Under the standard model of our universe there exists two types of particles...fermions which
are the building blocks of all the matter you see around you and bosons which govern
the fundamental forces that shape our everyday life.
Photons transmit the electromagnetic force which we see in the form of light.
W and Z bosons are responsible for the weak nuclear force which Occurs in events like
radioactive decay.
Gluons carry the strong nuclear force which is what keeps your atoms together.
But what about gravity?
Sure we have Newton's and Einstein’s equations but those only work when we look at the big
picture and break down when we enter the quantum mechanical realm leaving us with the question
are there particles of gravity?
Going by the standard model it makes sense that gravity should have a subatomic particle
responsible for its presence in our everyday life which scientists call the graviton .Now
There are 2 previous videos on this channel from about 3-4 years ago explaining the basics
of gravitons for those who are new to the subject.
Unfortunately since the release of both of those videos Gravitons have still not been
discovered.
Why is this?
Well let’s start off with the obvious.
Gravity is a very weak force.
In fact it is the weakest of the 4 fundamental forces of the universe.
Don’t believe it ? Consider how a magnet is able to pin a paper to a fridge surface
or how you can lift this book off the table despite the gravity from the entire Earth’s
mass pulling down on these objects.
Also in the past we’ve discussed how we need to build such large sensors and probes
to detect faint signals of stuff like gravitational waves from colliding black holes or the warping
of space due to the Earth’s gravity.
Just imagine the difficulty we’re going to face when trying to detect the effects
of gravity on something as small as an electron.
Keeping this in mind would it still be possible to detect even a single graviton?
To answer that we need to go back to 2015 where scientists at LIGO detected gravitational
waves.
To do this Ligo built an apparatus
in which an emitted laser beam was split and each half of the laser beam was then bounced
off a mirror.
Then the distance each beam of light travelled in its respective direction was calculated
to determine whether or not a gravitational wave had passed by and warped the space the
light traveled through.
The gravitational waves discovered by LIGO, an observatory with some of the most powerful
and sensitive instruments on Earth, are estimated to contain about 3*10^37 Gravitons, so if
we wanted to build an instrument to detect even a single graviton then we need to build
an apparatus which is 3*10^37 times more sensitive than what LIGO has, which is kind of impossible
given our current technology and the fact that radiation from the environment such as
neutrino particles emitted by the sun could prevent us from getting an accurate reading.
Also to make matters even worse, to detect an individual graviton using the described
ligo setup would in theory require mirrors so massive and heavy that they would collapse
and form a black hole.
Additionally the measurements for this experiment would also require an error smaller than a
planck length, the smallest possible length in the universe which is impossible by the
laws of nature.
However a simpler alternative proposed by physicists is to detect gravitons via the
gravito-electric effect which given our current technology is perhaps the most realistic way
we could detect gravitons that are passing by at a frequency of atleast 10^15 hz.
The idea is simple.
In the photoelectric effect when an electron in an atom absorbs light it gets more excited
and energetic and moves away from the nucleus and in some cases gets ejected from the atom
which we can detect.
The gravito-electric effect is similar except instead of photons its gravitons which are
absorbed by the electron putting it in a more energetic and excited state that we can measure
However in order for an electron or particle to be
excited by a graviton it needs to surpass the absorption cross section which is basically
the probability of absorbing a given particle expressed in terms of area with a smaller
number meaning its harder to detect since it doesn’t interact with Matter as much.
Neutrinos which are famous for hardly interacting with any form of matter have a cross section
of about 10^ -45 cm^2.
This is why trillions of neutrinos are passing through you right now without you feeling
a thing.
For the graviton this area is 8 x 10^-65 cm^2 which means gravitons are even harder to detect
than neutrinos because of how weakly they interact with Matter.
This is also why we have to make sure our graviton detector filters out.
neutrinos from the readings.
In fact because of the weakness of gravity as a force, any potentially viable solution
involves sensors which are way too massive or would take billions of years before detecting
even a single graviton.
Due to this, unfortunately the answer to whether gravitons exist or not is something that can’t
currently be answered simply due to the lack of technology to gather evidence but that
doesn't mean that future technological breakthroughs allowing us to do so may not happen.
So let’s say gravitons were discovered… what would happen.
Well it would not only be the greatest discovery in physics of all time but would fundamentally
change how scientists understand gravity on a classical and quantum mechanical level and
even open up the door to us taking a closer look at other possibilities in physics like
string theory.
And like any other advance in human history the discovery of gravitons would lead to further
research and development potentially leading to technology that manipulates gravitons which
would admittedly make for an awesome utopia.
Of course despite the scientific community’s deep desire to find the elusive graviton it’s
important we keep an open mind regardless of whether gravitons exist or not when learning
about the secrets of the universe as anything could be a possibility and always remember
what we want reality to be is different from what reality is...otherwise it wouldn’t
be reality.
As always don’t stop exploring and don’t forget to stay tuned for more science videos.