The Banana of Time

Time flies like an arrow. Fruit flies like a banana.

While this is interesting and for the most part, true. If you actually delve into the complexities and the metaphysical aspects of the statement, you'll find that it's not exactly correct.

fig 1: banana

There are three ways to tell time. Not by your watch, clock and computer, but rather by three "arrows" of time that point in a direction. These three arrows are mental, thermodynamic, and expanding universe. The easiest of the three to understand is mental. We experience time in one direction, forward. We can't suddenly say oh hey I want to go back to yesterday please. Take me there. That doesn't work (unless you're Hiro from Heroes), "forward" is the only direction we have ever lived with. 

fig 2: exception

The second arrow of time is thermodynamic. This is taking advantage of the second law of thermodynamics which states that the entropy in an isolated system NEVER decreases. We can imagine this as such. Say I have a wine glass in my hand and I toss it at the ground with great force. Unsurprisingly, it would shatter into hundreds of tiny tiny tiny pieces of glass that scatter everywhere (don't try this at home. unless you want to). The "disorder" in the system increased since the glass is scattered around the room instead of being a single, uniform entity. The second law of thermodynamics broadly states that in this case, it would be impossible to put the wine glass back together again without exerting some sort of energy upon it. We define the thermodynamic arrow of time to be pointing in the direction of increasing entropy. The universe is always continually getting more and more chaotic. Coincidentally (or maybe not) this is pointing in the same direction as our mental arrow of time.

fig 3: physics

The third arrow of time is a tricky one. The universe is expanding and we know this by observing various galaxies and calculating their speeds using the Doppler Effect. This arrow of time specifies that it is pointing in the direction at which the universe expands. In the early 1990's, scientists thought that theoretically the universe's expansion is slowing down and eventually it would stop expanding and being contracting. This is a mindbogglingly weird answer. If you think about it, all mass in the entire universe is attracted to everything else through gravity. Since everything is attractive in the universe, it would only be a matter of time before things being attracted to each other causes the universe to start shrinking right? So what happens to the expanding universe arrow of time then? Would it point in the opposite direction of our mental and thermodynamic arrows?

Things got even more messed up when the Hubble Space Telescope discovered something amazing. The universe's expansion isn't slowing down, it's speeding up. What? How? What's making it speed up? That's what scientists are dubbing as Dark Energy. 

fig 4: dark energy

Judging by the acceleration of expansion, it was discovered that about 70% of ALL the mass-energy in the universe is made up of dark energy. This dark energy is continually pushing the universe away from itself and countering the force of gravity felt by galaxies. For something that has such a huge impact on the universe, we know surprisingly little about it except that it exists.

Weird.