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…
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.
To be continued…
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