Time reversal is not the same as actual time travel, rather its pseudo time travel in the sense that time reversal is more akin to a thought experiment yet one you can often perform with a motion picture camera. I’ll explain…
Continued from yesterday’s blog…
Things left to the natural way of things tend to go downhill not uphill. That’s gravity for you. Things traveling uphill better be expending energy, if they aren’t then you are probably in The Twilight Zone. It’s the old saying ‘what goes up, must come down’, and thus Newton
Things that don’t have front-back symmetry left to the natural way of things tend to move in one direction, and if you see the reverse, it looks odd. Humans and companion animals tend to move forward, not backward. The same applies for most modes of transport – you don’t normally pedal a bicycle backwards just like you don’t drive to work in reverse, and aircraft certainly don’t fly backwards. But things with front-back symmetry can, like a balloon or a rocket taking off vertically. [But said rocket, once it turns on its side loses front-back symmetry and hence looks weird flying backwards.]
The key to all of this interpretation of time reversal, IMHO, is motion, or change (there can be no change without motion and vice versa). Filming something that’s standing still (no motion; no change) and playing the film backwards is a no-brainer for ‘there’s nothing strange happening here’.
Motion (or change) can be exhibited as left-right (LR), forward-back (FB), down-up (DU) or bottom-top (BT) or a mix of the three. Since time is a measurement of rate of change (or motion), all motion (or change) must have in addition a time dimension to it. If it didn’t have a time dimension to it, it’s rather silly to talk about a time reversal!
Motion of course can also be constant per unit of time, or variable per unit of time. Reversing the former is not as strange as reversing the latter.
Now for some general observations - first and foremost with respect to all things anomalous or weird, might I suggest that all else being equal, time reversing objects with high symmetry or no symmetry tends to be less weird than something in-between. So filming an amoeba, an asteroid rotating or a cloud forming, moving and dissipating (no symmetry) or a sphere in motion (a ball, a balloon) or a cube (say a dice), or a rotating tire or a Black Hole (high symmetry) isn’t likely to appear strange.
Time reversing things with LR and FB symmetry, or BT asymmetry, like rockets, pencils, trees/plants (in the abstract), dry spaghetti, mountains (in the abstract), tornadoes (in the abstract), and CDs/DVDs, don’t tend to appear too anomalous over the short term, at least if they aren’t moving. If they are moving, then things might look odd, but within bounds of reasonableness. Of course the CDs/DVDs might be rotating the wrong way and playing from outer edge to inner edge; the rocket might be landing with all motors firing instead of taking off. Of course if filmed for a long enough period, as in time-lapse photography, you’ll find it odd that plants shrink instead of grow, mountains don’t erode away but grow higher, and pencil points don’t become blunt but ever sharper.
Time reversing objects with only LR symmetry (vertebrates; most modes of transport like cars and planes and boats) tends to look odd. Land animals and automobiles don’t tend to walk/drive backwards; fish and boats don’t tend to swim/tread water backwards; birds and planes don’t tend to fly backwards.
Time reversing a change in symmetry from high to low is not overly anomalous, but low to high is usually considered strange. Examples: A punctured tire (originally LR, BF, BT) deflating to (LR, BF) isn’t strange; the reverse is. A pencil (LR, BF) falling over from the vertical (its radial symmetry broken) isn’t anomalous, but the view in reverse is. When a car (LR symmetry) crashes (zero symmetry), that’s normal – a car un-crashing most certainly isn’t.
Time reversing inanimate objects (still life – natural or man-made) tends towards the less anomalous, so filming, over the short term, a cliff face, or a painting, doesn’t look all that strange in reverse.
However, most artificial, man-made objects, tend to have a purpose(s) which for the most part doesn’t include just sitting there looking pretty, like a painting. If such objects have a purpose, then they tend to have a before and after aspect to them - if before and after tends to be the same as after and before, then things will look the same in reverse. However, the nature of the before tends to be different than the nature of the after, so reversing the two will probably be noticed – and odd. For example, place the white bread in the toaster and toast. The bread pops up toasted. Now reverse and you go from toasted bread to un-toasted bread. Or, throw your dirty dishes in the dishwasher and wash them. Take your nice clean dishes out. Reverse the film and you now put clean dishes in the dishwasher, apparently ‘wash’ the dishes, and take out dirty dishes!
Time reversing simple animate objects is less anomalous than time reversing complex organisms. If I viewed a bacteria’s behavior for an hour, and my cat’s behavior for an hour, filmed same and reversed same, I bet I could tell that the film of the cat had been reversed, but not nearly as certain about the bacteria.
Things get odd if you time reverse events dependent upon not space, but on time itself, so time reversing objects that by their nature are affected by the passage of time will be more anomalous than objects that aren’t. Film an electron for a million years and reverse – yawn. Film a rock for a million years and reverse. Over a million years, a rock will tend to erode, so the reverse is an apparent violation of entropy. Film a drop of blue ink placed and entering a glass of water at its surface and let time strut its stuff. Reverse the picture and you have a glass of pale blue water becoming clearer as all the blue bits come together at the surface in what appears to be a drop of blue ink. That’s odd. Film a hot cup of coffee and place a thermometer in it. The mercury in the coffee thermometer goes up and levels off. Place another thermometer next to the cup of coffee. Over time, the coffee’s thermometer registers lower and lower temperatures as the coffee cools; the outside thermometer gains a bit in temperature as heat from the coffee warms up the surrounding area. Now reverse. The coffee appears to get warmer and warmer; the adjacent area cooler and cooler. That’s also odd.
Time reversing objects moving LR or FB will tend to exhibit less anomalous behavior relative to DU/BT. So, two tennis balls, one each entering upper right and lower right move towards the centre, collide, and rebound each moving off stage towards the upper left and lower left. Reverse the film – nothing strange. But, drop a tennis ball and it goes top bottom, bounces bottom top, stops, goes top bottom, etc. until friction takes its toll and the ball comes to rest on the surface. Played in reverse, a motionless ball starts vibrating, jiggling, and then bouncing ever higher and higher. That looks very odd indeed. Or, Humpty Dumpty shuffles across the garden wall left to right – reverse and Mr. Dumpty shuffles right to left. But, if someone pushes Mr. Dumpty off the garden wall and Humpty falls down and shatters, the reverse looks odd. Why? Basically, I think its because top bottom/bottom to (or up down/down up) is associated with gravity and gravity is a one way (pull) force. Time reversal makes gravity a push force, something that’s well outside our day-to-day experiences.
Time reversal of steady-as-she-goes motion (constant velocity) is bound to be less anomalous than accelerated motion. A tennis ball moves at constant velocity left to right; ditto right to left in a reversal. A tennis ball hit by the server decelerates (due to friction) as it moves toward the opposite court. If the opposing player returns the ball in the normal course of events, then the ball will also be decelerating as it approaches the original server. Reverse the film, and when you get to the original beginning, the original server finds the ball accelerating as it approaches. How very odd.
Time reversal in the micro world is a lot less anomalous than in the macro. Drop a uranium atom and a piece of dry pasta in a pot of boiling water. Reverse the picture. Which now looks odd – the micro or the macro? In the world of the micro, an electron and a positron meeting up for a date will quickly find their relationship in strife as they mutually annihilate and give off energy. But, the reverse is also possible. The vacuum energy can spontaneously produce matter-antimatter particle pairs. Now look at a chemical macro situation. An acid plus a base will react to form a salt and water. For example, the base sodium hydroxide (NaOH), will react with hydrochloric acid (HCl) to form table salt (NaCl) and water (H2O). But note that salt water, brine – our oceans – don’t revert naturally back into sodium hydroxide and hydrochloric acid. Chemical reactions that proceed of their own accord in one direction can of course be reversed, but it takes a bit of prodding. Without evidence of that prodding, the reverse reaction looks odd. So, running a film of that chemistry experiment backwards will look odd.
Summaries:
There’s nothing strange happening here: Still life. Motion too can be within this category. Toss a ball up in the air and it comes down again. If you reverse that, you still get an up and then down trajectory. Dig a hole and make a pile of dirt; take a pile of dirt and drop it into a hole.
That’s possible: A rocket ship can blast off and head straight up; a rocket ship, all engines blasting, can descend and land. While that’s certainly not the way the Redstone, Atlas, Saturn rockets and space shuttle worked, it’s possible, as the Lunar Surveyors and LEM and how the Mars Viking surface craft, landed. It’s possible to film the launch of a V-2 and reverse the film and conclude that such an event was possible.
That’s odd: Events showing objects without front-back symmetry moving contrary to expectations is certainly odd, but possible. Humans can walk backwards. Cars can travel in reverse gear. Birds can fly backwards if the wind is strong enough; ditto fish if the current is strong enough. Seeing a plane fly backwards is probably odd enough as to send you heading off to see the optometrist, the shrink, or attend AA meetings.
That’s really weird and highly improbable: You’d place viewings of apparent violations of the 2nd ‘law’ of thermodynamics (like a box of air molecules all moving in such a fashion that the faster/warmer molecules congregate in one half of the box and the slower/cooler molecules get together in the opposite half); and events hinting at antigravity, in such a category. Another case might be relativities so called ‘twin paradox’. Take two identical twins. One stays at home and the other boldly goes and travels at near light speed to a faraway destination somewhere out there, then reverses direction and heads for home. Upon arrival home, the boldly going twin appears a lot younger than her stay-at-home twin sister. Reverse the chain of events and you’ll have the stay-at-home sister growing younger as a result of her twin gallivanting around the cosmos! Take an inflated balloon and pop it, or let go of the end and watch it fly erratically around. Reverse that image. You’re sure to bet your life’s savings which viewing is forwards and which is backwards.
That’s impossible: There’s only one real example here. Film a macro object entering a Black Hole. Reverse the film. Since nothing (except Hawking radiation) can escape from the gravitational pull of a Black Hole, any film showing such an event is not just suspect and highly improbable – it’s flat out impossible.
Conclusion: Watching film in reverse can be fun and if you think about what you’re watching – the behind the scenes – educational.
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