October 29, 2025 • 20 mins
Black Hole is a podcast series exploring the most extreme and fascinating phenomena in our universe. Hosted by Felix Mercer, each episode delivers deep dives into cosmic mysteries including supermassive black holes, neutron stars, the cosmic web's vast architecture, and the relativity of time itself. Through accessible explanations and compelling narratives, the series examines how gravity shapes reality, how dead stars create the elements necessary for life, and why time passes differently depending on motion and gravity. Without interviews or guest segments, Felix guides listeners through twenty-five-minute journeys into the heart of astrophysics, making complex concepts understandable and awe-inspiring.
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Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:00):
Welcome back to Black Hole. I'm Felix Mercer, your AI host.
Over the past five episodes, we've journeyed through supermassive black holes,
explored their complete anatomy, investigated neutron stars at the edge
of physical possibility, and map the cosmic web that structures
our universe. But today we're going to examine something even
(00:21):
more fundamental, something that affects every moment of your existence,
whether you realize it or not. Today we're talking about
time itself, and specifically about how time is not what
you think it is. Imagine two twins, identical in every way.
One stays on Earth while the other travels into space
(00:41):
at nearly the speed of light, journeying to a distant
star and back. When the traveling twin returns, they're younger
than their sibling who stayed home, not in appearance, not metaphorically,
but actually younger. Their cells have divided fewer times, their
heart has beaten fewer times, they have lived less time.
(01:01):
This isn't science fiction. This isn't a thought experiment that
breaks down when you examine it too closely. This is reality,
a fundamental feature of our universe, confirmed by countless experiments.
Time itself is flexible malleable, dependent on how fast you're
moving and how strong the gravity is where you're standing.
(01:21):
By the end of this episode, you'll understand why astronauts
age slower than people on Earth, why GPS satellites need
constant time adjustments to function properly, and why the universe
has no universal now no moment that's simultaneous everywhere. Time
is not what you think it is, and once you
understand time dilation, you'll never look at a clock the
(01:43):
same way again. The story begins in the early twentieth
century with a young patent clerk in Switzerland named Albert Einstein.
Einstein was obsessed with a problem that had been nagging
at physicists for decades. The speed of light seemed to
be constant all always the same value, regardless of how
fast you were moving when you measured it. This was
(02:05):
deeply weird. If you're on a train moving at sixty
miles per hour and you throw a ball forward at
twenty miles per hour, someone watching from the platform would
measure the ball moving at eighty miles per hour. Velocities
add that's how motion works in everyday life, but light
doesn't behave this way. If you're on a spaceship moving
(02:27):
at half the speed of light and you shine a
flashlight forward, someone watching from a stationary position doesn't measure
the light moving at one and a half times the
speed of light. They measure it moving at exactly the
speed of light, the same speed you measure it. Light's
velocity is absolute, unchanging, the same for all observers, regardless
of their motion. This fact had been confirmed experimentally, most
(02:51):
famously in the Mickelson Morley experiment of eighteen eighty seven,
but no one could explain why. Einstein realized that if
the speed of light is constant for everyone, something else
has to give. If velocities don't add normally for light,
then our assumptions about space and time must be wrong,
And in nineteen zero five, in his paper on Special Relativity,
(03:11):
Einstein showed exactly what was wrong. Space and time aren't
separate absolute quantities. They're intertwined relative dependent on the observer's motion,
Time itself can stretch or compress depending on how fast
you're moving. Here's the key insight, and it's worth taking
a moment to really understand it because it's not intuitive.
(03:32):
Imagine you're on a train with a laser on the
floor and a mirror on the ceiling. You fire the
laser and the light travels straight up to the mirror
and bounces straight back down. From your perspective on the train,
the light traveled the height of the train and back
simple vertical motion. Now imagine someone standing outside the train
(03:52):
watching this happen. From their perspective, the train is moving.
When the light pulse travels from floor to ceiling, the
train has moved forward, so the light has to travel
not just up, but also horizontally to hit the mirror.
The path is diagonal, longer than the simple vertical path
you measured from inside the train. But here's the critical point.
(04:13):
Light speed must be the same for both observers. If
the light traveled a longer path for the outside observer
but did so at the same speed, it must have
taken more time. This is time dilation. The clock on
the train from the perspective of someone outside, is running slower,
not because there's something wrong with the clock, not because
(04:34):
of any mechanical effect, but because time itself passes more
slowly for objects in motion. The faster you move, the
more pronounced this effect becomes. At everyday speeds. It's negligible, invisible,
too small to notice. But as you approach the speed
of light, time dilation becomes dramatic and impossible to it more.
Let's make this concrete with real examples, because time dilation
(04:58):
isn't theoretical speculation, it's measured, observed, required for modern technology
to function. Consider the International Space Station orbiting Earth at
roughly seven and a half kilometers per second. That's fast
by human standards, about twenty seven thousand kilometers per hour,
but it's still only a tiny fraction of light speed,
(05:18):
about twenty five thousandths of one percent. Yet even at
this modest velocity, time dilation is real and measurable. Astronauts
on the ISS age more slowly than people on Earth.
The effect is small, about point zero zero seven seconds
less per six months in orbit, but it's real and
it's been measured. If an astronaut spends six months on
(05:40):
the station, they return having aged seven milliseconds less than
they would have on Earth. Their clock and their biology
ran soer they experienced less time. This isn't an illusion
or a trick. The astronaut's heart actually beat fewer times,
their cells actually divide. They did fewer times they lived
(06:02):
less time. The effect scales with velocity. If you could
travel at fifty percent the speed of light, time dilation
becomes impossible to ignore. For every year that passes for
you on your spacecraft, roughly one point one five years
pass for someone's stationary at ninety percent of light speed.
(06:22):
The effect is dramatic. For every year you experience, about
two point three years pass for a stationary observer at
ninety nine percent of light speed. Every year for you
corresponds to about seven years for someone hot moving, and
at ninety nine point nine to nine percent of light speed,
every year you experience equals roughly seventy years for a
(06:43):
stationary observer. This is how the traveling twin could return
home younger. If they traveled at ninety nine point nine
percent of light speed for what they experienced is five years,
roughly two hundred and twenty three years would pass on Earth.
They would return to find their twin long dead, their
children aged beyond recognition, their world transformed by centuries while
(07:04):
they experienced only a few years of travel. This seems impossible,
paradoxical even How can both perspectives be correct? How can
the traveling twin age less, while the stationary twin ages more.
The resolution lies in understanding that there's no absolute reference frame,
no universal standard of rest. Each observer's experience of time
(07:26):
is equally valid in their own frame of reference. The
asymmetry comes from acceleration. The traveling twin had to accelerate
to reach high speed and decelerate to return home. That
acceleration breaks the symmetry and makes their experience different from
the stationary twins. But velocity isn't the only thing that
affects time decided. It sets and slow the dull windows.
(07:49):
Gravity does too, and in some ways gravitational time dilation
is even more profound than velocity based time dilation. Einstein
realized this in his Theory of General Relativity, published in
nineteen fifteen. Gravity isn't a force in the traditional sense.
It's a curvature of space time itself, and in curved
(08:09):
space time, time flows at different rates depending on how
deep in the gravitational well you are. The principle is straightforward.
Stronger gravity means slopper time. If you're standing at sea level,
your deeper inerts gravitational well than someone standing on a mountaintop.
Your clock runs slower than meres. The difference is tiny
(08:31):
measured and mannoseconds over a lifetime, but it's real and
measurable with precise enough instruments. Time genuinely passes more slowly
for you at sea level than it does for someone
at higher altitude. This effect becomes more pronounced with stronger
gravitational feels on Earth. The effect is negligible for everyday purposes,
but for GPS satellites orbiting about twenty thousand kilometers above
(08:54):
Earth's surface, where gravity is weaker than at ground level,
time runs faster. Each satellite's clock gains about forty five
microseconds per day compared to clocks on Earth's surface. This
gain is partially offset by velocity time dilation because the
satellites are moving, which makes their clocks run slower by
about seven microseconds per day. The net effect is that
(09:17):
satellite clocks gain about thirty eight microseconds per day relative
to ground clocks. This might seem like a trivial difference,
but GPS to hands on extremely precise timing. The system
works by measuring the time it takes for radio signals
to travel from multiple satellites to your receiver. Light travels
roughly thirty centimeters per nanosecond, so a timing eer of
(09:40):
just one microsecond translates to a position error of about
three hundred meters. Without accounting for both velocity and gravitational
time dilation, GPS would accumulate position errors of several kilometers
per day, rendering the system completely useless. The fact that
your phone can tell you your position to within a few
meters is direct practical proof that time dilation is real.
(10:04):
Every time you use GPS, you're relying on corrections for
relativistic effects, on adjustments that account for time passing at
different rates for satellites than for you. Einstein's theories aren't
abstract mathematical curiosities. They're essential for modern technology, built into
systems we use every day without thinking about the physics
(10:24):
that makes them work. But to really appreciate how extreme
gravitational time dilation can become, we need to look at
stronger gravitational fields. Consider a neutron star with surface gravity
billions of time stronger than Earth's. Time at the surface
passes measurably slower than time far from the star. If
you could somehow survive standing on a neutron star's surface,
(10:47):
for what felt like an hour. While your friend waited
in a spacecraft safely distant, they might experience several hours passing.
The exact ratio depends on the neutron star's mass and radius,
but the effect is profound. And then there are black holes,
where gravitational time dilation reaches its ultimate extreme. Mirror black
(11:08):
hole's event horizon, time slows to a crawl relative to
distant observers. If you fell toward a black hole while
a friend watched from a safe distance, they would see
you appear to slow down as you approached the horizon,
your movements becoming more and more sluggish, your pork ticking
slower and slower. In fact, from their perspective, you would
never quite reach the event horizon. You would appear to
(11:31):
freeze asymptotically, approaching the horizon but never crossing it. Your
image red shifted and dimmed until it faded from view.
But that's only their perspective. From your perspective, nothing strange
happens at the horizon. You would cross it in finite
time by your clock falling toward the singularity at the
black hole's center. The difference between perspectives is a consequence
(11:54):
of extreme time dilation. Near the horizon, time has slowed
so dramatically relative to the outside universe that events that
take finite time for you correspond to infinite time for
distant observers. This was depicted with remarkable accuracy in the
film Interstellar. The water planet scene, where the crew visits
(12:14):
a planet so close to a massive black hole that
every hour on the surface corresponds to seven years for
the crew member waiting an orbit is beas on real physics.
The numbers are correct for a planet orbiting near a
black hole of gargantuas depicted mass. The emotional impact the
crew returning to find decades had passed messages from home
showing years of aging and life lived in what felt
(12:36):
like hours. That's a real consequence of gravitational time dilation.
Taken to its extreme. Time dilation has implications that go
beyond just clocks running at different rates. It fundamentally changes
how we think about simultaneity, about what it means for
events to happen at the same time. In Newtonian physics,
simultaneity is absolute. If two events happen at the same moment,
(12:59):
they have at the same moment for all observers everywhere
but relativity destroys this notion. Here's why. Imagine a spaceship
passing Earth at high speed at the exact moment from
Earth's perspective that the spaceship is aligned with Earth, two
events happen, one at the front of the ship and
one at the back. From Earth's perspective, these events are simultaneous,
(13:22):
but from the space ship's perspective they're not. The event
at the front happens before the event at the back,
or vice versa, depending on the direction of travel. The
order of events what happens first and what happens second
depends on the observer's motion. This isn't an illusion or
a measurement problem. The events genuinely occur in different orders
(13:42):
for different observers, provided they're far enough apart in space.
There is no universal now, no cosmic clock that defines
what the present moment means. Everywhere in the universe now
is local, personal, relative to your frame of reference. Some
physicists take this even further, arguing for what's called the
(14:03):
block universe interpretation. If all moments are equally real, just
experience differently depending on your motion and position, and perhaps
the entire history of the universe past, present, and future
exists simultaneously. Time doesn't flow from past to future. Instead,
space time is a static four dimensional structure, and our
(14:25):
perception of time passing is just an artifact of our
consciousness moving along our world line through this lock. This
interpretation is philosophically challenging. It seems to eliminate free will,
suggest that the future already exists as concretely as the past.
Not all physicists accept it, but it's a natural consequence
of ticking relativities insights. Seriously, if time is relative, if
(14:49):
simultaneity is observer dependent, if there's no universal now, then
perhaps all moments are ontologically equivalent, all existing in the
eternal space time structure. Regardless of interpretation, time dilation points
towards something profound. Time travel to the future is not
only possible but inevitable. Every time you move, you travel
(15:10):
into the future. Relative to stationary observers, the effect is
usually too small to notice, but in principle, if you
could travel fast enough or descend deep enough into a
gravitational well, you could skip forward in time by any amount.
You could travel a year into the future, a century,
a millennium, simply by moving fast enough or hovering nearer
(15:31):
black holes of vent horizon for long enough. What you
cannot do, at least according to our current understanding of physics,
is travel to the past. Moving backward in time would
require exceeding the speed of light, which general relativity forbids.
It would also create causality paradoxes, situations where you could
potentially prevent your own time travel from occurring. Some exotic
(15:54):
solutions to Einstein's equations seem to allow closed timelike curves
paths through space time that loop back on themselves temporarily,
but these require conditions that likely don't exist in nature,
such as traversible wormholes held open by exotic matter with
negative energy density. So well, the future is accessible, the
past remains probably forever beyond reach. Time has a direction,
(16:20):
at least for practical purposes, even if that direction is
more complicated than simple universal flow from past to future.
Let me bring this back to something personal, something immediate.
Right now, as you listen to this, time is passing
differently for different parts of your body. Your head, slightly
(16:40):
farther from Earth's center than your feet experience is fractionally
less gravitational time dilation. Your head is aging faster than
your feet by about ninety billions of a second per year.
The difference is absurdly small, unmeasurable without atomic clocks, but
it's real. Every time you move, you affect how quickly
(17:01):
you age relative to people standing still. Take a flight
across the country at six hundred miles per hour, and
you age very slightly less than someone who stayed home,
though the effect is far too small to notice. Drive
to work, and your commute means you've experienced fractionally less
time than someone who walked. The universe is constantly adjusting
(17:22):
the rate at which time passes for you, based on
your motion and your position in gravitational fields. This seems
strange because our intuition about time is built on everyday
experience at low speeds and weak gravity. We evolved in
an environment where time dilation is negligible, invisible, inconsequential, So
(17:43):
we developed an intuition that time is absolute, universal, the
same for everyone. But that intuition is wrong. It's an
approximation that works well at human scales but breaks down
when we examine the universe more carefully. Time is personal,
time is relative. Your now is not mine now. If
(18:04):
we're moving relative to each other or experiencing different gravitational feels.
We each carry our own clocks, our own experience of duration,
and these clocks can and do disagree depending on our
paths through space time. The universe doesn't have a master clock,
a cosmic time keeper defining the correct rate at which
(18:24):
time passes each clock. Each observer has their own, equally
valid measurement of time. This realization, more than perhaps any
other discovery in physics, reveals how different the universe is
from our every day intuition. Time, which seems so fundamental,
so constant, so absolute, is actually flexible relative, dependent on circumstances.
(18:47):
It's one of the strangest features of reality, and yet
it's confirmed every day by experiments by GPS satellites, by
particle accelerators, by astronomical observations. Time dilation isn't speculation or science.
It's how the universe actually works. Right now is only
right now for you. Someone moving relative to you has
(19:08):
a different now. Someone in a different gravitational field has
a different now. The universe unfolds differently for different observers,
each experiencing their own path through space time at their
own rate, and understanding this really grasping What it means
for time to be relative changes how you see reality itself.
Time is not what you thought it was. It's stranger,
(19:31):
more flexible, more wonderful than the simple forward march of
seconds we imagine when we glance at our watches. Thank
you for listening to this episode of Blackhawn. If you
enjoyed exploring the relativity of time, please subscribe to stay
updated on future episodes. This episode was brought to you
by Quiet Please Podcast Networks. For more content like this,
(19:52):
please go to Quiet. Please dot ai Quiet, Please dot
ai hear what matters
Welcome back to Black Hole. I'm Felix Mercer, your AI host.
Over the past five episodes, we've journeyed through supermassive black holes,
explored their complete anatomy, investigated neutron stars at the edge
of physical possibility, and map the cosmic web that structures
our universe. But today we're going to examine something even
(00:21):
more fundamental, something that affects every moment of your existence,
whether you realize it or not. Today we're talking about
time itself, and specifically about how time is not what
you think it is. Imagine two twins, identical in every way.
One stays on Earth while the other travels into space
(00:41):
at nearly the speed of light, journeying to a distant
star and back. When the traveling twin returns, they're younger
than their sibling who stayed home, not in appearance, not metaphorically,
but actually younger. Their cells have divided fewer times, their
heart has beaten fewer times, they have lived less time.
(01:01):
This isn't science fiction. This isn't a thought experiment that
breaks down when you examine it too closely. This is reality,
a fundamental feature of our universe, confirmed by countless experiments.
Time itself is flexible malleable, dependent on how fast you're
moving and how strong the gravity is where you're standing.
(01:21):
By the end of this episode, you'll understand why astronauts
age slower than people on Earth, why GPS satellites need
constant time adjustments to function properly, and why the universe
has no universal now no moment that's simultaneous everywhere. Time
is not what you think it is, and once you
understand time dilation, you'll never look at a clock the
(01:43):
same way again. The story begins in the early twentieth
century with a young patent clerk in Switzerland named Albert Einstein.
Einstein was obsessed with a problem that had been nagging
at physicists for decades. The speed of light seemed to
be constant all always the same value, regardless of how
fast you were moving when you measured it. This was
(02:05):
deeply weird. If you're on a train moving at sixty
miles per hour and you throw a ball forward at
twenty miles per hour, someone watching from the platform would
measure the ball moving at eighty miles per hour. Velocities
add that's how motion works in everyday life, but light
doesn't behave this way. If you're on a spaceship moving
(02:27):
at half the speed of light and you shine a
flashlight forward, someone watching from a stationary position doesn't measure
the light moving at one and a half times the
speed of light. They measure it moving at exactly the
speed of light, the same speed you measure it. Light's
velocity is absolute, unchanging, the same for all observers, regardless
of their motion. This fact had been confirmed experimentally, most
(02:51):
famously in the Mickelson Morley experiment of eighteen eighty seven,
but no one could explain why. Einstein realized that if
the speed of light is constant for everyone, something else
has to give. If velocities don't add normally for light,
then our assumptions about space and time must be wrong,
And in nineteen zero five, in his paper on Special Relativity,
(03:11):
Einstein showed exactly what was wrong. Space and time aren't
separate absolute quantities. They're intertwined relative dependent on the observer's motion,
Time itself can stretch or compress depending on how fast
you're moving. Here's the key insight, and it's worth taking
a moment to really understand it because it's not intuitive.
(03:32):
Imagine you're on a train with a laser on the
floor and a mirror on the ceiling. You fire the
laser and the light travels straight up to the mirror
and bounces straight back down. From your perspective on the train,
the light traveled the height of the train and back
simple vertical motion. Now imagine someone standing outside the train
(03:52):
watching this happen. From their perspective, the train is moving.
When the light pulse travels from floor to ceiling, the
train has moved forward, so the light has to travel
not just up, but also horizontally to hit the mirror.
The path is diagonal, longer than the simple vertical path
you measured from inside the train. But here's the critical point.
(04:13):
Light speed must be the same for both observers. If
the light traveled a longer path for the outside observer
but did so at the same speed, it must have
taken more time. This is time dilation. The clock on
the train from the perspective of someone outside, is running slower,
not because there's something wrong with the clock, not because
(04:34):
of any mechanical effect, but because time itself passes more
slowly for objects in motion. The faster you move, the
more pronounced this effect becomes. At everyday speeds. It's negligible, invisible,
too small to notice. But as you approach the speed
of light, time dilation becomes dramatic and impossible to it more.
Let's make this concrete with real examples, because time dilation
(04:58):
isn't theoretical speculation, it's measured, observed, required for modern technology
to function. Consider the International Space Station orbiting Earth at
roughly seven and a half kilometers per second. That's fast
by human standards, about twenty seven thousand kilometers per hour,
but it's still only a tiny fraction of light speed,
(05:18):
about twenty five thousandths of one percent. Yet even at
this modest velocity, time dilation is real and measurable. Astronauts
on the ISS age more slowly than people on Earth.
The effect is small, about point zero zero seven seconds
less per six months in orbit, but it's real and
it's been measured. If an astronaut spends six months on
(05:40):
the station, they return having aged seven milliseconds less than
they would have on Earth. Their clock and their biology
ran soer they experienced less time. This isn't an illusion
or a trick. The astronaut's heart actually beat fewer times,
their cells actually divide. They did fewer times they lived
(06:02):
less time. The effect scales with velocity. If you could
travel at fifty percent the speed of light, time dilation
becomes impossible to ignore. For every year that passes for
you on your spacecraft, roughly one point one five years
pass for someone's stationary at ninety percent of light speed.
(06:22):
The effect is dramatic. For every year you experience, about
two point three years pass for a stationary observer at
ninety nine percent of light speed. Every year for you
corresponds to about seven years for someone hot moving, and
at ninety nine point nine to nine percent of light speed,
every year you experience equals roughly seventy years for a
(06:43):
stationary observer. This is how the traveling twin could return
home younger. If they traveled at ninety nine point nine
percent of light speed for what they experienced is five years,
roughly two hundred and twenty three years would pass on Earth.
They would return to find their twin long dead, their
children aged beyond recognition, their world transformed by centuries while
(07:04):
they experienced only a few years of travel. This seems impossible,
paradoxical even How can both perspectives be correct? How can
the traveling twin age less, while the stationary twin ages more.
The resolution lies in understanding that there's no absolute reference frame,
no universal standard of rest. Each observer's experience of time
(07:26):
is equally valid in their own frame of reference. The
asymmetry comes from acceleration. The traveling twin had to accelerate
to reach high speed and decelerate to return home. That
acceleration breaks the symmetry and makes their experience different from
the stationary twins. But velocity isn't the only thing that
affects time decided. It sets and slow the dull windows.
(07:49):
Gravity does too, and in some ways gravitational time dilation
is even more profound than velocity based time dilation. Einstein
realized this in his Theory of General Relativity, published in
nineteen fifteen. Gravity isn't a force in the traditional sense.
It's a curvature of space time itself, and in curved
(08:09):
space time, time flows at different rates depending on how
deep in the gravitational well you are. The principle is straightforward.
Stronger gravity means slopper time. If you're standing at sea level,
your deeper inerts gravitational well than someone standing on a mountaintop.
Your clock runs slower than meres. The difference is tiny
(08:31):
measured and mannoseconds over a lifetime, but it's real and
measurable with precise enough instruments. Time genuinely passes more slowly
for you at sea level than it does for someone
at higher altitude. This effect becomes more pronounced with stronger
gravitational feels on Earth. The effect is negligible for everyday purposes,
but for GPS satellites orbiting about twenty thousand kilometers above
(08:54):
Earth's surface, where gravity is weaker than at ground level,
time runs faster. Each satellite's clock gains about forty five
microseconds per day compared to clocks on Earth's surface. This
gain is partially offset by velocity time dilation because the
satellites are moving, which makes their clocks run slower by
about seven microseconds per day. The net effect is that
(09:17):
satellite clocks gain about thirty eight microseconds per day relative
to ground clocks. This might seem like a trivial difference,
but GPS to hands on extremely precise timing. The system
works by measuring the time it takes for radio signals
to travel from multiple satellites to your receiver. Light travels
roughly thirty centimeters per nanosecond, so a timing eer of
(09:40):
just one microsecond translates to a position error of about
three hundred meters. Without accounting for both velocity and gravitational
time dilation, GPS would accumulate position errors of several kilometers
per day, rendering the system completely useless. The fact that
your phone can tell you your position to within a few
meters is direct practical proof that time dilation is real.
(10:04):
Every time you use GPS, you're relying on corrections for
relativistic effects, on adjustments that account for time passing at
different rates for satellites than for you. Einstein's theories aren't
abstract mathematical curiosities. They're essential for modern technology, built into
systems we use every day without thinking about the physics
(10:24):
that makes them work. But to really appreciate how extreme
gravitational time dilation can become, we need to look at
stronger gravitational fields. Consider a neutron star with surface gravity
billions of time stronger than Earth's. Time at the surface
passes measurably slower than time far from the star. If
you could somehow survive standing on a neutron star's surface,
(10:47):
for what felt like an hour. While your friend waited
in a spacecraft safely distant, they might experience several hours passing.
The exact ratio depends on the neutron star's mass and radius,
but the effect is profound. And then there are black holes,
where gravitational time dilation reaches its ultimate extreme. Mirror black
(11:08):
hole's event horizon, time slows to a crawl relative to
distant observers. If you fell toward a black hole while
a friend watched from a safe distance, they would see
you appear to slow down as you approached the horizon,
your movements becoming more and more sluggish, your pork ticking
slower and slower. In fact, from their perspective, you would
never quite reach the event horizon. You would appear to
(11:31):
freeze asymptotically, approaching the horizon but never crossing it. Your
image red shifted and dimmed until it faded from view.
But that's only their perspective. From your perspective, nothing strange
happens at the horizon. You would cross it in finite
time by your clock falling toward the singularity at the
black hole's center. The difference between perspectives is a consequence
(11:54):
of extreme time dilation. Near the horizon, time has slowed
so dramatically relative to the outside universe that events that
take finite time for you correspond to infinite time for
distant observers. This was depicted with remarkable accuracy in the
film Interstellar. The water planet scene, where the crew visits
(12:14):
a planet so close to a massive black hole that
every hour on the surface corresponds to seven years for
the crew member waiting an orbit is beas on real physics.
The numbers are correct for a planet orbiting near a
black hole of gargantuas depicted mass. The emotional impact the
crew returning to find decades had passed messages from home
showing years of aging and life lived in what felt
(12:36):
like hours. That's a real consequence of gravitational time dilation.
Taken to its extreme. Time dilation has implications that go
beyond just clocks running at different rates. It fundamentally changes
how we think about simultaneity, about what it means for
events to happen at the same time. In Newtonian physics,
simultaneity is absolute. If two events happen at the same moment,
(12:59):
they have at the same moment for all observers everywhere
but relativity destroys this notion. Here's why. Imagine a spaceship
passing Earth at high speed at the exact moment from
Earth's perspective that the spaceship is aligned with Earth, two
events happen, one at the front of the ship and
one at the back. From Earth's perspective, these events are simultaneous,
(13:22):
but from the space ship's perspective they're not. The event
at the front happens before the event at the back,
or vice versa, depending on the direction of travel. The
order of events what happens first and what happens second
depends on the observer's motion. This isn't an illusion or
a measurement problem. The events genuinely occur in different orders
(13:42):
for different observers, provided they're far enough apart in space.
There is no universal now, no cosmic clock that defines
what the present moment means. Everywhere in the universe now
is local, personal, relative to your frame of reference. Some
physicists take this even further, arguing for what's called the
(14:03):
block universe interpretation. If all moments are equally real, just
experience differently depending on your motion and position, and perhaps
the entire history of the universe past, present, and future
exists simultaneously. Time doesn't flow from past to future. Instead,
space time is a static four dimensional structure, and our
(14:25):
perception of time passing is just an artifact of our
consciousness moving along our world line through this lock. This
interpretation is philosophically challenging. It seems to eliminate free will,
suggest that the future already exists as concretely as the past.
Not all physicists accept it, but it's a natural consequence
of ticking relativities insights. Seriously, if time is relative, if
(14:49):
simultaneity is observer dependent, if there's no universal now, then
perhaps all moments are ontologically equivalent, all existing in the
eternal space time structure. Regardless of interpretation, time dilation points
towards something profound. Time travel to the future is not
only possible but inevitable. Every time you move, you travel
(15:10):
into the future. Relative to stationary observers, the effect is
usually too small to notice, but in principle, if you
could travel fast enough or descend deep enough into a
gravitational well, you could skip forward in time by any amount.
You could travel a year into the future, a century,
a millennium, simply by moving fast enough or hovering nearer
(15:31):
black holes of vent horizon for long enough. What you
cannot do, at least according to our current understanding of physics,
is travel to the past. Moving backward in time would
require exceeding the speed of light, which general relativity forbids.
It would also create causality paradoxes, situations where you could
potentially prevent your own time travel from occurring. Some exotic
(15:54):
solutions to Einstein's equations seem to allow closed timelike curves
paths through space time that loop back on themselves temporarily,
but these require conditions that likely don't exist in nature,
such as traversible wormholes held open by exotic matter with
negative energy density. So well, the future is accessible, the
past remains probably forever beyond reach. Time has a direction,
(16:20):
at least for practical purposes, even if that direction is
more complicated than simple universal flow from past to future.
Let me bring this back to something personal, something immediate.
Right now, as you listen to this, time is passing
differently for different parts of your body. Your head, slightly
(16:40):
farther from Earth's center than your feet experience is fractionally
less gravitational time dilation. Your head is aging faster than
your feet by about ninety billions of a second per year.
The difference is absurdly small, unmeasurable without atomic clocks, but
it's real. Every time you move, you affect how quickly
(17:01):
you age relative to people standing still. Take a flight
across the country at six hundred miles per hour, and
you age very slightly less than someone who stayed home,
though the effect is far too small to notice. Drive
to work, and your commute means you've experienced fractionally less
time than someone who walked. The universe is constantly adjusting
(17:22):
the rate at which time passes for you, based on
your motion and your position in gravitational fields. This seems
strange because our intuition about time is built on everyday
experience at low speeds and weak gravity. We evolved in
an environment where time dilation is negligible, invisible, inconsequential, So
(17:43):
we developed an intuition that time is absolute, universal, the
same for everyone. But that intuition is wrong. It's an
approximation that works well at human scales but breaks down
when we examine the universe more carefully. Time is personal,
time is relative. Your now is not mine now. If
(18:04):
we're moving relative to each other or experiencing different gravitational feels.
We each carry our own clocks, our own experience of duration,
and these clocks can and do disagree depending on our
paths through space time. The universe doesn't have a master clock,
a cosmic time keeper defining the correct rate at which
(18:24):
time passes each clock. Each observer has their own, equally
valid measurement of time. This realization, more than perhaps any
other discovery in physics, reveals how different the universe is
from our every day intuition. Time, which seems so fundamental,
so constant, so absolute, is actually flexible relative, dependent on circumstances.
(18:47):
It's one of the strangest features of reality, and yet
it's confirmed every day by experiments by GPS satellites, by
particle accelerators, by astronomical observations. Time dilation isn't speculation or science.
It's how the universe actually works. Right now is only
right now for you. Someone moving relative to you has
(19:08):
a different now. Someone in a different gravitational field has
a different now. The universe unfolds differently for different observers,
each experiencing their own path through space time at their
own rate, and understanding this really grasping What it means
for time to be relative changes how you see reality itself.
Time is not what you thought it was. It's stranger,
(19:31):
more flexible, more wonderful than the simple forward march of
seconds we imagine when we glance at our watches. Thank
you for listening to this episode of Blackhawn. If you
enjoyed exploring the relativity of time, please subscribe to stay
updated on future episodes. This episode was brought to you
by Quiet Please Podcast Networks. For more content like this,
(19:52):
please go to Quiet. Please dot ai Quiet, Please dot
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