Howdy Stargazers and welcome to the season finale
of Star Trails. My name is Drew and I'll be your
guide to the night sky for the week of November
30th to December 6th. This week we're building
a time machine, or at least looking at the science
that it might take to create one. Along the way,
we'll look at some famous time machines in pop
culture, plow through Einstein's two theories
of relativity, which provide us with the equations
to calculate our time travel efforts, and we'll
verge into some wild theoretical territory that
reads more like science fiction than fact. The
winter's night sky is nearly upon us and in the
second half of the show I'll preview what you
can expect to see in the night sky for the next
month while we're on a holiday break. Whether
you're tuning in from the backyard or the balcony,
I'm glad you're here. So grab a comfortable spot
and let's see what the universe holds for us
this week. As a kid growing up in the 80s, it's
safe to say time travel movies were some of my
favorites. There was Back to the Future, which
taught us about the dangers of creating a paradox
by going back in time. There was Star Trek IV,
The Voyage Home, which showed us we could go
back in time by slingshotting around the sun
in a Klingon battle cruiser. Oh, and that humpback
whales are apparently an intelligent alien species.
Who knew? In middle school we read the original,
The Time Machine by H .G. Wells, and I also became
a fan of the 1960 film of the same name. That's
the one where Rod Taylor manages to go forward
in time some 800 ,000 years in a brassed out
steampunk sled. Once he gets there, he falls
in love with an Eloi girl and presumably brings
her back home to civilized Victorian era London.
Talk about a goal to aspire to. And later, as
a teen, Bill and Ted's excellent adventure. A
gonzo trip through time with two California goofs
who gain access to a time machine to help them
write a history term paper, of all things. I
also have to mention Disney's Star Wars knockoff,
The Black Hole. Not because there was time travel,
but because of its willful ignorance of general
relativity. More on that in a moment. The 1980s,
especially as a child, felt like an era where
anything was possible if you threw enough science
at it, especially with the rise of computers
and advancements in space travel, like the space
shuttle. Magazines of that era were wild. As
a kid, I read Omni, Discover, Astronomy, Popular
Science, and more. And one day, after reading
an article about the possibility of time travel,
I decided to grab a pack of graph paper and start
designing my own interstellar ship. One that
used theoretical propulsion that could either
take us to the speeds nearing that of light itself,
or at least provide us with the means to reach
the event horizon of a black hole. I was so naive,
but I had the science to back it up. Reading
those articles in Omnia and Discover taught me
a few important things. One, you can't go back
in time, only forward. Two, if you could travel
near the speed of light, you can travel forward
faster in time. Three, if you could orbit a black
hole, you could go even further forward in time.
Sounds pretty easy, right? These time travel
parameters were already old news even in the
1980s. See, Albert Einstein figured this stuff
out back in the 1930s. He was Doc Brown way before
Michael J. Fox jumped into a DeLorean and fired
up the flux capacitor. So in this episode, we'll
examine the two surefire ways to dilate time
based on Einstein's special theory of relativity
and his general theory of relativity. The former
deals with high speeds. The latter deals with
regions of massive gravity. We know these principles
are accurate because we use technology linked
to it every day. Those GPS satellites that float
overhead that we use for navigation, they're
time travelers in a sense. They orbit fast and
high above our planet. So much that if we don't
factor in their time offsets using super accurate
atomic clocks, we'd be sent off course down here
on Earth. So if the science actually works, what
would it realistically take to make a time machine?
One we can ride around in and fly forward ages
into the future. Let's find out. Before we get
any ideas of going back in time to play with
dinosaurs, assassinate Hitler, or play Johnny
B. Goode at a prom to make our parents fall in
love, we need to know that time is a one -way
street. We're not going back in time, sadly.
Time only moves forward. This isn't philosophy,
it's physics. The arrow of time points toward
increasing entropy, toward disorder. toward future
moments that haven't happened yet. You age forward.
Your memories accumulate forward. The universe
expands forward. Einstein's equations might be
symmetrical, but everything we observe in nature
has a preferred direction. Backwards simply doesn't
work. Yes, there are some theories where time
may go backwards, but as you'll find out later,
all these theories generally have a giant loophole
that breaks them. The big takeaway is simply
this. The rate at which time flows is flexible.
You can stretch it. You can compress it. And
if you're clever, you can make your journey into
the future faster. That's where relativity comes
in. Special relativity tells us that the faster
you move, the slower your clock ticks relative
to someone staying still. Go fast enough, really
fast, and your timeline becomes syrupy. A five
-year journey for you might be decades for the
people left on Earth. The problem is the speeds
required are insane. Take the Parker Solar Probe,
for instance, the fastest object humans have
ever built. It falls toward the Sun at about
.058 % the speed of light. That's blistering
for us and virtually meaningless for space -time.
After years of flying, the Parker Probe has accumulated
about as much time dilation as you'd get from
a long nap. To get dramatic effects, the kind
that movies show, you need to be going a significant
fraction of the speed of light. 50 to 99 percent.
Right now, we don't have any technology that
could even begin to approach those numbers. Traditional
rockets forget about it. One proposed method
that's been theorized since the 1960s involves
detonating a series of nuclear bombs off behind
your ship and riding the shockwave. That was
Project Orion, and I mention it because it could
get us in the theoretical range of 5 % of the
speed of light. This project was a real government
-funded proposal decades ago, with the idea being
we could travel to Mars in weeks and stars in
decades. This was published everywhere in science
magazines during the early space age and it echoed
through sci -fi art and articles for years after.
Omni especially adored Project Orion because
it embodied that mid -century utopian futurism.
What if we used nuclear bombs to explore the
stars? And even physicists like Freeman Dyson
worked on it seriously. Strangely, this is the
propulsion method used by my middle school graph
paper time machine. Evidently, I read about Project
Orion somewhere. I also augmented my design with
something called a Bussard ramjet. This was another
theoretical propulsion scheme that involved a
giant scoop at the front of a starship to collect
wandering hydrogen, convert it to energy via
fusion, and propel the ship forward using the
nuclear exhaust. Sounds great, right? Infinite
fuel for a long journey. Heck, even Star Trek
borrowed this idea once. The problem is, interstellar
space contains hydrogen, but just barely. The
typical density is around 0 .1 atoms per cubic
centimeter. So if we do some math, we learn that
the Boussard scoop would need to be tens of thousands
of kilometers wide, maybe millions for the concept
to work. It's not practical at any scale. And
another problem arises. Eventually, this massive
scoop would encounter a certain amount of drag
as it rams into these hydrogen atoms, becoming
more like a parachute than a stellar supercharger.
So, assuming we managed to get our Project Orion
engine fired up and we could achieve 5 % of the
speed of light, what does that really get us?
Let's set our sails for Proxima Centauri, the
closest star to us outside of our own solar system.
Because if we're pushing towards the speed of
light, we might as well go somewhere interesting.
Proxima Centauri is 4 .24 light years away. Cruising
at 5 % the speed of light, it would take nearly
85 years to get there, one way. We're going to
use something called the Lorentz factor to calculate
the time dilation. This equation tells us that
if it takes us 84 .8 years to get to Proxima
Centauri, we'd only experience about 39 less
days than clocks back on Earth. So at 5 % of
light speed, you're doing something insanely
fast by any human standard. And relativity basically
says, cool story, here's a one month bonus. Time
is stubborn until you get really close to the
speed of light. And that is some sad astrophysics.
To make this fun, let's assume we can move at
99 % of the speed of light. Because remember,
we can never achieve the speed of light itself.
The journey to Proxima would still take 4 .28
years, but things get more intriguing when we
plug that into the Lorentz equation. At this
rate, we're about 4 .5 years younger than people
back home. And because our local time is slowing
down, Our one -way journey feels more like seven
months. If you could achieve 99 .999 % of the
speed of light, the effect would even be wilder.
At that speed, Earth would see the trip take
just over four years, but from our perspective
on the ship, it would only take about a week.
So a week for us and four years for everyone
back home. do a round trip, and Earth ages eight
and a half years while we age about two weeks.
At that point, you're not just traveling through
space, you're surfing along the edge of time
itself. So speed is one way to bend time, but
it's not the easiest way. Gravity does something
extraordinary. It slows down time. The deeper
you fall into a gravitational well, the slower
your clock runs compared to someone farther away.
This means that technically your head ages faster
than your feet, really. Sadly, nothing in our
solar system is massive enough to give you sci
-fi levels of time dilation. If you orbit the
Sun closely for a year, you might gain a couple
seconds compared to interstellar space, and you'd
probably fry. Orbit Jupiter and you might gain
a few microseconds. Real gravitational time machines
need something stronger, preferably a black hole
or a neutron star, where we can achieve substantial
dilation, minutes to hours in a day. But let's
get crazy. Let's journey to the center of the
Milky Way to the supermassive black hole there,
Sagittarius A. Here we can seriously mess with
time. Sagittarius A is four million times the
mass of the Sun. It curves space -time so violently
that time practically sticks to it like glue.
If we park our spacecraft just outside the innermost
stable circular orbit, a place where the tidal
forces won't tear us apart, our personal clocks
would slow dramatically compared to clocks back
on Earth. We calculate it using the Schwarzschild
time -dilation equation. Depending on the specific
orbit, one hour for me might equal days, weeks,
or even years back home. If you've seen the movie
Interstellar, you've seen this type of dilation
accurately depicted when the crew momentarily
landed on Miller's planet, then returned to their
ship to find years had elapsed for the scientists
who remained behind. Of course, there's one tiny
detail. Sagittarius A is 26 ,000 light years
away. Getting to the thing that lets you time
travel efficiently requires a level of time travel
we don't have. Remember when I mentioned Disney's
Black Hole movie earlier? Well, what bugs me
about that film is that they're orbiting right
at the edge of a black hole, seemingly ignorant
to its effects on time. Remember, Einstein worked
this out nearly 50 years earlier. Not sure it
matters because they all fall into the black
hole at the end of the film and the bad guys
emerge in some existential version of hell and
the protagonists pop out near a nice -looking
planet, presumably to repopulate Eden or something.
So why does this happen? Why does moving fast
or floating in a gravity well affect time? To
the universe, time isn't an independent thing.
Time is woven into space. It's literally part
of the geometry. When you move or when gravity
bends space -time, you're changing that geometry.
Your path through space -time gets stretched
or squeezed and your clock ticks along that path.
When you approach the speed of light, you're
using more of your motion budget to move through
space and less to move through time. Relativity
says there's a fixed speed you move through space
-time, always the speed of light, but divided
between space and time. If you move faster through
space, you must move slower through time. So
your personal time that is the time your body
experiences slows down But you don't feel it
to you everything feels normal your heart beats
normally your brain fires normally It's only
when you compare with someone who stayed behind
that you see the difference Gravity slows time
because it bends space -time Remember, general
relativity redefined how science sees gravity.
It isn't just a force pulling downward. It's
a curvature of space -time itself. The closer
you are to a massive object, the more space -time
is bent. And the deeper you sit in that curvature,
the slower time passes for you. That's why clocks
tick slower at sea level than on mountaintops.
It's why GPS satellites tick faster than your
phone. And near a black hole, space -time is
curved so violently that time practically congeals.
Even light struggles to escape. We understand
the how. The equations match experiments perfectly.
GPS, particle accelerators, atomic clocks flown
on planes, or nuclear -powered clocks on mountaintops,
and so on. But the deeper why is more philosophical.
The best answer physics gives us is because space
-time has geometry, and time is one of its dimensions.
Einstein didn't explain time dilation so much
as he showed that it's a natural consequence
of living in a four -dimensional universe, where
the speed of light is constant and mass bends
space -time. Once you accept those two facts,
everything else follows. Time slows near gravity.
Time slows at high speeds. It's all a form of
geometry. We don't know why the universe has
this geometry instead of some other one, but
we know that once you plug the geometry in, everything
behaves exactly as expected. So what use is a
time machine if we can't go back in time? There
are theories that enable this, but so far quantum
physics has sort of put its foot down. In the
last 15 or 20 years, physicists and quantum information
theorists have been exploring what happens if
you allow quantum bits to interact with versions
of themselves from the future, along a so -called
closed time -like curve. This isn't a proposed
machine, more of a thought experiment to test
the boundaries of causality, and the punchline
is always the same. The quantum math protects
itself from paradox. If you try to send contradictory
information into your own past, the equations
smooth it out. so no paradox forms. It's like
the universe is auto -correcting itself. Even
at the smallest scales, the quantum world, the
universe seems allergic to backward time travel.
One modern idea, the Novikov self -consistency
principle, says that even if you could go backward
in time, the universe wouldn't let you create
a paradox. Every choice you try to make in the
past loops around to become exactly the thing
that made time travel possible in the first place.
It's cosmic predestination. The past is locked,
the future is open, and time refuses to contradict
itself. Another modern quantum theory leaves
a tiny conceptual crack open. You might not be
able to visit your own past, but you could, in
theory, visit the past of a different branch
of the universe, one where your arrival doesn't
break causality. It's not time travel in the
traditional sense. It's more like switching tracks
on a cosmic railroad made of probability. So
far, this idea has been untestable. I can't imagine
why. For those of us stuck down here on Earth,
the best time travel we can do is simply observe
the universe. Feel like I've been saying this
a lot lately, but every time we look up we're
looking back into the past Sometimes the recent
past like the light leaving Jupiter is 40 minutes
old Light from our Sun is eight minutes old Sometimes
we look at the deep past like the ancient glow
of galaxies whose first stars fired long before
Earth even formed The night sky is a map of moments
that have already happened, still streaming towards
us across the vastness. We may never build a
starship that reaches 99 .999 % of the speed
of light. We may never orbit a supermassive black
hole and return home decades ahead of the people
we left behind. And we certainly won't be sitting
on Victorian chairs with bejeweled levers racing
off to fight the Morlocks, ducking into a police
call box or driving a DeLorean like we stole
it to rewrite history. It's not about a machine
so much as a perspective. Relativity teaches
us that time isn't a singular river, but more
like thousands of branching streams. And when
speed or gravity gets involved, those streams
can meander. Ultimately, they still all run forward.
So if you really want to experience time travel,
go outside tonight. Look up. Find a star. The
moment you're seeing left that star years ago,
sometimes centuries, sometimes millions or billions
of years ago. You're reaching backward with nothing
but your eyes and the physics of the cosmos.
those are ancient photons from the past hitting
our retinas in the present. In the second part
of this episode, we'll explore what's going on
overhead for the month of December. That's coming
up after the break. Stay with us. Welcome back.
Star Trails is taking a breather in December,
but that doesn't mean the sky is taking a month
off. Far from it. December is one of the richest
observing months of the entire year. So here's
your December sky strategy. Think of it as a
guidebook for the whole month. Three simple beats
to follow. Bright moon season, meteor season,
and deep winter season. December starts with
a burst of brightness. We get a full supermoon
on December the 4th, the final supermoon of the
year, traditionally called the cold moon. It'll
look big and bold, climbing the eastern sky just
after sunset. A week later, on December 11th,
the moon hits third quarter and lights up the
sky. But stick with it, because the sky gets
wonderfully dark by December 19th, when we hit
the new moon. The new moon window will be your
best opportunity for deep sky observing. And
then the month closes out with a first quarter
moon on December 27th, a bright half moon riding
high in the afternoon and evening sky. December
also gives us two meteor showers. One is spectacular.
The other is subtle, but atmospheric. First,
the Geminids. They're active from December 4th
through the 20th, and they peak on the night
of December 13th, into the early morning of the
14th. The moon will be just a thin waning crescent
rising late so the sky will be beautifully dark.
Under good conditions, the Geminids can deliver
more than 100 meteors per hour. If you're going
to commit to one night this month, make it Geminid
night. Face east, get comfortable, and start
watching around 10 p .m. The ursids are the second
meteor shower of the month. These are active
from December 13th through the 26th, and they
peak right around the time of the solstice. The
ursids are gentle, maybe five to ten meteors
an hour. Look north toward the Little Dipper,
and let the longest night of the year do the
rest. December's planetary lineup is straightforward.
Mercury is the prize for early birds. It has
one of the best morning showings of the entire
year. Look low in the southeast about 45 minutes
before sunrise from December 1st through the
23rd with the greatest visibility around December
7th. Jupiter is the showstopper of the month.
Bright, obvious, and unmistakable in Gemini in
the early evening. If you have a telescope, Jupiter
is your December anchor. Belts, zones, storms,
and the dance of its four big moons every single
night. Saturn hangs in the southwestern sky during
the early evening hours. It's fading, sinking
earlier each night, but still makes a wonderful
warm -up target in Aquarius before you turn your
attention to the winter constellations. Venus
and Mars are both tucked too close to the Sun
this month. If you're hunting planets, your trio
is Mercury at Dawn, Saturn at dusk and Jupiter
all night. Now for the real magic, the winter
sky itself. By 9 or 10 p .m., the whole eastern
sky is glowing with the constellations that define
the season. There's Orion rising with his belt
leading downward towards Sirius and Canis Major,
the brightest star in the night sky. Above Orion
is Auriga with brilliant Capella. To the left
are Castor and Pollux in Gemini. overhead, drifting
westward, the Pleiades sparkle like a tiny jewel
box. If you want to keep things simple during
December, here's an easy plan. Session 1, early
December around the full supermoon. Watch the
cold moon rise. Use its glow to trace the winter
constellations. Let Jupiter and Saturn orient
you in the evening sky. Session 2, the Geminid
weekend, December 13th and 14th. Block out one
night, go somewhere dark if you can, bundle up,
lean back, and let the meteors pour out of Gemini.
Session 3 Solstice night, December 21st and 22nd.
Take advantage of the new moon darkness. Check
out the Orion Nebula, the Pleiades, the clusters
in Auriga, and then, after midnight, turn north
for the soft drizzle of the Ursids on the longest
night of the year. If you manage even one of
these sessions, you'll have a memorable December.
We'll see you again in January. I hope you all
have a great holiday and maybe some of you will
unwrap a new pair of binoculars or a smart scope.
If you do, let me know. If you found this episode
interesting, please share it with a friend who
might enjoy it. The easiest way to do that is
by sending folks to our website, StarTrails .Show.
And if you want to support the show, use the
link on the site to buy me a coffee. It really
helps. Be sure to follow Star Trails on Blue
Sky and YouTube. Links are in the show notes.
Until we meet again beneath the stars, be excellent
to each other. And party on, dudes!