Neutrinos are really speedy.
These little particles can travel straight through the entire Earth
in a fraction of the time it takes you to blink.
Pound for pound, of all the particles with mass, they are the fastest, almost
traveling at the fastest possible speed in the universe, the speed of light.... but not quite.
So, exactly how fast are they?
That's tougher to say.
Today on Even Bananas, I feel the need, the need to talk about speed.
You may have heard that light from the sun takes eight minutes to reach Earth.
When you look up at the sun, you're actually seeing it as it was eight minutes ago,
before all those photons traveled 149.6 billion meters to reach the Earth.
But some weird things happen when you travel at the speed of light.
And if you're a photon, it seems like the journey takes no time at all.
I've invited Jessica Turner to help explain it.
Jessica is a neutrino theorist at Durham University.
Hi, Jessica!
>Hi, Kirsty!
You're correct. To a photon traveling at the speed of light, the journey happens instantaneously.
We move really slowly compared to photons, so our experience of time is immensely different.
From Einstein's Theory of Special Relativity, we know that the faster something travels the more
time slows down, to the point that for a photon traveling at the speed of light time stops.
So a photon experiences the entire universe's timeline all at once.
>I mean - what? Think about that for a second.
But why are we talking about photons, when we're supposed to be discussing the speed of neutrinos?
The short answer is, on a practical level, neutrinos travel so close to the speed of
light, you can basically just round up. That's right. We've tried to measure the
speed neutrinos travel by measuring how long it takes for them to travel long distances.
For example, the MINOS Experiment timed how long it took neutrinos
to travel 735 kilometers from Fermilab to Northern Minnesota.
But we don't have the resolution even in our most precise experiments to measure the
speed of neutrinos being noticeably different from the speed of light.
I mean, you know, to the extent that we measure neutrinos at all.
Now, let's get a bit more specific.
We can calculate the speed of a neutrino from its energy using Einstein's Special
Relativity equations, if we know it's mass. Let's run the numbers for a neutrino we'd make
at Fermilab's particle accelerators and give it an energy of one GeV.
The other part of the equation is harder because we don't actually know the mass of the neutrino.
We've never been able to measure it.
There are some experiments that have helped narrow it down, including KATRIN,
seen here moving at a very leisurely non-neutrino pace through the streets of Germany to its lab.
KATRIN recently showed that the mass of
the neutrino has to be less than 0.8 eV over c squared.
So let's say Katrin is just shy of finding the neutrino mass.
We'll plug in 0.7 eV over c squared into our equation.
That tells us that our neutrinos would travel at:
0.9999999999999999995 times the speed of light.
To put that in context, the Andromeda galaxy is 2.5 million light years away.
Meaning it takes a photon two and a half million years to travel here. At least, according to us.
If a photon and a 1GeV neutrino set off at the same time from the Andromeda galaxy,
the neutrino would arrive just 0.0004 seconds after the photon.
That is how close to the speed of light they travel.
So you can see why we have trouble measuring the difference.
But then why are we so certain that neutrinos are traveling slower?
One of the special things we know about neutrinos is that they change flavor as they travel.
The fact that they can change tells us that, unlike photons, neutrinos experience time.
And that tells us that they aren't quite traveling at the speed of light.
But if you thought it was that easy to calculate the speed of neutrinos, just wait, there's more.
Let's get into some of the weirdness of quantum mechanics.
In an earlier video, we talked about how neutrinos aren't separate.
They actually travel as a mix of three different mass states: mass one, mass two and mass three.
And each one has a probability of interacting as a different
flavor of neutrino: electron, muon or Tau.
This is an example of quantum superposition and it is pretty wild stuff.
Here's Jessica to explain it.
>So, an important thing to remember in this discussion is that particles are also waves,
and the length of the wave is not infinite.
So you can think of particles as little sections of waves or as a "wave packet".
When a neutrino was born at an accelerator,
it starts out as a superposition of all three mass states.
This means that the wave packets for all three mass states are overlapping.
And as long as the wave packets are overlapping, you have a coherent neutrino.
Any one of these Mass States could be the one to interact in your detector,
and you wouldn't be able to tell the difference.
This leads to the phenomenon of neutrino oscillation.
>But as many of you pointed out in the comments,
things with different masses should travel at different speeds.
So what does that mean for the neutrino?
>We do think that the different mass states of neutrino should travel at different speeds.
If the neutrino traveled for long enough, the three wave packets for the three different mass
states would slowly spread out over time until they no longer overlap.
This would cause what we call decoherence.
And in that case, we wouldn't see neutrino oscillation.
We'd always have the same proportion of electron, muon and Tau neutrinos showing up each time.
But to get these decoherent neutrinos, they'd have to have traveled an incredible distance,
way further than any experiment we could ever build on Earth.
The only neutrinos we know of that might experience this phenomenon
are the ones that have been traveling since the Big Bang.
Unfortunately, they're so low energy at this point
that we don't have detectors sensitive enough to see them.
Thanks, Jessica!
As you can see, examining the speed of neutrinos is anything
but simple. And there's still more to uncover.
Big Bang neutrinos, also known as Relic neutrinos or the cosmic neutrino background,
are one of my favorite neutrino physics topics.
We're going to talk more about them in a future episode, so be sure to like,
share and subscribe to stay up to date as we dive deeper into the neutrino universe.
>Fun fact: When a neutrino is created, it's immediately going full speed.
There's no acceleration period like your car.
And while light can slow down while traveling through material,
like water or air, neutrinos are basically always traveling at the same speed.
This is because they aren't interacting with anything the way that light does,
through electromagnetism.
So it takes the very expansion of space-time itself to stretch
their wavelengths and slow them down.
>Bonus fun fact: In 2011, neutrinos made headlines when the OPERA Experiment reported
a measurement that seemed to show neutrinos moving faster than the speed of light.
Investigations showed that it was actually an equipment error
in the experiment and neutrinos do obey special relativity after all.
Interestingly, one of the experiments that cross-checked the OPERA result was ICARUS,
which measured the same neutrinos as OPERA with a different timing system.
Everything's coming full circle because ICARUS was
moved to Fermilab in 2017 and is now taking data in Fermilab's booster neutrino beam.