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When most of us see the world around us, we see it in a myriad of different colors. Eyesight is
the most dominant sense that most humans have. But this light that we can see is just a tiny
fraction of all the electromagnetic waves present in the universe - the part where
the electromagnetic wave has a frequency between 4.3x10^14 Hz to 7.5x10^14 Hz.
This spectrum also contains any other imaginable frequency - for example light that we can’t see,
such as radio waves, microwaves, infrared light, ultraviolet light, x-rays and gamma rays.
Light is energy in the form of an electromagnetic waves governed by this simple energy equation,
where E is the energy, h is Planck’s constant and nu is the frequency,
or how fast the wave oscillates in spacetime. The electromagnetic waves that we can see are
nothing special, as this simple energy equation states. All the different
wavelengths are basically equivalent, except for the fact that waves with higher frequencies
are more energetic. In principle that’s it. The speed of all these electromagnetic waves,
however, is special. All light is made of massless objects called photons.
Photons happen to travel at the maximum speed allowed in the universe,
as do all massless objects. The universe has set an upper limit to
the speed with which information can flow from one part of the universe to another. A finite limit
on the rate of information transfer assures that causation is preserved in the universe.
And that's the speed at which light travels.
Einstein showed that we live in a relativistic universe, where the world can look very different as
you approach this maximum speed, and depending on how fast you move compared to another person. We
don’t see this world because the speeds at which we travel, say on a car or airplane are very low
compared to this maximum speed. But what would happen if this maximum speed
was closer to the terrestrial speeds that we are used to, or equivalently what would we see if we
traveled much closer to the speed of light. Well we would enter a universe that would be
very strange where thing would appear not as they are when we are standing still. What would we see?
And why would things appear this way? That’s coming up right now…
You have probably heard an ambulance or fire truck drive by outside. As it approaches you,
the siren makes a high pitched noise, but after it passes you, it changes to a low pitched noise.
Why is that? Sound is mediated through the air,
at a finite speed. As the siren sounds, it sends out a circular wavefront. But because the
emergency vehicle also moves, each wavefront is sent out from a different position as it’s moving.
This means that the wavefronts get packed together towards the direction of movement.
This results in a higher frequency wave – so you hear a higher pitched sound.
Similarly, the wavefronts get further apart in the opposite direction. This results in a
lower frequency, or lower pitched sound. The sound waves are simply stretched in
the backwards direction and compressed in the forward direction. The resulting
frequency is higher up front, and lower in back. This is called the doppler effect.
Even though the source of the noise is not changing, it sounds different because that
source is moving relative to you, the observer. Something similar happens with light, but on
much larger scales. Because light is a wave just like sound, it works in a very similar fashion.
But this doppler effect is only noticeable as the source of the light compared to the observer,
approaches very high speeds. We don’t see it on earth because
our everyday speeds such as driving or flying is a tiny fraction of the speed of light. But we see it
in cosmos. If we were to look through a powerful telescope, we would see that most galaxies are
redshifted, thus the wavelength is stretched, just like when the ambulance is moving away from you.
Similarly, this redshift happens because the galaxies are moving away from us.
Now, I should mention that while most galaxies are redshifted, a few are
moving towards as well, that are blue shifted. This is called the relativistic doppler effect,
and it happens thanks to relativity. We don’t notice this at terrestrial
speeds, but what if we could build a superfast spaceship to travel close to the speed of light?
In this case a lot of interesting things will happen, as this relativistic doppler effect
will actually start to change how you see the world compared to person who is not moving!
What would you see? Well, as I mentioned, light is a wave. And our
eyes and brains have evolved to interpret light waves of certain wavelengths as the colors that
we a familiar with. Thus, if we have a light wave with a frequency of 4.3x10^14 or a wavelength of
700 nm, then our brain interprets it as red light. But, let’s say you are travelling in a spaceship,
and there is a traffic light in space. Maybe it’s a very busy strip of space. If you approach it
slowly, you would see the stop light normally. But what if you’re going very fast approaching
it at a significant fraction of the speed of light, let’s say 20%?
What should you do if you see a yellow light? The correct answer is, stop. Why?
Well, what happens is that if you’re moving 20% the speed of light, the colors you see
would be different than the colors standing still. The red light from the traffic light will
actually look yellow to you, as you approach it. Your eyes are still the same. The light source
is still the same. But the relativistic doppler effect changes what you see.
We can calculate exactly what color light you would see by using this formula:
Where lambda r is the wavelength of light we see, and lambda s is the wavelength of the traffic
light for someone standing still. Beta is your velocity divided by the speed of light. Lambda s
would be 700 nm, in this case, which corresponds to red light. If we plug in the numbers, we find that
lambda r is around 572nm, which corresponds to yellow light. But since it’s red to the
stationary observer, you should definitely stop. Something even more dangerous would happen
if you approached the traffic light just a little bit faster - 25% the speed of light.
Lambda r in this case would be 542nm, which corresponds to green light. So
the red stop light would look like a green go light!
Now what would happen if we passed the traffic light at the same 20%
the speed of light? How would the red light look from the rear window? We can use a similar
formula to figure out the color of the traffic light we would observe from the back window:
The equation is the same but the signs change. Lambda r in this case would be 857 nm,
which is in the infrared range. So in this case, we would not see the red light at all, because our
eyes can’t detect infrared light. So the traffic light would just appear to be non-functional or broken.
In a similar fashion, some of the low energy infrared light that is naturally present in
the universe, but invisible to the stationary observer’s eyes, would now appear to be colorful
to the moving observer on the spaceship, because just like red light shifts to higher frequency
yellow and green, infrared would shift to the higher frequency visible light spectrum. Likewise,
natural high energy ultraviolet light which is also invisible to the stationary observer, would
shift to the visible range if we were to view it from our rear window. So we might see unexpected
objects and colors both in front and in back our spaceship. Things that were invisible to us before
could now be clearly seen from the spaceship. This doppler effect is however only ONE of the
weird things that would happen as we move at speeds close to the speed of light.
Another strange effect you would see looking out of the side window of the
spaceship is what is called relativistic aberration, or the "searchlight effect."
When looking out of the side window of the spaceship, you would not only see the colors
distorted, but you would also notice that the light is bright towards the direction of motion,
and less bright away from the direction of motion. This might initially seem strange, but here’s why
it makes sense. Light intensity is literally just the number of photons hitting your eyes,
and if you are going very fast forward then you are racing into more photons.
So naturally as you hit more photons in the forward direction, things will appear brighter!
On the other hand, behind you, you are racing away from photons, so things appear dimmer.
But there are even weirder effects that you would experience because Einstein’s theory
of special relativity shows us that things like length and time are also different for a moving
observer, compared to a stationary one. So moving at 60% the speed of light,
time will slow down for you compared to a stationary observer, but you will not
experience anything different. From your frame of reference, time will tick for you on the
space ship just like it does on earth. Moving at 60% the speed of light though,
what is 1 meter for a stationary observer is only going to be 80 centimeters for the moving observer
according to this equation. This is called length contraction.
This should mean that at these speeds, you should start to see objects more squished together.
And this is in fact what’s happening. However, this is not what you will see. Instead,
what you see is objects curving away from you, and rotating towards you.
So if you were traveling at relativistic speeds in a car for example, the buildings would rotate slightly and
curve away from you. The reason this happens is because the photons from the front of the building
are reaching your eyes earlier than the photons in the back of the building, so you see it curved.
This is an optical illusion. the object in fact contracted but you don’t see it that way.
This time difference of photons from different part of the object reaching your eyes also
means that in space, spherical objects like earth don’t appear squished like a pancake,
even though they are contracted. But they remain spherical-looking. The only difference is that
at relativistic speeds, they will appear rotated towards you, so that you begin to
see the side and back of the sphere. This is not something that would happen at slower speeds.
This is again an optical illusion, because the reality is that the object is in fact contracted.
You just don’t see it that way. What you will experience though is that distances are reduced,
so for example, a trip to Proxima Centauri, the closest star to us,
which is 4.5 light years away, will only be 3.6 light years away.
The interesting thing is, however, that even though things would be closer to you,
they would not appear that way. They would actually look farther away when you viewed them
from your front window. Likewise looking out the back window things will appear to be a lot closer.
This seems to be a paradox. Shouldn’t things look closer in the front window since there
is length contraction when moving so fast? Here’s the simplified explanation for that.
Let’s look at what’s happening on this on this graph. B is our spaceship. The angle theta
represents the angle of a photon reaching you on the spaceship. But as the spaceship moves faster,
length contraction in the direction of motion means that the length on x- axis
becomes shorter, so the angle theta becomes larger. Another way to put this is that,
you would view the photon from the side of the ship as being in front of the ship.
In fact, all the light from the side and even behind the ship would shift forward.
This effect becomes greater and greater as the ship moves faster and faster,
shifting more and more of the light from the side and behind the ship to the forward direction.
We can illustrate this with a circle. If you are in the middle on the spaceship, for the
case where you are not moving, photons reaching you from all directions would be the same angle.
There is no length contraction in this case. Now as you start moving towards the speed of
light, this changes so that you start seeing things that are behind you
in your forward view. This means that your field of view increases in the direction of movement
and decreases behind the spacecraft. And this effect becomes even more pronounced
As you get closer to the speed of light.
This does two things. It make things
look crooked or warped, as if you are looking at things through a wide angle lens on a camera.
And the other thing that happens is that since the entire field of view
from the side and the back of the ship now shifts to the forward window, in order to fit in the field of view, everything
looks smaller. This causes the optical illusion of things being further away. But you have to remember
that this is only an optical illusion because in reality, length contraction will have caused things
to be closer to you, contrary to how it appears. Looking forward in the spacecraft would be like
looking through a fisheye lens and looking out in the back would be like looking into a zoom lens.
The lesson here is that the universe is truly relativistic. What you see at terrestrial speeds
in your everyday life is nothing like how the universe would look if you were moving very fast.
Not only would you see different colors, but things that are invisible to you now,
would become visible. Time, distances, angles and views would all be different and bizarre.
This calls into question, what is really true? Is there an objective reality? You may argue
that objective reality is what we would see with zero movement. But what is zero movement?
Everything on earth and the universe is moving relative to something else. There may be aliens
living on planets moving very fast around a super heavy star? Time and light would be different for
them – but they might think of that as their objective reality. If in the future, we develop
the technology for interstellar travel at close to the speed of light, then the world I described in
this video would be the objective reality of any humans born or brought up in those ships. There
is no absolute objective reality in the universe. What’s really true is that everything is relative.
If the subject of light fascinates you
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Brilliant has a special offer for Arvin Ash viewers right now.
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And if you have a question for me or any of the other viewers, please leave them in the comments section.
I'll see you in the next video my friend!