 Thank you to Curiosity Stream
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Studios.
 We are currently being
visited by a traveler
from outside our solar system.
This is the first
time we've ever
seen an asteroid
like object that came
to us from interstellar space.
Today, on Space
Time Journal Club,
we'll see what
mysteries it can unlock.
On October 19th this
year, astronomers
spotted an unusual object moving
rapidly away from the sun.
The discovery was made by Pan
Starrs, the panoramic survey
telescope and rapid response
system, a group of telescopes
that constantly monitors
the sky for moving
or variable objects like
asteroids or comets.
This object was initially
thought to be a new comet.
But after 34 days of
follow up observations,
and a bit of orbital mechanics,
it became abundantly clear
that we were looking at the
first object ever observed
that came from outside
our solar system.
The mysterious object
has been dubbed
1I, the first object in a brand
new International Astronomical
Union class, with I
standing for interstellar.
1I also scored a
more poetic name.
Oumuamua, from the
Hawaiian meaning
a messenger from
afar arriving fast.
Now this object was first
spotted several weeks ago.
But by now, we've
had time to study it
with many of the world's
great telescopes.
For this Space
Time Journal Club,
we'll be discussing what
we've learned about Oumuamua
since its discovery.
Our focus will be on a
paper by a Dutch team
from Leiden Observatory,
Portegies Zwart, et al., 2017,
PZ 17 for short.
This paper, which is still
in the process of being
peer reviewed, looks
at the possible origins
of this strange object.
Before we get to its origins,
let's look at the object
itself.
It's pretty weird.
It's highly elongated, like
a cigar or the monolith
from "2001: A Space Odyssey."
Hundreds of meters long,
it's size and shape
can't be determined
directly from the images.
It just appears as a faint dot.
Even to our most
powerful telescopes.
However, it varies in brightness
with an irregular period.
And astronomers
realized that this
is consistent with an elongated
body with a tumbling motion.
We've seen such tumblers
in our solar system.
And it suggests a major
collision in the object's past.
Oumuamua doesn't have
a tail like a comet,
so its surface isn't vaporizing
in the sun's radiation.
That suggests the
surface to at least one
meter depth is rocky
like an asteroid
rather than icy like a comet.
Or it could be the flickering
exhaust jets of an alien scout
ship returning to report a
planet ripe for invasion.
But no, it's never aliens.
The weirdest thing about
Oumuamua is its motion.
Every previously observed
asteroid-- indeed,
every previously observed
everything in our solar
system--
moves in elliptical orbits
as governed by Kepler's laws.
The eccentricity of
the orbit measures how
stretched out the ellipse is.
Eccentricity 0 is
a perfect circle.
And eccentricity is less than
one, our elliptical orbits.
Meaning the object is
gravitationally bound
to the sun.
The earth, for example, has an
eccentricity of 0.0167, giving
us a nearly circular orbit.
While Halley's comet has
an eccentricity of 0.967.
That's an extremely
stretched out ellipse.
Objects with eccentricities
greater than 1
follow unbound hyperbolic paths.
And they never
actually orbit the sun.
Rather, they're deflected by the
sun's gravity as they pass by.
Oumuamua has an
eccentricity of 1.2,
meaning its hyperbolic
path will take it out
of the solar system.
Another way to think of this
is in terms of escape velocity.
That's the velocity
an object would
need to have to escape
a gravitational field.
Oumuamua had a maximum speed
of 87.7 kilometers per second
at its closest approach
to the sun, which
is well inside Mercury's orbit.
The escape velocity at
that closest approach
was a little over 80
kilometers per second.
That means Oumuamua
has enough speed
to climb out of the
sun's gravitational well
and escape back to
interstellar space.
So where did this
object come from?
And why is it moving so fast?
PZ 17 investigate
three hypotheses.
The first is that it originated
in our Kuiper belt Oort cloud.
Now objects falling in from
these regions, like comets,
only pick up enough
speed to bring them
back to where they started.
But perhaps Oumuamua got some
sort of gravitational kick
from an unknown planetary body.
PZ 17 performed
computer simulations
to find the frequency of
such kicks sending objects
close to the earth
at such high speeds.
They conclude that it's
exceptionally unlikely.
The simplest explanation
for Oumuamua's solar system
escaping speed is that
it gained that speed
by falling into the solar
system from outside.
So, hypothesis number
two is that the object
was ejected and flung towards
us from a nearby star system.
PZ 17 performed more
computer simulations
to rewind the motion of both
Oumuamua and the 3,700 stars
within 100 light
years of the sun.
They found that the object
passed through the Oort
cloud of another
star, the unpoetically
named TYC4742-102701 around
1.3 million years ago.
But its speed
relative to that star
would have been over 100
kilometers per second, higher
than the escape velocity at the
distance of closest approach
to that star.
So Oumuamua was probably only
a visitor to that solar system
too.
Unless of course there are
aliens in that solar system
throwing rocks at us?
No, it's never aliens.
A third possible
origin for Oumuamua
is that it's been traveling
for a very very long time.
It may come from a vast
population of random debris
floating around in
interstellar space.
People have hypothesized about
the existence of such objects.
And with good reason.
In planet formation models, lots
of chunks of matter, and even
some planets, get ejected
from the relatively violent
protoplanetary disk.
Given how many stars
there are, there
should be a ton of asteroidal
objects floating around
in interstellar space.
Our sun, as it moves
around the galaxy,
passes through this
field of debris.
PZ 17 conclude that this
is the most likely origin
for Oumuamua.
They dubbed this type
of unbound non cometary
object a lonely
rock, or sola lapis.
In their paper, they hopefully
give the Klingon translation,
mob nagh.
Based on this one possible
chunk of interstellar debris
that we've found so far,
and on the volume of space
scanned by Pan Starrs,
PZ 17 extrapolate
to estimate the density
of the debris field.
They get that in order for
us to have seen this one
object in the five years we've
been watching with Pan Starrs,
there must be roughly
700 trillion objects
per cubic parsec in
the solar neighborhood.
Now that's higher than
the density of comet
like objects in the Oort cloud.
And based on this, PZ 17
predicts that two to 12
of these interstellar
objects should
pass through our solar
system inside Earth's orbit
every year.
The main reason we've
only spotted one so far
is that most don't get close
enough to the earth for Pan
Starrs to detect.
Oumuamua got pretty close.
Only 18 million kilometers
at closest approach.
But even then, Pan Starrs was
only just able to spot it.
Anything smaller or more
distant would be missed.
The good news is that
future telescopes
will be much more sensitive.
For example, the Large Synoptic
Survey Telescope, LSST,
which is currently under
construction in the Anacondian
Andes in northern Chile, with
first light planned for 2019.
It'll photograph the entire
night sky every few nights.
LSST will be able to see
objects around 14 times fainter
than Pan Starrs.
So we should expect to find
many more of these sole lapids.
What, then, of our
visitor, Oumuamua?
Is this the end of its story?
Well, it'll leave our
solar system behind
in roughly 20,000 years.
But, it'll be invisible
to our telescopes
within a month or two.
Its trajectory we'll send
it towards the constellation
Pegasus, perhaps to find a new
star system there to visit.
In the meantime, it joins its
countless interstellar cousins,
orphaned planetary debris,
stretching across the reaches
of space time.
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Time during the login process.
[INAUDIBLE] Leonard
asks whether a particle
can have momentum
higher than its mass
times the speed of light.
In other words, shouldn't there
be an upper limit to momentum
if the speed of
light is limited?
Well actually, momentum
can be arbitrarily high,
even when speed is limited
to the speed of light.
The equation for
momentum, P = M times V,
only works at low speeds
approaching the speed of light.
And you have to divide that
MV by the Lorentz factor.
That factor, the
square root of one
minus V squared on C
squared, approaches 0
as velocity approaches
the speed of light.
That means momentum
approaches infinity
for any object with mass that's
approaching the speed of light.
William Smith wrote
a good question.
How can a photon's frequency
be generalized as momentum?
Does frequency then
include information
about direction of motion?
Well we generalize
frequency as momentum
because the Heisenberg
uncertainty principle
applies to momentum in general.
In the case of matter,
uncertainty in momentum
can manifest in both
velocity and mass.
But photons have constant
speed and no mass.
So that uncertainty is
all in their frequency.
And yes, also in their
direction of motion, which is
separate to frequency.
That last fact explains
the increasing spread
in the direction of
photons after they
pass through a narrowing slit.
By increasing our
certainty in the location
of the photon passing
through the slit,
we increase the uncertainty
in its momentum and hence,
its direction of motion
after it exits the slit.
[INAUDIBLE] 777 asks, if
you try to do a Fourier
transform of PBS Space Time,
do you get PBS infinite series?
Actually, I think you need an
infinite series of PBS Space
Time to get a Space
Time localization of PBS
infinite series.
But I'm not certain.
