The concept of time travel has fascinated humans for centuries. The idea that we could journey to the past or future holds an irresistible allure. Time travel captures the imagination, featuring in countless works of fiction. It also plays a major role in many theories about the nature of the universe. However, most scientists believe backwards time travel is almost certainly impossible.
What is time travel?
Time travel refers to the concept of moving between different points in time, analogous to moving between different points in space. This could involve traveling into the future, into the past, or sideways in time to parallel timelines.
Some key time travel concepts include:
- Traveling to the future – Accelerating to near light speed causes time dilation, allowing travelers to experience less passage of time.
- Traveling to the past – Going backwards in time, violating normal causality.
- Alternate timelines – Accessing alternate versions of history or parallel realities.
Much speculation also exists about potential technologies, such as traversable wormholes and cosmic strings, and time travel paradoxes they might cause.
Why is time travel so appealing?
The appeal of time travel is easy to understand. Here are some of the main reasons:
- We can’t help imagining ‘what if’ scenarios – What if major events turned out differently? What if you could change mistakes in your past?
- Reliving happier times – Going back to when you felt younger, healthier and less stressed or reliving fond memories.
- Gaining knowledge of the future – Knowing what will happen could let you profit or avoid disaster.
- Righting wrongs – Preventing tragedies by changing the past.
- Exploring history firsthand – Witnessing important historical events for yourself.
- Eternal life – Traveling into the far future to extend your lifespan.
These powerful ideas help explain why time travel is such an endlessly intriguing concept.
Theories of time travel
Several theories exist about how time travel could occur. Here are some of the most prominent:
Wormholes
Wormholes are theoretical ‘shortcuts’ through space-time. In theory, traversing a wormhole could allow you to emerge at a different time. While wormholes are consistent with Einstein’s theory of general relativity, none have ever been observed. The types that could permit time travel would likely be highly unstable.
Cosmic strings
Cosmic strings are hypothetical 1-dimensional ‘fault lines’ in space-time. It’s been proposed that two parallel cosmic strings could bend space-time so severely that the resulting distortion might create a path into the past or future. However, the existence of such cosmic strings has also never been confirmed.
Exotic matter
A substance known as exotic matter with ‘negative energy density’ could theoretically amplify certain time travel effects in wormholes or cosmic strings. But this strange form of matter has also only been hypothesized.
Spacetime rotation
In this model, proposed by physicist Frank Tipler, a massive cylinder spins at an immense speed, twisting spacetime and allowing a ship to move backwards in time along a spiral path inside it. But constructing such a cylinder would likely be practically impossible.
Infinity cylinders
An infinite rotating cylinder could enable time loops within it, according to some interpretations of general relativity. However, its infinite nature makes the concept metaphysically problematic.
Traversable acausual retrograde domains in spacetime (TARDIS)
This is a complex theoretical concept where direction of causality can be manipulated, allowing time travel into the past. Its name is also clearly an homage to the time traveling ship in Doctor Who.
Time travel in physics
While time travel remains theoretical, many key aspects of modern physics do touch on ideas relevant to it. Here are some of the main examples:
Time dilation
Einstein’s special theory of relativity showed that time is relative and not absolute. The passage of time is different for observers moving relative to each other.
Timelike and spacelike separation
Events with ‘timelike separation’ have a causal relationship, while ‘spacelike separated’ events cannot influence each other. This influences ideas about how time travel might work.
Closed timelike curves
A theoretical type of space-time where an object’s world line forms a closed loop, allowing travel back to its starting point. This is crucial to many wormhole and other time travel proposals.
Quantum entanglement
The quantum effect where interacting particles remain connected across space and time. Researchers have speculated that entanglement could power time travel to the past.
Black holes
It’s theorized that sufficiently advanced civilizations might use black holes to create wormholes suitable for time travel.
Time travel paradoxes
Many complex paradoxes emerge when considering time travel to the past. These are some of the key ones:
Grandfather paradox
What if you went back in time and accidentally caused your grandfather’s death? You would no longer exist, making your original time journey impossible.
Bootstrap paradox
Also called an ontological paradox – an object is sent back in time and becomes the source of itself. Like receiving future technology from yourself that you then use to invent the technology.
Fermi paradox
Asks why we don’t see evidence of alien time travelers if time travel will be invented in the future.
Polchinski’s paradox
Quantum mechanics appears to forbid time-like loops in quantum states, throwing doubt on some proposed time travel mechanisms.
Resolving these paradoxes may require approaches like alternative timelines, divergent realities or self-consistency principles to avoid contradictions.
Why most scientists think backwards time travel won’t happen
While ideas about time travel continue to be hypothesized, the consensus view among scientists is that backwards time travel is extremely unlikely if not fundamentally impossible. Some key reasons for skepticism include:
Physics argues against it
Current theories of spacetime in physics appear to rule out going back in time – the mathematical models simply don’t work.
No evidence it’s possible
There’s no experimental evidence that time travel to the past is possible – no signs of time travelers from the future or any technologies with the potential to make it real.
Time may not physically exist
A number of physicists propose time itself is not a fundamental property of the universe, which would preclude ‘traveling through’ it.
Paradoxes cause logical problems
Many times travel paradoxes like the grandfather paradox cause logical contradictions that can’t be resolved within our current understanding of physics.
Black holes don’t work
The laws of thermodynamics appear to rule out using black holes to construct wormholes suitable for backwards time travel.
Wormholes are likely impossible
Exotic theoretical wormholes require speculative physics we’ve never observed and may not even make mathematical sense.
How far can we realistically time travel?
While time travel to the past seems extremely unlikely, some limited forms of ‘time travel’ may be achievable:
Time dilation
Traveling close to light speed or in intense gravity would allow people to experience slowed down time relative to others not traveling.
Hibernation
Hibernation or freezing yourself could effectively transport you to the future one day at a time by skipping over intermediary time.
Wormholes into the future
While moving backwards would be difficult, wormholes that moved mouths forward in time might be possible and allow limited time jumps into the future.
Cylinder of dense matter
A hypothetically possible arrangement of dense matter spinning incredibly fast could slow time within the cylinder relative to outside.
When will we know for sure?
Right now some physicists think a theory of quantum gravity merging general relativity and quantum mechanics will be needed to resolve questions around time travel. Potential events that could shed light include:
- Discovery of particles like gravitons that carry the gravitational force.
- Development of a complete unified theory explaining all fundamental forces.
- Better understanding of the relationship between quantum entanglement, wormholes and time.
- Observing macroscopic quantum behaviors that allow time dilation effects.
- Finding convincing evidence that time doesn’t really exist as fundamental property.
However, many physicists believe backwards time travel will remain impossible even with a theory of everything. The future is uncertain, but for now the fundamentals of physics appear firmly stacked against ever traveling to the past.
Conclusion
Time travel to the past captured the imagination for good reason – who wouldn’t want to change past mistakes? But unfortunately, actually building a real life ‘time machine’ remains firmly in the realm of science fiction. The deeper we understand fundamental physics, the less plausible backwards time travel seems. While movie characters continue enjoying the benefits of time machines, real science will likely never allow these expeditions to history.