In the early 1960s, the well-known physicist
Richard Feynman said something cool in a lecture.
I mean, he said a lot of cool things in his
lectures.
But what he said this time, according to a
transcript published later, was that the center
of the Earth should be a day or two younger
than the surface.
As strange as that sounds, the basic idea
behind it makes sense, if you know something
about the theory of relativity.
And it was Feynman saying this, so people
kept quoting him in lectures and textbooks,
and nobody really questioned it.
That is, until April 2016, when three Danish
physicists actually did the math, and realized
that Feynman was wrong.
The center of the Earth isn’t a day or two
younger than the surface.
It’s two and a half years younger.
And the center of the Sun?
It’s 39,000 years younger than its surface.
All because of general relativity.
General relativity is basically the science
of the very fast, and the very massive.
It takes the big-picture universe — like
planets, stars, and galaxies — and describes
it using math.
That math is built around a few fundamental
ideas.
One of those ideas is that the speed of light
in a vacuum is always the same — it’s
about 300,000 kilometers per second, no matter
what perspective you’re looking at it from.
And that rule — that the speed of light
is set — can seriously mess with both time
and space, if you start to consider situations
that are way more extreme than your everyday
life on Earth.
For example, when you’re traveling close
to the speed of light — say in some hypothetical,
ridiculously fast train — time will pass
more slowly for you than for someone standing
on the ground watching you zoom past.
Relativity shows that space and time are so
closely interconnected that you can really
think of them as parts of the same thing.
That’s spacetime.
And another thing relativity shows is that
something with a lot of mass, like Earth or
the Sun, will warp spacetime.
One of the effects of this warping is that
the closer you get to the center of one of
these massive objects, the slower time will
pass.
It’s a brain-bendy idea, but we know it’s
true.
We have to correct GPS systems because of
it.
GPS satellites orbit Earth from about 20,000
kilometers up, so they’re much farther from
Earth’s center than we are on the ground.
That means that time passes a little more
quickly for a GPS satellite than for us.
It’s a tiny difference, which adds up to
only about 38 millionths of a second per day.
But for the satellites to keep track of your
position, their clocks need to be synced up
pretty much exactly.
So GPS satellites are designed to take relativity
into account and correct their clocks.
And this is the same basic idea Feynman pointed
out in his lecture:
If something with lots of mass warps spacetime,
and if time passes more slowly the closer
you are to Earth’s center, that means the
center of the Earth must be younger than the
surface.
And that part was right.
The thing is, he said that in the 4.5 billion
years or so that Earth’s been around, the
center would be younger than the surface by
a day or two.
And no one checked his math for more than
50 years, until that group of Danish physicists
decided to look into it, in a paper published
in the European Journal of Physics.
First, they did a simpler version of the calculation,
using equations that treated Earth like a
uniformly dense sphere.
Which it’s not, by the way, because of all
those different layers of rock.
But it’s still a pretty good way to get
a sense of the physics, and that’s probably
how Feynman would’ve done the calculations..
They found that the center of the Earth would
be about a year and a half younger than the
surface, which was their first sign that Feynman’s
famous fact was wrong.
Next, the team did a more detailed calculation,
using a model that takes into account the
variations in Earth’s density.
That gave them a more exact answer: Earth’s
center is about 2.5 years younger than its
surface.
It’s hard to know whether Feynman himself
was wrong, or whether the people transcribing
his lectures just wrote down “day” instead
of “year”.
But, either way, the fact that people kept
repeating in all those lectures and textbooks
was wrong.
While they were at it, the Danish researchers
did the same detailed calculation for the
Sun, since there’s a similar model of the
Sun’s density that they could use.
And they found that the Sun’s center is
about 39,000 years younger than its surface,
which is a much bigger age difference than
for Earth.
A lot of that is because much more of the
Sun’s mass is concentrated close to its
center, and warps spacetime so much more.
Practically speaking, the fact that time passes
more slowly at the center of the Earth and
the Sun doesn’t really matter that much.
But relativity is one of our most useful tools
for learning about the universe, so it’s
important to understand the way it affects
everything around us.
And for scientists, the fact that Feynman’s
numbers were wrong is a reminder of one of
the most fundamental aspects of scientific
thinking: question everything.
Thanks for watching this episode of SciShow
Space, and thanks especially to our patrons
on Patreon who help make this show possible.
If you want to help us keep making episodes
like this, you can go to patreon.com/scishow
to learn more.
And don’t forget to go to youtube.com/scishowspace
and subscribe!
