When Einstein first began telling the world of his ideas of relativity, pretty much everyone was floored.
Relativity describes a world which continues to excite and baffle many people today. It’s a world of forward time-travel, a world of shrinking objects and one where the path of light is bent along the curvature of gravity wells.
Broadly speaking, there are two branches of relativity: special and general. Special relativity can be thought of as the study of the structure of spacetime or the physics of objects moving extremely fast.
For the layperson, you might be wondering what exactly special relativity means and how you can think of it. It’s actually surprisingly easy to get. The theory manages to marry space and time together so that things that physics equations which have to do with space have consequences with regards to time as well (and vice versa).
Let me lay out a scenario for you which illustrates quite how special relativity will play into our lives in the far future (assuming Joe Haldeman is right):
Let’s say we discover intelligent and hostile extraterrestrials in the Alpha Centauri star system (which is 4.37 lightyears away). We deploy a force of elite “space soldiers” to travel to Alpha Centauri and fight these aliens.
On Earth, soldiers have 1 year deployments. If we assume these space soldiers had a ship which could carry them at 99% the speed of light then by the time they reached their faraway battlefields, fought their war and returned home they would have only aged 1.25 years - about as long as a soldiers deployment can go for today.
On Earth, however, 8.83 years will have passed.
-> 8.83 years = γ*1.25 years
Kids will have grown up, family and friends will have passed away, politicians will have risen and fallen and, who knows, the country that sent you to war may have collapsed and been replaced with a new one years ago.
Perhaps the governments will find new ways of avoiding paying their soldiers by giving them high interest funds that will grow equivalently in their time gone that they would’ve had to have been paid.
This is called a Lorentz Transformation and it can be used to calculate many of the strange phenomena predicted in special relativity. Often you can solve a LT by simply introducing the Lorentz Factor into an otherwise plain equation. If the above is the LF than here is what it looks like:
EarthTime = (1/√(1-v^2/c^2))*SoldierTime
Where (1/√(1-v^2/c^2)) = γ, v is velocity and c is the speed of light.
Equivalently, objects moving extremely fast shrink in the direction of their movement (this is called length contraction). Say the goddess Athena was in a javelin throwing competition and she throws a 6-foot long javelin at 99% the speed of light, the javelin, while in mid-flight, would shrink to 0.85 feet in length. Perhaps it would fit in Pandora’s Box?
Alternately, there is general relativity. This is the study of how gravity relates to space and time. It turns out, it does, and it does so a lot.
The mathematics behind general relativity is beyond me at the moment and therefore beyond my ability to teach to you. However I can still tell you about some well-known consequences of the theory:
Time dilation! Again! Time goes slower in deep gravitational wells. What this means is that near a very massive object (and more mass = more gravity -> general relativity is a gravitational theory) an object ages slower. This is why, were you to fall into the event horizon of a black hole time would slow down so much for you that you could turn around and watch the life of the Universe. You would see stars born and die as you drift into singularity (Maybe? That’s another post).
This is also why our GPS satellites need to have a mathematical correction in them. The time difference generated by gravitational time dilation would throw them off and we wouldn’t be able to ever get to work on time.
To me, one of the most exciting aspects of general relativity is that gravity bends the direction of light.
The fabric of spacetime warps as a function of gravity. Very massive objects (like, say a galaxy or the sun) can have visible influence on what we actually are looking at.
For example, a galaxy can warp the image of things behind it so that their image is distorted and sometimes simply magnified!
(Gif credit: G. Mikaberidze)
This has two really profound consequences:
1) We have incredible images of massive galaxies warping spacetime so much that the light of objects around it are skewed and it’s almost like we’re staring at the stars through a rippling pond:
(Image credit: NASA and ESA)
2) When done carefully, this effect can be used to bend light in such a geometrically precise manner as to essentially be cosmically large telescopes. If we can properly use the gravitational well of the sun, for example, we would be able to see surface features on exoplanets! This is known as gravitational lensing.
Gravitational lensing was famously proven by Arthur Eddington when he saw that, during the solar eclipse of 1919, the constellations and stars of the sky near the sun weren’t where they were supposed to be!
Now, they were of course, but their images were distorted as they traveled through the sun’s gravitational well.
(Image credit: F. W. Dyson, A. S. Eddington, and C. Davidson, “A Determination of the
Deflection of Light by the Sun’s Gravitational Field, from Observations
Made at the Total Eclipse of May 29, 1919” Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character (1920): 291-333, on 332.)
