Is Time Travel Possible In Our Universe?
It was a cold moonless night with a heavy mist in the air. As you crossed the road, a car came out of nowhere and an elderly stranger saved you from being hit, sacrificing their own life. They left behind an envelope addressed to you with a note that simply said "study physics." Following their advice, you become a celebrated physics professor and eventually invent a time machine. You use the time machine to go back in time and save your younger self from the car accident, but this story raises profound questions about time travel and paradoxes.
The Bootstrap Paradox
Who wrote the letter that saved your life? The older version of you simply takes the letter they had already been given as a teenager back in time. The letter seems to have no origin and is stuck in a time loop.
The Restoration Paradox
Does the letter age and become more worn and tattered each time it is taken back in time? This paradox questions the preservation of objects in time loops.
The Exniello Paradox
Where did the idea to study physics come from? It seems to appear out of nowhere, without any origin. This paradox questions the source of thoughts and ideas.
The Predestination Paradox
If events like saving your younger self can happen, then it suggests that your future is already fixed and predetermined. This paradox challenges the concept of free will.
The Auto Infanticide Paradox
If you change your mind and decide not to save your younger self, how could you be there to have second thoughts in the first place? This paradox explores the impossibility of killing a younger version of yourself.
Despite the complexities and paradoxes, time travel stories like this one capture the imagination and raise interesting philosophical questions.
Possibilities of Time Travel
While freely traveling through time is currently impossible, there are theoretical possibilities lurking at the edge of physics. Temporal loopholes and slivers of possibility exist, challenging our understanding of time.
Time as a River
Jack Finney compared our experience of time to a river, where we are carried downstream from the past to the future. But what happens to the parts of the river we've left behind? The concept of eternalism suggests that the past, present, and future exist alongside each other.
Eternalism and Presentism
Eternalism argues that all moments in time are equally real, while presentism argues that only the present moment exists. This debate has been ongoing since ancient Greece, and Einstein's theory of relativity supports the concept of eternalism.
Einstein's Theory of Relativity
In 1905, Einstein published the special theory of relativity, which revolutionized our understanding of time. It introduced the idea that time is relative and depends on an observer's motion through space. The theory also showed that space and time are interconnected.
How Space and Time are Connected in the Block Universe
In the block universe, space and time are intertwined into a single fabric called spacetime. This concept was proven through tests of Einstein's general theory of relativity. In the 1840s, astronomers used Newtonian gravity to discover Neptune, but also realized that Mercury's orbit did not follow the predicted path. Einstein later explained this discrepancy by proposing that the presence of a massive object, like the sun, creates a dip in the fabric of spacetime, causing the curvature that affects Mercury's orbit. This idea has been tested and confirmed through various experiments.
The Block Universe and the Big Bang
The block universe is often compared to a book, where the past, present, and future are all written and unchangeable. Every choice and action is predetermined, just like the path of a train on its tracks. Scientists believe that the big bang gave birth to all of spacetime, meaning that all of space and time, including the past, present, and future, exist simultaneously. This allows for the existence of time travel, where a person can travel to different points in time, just as they can travel to different locations.
Time Travel and Cosmic Rays
One example that supports the possibility of time travel is the study of cosmic rays. Bruno Rossi, an Italian physicist, studied muons created by cosmic rays and discovered that more muons were reaching the ground than expected. This phenomenon was explained by Einstein's special theory of relativity, which states that objects moving close to the speed of light experience less time passing for them. This means that from the perspective of the muons, fewer halflives have elapsed, allowing them to "time travel" into the future.
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