The human brain is one of the smartest on
the planet. But there are some things we just can’t
wrap our minds around. One of those is the paradox. We’ve evolved to think of reality in a specific
way, but there are paradoxes out there that suggest reality doesn’t work the way we
think it does. And now some physicists think they have solved
a 50 year old paradox...but have they? And what are the other strangest paradoxes? Get ready to find out! [Fermi Paradox] A lot of you are probably familiar with the
Fermi paradox. Named after Italian physicist Enrico Fermi,
who is famous for creating the first nuclear reactor, this paradox seeks to answer the
question: Where are the aliens? Given that our star and Earth are part of
a fairly young planetary system compared to the rest of the universe, and that it’s
possible for a civilization to achieve interstellar travel during this time, it seems we should
have been visited by some form of extra-terrestrial intelligence by now. Now some say it’s not a real paradox, because
we can only guess that there is intelligent life out there. But the Drake Equation is used in this so-called
paradox to estimate the number of possible civilizations in our galaxy. The Drake Equation uses seven variables to
estimate the number of detectable civilizations in the Milky Way. Voice Actor - You do not need to narrate this
because it will go on the screen. This gave us an idea that there could be millions
of Earth-like worlds with civilizations out there. But this was back in 1961, and no one knew
of any worlds orbiting stars other than our own. It was only recently that we got a good idea. In 2020, astronomers using data from the Kepler
Space Telescope found there are more than 300 million worlds with similar conditions
to Earth scattered throughout the Milky Way. The analysis concluded that roughly half of
the galaxy’s sun-like stars host rocky planets in habitable zones where liquid water could
be on the surface. In fact, planets are extremely common and
outnumber all the stars in our galaxy. And very soon, the James Webb Space Telescope
will be headed into space to look for new potentially habitable worlds. We’ll have a new video on that soon! So make sure to stay tuned here! Now that we know how many worlds there are,
and how many possibilities there could be advanced civilizations, the question remains. Why is the universe silent? Maybe we’re about to find out soon as technology
advances, or perhaps we've already been visited by some far-away civilization, and just don’t
know it yet. There have been many unexplained UFO sightings
recently, and some believe extraterrestrials are already here. Solid proof of that would put an end to this
paradox. [The Bootstrap Paradox]
The Bootstrap Paradox is a paradox of time travel that questions how something that is
taken from the future and placed in the past could ever come into existence in the first
place. It’s a common theme used by writer's inspiring
plotlines in many science fiction films; such as the Bill and Ted movies, Terminator, and
Doctor Who. So let’s look at one of the examples of
this paradox. Imagine that you are a time traveler, but
before you go on an excellent adventure, you go into a bookstore and buy a copy of Hamlet,
written by Shakespeare. You then travel back in time to London during
the Elizabethan Era, and give the book to Shakespeare. William S. then copies the book and claims
it as his own work. Centuries go by, and during this time, Hamlet
is printed and reproduced countless times until a copy of it ends up back in the same
bookstore that you bought it from. The question then becomes: Who wrote Hamlet? By the way, for those who missed our video
on parallel universes, check out the link in the description. [The Grandfather Paradox]
This is another famous paradox, which involves you going back in time to ‘take out’ your
grandfather… by that we mean, erasing his existence. Now, we know everyone here loves their grandparents,
but this is only an example. Once again, you are a time traveler, and you
pop back in time to do the deed, and erase your grandfather’s existence. You then return to present time, but the thing
is, with your grandfather gone, your father wasn’t born, and you now realize you never
even existed. Everything about you has now been erased,
including all your family, friends, all your possessions, and your history. So you wouldn’t have been born in the first
place, so it would be impossible to do this. Now some scientists believe that if this were
to happen, you would have now created an alternate timeline or even entered a parallel universe. By the way, for those who missed our video
on parallel universes, check out the link in the description. [Taking Out Hitler Paradox]
Another interesting variant to the grandfather paradox, going back in time to eliminate Hitler
to stop World War II. This would have some interesting consequences. Let’s say you have a shiny time machine,
and you’ve got a plan to go back before things get out of control during the war,
and put things right. The problem now is that the action removes
any reason to travel back in time, along with any knowledge that the reason to time travel
back ever existed. Acting as a time traveling executioner simply
creates a paradox, and along with the many world's idea, traveling back in time might
create a new timeline without der Führer, but the old timeline would also still exist. You might even create a new timeline that
is even worse. But what would happen if you sent something
back through a wormhole? [Polchinski's Paradox]
The late, great Joseph Polchinski is the famous theoretical physicist who wrote the book on
String Theory. But Polchinski also came up with a potentially
paradoxical situation involving a billiard ball sent through a wormhole that travels
back in time. In this scenario, the billiard ball is fired
into a wormhole at such an angle that, if it continues along the path, it will exit
the wormhole in the past at the right angle to collide with its earlier self, thereby
knocking it off course and preventing it from entering the wormhole in the first place. However, some physics students came up with
solutions which avoid any inconsistencies by having the ball emerge from the future
at a different angle than the one used to generate the paradox, and deliver its younger
self a glancing blow instead of knocking it completely away from the wormhole, a blow
which changes its trajectory in the right way, so that it will travel back in time with
the angle required to deliver its younger self this glancing blow. Now here is something that will make you think
about reality differently... [Observer's Paradox]
The observer's paradox is something very strange indeed, and of all the bizarre facts of quantum
theory there are fewer stranger than Schrödinger’s famous fable about a cat that is neither alive,
nor deceased. For the record, this is a thought experiment
only, and no animals have ever been harmed. The paradox describes a cat that is locked
inside of a windowless box along with some radioactive material, a Geiger counter, a
hammer, and a container of deadly poison. The radioactive material has a 50% chance
to decay. If the Geiger Counter records an emitted radioactive
particle, it then triggers a hammer that smashes a vial of poison that will be fatal to the
cat. However, you would not know if the cat was
alive or deceased until you opened the box. And so until the box was opened, the cat would
be both alive and deceased at the same time. 'How is this possible', you ask? This is because simply looking at matter actually
changes the outcome of what happens to it. You can’t know something is there unless
you see it. Now you’re probably saying this whole thing
is very strange. But consider another observer’s paradox
called the double-slit experiment. It is the most famous physics experiment of
all time. Imagine a wall with two slits in it, and then
throwing tennis balls at the wall. Some of them will bounce off the wall, but
some of them will travel through the slits. If there is a wall behind the first one with
slits, some of the tennis balls that made it through will hit it. Now if you mark where the tennis balls hit
the second wall, you should expect to see two strips of marks roughly the same size
as the slits. Sounds pretty straightforward. But in the double slit experiment, something
awfully strange happens when you shine a light through the slits. Light isn’t just a wave, it’s also a particle
called a photon. Now if you shoot a single photon at the double
slits, it forms an interference pattern on the back wall as if it is interfering with
itself. It’s like the photon went through both slits
at the same time. But this is where it gets stranger. Simply by looking at the double-slit experiment,
the behavior of the photons changes, as if the photons are alive and know you are watching
them! We know this because if the experimenter tries
to find out which slit the photon is going through, the interference pattern doesn’t
show up at all. The bottom line is that observing a photon
can change events that have already happened. How is that possible? No one has figured it out yet, maybe YOU might
be the one to solve this puzzle. [The Black Hole Information Paradox] One of the biggest paradoxes in physics is
the black hole information paradox - a puzzle that results from the combination of quantum
mechanics and general relativity. Calculations show that physical information
could permanently disappear into a black hole, allowing physical states to devolve into the
same state. But this is controversial because quantum
mechanics states that information can never be destroyed. Let’s say you burned two different letters
written on paper. Putting them back together from ash would
be nearly impossible, but not entirely. The small differences in smoke, temperature,
and the amount of ash would still retain information about the two different letters. The problem with black holes is they suck
things up and then, over a very, very long time, radiate what they have swallowed back
out in the form of Hawking Radiation. Unfortunately, unlike the smoke, temperature,
and ash from burning a letter, Hawking radiation contains no information about what the black
hole ate. This is because all Hawking radiation is the
same, which implies that black holes destroy information about the universe. So do evaporating black holes really destroy
information, or does information escape as the black hole evaporates? A new generation of physicists say that information
does indeed escape a black hole by their radiation, and they have identified an invisible surface
that lies inside a black hole's event horizon called the ‘quantum extremal surface’. This surface appears to encode the amount
of information that has radiated away from the black hole, evolving over the black hole's
lifetime exactly as expected if information escapes. Apparently, something can escape a black hole. It’s a problem not completely solved yet,
and is a work in progress. When we find out for sure, stay tuned here
and we’ll let you know!
