggested, maybe we would need to looking at time travel from a different perspective (or perspectives).
Wormhole and the grandfather paradox
Wormhole, a frequently used term in popular sci-fi fantasies, is always a go-to option for space and time travel in the imaginary world. A wormhole is a theoretical "tunnel" that opens at two different ends of space-time locations, first predicted by Albert Einstein in his theory of General Relativity.
As crazy as it may sound, at the smallest scale our world is full of tiny, transient wormholes. The flow of time is symmetrical in this microscopic domain dictated by quantum physics (see Part II of the series). But the problem to utilize wormholes for time travel couldn't be more obvious: how do we send human or exploratory vehicles of realistic sizes through such small tunnels? One has to assume that one day we will have advanced technologies to generate stable wormholes of a reasonable size with openings at the correct destination.
Even when the above conditions can be satisfied, there's still time-related paradox to resolve, the most well-known being the grandfather paradox—altering the past brings inconsistencies for the future. A person's journey to the past could result in the death of one's grandfather before the conception of their father or mother, which eliminates the existence of the time traveler.
So after all, wormhole-based time travel is not as feasible or safe as one can imagine.
How Scientists Created A Wormhole In A Lab (Seeker)
Gravity and black holes
Gravity is not a simple force. On the surface, you are stuck on Earth by your body is drawn by the Earth's gravity. But as relativity physics reveals, your body is trapped in the space-time curvature created by our planet's gravitational field, meaning that gravity works by bending both space and time.
The Global Positioning System (GPS) satellites that our society heavily relies on would serve as the best example. Flying at an orbit that's 20,000 km (12,427 miles) above, the satellites experience time faster (about 38 microseconds per day) than any object on the Earth's surface. Hence, their clocks need to be corrected to match with the time on Earth.
Imagine a supermassive body, such as the black hole at the center of our galaxy. If a spaceship can fly to a close yet safe distance to the space monstrosity, the astronauts on board will experience time much slower than anyone back on Earth. Hawking proposed that passengers in a round trip to a black hole will experience time travel, in the sense that time slows down significantly for those under the influence of the massive gravitational field.
But again, how practical is black hole-based time travel? It is paradox-free because it's essentially a time-slowing process for those involved. However, traveling toward any black hole is a long and dangerous journey on its own, not to mention that we have not yet developed sufficient capacity to send anyone outside of our solar system.
What Happens at the Event Horizon? (PBS Digital Studios)
Light speed and time travel
Another approach, hinted at Einstein's theories of relativity, involves boarding a vessel that can travel at close to light speed. The speed of light, 1,080,000,000 km or 671,000,000 miles per hour, is the speed limit of our universe. As an object's speed is getting closer and closer to light speed, the time around it gets slower and slower.
Inside the CERN's Large Hadron Collider, tiny charged particles whip around inside at a 99.99% of light speed, before they collide with one another. Scientists had trouble observing some of the exotic particles in their natural state because they only exist for a few billionths of a second. However, when accelerated to near-light speed, they live 30 or more times longer before decaying, therefore creating a much better window for scientific observation.
The spacecraft used in the Apollo 10 mission is the fastest manned vehicle in the recorded history, with a maximum speed of 25,000 miles per hour. But to travel in time, Hawing suggested, we will need to go way beyond that speed.
With the current rocket technologies, most of which are powered by chemical fuels, the race to light speed will run into the Tsiolkovsky rocket equation dilemma: to be able to continue speeding up, a spacecraft would require a lot of fuel. But the more fuel it packs, the harder for it to accelerate. Scientists and engineers will either have to build an enormous vessel or find a smarter way to edge close to light speed.
An ion-propelled spacecraft can slowly but effectively accelerate to light speed without being overpacked with fuel material. Last October European Space Agency launched the BepiColombo mission, sending a carrier and two orbiters to Mercury. The spacecraft is powered by twin ion thrusters, which relies on ejecting positively charged xenon atoms to create thrust.
Needless to say, it would take up to years for our spacecraft to reach near-light speed, no matter whichever engine it utilizes. If the purpose of the trip is to visit the future Earth, meaning it would take the vessel the same amount of time to decelerate before safely arriving back on Earth.
Admittedly, manipulation of wormholes are too far from reality, and the "time travel" by super-strong gravity or ultra-fast spaceship is neither satisfying or feasible yet. But hey, just over one hundred years ago we have no idea that time is relative, bound to space, and even symmetrical in the sub-atomic world.
The evolution in physical science has brought us unprecedented insights into the nature of time, a phenomenon we thought we knew so well. As scientists continue exploring spatial and temporal boundaries across different disciplines, who knows where the request to understand time will take us to next?
How To Time Travel: Stephen Hawking's Universe (Discovery Science)