19th May, 1919 was one of the most momentous days in human history for astronomers and physicists alike, as Albert Einstein's very own Theory Of General Relativity was put to its first serious test.

Not what it seems

This day marked the occurrence of a total solar eclipse that was observable over parts of Brazil and West Africa, making a grand opportunity to put Einstein's theory to the test.

You're probably wondering what on Earth a total eclipse in 1919 has got to do with Einstein and his theory, which made the extremely controversial claim at the time that gravity was not a force, but rather a curvature of space-time due to mass.

Well, let's talk about it...

A simulation of the event in Universe Sandbox - check it out here to support us and the community at no additional cost to you!

Just search for "Universe Sandbox" - would highly recommend it if you're interested in astrophysics

During the day, as you're probably aware of, you can't really see any stars.

Close to sunset and sunrise, you might be able to see some bright planets like Venus, Mercury, Jupiter and Saturn, but other than that, you can pretty much just see the sun and/or the moon.

As a quick side note, the reason why you can only ever see Venus and Mercury near sunset or sunrise is because at night time you're looking away from those planets (as they're closer to the sun), but at daytime, the light from the sun is too strong for you to be able to distinguish that light from that of the sun's.

This isn't all that helpful when trying to prove GR (General Relativity), as the light coming from Venus or Mercury when you can see them isn't closely passing the sun.

Don't worry, there is a point to this...

So to summarise, during day-time, you can't see light that's passing closely by the sun, and at night-time, you're facing away from the Sun, therefore all sources of light that can be seen ie from stars, planets, or nebulae/galaxies if you're lucky enough to be in a dark area, are not passing by the sun at all.

As the Earth orbits around the sun, we can collect light from all different sources as we face in different directions throughout the year, but we can never view light sources that are passing closely enough by the sun to be affected by its gravity, or rather it's space-time curvature; and this is what makes this eclipse so special, as we are about to talk about, but you might already think you know why at this point....

Just to demonstrate how you can never see Venus and Mercury at night time (I'm talking midnight-4am), since by virtue of you facing away from the sun, you're facing away from those 2 inner planets.

During a total solar eclipse, the moon passes between Earth and the sun and completely blocks the light coming from the sun in a specific place on Earth (the moon isn't very big so it's shadow casted on the Earth isn't all that big either, but big enough to cover a decent proportion of Brazil and West Africa in this 1919 example.)

What this means is that, in the absence of the sun's light, we can actually see the light coming from stars, planets, galaxies etc that's passing very closely by the sun.

But why is that important?

Light has no rest mass.

It is but a photon which is essentially a packet of energy that oscillates up and down as it traverses through space-time.

Because it has no rest mass, it cannot be accelerated by forces in an empty vacuum, such as space.

The speed of light can change slightly depending on which medium it travels through (water, air, glass etc), which is in fact the reason for light refraction, but we won't go into the specifics of that here.

However, light cannot be accelerated in any direction when travelling through an empty vacuum; it can't speed up, it can't slow down, and it can't change direction.

This is in accordance with Einstein's theory of special relativity, of which proposes two postulates:

• Physics is the same everywhere in the universe

• The speed of light is constant, c

If gravity is in fact a force, then it should have no interaction with light as described above, and the light should appear to be coming from the same place, no matter whether it travels near a massive body such as the sun or not.

But in 1919, this proved to not be the case.

During the total eclipse, measurements were taken of the origin of the light that was passing very closely by the sun.

When these measurements were compared to the measurements that had been previously taken of the same objects but at different times of the year, when they could be seen at night time, the results were ground-breaking.

The measurements did not match.

In fact, the difference, although extremely minimal (a mere 1/2000th of a degree!), proved that the interaction of gravity when the light passed closely by the sun caused the light to change direction.

This proved that gravity is in fact not a force, validating the work of Einstein's Theory of General Relativity....

So what's the alternative?

Well, I don't want this post to be to go off on a tangent, so if you're interested in learning about what Einstein proposed gravity really is, then I'd recommend checking out this article.

Thanks for reading!