The laws of physics do not recognize the arrow of time, at least on the micro or quantum scale. The fundamental particles do not age. Their interactions can proceed in either time direction - past to present to future, or, future to present to past. Since the macro (like us), is made up of the micro, how come we age? Macro relationships often tend to be unidirectional, past to present to future only, what we call time's arrow or the arrow of time.
Make a film. Run it backwards. That's what I call a pseudo time reversal thought experiment. We've all seen them. They are usually funny, not so much funny ha-ha, but funny peculiar. So, at what point do you go from a viewing where there's 'nothing strange happening here', to 'that's possible' to 'that's odd' to 'that's really weird and highly improbable' to ultimately 'that's impossible'. Or, at what point do you go from acceptance of what you are seeing, albeit back-to-front, to suspecting a time reversal, to knowing you're watching a film backwards, and are their any generalities that can be drawn from this? Let's look at some ordinary examples.
There's a man sitting in his living room with the lights on, crossing and uncrossing his legs. Reverse the film - no anomaly. The man now picks up a book and reads it from front to back. Reverse the film - It's now slightly odd in that the book is being read from back to front. The lights go out. Reverse the film and the lights come on - nothing strange. The lights go out and the man picks up a flashlight and turns it on. Reverse the film - this is very strange as the light from the flashlight is instead now entering the flashlight from somewhere stage left (or right). The lights come back on and the man lights up a cigar and smokes it. Reverse the film - Now you absolutely know the film is the reverse because the cigar smoke and ash turns into cigar which grows ever longer!
Let's film that same man sitting in his chair for one hour. Run the film backwards and the odds are you won't see anything too strange. Now film the man for one second each day over the course of a decade - roughly the amount of film. Reverse the latter film - you should now certainly see something strange. The man is getting visibly younger!
Let's film a military person standing at attention for say ten minutes. Now that's not exactly an exercise in still life because the person is still moving - the eyes are blinking, the person is breathing, the heart is beating, etc. So, run the film backwards and there's nothing overly unusual. The eyelids open and close; the chest expands and contracts, the heart beats. But if you could look a bit deeper, beneath the surface, you'd note a lot of strange activity. Photons aren't entering the person's eyes; they are exiting (so in the reverse film, the person is blind). The person is breathing in lots of carbon dioxide and exhaling lots of oxygen. The heart may be beating, but the blood is flowing in the wrong direction! Food in the stomach is becoming undigested. Those are odd happenings indeed.
You're standing on the railway platform filming as the express train, the '4:50 from Paddington' (thanks Agatha) thunders by blowing its whistle. Reverse the film. There's nothing odd to be seen here. Well, trains can travel in both directions (especially if they have an engine at both ends) and you can't tell standing on and filming the platform which direction Paddington is in as there is no signage to that effect visible. But, the change in pitch of the train's whistle (the Doppler Effect*) will not be the same regardless whether it thunders by left-to-right or right-to-left. As you are filing, as the train approaches and passes you, you go from hearing a high pitch (the sound waves are compressed) to a low pitch (the sound waves are now spread apart). Reverse the film, and the order is low pitch to high pitch, even though the train is approaching (in the reverse film viewing) and receding (in the reverse film viewing). That's contrary to expectations. Ah, but also in the film frame is the station's clock, the hands of which are now going counter-clockwise! That gives the game away - unless you assume that's due to an electrical fault! [*Contrast with the filming of a galaxy moving ever faster away from us - the expanding Universe - with insert of the galaxy's spectrum lines shifting ever further towards the red. Reverse the film and the galaxy is now approaching us and the spectrum is moving in the other direction - towards the blue. That's exactly according to expectations.]
Say there's a tree swaying in the breeze. Reverse the film - no anomaly. Keep filming, and note the clouds are moving from right to left. Reverse the film - still nothing unusual. Keep rolling the film and watch the light get dimmer and dimmer as dusk approaches. Reverse the film - okay, so it's now dawn dawning. You see a bird fly away from one of the tree branches. Reverse the film - now that's odd. The bird is flying toward the tree branch, but tail first. Still, maybe it's just a very strong gust of wind blowing the bird backwards. The bird is flying away from the tree branch because the wind has cracked and broken the branch and it falls to the ground. Reverse the film - now you know its time reversed as the branch moves from the ground up and attaches itself to the tree.
Film the city late at night. The office lights go on and off; the neon signs are flashing, and the traffic lights are doing their red-green-amber-red-green-amber-red thing. Reverse the film - it's now slightly odd in that the traffic lights are going red-amber-green-red-amber green-red. That's not the normal sequence. Still, maybe its an electrical fault. Resume forward filming and see this lone automobile cruise on up the street. Reverse the film - so the car is now moving backwards. Well, it's odd but cars can move in reverse. Resume normal filming. The car is actually being driven by a drunk driver and he hits a pedestrian killing same before crashing and totally destroying his car by driving at speed into a building. Now reverse the film. Now you know it's a reversal because cars don't un-crash and the dead don't come back to life.
Film a clear sky that slowly over time shows a cloud developing and turning into a traditional storm cloud with anvil top. Reverse what you just filmed. There's no anomaly as clouds can dissipate as well as form. Continuing filming and watch the storm cloud develop a funnel and turn into a waterspout hitting the water before running out of puff and returning to its parent cloud. Reverse - nothing unusual to be seen, the water looks the same before and after regardless of time flow. Resume filming and note the funnel reforming and becoming a waterspout again, but there's this unfortunate ship that's caught up in the event and sinks. Reverse the film and because you rather suspect that ships don't un-sink, you know it's a time reversal image you're watching. Continue normal filming and see this cloud produce ball lightning that dances and floats around. Reverse the film, and I'd suspect you wouldn't suspect anything. Resume filming, and see the ball lighting go 'poof' and disappear. Now reverse the film and see the ball lighting 'un-poof' and appear from out of nowhere and out of nothing. That's very odd, but you're no expert on ball lighting so maybe it's possible. Now our cloud, in normal filming mode, produces a traditional sideways, cloud-to-cloud lighting bolt and crash of thunder. Reverse the film. What's odd, and a giveaway that the film is running back-to-front, is that you'll hear the thunder before you see the lightning. [It's also slightly odd in that the thunder starts off faint and builds in intensity, finally cutting out fairly abruptly - not in a fade-away fashion as per norm.] If that doesn't cinch it, continue filming while the cloud produces a hailstorm that batters the waters below. Reverse the picture - hail doesn't go from the ground up (hail can go up and down and up and down again and again within its parent cloud due to updrafts and downdrafts, but once it reaches the surface, that's the end of the journey).
Finally, film a symphony orchestra playing a concert. Reverse the film. There's probably nothing unusual to be witnessed. Now add the sound of the music and reverse the film - now that's weird (the 'music' that is), but maybe it's just some of that highly modern experimental atonal 'music' - except the concert program said it was to be an all Mozart concert, not that that is definitive proof - maybe it's a different Mozart! Now resume filming in the normal way and get the audience's applause. Reverse the film. There's nothing too odd here except, like the thunder example above, the applause started off softly, softly, builds up in intensity, and cuts off fairly abruptly.
One can easily come up with thousands or ordinary everyday scenarios that can evolve from being time reversible without question to 'whoa' - that's funny peculiar.
Things left to the natural way of things tend to go from order to disorder. If you see the reverse, it's suspect, probably highly improbable, but not impossible. It's just the second 'law' of thermodynamics in operation, but it's a statistical 'law' that can be broken, but rarely is, due to chance. There are lots of possible arrangements for disorder; few arrangements for something in a ordered state, so statistically, the odds favor disorder and thus you have to go to a lot of time and effort (energy expenditure) to go from disorder to order. Thus, seeing a film of a nuclear explosion in reverse, or a firearm shooting bullets in reverse or a plane un-crashing is suspect.
For example, pour out a really hot cup of coffee then start filming. Reverse the film. Apparently there's nothing odd to look at - just a cup of coffee - except, if you've filmed in infrared. If you run that infrared film backwards, you'd have the coffee get ever warmer, and the surrounding environment get ever cooler. In real life, the coffee would cool down and the immediate environment would warm up slightly. So, this is a case of things appearing normal, backwards or forwards in the visible spectrum, yet odd indeed with viewed backwards in the infrared.
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's apple bonked him on the head instead of doing a Superman imitation. Film a waterfall. Reverse the film. Oops!
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.
