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To repeat the Space Time maxim: It's never aliens...
... until it is!
So, let's talk about 'Oumuamua.
Last year, a visitor came to our inner Solar System.
To us, it looked like a faint spot of light, moving quickly relative to the fixed stars.
We'd seen similar things countless times before:
lonely rocks, remnants of the formation of our Solar System,
drifting on their endless orbits around the Sun, and faintly glimmering with its reflected light.
Nothing unusual about such asteroids and comets,
but what made this object special was its trajectory.
It was not in orbit around the Sun at all.
Rather, it appeared to have fallen in from interstellar space, and was on its way back out again.
The object is 1I/2017 U1 -- better known as 'Oumuamua.
It's the first chunk of interstellar space debris we've ever spotted passing through our Solar System.
It caused a justifiable stir at the time -- hell, even we did an episode on it!
But 'Oumuamua has been making the news again.
This time, it's for a new paper by Schmuel Bialy and Avi Loeb that investigates the possibility that 'Oumuamua is...
... aliens!
Or, more accurately, an artificial light sail, presumably attached to an alien probe.
Today on Space Time Journal Club, I want to do two things.
First, assess the plausibility of this proposal, and second, talk about the media hype around it.
First up, here's what we know about 'Oumuamua:
Its changing brightness suggests it's a highly elongated object, tumbling through space.
It appears to be a few hundred meters long, but only a few tens of meters across.
It may or may not also be flat.
It's a space baguette, perhaps after going through a space sandwich press.
Initially, its reddish color looked comet-like, but then it didn't produce a cometary tail,
so it was assumed it must be more rocky or asteroid-like, at least on the surface.
A couple of strange things right there!
The geometry is very unusual for an asteroid or comet.
It's also a little surprising that we should have seen an interstellar object at all.
We certainly expect there to be a bunch of space junk floating between the stars,
probably ejected in the violent early stages of formation of planetary systems.
But in order for our encounter with 'Oumuamua to have been likely, that population needs to be huge --
much more interstellar debris than our theoretical models predict.
Those things are odd, but not odd enough to start assuming aliens.
But now that 'Oumuamua is on its way out of the solar system, it's doing something even weirder:
It appears to be accelerating.
Or, at least it's not slowing down as much as you'd expect due to the Sun's gravitational pull.
This non-gravitational acceleration was reported in a nature paper in June.
The extra acceleration is tiny, but it's clear.
Something is pushing the body outwards.
By itself, this wouldn't be anything especially weird.
Comets show the same sort of acceleration on their way out from the Sun.
In their case, it's a result of outgassing.
As the Sun heats the rear surface of the comet, water ice and frozen gases -- so-called volatiles -- evaporate.
The ejected gases act like jets to propel the comet forward.
In regular comets, the outgassing is typically visible as sunlight reflects off dust carried along with the jets,
and we see it as the comet's coma and tail.
But no coma or tail was observed in 'Oumuamua.
Also, outgassing should probably change the way the object is tumbling, but 'Oumuamua's tumbling appears consistent.
So... duh... aliens, right?
So proposed Bialy and Loeb of Harvard University.
To be fair, in the paper itself, these authors don't say it's aliens, or even that it's probably aliens.
They just do a few calculations to show the plausibility of one particular explanation:
that 'Oumuamua is a light sail.
This is the argument:
Something is accelerating this object with no visible coma or tail.
It doesn't look like outgassing, so maybe it's radiation pressure.
The Sun's own light could be pushing on the object, speeding it up.
In fact, the observed acceleration seems to be following an inverse square law.
The acceleration is decreasing with the square of the distance from the Sun.
Now, that's exactly what you'd expect for solar radiation pressure.
Incidentally, this inverse square acceleration is also what you'd expect from outgassing,
because outgassing also depends on the strength of the Sun's radiation.
But the radiation pressure thing would only work if 'Oumuamua is incredibly thin and light --
thinner and lighter than any conceivable natural space object.
Ergo... aliens!
Bialy and Loeb calculate that a 50 meter wide sheet less than a millimeter thick
and made of a highly reflective material would produce the observed brightness and acceleration.
That sounds like a light sail.
This is technology that we are playing with already.
The Japanese IKAROS light sail flew to Venus in 2010
and the Breakthrough Starshot program is planning to use a light sail as our first interstellar probe.
The light sail hypothesis potentially explains the acceleration and the weird shape.
It may also explain why we saw something at all.
Given that natural space debris the size of 'Oumuamua should be much rarer between the stars,
what if this wasn't a chance encounter?
What if aliens sent this probe to us intentionally?
Let's review the evidence.
'Oumuamua appears to be accelerating out of the solar system in a way completely consistent with outgassing,
except there's no visible outgassing.
'Oumuamua is weirdly shaped for a natural body,
and there shouldn't be enough interstellar space junk for something like 'Oumuamua to have been spotted by chance.
Regarding these unusual qualities, co-author Avi Loeb is fond of quoting Sherlock Holmes:
"When you have excluded the impossible, whatever remains, however improbable, must be the truth."
The implication of that statement is that all other possible explanations for 'Oumuamua's weirdness have been ruled out.
So, have they?
Let's look at the non-alien explanations of 'Oumuamua's behavior.
The acceleration is the weirdest thing, so we'll start there.
Outgassing is the obvious alternative, but then, why don't we see it?
Well, I mentioned that in regular comets the coma and tail are seen through sunlight reflected off dust --
tiny mineral grains that are carried out with the vaporizing ice.
Without reflective dust, outgassing would be invisible to our telescopes, but could still cause the observed acceleration.
The paper that first reported the unusual acceleration proposes some plausible scenarios.
Perhaps the object has a low abundance of dust.
Or, perhaps it has unusually large dust grains that'd reflect less light than lots of smaller grains.
Or, perhaps the surface material has been fused into a crust by millennia of exposure to interstellar cosmic rays.
The lack of visible outgassing is weird, but frankly, it's not that weird.
Slightly weirder is that the tumbling motion doesn't seem affected by outgassing.
But until someone does a proper calculation to show that tumbling must change in an observable way,
this is not a serious argument.
Then there's 'Oumuamua's shape. How do you get such a weird shaped rock?
Well, different authors have suggested different scenarios.
Perhaps it was tidally disrupted -- pulled apart by the star or a gas giant from its home system,
much like the comet Shoemaker-Levy.
And then, it may have gravitationally reassembled itself into an odd shape on that very long interstellar journey.
Loeb himself proposes a similar explanation in an earlier paper,
adding that perhaps it broke itself apart due to its rapid rotation before reassembling.
And finally, what about the sheer unlikelihood of a random encounter with an interstellar space rock?
Well, in order for our spotting of 'Oumuamua to have been likely,
interstellar space needs to contain something like 10 to 100 times the number of 'Oumuamua-like space rocks than we expect.
And by that, I mean the expected density of space rocks that are ejected during the planetary formation process.
There are three counter-arguments:
First, there could be more space debris than we expected.
For example, it may be that stars release their Oort clouds when they die.
If every star that passed before us shed its vast cloud of comets into the galaxy,
that would make plenty enough interstellar trash to explain 'Oumuamua.
Second explanation:
Maybe we just got lucky with 'Oumuamua.
Maybe interstellar space junk is rare, but we caught a 0.1 or 1 or 10 percent chance.
And finally, perhaps 'Oumuamua isn't from outside our solar system at all, but rather comes from our own Oort cloud,
and was socked from its orbit by impact and/or gravitational interaction into a trajectory that will send it to the stars.
We can comfortably say that natural explanations for 'Oumuamua are still plausible.
Can we really say the same about the unnatural explanation of an alien light sail?
One thing the light sail doesn't explain is the tumbling motion.
Certainly, a working probe shouldn't be spinning end over end, so perhaps it's a broken probe.
But if so, then we have the same abundance problem as with natural interstellar debris.
Interstellar space would need to be filled with broken light sails -- something like 10^15 probes per star in the Milky Way.
The other issue with the light sail hypothesis is the speed.
'Oumuamua was traveling at 26 kilometers per second before it entered our Solar System,
and that's a pretty typical speed for a random object orbiting in the Milky Way.
On the other hand, the Breakthrough Starshot light sails are aiming at 20% light speed,
for a 20 year journey to the Alpha Cent. system.
It's been a million years since 'Oumuamua's last stellar encounter.
So why build a probe that travels at such a snail's pace, surely beyond the lifespan of any aliens who sent it?
Especially when the technology to accelerate a light sail to a good fraction of the speed of light
is within the grasp of even us primitive humans.
It's pretty safe to say that any unnatural explanation for 'Oumuamua is still less likely than a natural explanation,
and honestly, I suspect that Bialy and Loeb know that.
In their paper, they don't claim that an alien light sail is the most likely explanation -- just that it's a plausible one.
The issue isn't that they did the analysis.
Scientists should be free to pursue fringe ideas, as long as they do it scientifically.
The issue is how this idea ended up being communicated to the public.
So let's talk about that.
The light sail hypothesis for 'Oumuamua was first presented by Avi Loeb in late September, in a Scientific American article.
A month later, the Bialy and Loeb paper appeared on the preprint arXiv,
and a few days after that, Centauri Dreams and Universe Today wrote about it.
Over the next couple of weeks, many media outlets picked up the story.
Some of those articles were measured, and some were even critical,
but most were giddy at the possibility that we'd discovered an alien craft.
The credentials of the authors were blared in the headlines:
"Harvard astrophysicists... blah blah blah... alien probe."
While nothing in these articles was untrue, it's arguable that there's some dishonesty here.
The appeal to the authority of the authors seems designed to provoke the reaction,
"Well, if Harvard scientists say so, then it must be true."
The author responses in media interviews do not downplay the likelihood of aliens.
For example, that Sherlock Holmes quote.
So, my question to you:
Is it harmful to emphasize an unlikely scientific interpretation beyond its probable veracity,
just for the sake of clicks and attention,
or is any engagement of the public with science ultimately a good thing, as long as there's no clear harm?
Honestly, I don't know the answer,
but I do know that 'Oumuamua is one of the most fascinating discoveries in astronomy in recent times.
Whether or not it's aliens, it's likely our first encounter with the natural or unnatural denizens from interstellar space time.
Are you interested in learning more about astronomy?
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Last week, we talked about the possible detection of what may be the first supersymmetric particle.
Now, before I get to comments, I need to make one big correction.
Thanks to Gustavo Valdiviesso for pointing this out.
I said that extremely high energy neutrinos aren't expected to make it all the way through the planet.
Hence the surprise at seeing the decay products of a high-energy tau neutrino
that had appeared to pass right through the center of the Earth.
This ain't right!
Neutrinos have no trouble passing through the planet, whatever their energy.
What changes is neutrino oscillations.
As a neutrino travels through empty space, they [sic] oscillate between the three types:
electron neutrino, muon neutrino, and tau neutrino.
The rate of oscillation depends on the tiny mass of the neutrino.
When passing through matter, interactions with the electromagnetic field change the effective mass of the neutrino
by a process analogous to the refractive index, which changes the speed of light in matter.
This, in turn, changes the probabilities of oscillation, and suppresses oscillations to tau neutrinos.
This is the Mikheyev-Smirnov-Wolfenstein (or MSW) effect.
So, it's not that neutrinos don't make it through the planet,
it's just that they don't oscillate into tau neutrinos or stay as tau neutrinos.
So seeing a tau neutrino coming from directly below is incredibly unlikely.
Hence, the hypothesis that it was an electron or muon neutrino that interacted to produce a supersymmetric stau particle.
tu-nonna-emiliana asks: if SUSY is easy (just add an S to the start), then how about Sstring?
Well, actually modern string theory relies on supersymmetry, so it's already included.
Was it called "ting theory" before that?
Actually, the name string theory originally applied to what we call bosonic string theory.
It just explained the force carrying bosons like the graviton and the photon.
To encompass fermions, supersymmetry is needed. The result is superstring theory,
and these days some form of superstring theory is implied when people talk about string theory.
Many of you noticed a typo in the segment of the Wikipedia text that we put on screen for the hierarchy problem.
It appeared to say that the weak force is 1,024 times stronger than gravity.
Some of you took that to be evidence for the simulation hypothesis.
But actually, it's just that the exponent got lost in the cut and paste.
It should have been 10 to the power of 24 times stronger.
Hiccup Haddock asks:
Why ice?
As in: Why does IceCube use photon detectors in ice instead of any other material?
Well, the answer's simple enough: Ice is transparent(ish).
Neutrinos are detected by the Cherenkov radiation produced by particles produced in neutrino collisions.
That radiation travels a short distance through ice to the nearest photon detector.
The same interactions happen in rock, but the Cherenkov radiation doesn't get very far.
You could use transparent materials other than ice.
Liquid water is common, for example, in the Super-Kamiokande neutrino detector in Japan.
I guess you could use a giant cube of glass or similar, but that sounds expensive.
Many of you are impressed by Ice Cube's incredible contributions to particle physics,
and how the guy has had such a diverse career.
Thanks to infectedstyles and David M for reminding me of Ice Cube's classic,
"Straight Outta Lepton", during his "Neutrinos With Attitude" years.
And some of you thought my "or is it" was very Vsauce.