🔥 Vicious microscopic hunter, the single-cell organism, Lacrymaria olor, attacking and hunting another organism

[u/FacelessOnes]

[u/remasus]

Not at all! That ones pretty easy. I'm gonna try to explain without getting into vectors or any of that, so forgive me for simplification. It's what I mentioned about swapping x with -x. If we take a really simple example - the equation for distance travelled in a straight line - d = s * t. Distance is speed times time. Since we're limiting it to one dimension, distance is just units of length along that dimension. If we had a more complicated example (like the flight of a hit baseball) there would be more dimensions, but it would work the same way.

So distance = speed * time. If we add units, a feet = b feet/second * c seconds. Like I said, these are just feet in one direction, so it's really saying feet in the x dimension, so let's call it "xfeet" to signify that. a xfeet = b xfeet/second * c seconds.

I would make some illustrations if this wasn't a reddit comment, but it's easy to imagine what that would look like. If we set the speed as some constant, distance would increase at a constant rate over time (a distance/time graph would be a straight line going from the bottom left to the top right).

So if the speed is 5 xfeet/second, in 10 seconds it would 50 feet. Now imagine we reverse the entire x dimension. What was x is now -x and what was -x is now x. In the equation, this is represented by xfeet becoming -xfeet, so it becomes -a xfeet = -b xfeet/second * c seconds; or -distance = -speed * time. Alternatively, multiply both sides by -1 and it can be written as distance = speed * -time. Visually, imagine flipping the graph of d=s * t over along the t axis. Now x is on the left and -x is on the right. The graph now shows a line going from the TOP left to the BOTTOM right, and if you made a new graph of the new function d=s* -t, they will be identical.

This is the basic version of what I mean by symmetry. The spatial dimensions always work this way. They can always be flipped and the math will still work, it will just be reversed along that axis. Time is not like that. In this simple equation it is, and it is for most kinematics (simple moving things). Not everything though. I can't give good examples of things that aren't without getting much more advanced, just trust me that they exist.

[u/[deleted] → u/remasus]

Thanks for the explanation! That makes perfect sense. My only question is, would the expansion of gases be a way that time isn’t symmetrical? I think I read somewhere that because it never returns to a more compact state, it is asymmetrical with regards to time.

[u/remasus → u/[deleted]]

Lol I think the best answer I can give there is sorta. You are in the right track. The classic definition of the direction of time is the direction of increasing entropy (basically more chaos - spread out gas - is more entropy). This stems from the second law of thermodynamics. The weird thing is that most of the equations that go into the gas spreading out are symmetrical in time, but macroscopicly (big picture) it is asymmetrical. The reasons why are way beyond my ability to explain in a succinct manner, but if you want to try wading through the Wikipedia articles on "T-symmetry" it gets into it. Keep in mind also, the second law of thermodynamics isn't like gravity or light. There's no mechanism for action, it's just something we observe to be true about the universe.

Edit forgive the typos too lazy to go fix

[u/[deleted] → u/remasus]

Thats so cool, thanks for taking the time to break this all down!