As you may have read somewhere, light only
takes 8 minutes to cover the distance between
the Sun and the Earth.
Yes, that’s an insanely short time, considering
that we are talking about 150 million kilometres!
But the light that reach us is actually produced
at the centre of the Sun, in its core, where
nuclear reactions occur.
So, have you ever asked yourself how long
does light take to reach the surface of the
Sun, starting from its centre?
Well, we may attempt to find a rough estimate.
Consider that the Sun has a radius of “only”
696,000 kilometres (compared to the 150 million
kilometres that separates it from the Earth)…
so we would expect light to reach the surface
in, let’s say, a few seconds, is it?
…you may be surprised to hear that this
estimate is totally wrong!
In fact, light takes… (drums)….
Thousands of years to reach the surface of
the Sun!
Wait, what?
How is that possible?
…curious to know how?
Stick with us and we will tell you the answer
in this video!
Before talking about the journey of light
from the centre of the Sun to the Earth, we
need to do a short digression about light.
Stick with us, it won’t be that long.
What is light?
Well, light is something fascinating: in fact,
it has a “dual” nature.
What does it mean?
It means sometimes it behaves as a wave, and
sometimes it behaves as a particle.
Let me tell you more.
In the past centuries, scientists believed
that light was a wave.
And they were right, but.. that was only half
of the story.
At the end of the 19th century, and precisely
in 1900, in order to explain some weird phenomena
that were puzzling scientists, the German
physicist Max Planck postulated that light
was actually consisting of particles, called
“photons”.
Planck’s idea found confirmation five years
later, in 1905, when Albert Einstein (this
guy again!) managed to successfully explain
the “photoelectric effect” by using Planck’s
idea.
The photoelectric effect is a well-known phenomenon
that occurs when light is shone at a material
(metals, in particular) and electrons are
emitted from its surface as a result.
By thinking light as a wave, some aspects
of this phenomenon could not be explained.
For instance, if the frequency of light was
under a certain threshold, no electrons were
emitted from the material, no matter how strong
the intensity of the light beam is.
However, by thinking light as a “bunch of
particles”, the photons, this effect could
then be explained.
In fact, each photon in the light beam carries
a certain amount of energy, and when the photon
hits an electron in the material, it gives
all its energy to the electron, which is now
able to “escape” the material.
The energy of the photon depends on the frequency
of the light beam: so if this frequency is
lower than a certain threshold, the photon
has not enough energy to allow the electron
to “escape” the material.
The intensity of the light beam only determines
the number of photons in the beam: so it doesn’t
matter if we increase the intensity, we will
only have more photons, but none of them will
be able to “free” electrons from the material,
because none of them has enough energy to
do that.
For realizing this, Einstein won the Nobel
Prize.
Now that we have realized that light consists
of photons, let’s go back to our original
topic: why does light take so long to reach
the surface of the Sun?
To answer this question, we have to start
by looking at what happens in the core of
the Sun.
As in every other star, the core of the Sun
is the “motor” that produces all the energy
and the light emitted by our star.
The mechanism by which this occurs is called
“nuclear fusion”, which occurs only in
the core of the stars, since it requires extreme
conditions of temperature and pressure that
are reached only here.
In this process, two small nuclei of hydrogen
fuse together, producing a nucleus of helium
alongside with a bunch of photons: this is
the beginning of their journey.
Let’s now focus on the exciting life of
one of these photons.
After this photon has been produced, the path
it will follow is not straight: on the contrary,
it is quite messy.
The reason is that the Sun is not empty inside;
instead, it is very dense, so the photons
have to travel through a lot of matter before
reaching the surface.
In fact, our photon will travel only a microscopic
distance, before being “captured” by one
of the many atoms (ouch!).
Sorry photon, too bad.
But, wait.. so, this is the end of the story?
Not at all!
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So, we left our poor photon when it has been
absorbed by an atom.
What happens now?
When the atom absorbs our photon, the atom
gets into a particular state called “excited”.
Think to this as a state in which the atom
has an “excess” of energy, so it cannot
bear it for a long time.
In fact, very soon the atom will “de-excite”,
re-emitting another photon (which is technically
not the same photon as before!
That one is long gone… ouch).
So now we ended up with a new photon.
And then, what happens?
Well, this new photons has not a long life
either.
In fact, its faith is exactly the same as
the original photon: it will only travel a
very short distance, before being “captured”
by another atom.
This will cause this second atom to excite,
and after a while, to re-emit a new photon.
This process will continue for several thousand
times, and it happens much more frequently
in the core of the Sun, which is the most
dense zone of our star.
As the photons proceed towards the outer layers,
the density of matter decreases, which means
there are less atoms able to “capture”
the photons, and so photons are able to travel
longer and longer distances before being absorbed
by atoms.
Eventually, when they reach the outermost
layers of the Sun, the density of matter is
so low (compared to the core) that photons
are finally able to travel several metres
or kilometres, and they can finally escape
the surface of our star.
Now, let’s imagine for a moment that the
photon emitted at each step is actually the
original one.
By doing so, we can track down the path of
the photon and see how it looks like.
The result is quite bizarre: we will see that
the photon follows a very chaotic track, going
randomly up, down, left, right, and changing
direction all the time… in fact, it does
not follow any particular direction at all.
So now we can understand a bit more why the
photon takes so long to reach the surface.
It’s even easier by using an example.
Imagine you have to drive from your home to
your workplace, and imagine that the shortest
route is by going, let’s say, north for
5 kilometres.
Now, what happens if you keep changing direction
randomly at every crossroad instead of going
straight?
The answer is pretty obvious: It will take
you forever to reach the workplace!
Maybe eventually you will reach it, but after
a very, very long time.
Hope for you that you don’t have to attend
an important meeting…
This is exactly what happens to our photon:
since it decided to take this very bizarre
path, it will take a lot of time to “escape”
the Sun.
By comparison, if the photon could go straight
to the surface, it would take only less than
3 seconds to reach it.. but in reality, it
takes 10 million years on average!
And then… what?
Is its journey over?
Not at all!
In fact, the photon is now “free” to travel
and reach the Earth.
Since the interplanetary space between the
Sun and the Earth is almost empty, there are
now no atoms able to “capture” our photon,
which is therefore free to move at its original
speed: the speed of light.. about 300,000
kilometres per second!
Being that fast, our photon is able to reach
our planet in just 8 minutes.
Think about it: thousands of years to escape
the Sun… and just a few minutes to reach
the Earth!
But for some of the photons that were produced
in the Sun, this is not even the end of the
story.
In fact, many of them (those who don’t reach
the Earth) continue their journey through
the interplanetary space, and then even further,
beyond the Solar System, towards distant stars
and galaxies.
In fact, since the space is almost empty,
these photons don’t find anything to interact
with, and so there’s nothing stopping them,
so they keep travelling, free and happy…
However, at this point distances become so
huge that even at their crazy speed (remember:
300,000 kilometres per second!), these photons
will take thousands, or even millions of years
to reach other stars or galaxies.
At this point, it’s actually interesting
to look the other way around: what about photons
emitted by distant stars, and reaching us?
Let’s take, for example, a photon emitted
by Proxima Centauri, the closest star outside
the Solar System.
The star is located approximately 4.2 light
years from us: it means that a photon on the
surface of Proxima Centauri will take 4.2
years to reach our planet.
Of course, we have to add the time it takes
for the photon (which has been produced in
the core of the star, remember?) to reach
the surface, which can be thousands of years,
as we explained previously for the Sun.
But right now, let’s just take a photon
starting its journey from the surface of Proxima
Centauri: this photon will reach us 4.2 years
later.
…Do you realize what are the implications
of this?
Yes, that’s right!
It means that when we look at Proxima Centaur,
we are actually observing how the star looked
like 4.2 years ago.
So, it is actually correct to say that we
are looking… back in time!
In fact, the image that we have of Proxima
Centauri is the image of the star 4.2 years
ago.
And if you think carefully, this is even more
dramatic for very remote objects.
For example, take the Andromeda Galaxy, which
is located 3.4 million light years from us:
it means that photons coming from a star in
Andromeda will take 3.4 million years to reach
us… so, we are looking at how that star
looked like 3.4 million years ago!!
It’s crazy, isn’t it?
It is even possible that that star has exploded
in the meantime, but we simply don’t know
yet, because the photons produced in the explosion
haven’t reached us yet, and so we are just
looking at how the star was before the explosion…
This is valid for every object in the Universe:
when we are looking at it, we are looking
at how it was in the past.
Think about it next time you will like a starry
night sky: you are literally “looking into
the past”!
So… this sounds something like “going
back in time”!
Wait, what?
Now you want to talk about time travels?
Yes, why not… but perhaps in another video!
"This video ends here!
Thanks for watching everyone!
Did you find the photon’s journey exciting?
Is there something more you want to know about
photons and their journey through the Universe?
Let me know in the comments below, be sure
to subscribe, and I'll see you next time on
the channel!"
