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00:00If you put this helmet on your head and you can visit any year you choose, you could even travel
00:07into the far future!
00:10Time travel is a concept that has been around for centuries. It is the idea of traveling to the past
00:15or future.
00:17Before we proceed any further, let's lay the groundwork.
00:20According to Einstein's relativity, the perception of now is always changing and it moves along with time.
00:26We perceive the passage of time because we all accumulate memories and knowledge of past and present events, which creates
00:33a sense of flow over our timeline.
00:35This is because it arises from human consciousness and the way our brains are wired.
00:40As Einstein himself remarked, people like us, who believes in physics, know that the distinction between past, present and future
00:47is only a stubbornly persistent illusion.
00:50What if everything we know about time is merely an illusion?
00:54Could we find a way out of this by breaking the construct of how the universe progresses?
01:00Theoretically, would it be possible to break the natural flow of time?
01:04In this video, we will explore just that and beyond.
01:15Often when people hear the term time travel, it is in relation to a person going either forward or backward
01:21in time, without any physical changes to their body.
01:25It would be funny if you travel back in time to see a baby version of you 20 years ago.
01:30But instead, when you jump out of the machine, you are in the shape of a baby, just like the
01:34one you'd like to see in the first place.
01:37In our understanding of the casual flow of time, we are actually moving from the past to the future.
01:42There are different interpretations of this, but what people usually refer to as time travel usually mean either going to
01:49the future faster or traveling into the past.
01:52In any case, instead of the normal flow of time, we change the rate by X amount.
01:59According to Newtonian mechanics, we perceive time as linear and deterministic.
02:04But ever since Albert Einstein published the theory of relativity, the Newtonian conception of absolute time and space has been
02:11replaced by the idea that time has a subjective significance to everyone.
02:16In this case, time is one dimension of spacetime in spatial relativity, or SR, and of dynamically curved spacetime in
02:24general relativity, or GR.
02:26From here on, the possibility of moving backwards or forwards in time might not be as far-fetched as one
02:32might think.
02:34In this case, it might be just as simple as ordinary beings can move between different points in space.
02:40There are many different hypothetical answers for time travel, and over the years, those scenarios have been put forward by
02:47notable scientists.
02:48Essentially, these scenarios are different from the notion of physical machine with levers and dials, the one that HD well
02:55proposed.
02:56Einstein also previously showed that time actually runs differently in different situations.
03:01This basically implies that time passes at different rates for different observers based on their traveling speed.
03:08Here is a very interesting example.
03:10The astronauts onboard the ISS orbit the Earth at 28,000 kilometers per hour.
03:16Taking into account the conflicting factors of GR and SR in this case, will get the net time dilation effects
03:22on the astronauts with respect to an observer on the planet's surface.
03:26I don't want to bore you with the details, you could pause this video if you want.
03:30But the conclusion is, the astronaut onboard the spaceship will get this amount of time dilation per day.
03:37For a typical crew member who stays on the ISS for 6 months, the total time dilation would be about
03:435 milliseconds.
03:44In such a case where he has a twin, the astronaut has experienced less passage of time than his twin
03:49counterpart, who remained on the Earth's surface.
03:53Giving the option to fast forward through time is a pretty cool idea, but it would be a one-way
03:58process where you wouldn't have the chance to go back.
04:02However, there might be some scientific basis within the theory of relativity, supporting the possibility of time travel under certain
04:09scenarios.
04:10There was a scientist named Kurt Gödel in the early 19th century.
04:14He showed that there are some solutions to the field equation of GR.
04:18It describes space-time so warped that they contain close time-like curves.
04:23This is where individual time cone twists and closes in on itself.
04:26It allows a path from the present to the past or to the future.
04:30Kurt Gödel's solution was the first challenge in centuries to one idea of linear time.
04:36Since we're on the topic, we'll mention a few notable attempts by popular scientists on time travel.
04:43Theoretical physicist Miguel Alcubier proposed a speculative scenario.
04:47This involves a spacecraft that would contract space in front of it and expand space behind it.
04:53If this shortcut is taken, it's going to result in an effective faster-than-light travel.
04:58You might even be able to use it for time travel.
05:01Other theoretical physicists have been thinking about this kind of shortcuts as well.
05:06For example, Kip Thorne and Paul Davis think it's possible to use a wormhole and travel through space-time.
05:13In a similar way as GR could be used to make instantaneous spatial travel plausible.
05:18The concept of wormholes has been talked about quite a bit, especially in science fiction films.
05:23They are theoretical threads that connect two separate points in space-time.
05:28The thread is so twisted that these two points are virtually similar behind the space-time it bends between.
05:35To bring about such a wormhole, unimaginable amounts of energy would be required.
05:40Scientists are attempting to create mini-wormhole-style conditions by speeding up electrons to near light speed and colliding them.
05:47The experiments are taking place here, at CERN's particle accelerator.
05:53Scientists think that such a wormhole would collapse immediately and become a black hole,
05:57unless some way of holding it open were found.
06:00In negative energy, for example.
06:03Unfortunately, it is theoretically possible, but not yet practical.
06:08Given all this, the quest to time travel poses a compelling question, especially from a practical standpoint.
06:15Not only that, when you dig deeper into the causality of events, it becomes clear that time travel is close
06:21to impossible,
06:21and we would encounter a number of logical obstacles in doing so.
06:25We'll discuss that in the next chapter.
06:33The likelihood of traveling backwards in time is generally believed to be much smaller than that of traveling forward in
06:39time.
06:40We often consider how temporal paradoxes are caused by the violation of laws of causality.
06:46These are cases of so-called causal loops.
06:49For our purposes, a causal loop is a series of events that loops back in time.
06:53It includes the event as one of its own causes.
06:57Take a look at this example.
06:59Suppose a woman goes to a bookstore and gets a copy of a book about time travel.
07:04She then goes to a time machine and travels back to a year, in the distant past.
07:08She meets somebody there and gives that copy to him.
07:11The man is really impressed by the content inside, and he rewrites the book as if he wrote it.
07:17People enjoy the book, and fast forward, it is massively published, generations afterwards.
07:24Let's summarize it.
07:25The bookstore got the book from the production, the woman got the book from the bookstore, and the production got
07:31the book from the man.
07:32But who exactly wrote the book?
07:34Where is the entry point for the information into the loop?
07:38Each event in the chain has a sufficient cost, but the chain itself doesn't.
07:43The full loop is inexplicable, despite each part being explicable.
07:48Questioning where an event originates from is one thing.
07:52However, it is another thing to ask where an entire chain of events actually comes from.
07:57Quite often, it can be useful to interrogate the underlying causes of an event, like what caused it in the
08:03first place.
08:04I mean, suppose you wanted to know how you came to be born.
08:08You might appeal to earlier events, facts about your parents, how they met, facts about your grandparents, or facts about
08:15humans in general.
08:17Soon enough, you'd find yourself thinking about how our place in the universe evolved.
08:21So you might be able to trace the chain of causes and effects back and back and back, arbitrarily far.
08:27And there's no answer to the question, where does the whole chain come from?
08:32Suppose on the other hand, and this is a prospect that many physicists take very seriously,
08:37that there are causal chains that just appear from nowhere.
08:40Quantum physics and Big Bang cosmology take very seriously the idea that the laws of physics allow events to happen,
08:46without any pre-existing conditions.
08:49Standard Big Bang cosmology says that the Big Bang isn't the first event in time.
08:53Instead, it is the beginning of time.
08:56As Stephen Hawking said, the question of what came before the Big Bang is meaningless, because there's no answer.
09:03Similarly, just like asking the question of what's north of the North Pole, there's just simply no good answer.
09:09There comes a point beyond which you simply can appeal to pass events.
09:13Yes, causal loops, events that are among their own causes, information seemingly generated from nothing, sounds very, very counterintuitive.
09:22There might be a good explanation as to why each event occurred, but there's no explanation for the chain of
09:27events as a whole.
09:29Perhaps causality is so sacred, that the universe would try its best to safeguard raw information, by simply giving no
09:36further explanation.
09:37That it is fundamentally impossible for us to see the origin with our own eyes.
09:43To experience and jump into different timelines, let alone change any event that has happened before, in the hope of
09:49a better future.
09:51Perhaps this is our current best understanding of how time works.
09:55More importantly, how the universe works.
09:58Space and time, they are inextricably linked and determined by the laws of physics in our universe.
10:04It is an expression of how they relate to one another.
10:12The state, was being recorded foreded om Jikemverickly.
10:15To investigate and determine which is appropriate for us.
10:15Which means bravely!
10:16You
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