Is Time Travel Possible In Our Universe?

This paragraph introduces the block universe theory, which states that the past, present, and future all exist simultaneously alongside one another. It compares our experience of time to a river that we travel along, unable to slow down, speed up, or turn around. It questions where yesterday goes when it becomes today, arguing that the past continues to exist like upstream sections of a river.

This paragraph discusses how Einstein's special theory of relativity did away with the notion of absolute time, instead showing that time is relative based on an observer's motion through space. His general theory of relativity then revealed that gravity is actually the result of curved spacetime, not a force as Newton thought. Experiments testing general relativity, like observations of Mercury's orbit, provide evidence that support the block universe theory wherein all moments in time exist simultaneously.

This paragraph discusses physicist Bruno Rossi's experiments with muons created by cosmic rays. He observed an excess of muons reaching the ground due to time dilation from their relativistic speeds. This phenomenon of time passing slower for fast-moving objects provides experimental proof that time travel into the future is possible.

This paragraph explores additional possibilities for time travel into the future, like taking a high-speed trip through space and returning to find more time has passed on Earth. It also discusses time dilation due to gravitational effects, like atomic clocks running at different rates based on miniscule differences in height above Earth's surface. GPS satellites have to account for both special and general relativistic time dilation effects.

This paragraph recounts the brief excitement in 2011 over neutrinos that seemed to travel faster than light, and how Einstein's theories forbid this. It introduces physicist Willem van Stockum's 1937 paper showing that Einstein's equations allow for closed timelike curves under certain conditions, raising the prospect of backwards time travel.

This paragraph discusses proposals from physicists like Frank Tipler and Stephen Hawking regarding using rapidly spinning massive cylinders to warp spacetime in a way that could enable backwards time travel. It also introduces the concept of wormholes as potential shortcuts through spacetime that could allow effective travel back in time without exceeding light speed.

This paragraph suggests that the many worlds interpretation of quantum mechanics, wherein new branches of reality form at every quantum event, could resolve classic time travel paradoxes. If a time traveller enters a separate parallel worldline/timestream when journeying into the past, they avoid paradoxes like killing their own grandfather or giving their younger self an inexplicable letter.

This paragraph examines a color-changing billiard ball time travel paradox that challenges Novikov's self consistency principle, but can be resolved through many worlds by having the time traveller emerge in a different quantum timestream upon their journey into the past where events play out differently.

This closing paragraph muses poetically on the staggering implications of the many worlds theory, wherein new branches of reality form at every quantum event across all of existence through eternity. It notes skepticism from some physicists about whether this endless array of parallel worlds is truly necessary or merely adds unneeded metaphysical baggage when contemplating time travel.