Physicist: Reversing time seems to reverse how things work. Instead of growing, plants shrink. Instead of going forward, airplanes fly backward. And, “intuitively”, instead of falling down, things fall up. If you have a video of someone jumping into water, then they’ll always fall downward if the video is played normally, and they’ll always fly out of the water and upward if you play the video backward.
If you “rewind time” a couple of seconds you’ll find everything as it was a few seconds ago. So the problem here is that several seconds ago you were not falling from the sky. If you’ve been walking around here on the surface of the Earth recently, then in reverse-time you’ll still be walking around on Earth. Just backwards.
So, if you reverse time, things will not fall up.
Turns out that, in general, physical laws can’t tell the difference between time running forward or backward. The one very big exception is entropy.
As long as things aren’t slamming into each other gravity is time-reversal-invariant.
The second (and most awesome) law of thermodynamics says that entropy increases in time (technically, it just doesn’t decrease). Gravitation is a beautiful example of time-symmetry. So long as the gravitational interaction doesn’t increase entropy, you’d never be able to tell whether or not time is running forward or backward. A good way for gravity to increase entropy is to make things hit each other. In that case you’ve got heat being generated, stuff breaking, things flying around, that sort of thing.
So, orbits (which don’t increase entropy) are the same forwards and backwards, but cannonballing into a pool (which increases entropy a lot) only makes sense forwards.
Reversing time does flip the direction that things are moving, but the universe couldn’t possibly care less about how what direction things are moving (this is a less elegant way of describing relativity). But weirdly, reversing time does not change the direction of acceleration, which the universe does care about. So an orbiting planet switches direction, but the force of gravity (the acceleration) still points inward. Isn’t that cool?
You (and many other writers on the subject) assert, “Turns out that, in general, physical laws can’t tell the difference between time running forward or backward. The one very big exception is entropy.”
There is also quantum wave function collapse. But measurement impacts future values of a wave function, not the prior ones. (Depending on whether we figure out what it is, and whether it’s even real…. The current “in” interpretation, decoherence, may or may not make this a symptom of the thermodynamic arrow of time, the jury is still out.)
On a different note, say we have a closed system that can be modeled as two closed systems (A & B) up to t[1] as well as after t[2] — only connected in the time between t[1] and t[2]. The state of B after t[2] does depend on A, but the state before t[1] does not.
In English, explaining by example. A boy walks past a pond, noticing the waves on it. He picks up a stone and skips it on the pond, and walks away. The waves on the pond before he skipped the stone would be the same whether or not the boy came by. The waves on the pond afterward, does not.
The skipped stone represents a boundary condition for the waves after it skipped, but a discontinuity for the waves before.
How does entropy explain this kind of assymetry?
(And since all of physics are differential equations, starting with F = ma [acceleration being a 2nd order derivative of location], we could state any example of causality in terms of this assymetry.)
I know that’s a big question, I hope you have the time to respond, and would appreciate it if you did. (Or pointed me to someone who does.)
“But weirdly, reversing time does not change the direction of acceleration, which the universe does care about.”
Would this be sufficiently explained by the fact acceleration is twice the time derivative of position? So you end up with (-1)^2 factors, hence, no sign change.
Would it be possible in theory at least, that an imaginary component of Time pointed backwards? In other words, some version of “doppelganger” of mine would experience in his future what I experienced in my past, and what I experience in my future, my doppelganger would have already passed through? Just weird thoughts, I admit, not backed up by solid proof, still…
Is it theoretically possible to reverse time? Or even causing time to stop for even one very very small period?
This is interesting question and I think time and gravity are two different things like themperature and for exaple speed the objetct move, or mass and shape
Ofcourse we can make them dependeble if we connect them and we can do it in many ways but in general they dont have the same relation.If we can make time go slower than object will fall down slower but the gravity force acting on it will be the same.And if we can reverse time than i think earth would spin on the oposite direction than now and to go on the work in the morning you shoul go to oposite direction than today.Nature always find way not to disturb its laws.
Thank you Physicist, this is absolutely fabulous!
I have a question slightly relating to the above: Supposing the universe was an oscillating one (which most scientists are agreed it isn’t, worse luck!), what would happen when all the matter in the universe reached its limit and started to contract? Would time “run backwards”? How would we as a life form experience this contraction? Let’s suppose for the sake of argument that we lived long enough to witness most of it.
If the expansion of the universe was chaotic and it was, you can never go precisely back into the past due to the sensitive dependence to initial conditions. Trajectories in space-time, even backwards, cannot be repeated.
The example with gravity, i.e. a planet orbiting a star is not a good one to prove the time-reversal-invariance as just the opposite is true – a planet NEVER really goes the same path twice around the star. The cross section of the trajectory set (with large enough number of rotations) will prove to be fractal!
@Lucas:
Basically yes! That’s the quick, back-of-the-envelope way of showing that.
@anton:
Not even remotely possible (using any kind of technology, even in theory).
@Maria:
There are some people who think that the Arrow of Time, the increase of entropy, and the expansion of the universe are all related or somehow the same thing. Personally, I’d guess that a shrinking universe would seem about the same as the universe we live in now, just getting smaller.
I don’t think the phrase “reverse time” even has meaning. If everything runs the other direction, what makes it “the other direction”. The Physicist discusses a question that boils down to whether a movie when watched in reverse would defy the laws of physics. But if everything were reversed, so would the watcher — and so everything would look like it’s running forward. The question requires a frame of reference; time was reversed compare to what?
This is a slightly related question, but can gravity also lower entropy, at least locally? Although it can cause things to hit each other, which causes extra heat and disorder, it also is arranging them in a smaller area of space in the case of things like a planet, which seems to increase order.
a very nice video I used to illustrate an article on this topic :
http://www.youtube.com/watch?feature=player_embedded&v=YDKmFipygWY
(my article on CPT symmetry, in french if anyone cares : http://www.drgoulu.com/2009/04/04/miroir/ )
The only way to reverse time is to reverse the entropy.One can imagine reversing the direction,momentum,energy etc of these particles.Then by doing so reversing time can be fullfilled.
If gravity was reversed would time run backwords because gravity stretches time so amplified gravity amplifies time would that mean reversed gravity reverses time in other words if scientists created a machine that reverses the gravity of an object such as a plant not just would it fly upwards would it age backwords would this happen or not maybe under the right sircumstances?
Looking at the question from a physical dimensions point of view,
acceleration, including gravitational acceleration, has units of [m/s^2], therefore sign of time doesn’t matter and will allways produce positive results, that can help to explain why “things will not fall up”.
However, other physical quantities, such as velocity for example, which has the units of [m/s], clearly, this is sign variant, so negative time will produce negative velocity, which can help to explain why people and animals and planes will be moving backwards.
A more interesting question (in my eyes) what can happen to electricity??
Knowing that electric charges have dimensions which linearly depend on time, this [sA] (amper second). What will happen with negative time? reversed charges lead to reversed electric forces? Lightbulbs “taking away light”? Computers that require computation power to function?
Think about it. mind blowing.
Lucky for us we can’t really imagine what will happen if time is reversed, since time is a dimension we don’t fuly understand. Is it proper? Does it have a reference? Where is the zero point of time? As long as we don’t know the answers to theses questions, any further discussion on reversing time remains strictly theoretical.
So if a parachutist was falling from two miles high, and was about to reach the ground, and the time is reversed at that moment, he won’t go up, he still will fall to the ground, so laws of physics are not time reversible.
If you agree to also reverse gravity, you’ll have bigger problems.
Laws of physics are not time reversal, clearly.
I don’t understand why physics are so blind on this subject.
I prigogine and others proved that all the real physical processes are the dissipative ones and thus irrevercible.This defines the time arrow.
I think it’s worth mentioning that events like the time reversal of diving into a pool can be explained as the water suddenly moving in just the right way and with just the right amount of force to propel the person out of the pool back onto the diving board.
Would it reverse entropy? Yes, absolutely. This will never actually happen, anywhere, ever. But it doesn’t violate any physical laws of motion.
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