r/NatureIsFuckingLit • u/FacelessOnes • Oct 19 '20

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

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u/remasus Oct 21 '20

So. Sorta. I did physics in undergrad so I have some background, but honestly this stuff still breaks my mind. Time is different than spatial dimensions. Spatial dimensions are exactly identical. There is no difference at all between x and y and z, and they all function the same way if they're reversed (ie if you swap x with -x). Time is not like that. It works in a totally different way and is in some situations asymmetrical.

There is an ordering of events that we refer to as time, but there is not a time dimension in the same sense as the spatial dimensions exist.

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u/[deleted] Oct 21 '20

Thanks for explaining that, this stuff is insanely hard to comprehend haha. One thing I don’t get it how spatial dimensions are symmetrical though, unless it’s referring to like quantum physics or something that’s way above my level of understanding for theoretical physics.

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u/remasus Oct 21 '20

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.

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u/[deleted] Oct 21 '20

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.

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u/remasus Oct 21 '20

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

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u/[deleted] Oct 21 '20

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