In the perhaps not too distant future, every radio telescope on Earth receives the same massive data
dump from what can only be an advanced alien species. It seems to contain the detailed
instructions for building a faster-than-light propulsion device, along with a reference manual
containing what amounts to all physics known by the aliens - including the equations of
their laws of physics and a list of all elementary particles and their properties and interactions.
If we can correctly decipher this stuff, we can build warp-ships
and join the galactic civilization. Of course if we get it wrong we might blow up the planet.
How hard can it really be to decode alien physics and engineering?
It’s gotta map to our own physics - I mean, we live in the same universe after all.
We start by noticing that the alien technology seems to use good ol’ fashioned electronics,
even if it is insanely complex. We know this because the particle carried by the
alien circuitry looks like the electron. We decide this through a process of elimination.
The aliens describe two sets of spin-1/2 particles, each with 3 generations - these
must be the quarks and the leptons, which are distinguishable from each other by
their relative masses, color charge, etc. The particle running through the alien circuitry
is the lightest lepton and has negative electric charge. Obviously the electron.
Actually, no. Unbeknownst to us, these aliens use positronic circuitry, not electronic. It’s just
that they define electric charge in the opposite way - electrons positive, positrons negative.
But how could we make such a blunder? Surely, a quick glance at the alien Maxwell’s equations
which govern the interactions of charged particles will reveal that they use a flipped
sign convention. Nope. It turns out that not only is th e sign convention arbitrary,
but there’s no way to tell which was chosen based on the laws of physics alone.
So we build the device with its incorrect electronic circuitry.
The President exerts some pressure to switch it on already because
an election is coming up. But just before we flip the switch and destroy the planet,
a few cautious scientists take one last look at the alien documentation. So what do you think?
Is there anything in the way the aliens wrote down their laws of physics that might reveal our error?
Before we figure that out, let’s recall how we humans decided on our own electric charge sign
convention. It was a pretty arbitrary choice based on a mistake. Made by this guy, Benjamin
Franklin. The nature of electricity was just one among Franklin’s many scientific interests.
By Franklin’s time, we already knew that if you rub a glass rod with a piece of cloth,
both gain an electric charge. Franklin was the first to guess
that both rod and cloth gain the same type of charge, just with opposite signs to the charge.
He imagined an electric fluid that flowed from one to the other, so that in one you
have an excess of fluid and in the other an absence. Then the direction of the force
comes from multiplying the signs of the density of this fluid together - bring together two positive charges or
two negative charges and you get a net positive force - which means a repulsive force by the way. But
a negative and positive charge brought together leads to a negative force - and that is an attractive force.
The choice of sign convention actually doesn’t matter as long as you stay consistent. Which
is lucky, because the reasoning behind Franklin’s choice was wrong.
He noticed that when you rub the glass rod with a piece of cloth, you can get a
more powerful shock from the rod compared to the cloth. He imagined the electrical fluid
building up in the rod and flowing into your hand - so he defined the rod's charge as positive.
We now know that it’s the other way around. Electrons from the rod are actually
rubbed off onto the cloth, so there’s a flow of matter in the opposite direction.
You get a stronger shock from the rod because the charge moves around more easily inside the glass.
That means a large charge difference builds up in the rod near your point of contact,
and that causes electrons to jump from your hand into the rod.
This was in the early part of the 1700s. It was a century and a half later before we actually discovered that
the electron existed. That discovery finally allowed us to figure out which way this “fluid” was really flowing.
But by then it was too late; what we now call the Franklin convention was fully entrenched.
Electrons came to be defined as having negative charge, and that defined the
relative charge of everything else - including the positive charge for protons and positrons.
Happily, Benjamin Franklin’s misguided choice doesn’t make any difference to the laws of
electromagnetism - the Maxwell’s equations. Those equations tell us how particles with
electric charges respond to each other. They don’t care what names we give those particles.
Had Franklin defined charge the other way around, Maxwell’s equations would be unchanged.
But this is bad news for our fictional scenario because we can’t rely on the equations describing
the laws of physics to determine the alien sign convention for electric charge.
Our problem is compounded by the fact that matter and antimatter seem to be identical except for
having exactly opposite charges. The universe is mostly symmetric under charge conjugation - switch
all matter with antimatter and vice versa and the universe would keep ticking along almost exactly
the same. But not quite exactly. It’s clear that the universe itself knows the difference between
matter and antimatter; after all, it contains almost exclusively the former. And that fact
tells us that somewhere buried deep in the laws of physics is a subtle breaking of charge symmetry.
So in order to save the world from poorly assembled alien technology, we need to find a single
manifestation of this symmetry breaking. The classic example is the decay of the kaon particle,
which we’ve talked about in detail previously. The long story short is that this unstable
meson decays very differently compared to its antiparticle. So as long as our compendium
of alien physics describes the interactions of specific particles, we can distinguish
matter from antimatter in the case of kaons. That breaks the frustrating symmetry of the
entire alien system of physics, allowing us to identify its charge sign convention,
distinguish positrons from electrons, and hopefully not explode the planet.
But maybe we’re not quite there yet. The Franklin convention for the sign of electrically charged
particles is not the only arbitrary choice we’ve made in our development of the laws of physics.
What if alien physics made other choices differently? Let's say that this rather finicky device
also cares about the handedness of the positron. Weirdly, there are right-handed and left-handed
versions of all particles with quantum spin. If we build the device with positions that
are spinning the wrong way then, I dunno, maybe we create a black hole that eats the
planet. But the definition of handedness also depends on an arbitrary convention,
just like the sign of electric charge - in this case, something called the right-hand rule.
Let’s put aside quantum spin for a minute to define this rule. Say you have an electron moving
through a magnetic field. That field is going to apply a force in some direction to that electron.
To find that direction you just point the index finger of your right hand in the direction the
electron is moving and your middle finger in the direction of the magnetic field, then your thumb
will be pointing in the direction of the force. ￼
Mathematically we calculate the force using something called the cross product. Force equals
charge times velocity cross magnetic field. This is a type of vector multiplication where
the direction of the final vector - in this case force - can be found using the right hand rule.
Cross products are everywhere in physics, for example in calculating torque, angular momentum,
and through Maxwell’s equations.
The right hand rule is another arbitrary convention, and its choice also determines how we
define particle handedness or partical chirality. What if our aliens were predominantly left-handed
or left tentacled or whatever? They might define chirality in the opposite way,
with devastating existential consequences. Fortunately,
this switch in convention does seem to change the actual equations of the laws of physics. To make
a LEFT-hand-rule work you need to either flip the order of the two vectors in the cross product,
so field cross velocity instead of the other way around, OR you need to add a minus sign in front
of the equation. So it sounds like we could figure out which convention the aliens used just by
looking at their Maxwell’s equations. But actually no - at least not without other information.
To understand why, imagine that all right hands in the universe were transformed into left hands and
vice versa. In other words, imagine reflecting the whole universe in a mirror. We call such
a transformation a parity inversion. Parity is another fundamental and broken symmetry of nature
like charge, and just as with charge symmetry, parity is related to an arbitrary convention - the right hand rule.
In fact charge and parity are intimately tied. I just told you that you can undo the effects
of using the left-hand rule by placing a minus sign in front of your magnetic force equation.
Well that minus sign can equally be interpreted as a flip in the sign of the electric charge in that equation.
If you change the convention for parity by using the left-hand-rule AND you change the convention
for charge by flipping its sign then the differences cancel out and you’re left with the
same laws of physics. So can we figure out what parity convention our alien benefactors are using?
Well, only if we also know their sign convention. But if we don’t know that then
there’s no way to tell from the master equations themselves. But
just as with charge, if we dig into the way particles interact we can find a clue.
Parity is the symmetry between left and right handed chirality for particles with quantum spin,
and in our universe P-symmetry is broken in much more obvious ways than charge symmetry.
That’s because the Weak Force only interacts with left handed particles and right handed
anti-particles, not at all with right-handed particles nor left-handed antiparticles. If we search the
alien user manual for the part that describes weak force interactions we should be able to figure out
which handed rule the aliens are using. But again, this depends on our ability to distinguish matter
from anti-matter, so we still need to know which sign convention they use for electric charge.
The combination of charge and parity transformation - appears to bring our equations
back to where they started. This is CP symmetry. A mirror-reflected, antimatter universe has the
same laws of physics as ours. But actually, such a universe works differently in very,
very subtle ways. That universe works just like our universe if you reversed the flow of time.
The final symmetry and the final convention that we need to figure out is how the aliens define
the flow of time. Do they define the future as an increasing progression of positive times like we do,
and the past as negative? Or do they do it the other way around? The laws of physics shouldn’t care
which you choose - so we say those laws are time reversal symmetric.
But if time reversal is equivalent to charge-parity inversion, and the latter
is a broken symmetry, then time symmetry must also be broken. The grand, unbroken
symmetry - CPT symmetry - is a combination of all three. The laws of physics and the behavior
of nature is unchanged under simultaneous inversion of charge, parity and time.
And here we might have our final, insurmountable problem.
What if the aliens use reverse conventions for the sign of electric charge, the handedness of parity,
and the direction of the flow of time? The change in the time-flow convention along with the changes
in charge and parity would change how they represent their particle interactions in a way that we couldn’t
distinguish. We might build a time-forward, right-handed, electronic device instead of
the alien’s intended time-reversed, left-handed, positronic doodad. And guess what - that should be
totally fine. The unbroken nature of CPT symmetry means the thing should work just as intended.
Like assembling a bookshelf upside down because you inverted the instruction manual - the thing
still holds books just fine. It may be embarrassing when we warp into the galactic
civilization with our spaceship on backwards, but the important thing is we get there.
The takeaways from all of this: 1) check if your alien instruction manual is upside down, and 2)
remember that whenever we measure something, anything, we are making choices. Those choices
can seem inconsequential, but every seemingly arbitrary choice reveals something powerful.
It points to the existence of an underlying symmetry of space time.
Thanks to everyone who supports us on Patreon. Every little bit really helps
give us the stability we need to keep making this show. Today’s special Patreon shoutout goes to
Bryce Fort, who’s supporting us at the big bang level. Bryce, we realised it was selfish to just
sit waiting for aliens to send us their advanced technology, so we’ve used your contribution
to blast our own radio beam to hundreds of nearby stellar systems. It contains our own confusingly
incomplete laws of physics. We actually ran out of space to include a lot of technology. We had to ditch the space shuttle blueprints.
In fact we only had just enough bits remaining to ad instructions for making some chocolate brownies.
Which some would say are our most advanced achievement as a species anyway.
And the good news is that if they screw it up and accidentally bake time-reversed antimatter brownies
they don't blow up their planet. In fact those things are freaking delicious.
Bryce, thanks for helping us enhance the collective wisdom of the galaxy.
A quick announcement before
we get to comments: we’ll be taking a two-week break from releasing episodes. Don’t worry,
we’ll be making episodes extra hard during that time, and this gap will give us the time we need
to make some extra amazing stuff. So today we’re answering comments for our episode on whether
interstellar travel is even possible for living beings - in particular given the extreme hazards
of traveling at relativistic speeds through the dust and gas that lives between the stars.
It occurs to Greg N that we’ll most likely encounter the machines built by intelligent
civilizations than the lifeforms themselves. So you know who else that occurred to? One of the
greatest savants of 20th century math and physics, John von Neumann. He came up with the concept
of the von Neumann probe - a self replicating device designed to propagate across the galaxy.
Even if interstellar travel is near impossible for living things, the fact
that the galaxy could be completely filled with von Neumann probes from a single civilization
in a million years or less is a strong factor to consider when pondering the Fermi paradox.
We did an episode on this once upon a time.
Knurlgnar24 reminds us that the thing that’s even harder than accelerating to relativistic
speeds - and that’s stopping, because the ship itself needs to carry all the energy that it needs to stop.
Where as the energy to accelerate can be based back on Earth.
And an example is something like a giant laser array pushing a light sail.
So this concern is very valid. Slowing down takes as much energy as speeding up.
If your destination isn’t already set up with a nice decelerating laser or similar,
how do you slow? The problem is worse than that actually. Without an external source of momentum
exchange like with a light sail, you need to to carry your entire deceleration reaction mass with you.
You need to throw as much stuff ahead of you as you threw behind you in speeding up,
and all of that reaction mass needs to be accelerated with your ship in the first place.
Fortunately there are some ideas for
non-reaction-mass deceleration that don’t require you to have already got to your destination.
Examples include ridiculously vast light sails that decelerate in the light and wind
and magnetic field of the destination star, or Bussard ramjets - vast scoops that collect
diffuse interstellar material as reaction mass and fusion fuel for the slowdown phase.
Actually Tony Tee
seems to have independently invented the Bussard ramjet, by the way. Shame you
didn’t get to it 60 years earlier, Tony - the Tee Jet has a nice ring to it.
Regarding shielding against interstellar material,
Banoffey asks whether an electromagnetic shield would be sufficient. Well actually a magnetic shield may be
enough to block many charged particles, but neutral particles like dust would pass right
through. There are proposals for ionizing these particles before they hit your magnetic shield,
or for generating a hot plasma in that magnetic field to vaporize particles. There are lots of
novel shielding ideas that we could’t survey in this episode - the point was really to assess
whether interstellar travel was a fundamental no-go, which it seems to NOT be the case.
Cessationoftime has a
beautiful vision of a galactic “humanity” in which we’ve spliced our DNA with that of the the naked
mole rat and the tardigrade to grant us radiation resistance for these long interstellar journeys.
So I knew about tardigrades, but thanks for sending me down the naked mole rat rabbithole. Those
things are amazing and I’m completely on board with joining genetic forces. With luck we’ll also
inherit other traits, like proportional longevity, hibernation in extreme low-oxygen environments,
and the ability to absorb parenting hormones through eating the queen moles feces.