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ETpro's avatar

Why does time only run in one direction?

Asked by ETpro (34503points) August 15th, 2010

We all are familiar with the Arrow of Time. Even those who have never studied time as a concept in physics would find it quite strange if one day they found themselves walking backwards on a street, and as they backed past the china shop, they noted that a bull was backing around in it. As the bull backed up, various shards of china would fly from here and there to assemble themselves into complete tea kettles and cups, then bound from the floor back onto the shelves as perfect tea sets and other assorted pottery.

Finally, the china shop now orderly, the bull would emerge from the broken glass of the china shop door, butt-first of course, and as his horns cleared the opening broken shards of glass would fly up from the china shop floor and the sidewalk outside, and assemble themselves into an unbroken pane of glass in the door.

Any of us, no matter what our scientific training, would think we had either gone quite insane, or somehow fallen into a movie running in reverse. Outfielders simply don’t catch a pop-up fly before the batter swings at the ball.

And yet there is nothing about Newtonian mechanics or quantum mechanics that doesn’t work just as well when calculated backwards in time as forwards. If you know the conditions now, you can calculate previous states just as easily as you can future states. So if the laws of physics describe how the world we live in works, why is there an Arrow of Time?

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13 Answers

Dr_Lawrence's avatar

I suggest you read Stephen Hawking’s A Brief History of Time . You may find your answer there.

mrentropy's avatar

Would we notice if it did?
I read another book (wish I could remember the title and author) that put forth the idea that all of time has already happened, it’s how our consciousness moves through it that makes it look like everything is in motion. Light a point of light going over a series of pictures on a certain path.

Finally, the Red Dwarf episode, Backwards, is one of my favorites.

Winters's avatar

Ever hear of the tachyon particle (I hope I spelled that right)? Supposedly its a particle that goes backwards in time for a little bit. So technically you could detect a nuclear explosion shortly before it happens.

ETpro's avatar

@Dr_Lawrence Thanks. It’s actually on my reading list, so I will be doing so—when I get the time. :-)

@mrentropy I think we would notice, just as certainly as we notice when a video is run in reverse. Even if we accept that we are always moving backward in time, we would then notice if something moved forward. We can see the odd man out.

@Winters Tachyons are hypothetical particles that move faster than the speed of light—if they exist, that is.

Winters's avatar

@ETpro thats why I said, “supposedly,” lol.

Trillian's avatar

As I understand it, time is not linear, but the limitations imposed on us by our physical bodies only allow us to perceive time flowing in one direction. I undersand time to be happening all at once. This, supposedly, is why some people have precognitive flashes. When we step out of the physical body, the limitations are no longer imposed on us and sometimes we can remember what has not yet happened in our perception but of course has/is happened/happening

chocolatechip's avatar


If time is running in reverse, then your brain functions are running in reverse also, and so I would speculate that to you, nothing would seem out of the ordinary.

ETpro's avatar

@Trillian I have never seen any credible evidence people actually do have premonitions, or that all of time happens simultaneously. Seems the stuff of science fiction or pure fiction.

@chocolatechip Whichever way time is actually flowing, we do notice reverses in that direction. Running a motion picture film in reverse shocks our senses. We do not just fall into believing that watching a baseball game in reverse, from the final score in the 9th inning to the opening pitch, is perfectly normal.

chocolatechip's avatar

@ETpro “Running a motion picture film in reverse shocks our senses. We do not just fall into believing that watching a baseball game in reverse, from the final score in the 9th inning to the opening pitch, is perfectly normal.”

That’s because running a motion picture film in reverse is not reversing time. Time is still flowing “forward”, only you’re playing the frames in reverse order. If you were to reverse time, everything would be reversed, not just movement. That means your thoughts brain functions, including your thoughts, would be reversed.

ETpro's avatar

@chocolatechip We atrated with an unsupported postulate that time is one giant loop and repeats. Now it not only repe4ats, but some mysterious, undetected force causes it to see-saw backward and forward, all completely undetected. This is an awful lot to swallow simply to dispense with answering why time seems to inexorably proceed in a single direction. What observed phenomenon support either the continuous loop or the constantly reversing postulates? How are the falsifiable?

Trillian's avatar

@ETpro I knew you were going to say that.~

daytonamisticrip's avatar

Is there an end to space? what is beyond space?

ETpro's avatar

@daytonamisticrip That’s another question that can get us all wrapped up in conundrums. It would seem that spacetime is defined by the matter or particles it stretches between. That being so, then there is nothing beyond spacetime. But that nothing is permeable and when particles invade it, it then becomes part of spacetime. In an expanding Universe, this process of claiming new spacetime is continuous.

Some have postulated that the curvature of spacetime makes it a closed system floating in a vast sea of nothingness. But this doesn’t negate your question. Nor does it jibe with observed data on the mass in the Universe, which is insufficient, per the math of general relativity, to produce such a dramatic curvature.

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