This view of time does not look encouraging for time travelers

April 25, 2011 2:01 PM

The concept of time as a way to measure the duration of events is not only deeply intuitive, it also plays an important role in our mathematical descriptions of physical systems. For instance, we define an object’s speed as its displacement per a given time. But some researchers theorize that this Newtonian idea of time as an absolute quantity that flows on its own, along with the idea that time is the fourth dimension of spacetime, are incorrect. They propose to replace these concepts of time with a view that corresponds more accurately to the physical world: time as a measure of the numerical order of change.
posted by finite (127 comments total)

[readability link]
posted by Foci for Analysis at 2:12 PM on April 25, 2011

Time is an illusion. Lunchtime doubly so.
posted by joannemullen at 2:13 PM on April 25, 2011 [19 favorites]

Ack! I'm late!
posted by pianomover at 2:16 PM on April 25, 2011

An example of an experiment in which time is not present as a fundamental entity is the Coulomb experiment; mathematically, this experiment takes place only in space.

Erm, I thought Coulomb's constant was thought to be changeable over very long periods of ... time?
posted by ZenMasterThis at 2:19 PM on April 25, 2011

“In our view, time travel into the past and future are not possible,” Sorli said. “One can travel in space only, and time is a numerical order of his motion.”

If this puts a stop to all the bullshit time traveling in science fiction - I AM LOOKING AT YOU, STAR TREK - the researchers deserve the nobel prize.
posted by Foci for Analysis at 2:21 PM on April 25, 2011 [9 favorites]

This is fantastic, can't wait to read it.
posted by empath at 2:22 PM on April 25, 2011

Oh, goodie. Another physics FPP I can not understand.
posted by Zed at 2:23 PM on April 25, 2011 [2 favorites]

My Timex is never wrong
posted by Postroad at 2:25 PM on April 25, 2011

Sigh, I never could get the hang of Thursdays.
posted by maryr at 2:26 PM on April 25, 2011 [4 favorites]

Time is just a construct of the human mind that prevents everything from happening all at once.
posted by rocket88 at 2:26 PM on April 25, 2011 [6 favorites]

But, as they note, we never really measure t. What we do measure is an object’s frequency, speed, etc.

And something to note -- for a great many systems, it's possible to translate from time to frequency -- to essentially make time disappear. That is, you can take the transformation of a system over time and change it into the sum of an infinite number of oscillations.
posted by empath at 2:28 PM on April 25, 2011

Does anyone have a link to the actual paper?
posted by empath at 2:28 PM on April 25, 2011

Ah, here it is...

Physicists from Northeastern University believe that a fundamental force of nature, the bond between electrons and protons, has been strengthening since the Big Bang. In fact, they believe it might have been 200,000 times weaker ten billion years ago ...

Personally, I think all these physicists have just been making this shit up so we'll keep funding them. And don't get me started on fuckin' magnets...
posted by ZenMasterThis at 2:29 PM on April 25, 2011 [5 favorites]

Time is just a construct of the human mind that prevents everything from happening all at once.

Everything did not happen at once before the first Homo Sapiens. The world does not stop existing when I close my eyes.
posted by justsomebodythatyouusedtoknow at 2:29 PM on April 25, 2011 [1 favorite]

The world does not stop existing when I close my eyes.

Don't be goin' all Ayn Rand on us in this forum, pal.
posted by ZenMasterThis at 2:31 PM on April 25, 2011 [5 favorites]

“Einstein said, ‘Time has no independent existence apart from the order of events by which we measure it,’” Sorli told PhysOrg.com. “Time is exactly the order of events: this is my conclusion.”

Why are events ordered at all? This assertion, that time is nothing but the order of events, is tautological. He says, time allows for events to be ordered but the ordering of events is nothing but time. So I'm not sure what is being said here.
posted by kuatto at 2:31 PM on April 25, 2011 [1 favorite]

One of the consequences of relativity theory is the relativity of simultaneity, which does what it says on the can. There is no absolute present, only an immediate present.

The world does not stop existing when I close my eyes.

The answer to this is, well, maybe.
posted by mek at 2:33 PM on April 25, 2011

So while 4D spacetime is usually considered to consist of three dimensions of space and one dimension of time, the researchers’ view suggests that it’s more correct to imagine spacetime as four dimensions of space. In other words, as they say, the universe is “timeless.”

Isn't this a really old view? I thought nearly everyone thought this. What are these guys suggesting that is different?
posted by painquale at 2:34 PM on April 25, 2011 [1 favorite]

Why are events ordered at all?
They aren't. Our experience of the events, though, is.
posted by Thorzdad at 2:34 PM on April 25, 2011 [1 favorite]

Why are events ordered at all? This assertion, that time is nothing but the order of events, is tautological. He says, time allows for events to be ordered but the ordering of events is nothing but time.

I think he's arguing against the concept of 'space-time', not against the concept of time.
posted by empath at 2:35 PM on April 25, 2011

I'm just a biologist, but this whole article seems very foolish to me. It's an old idea & I don't think the physorg people are portraying it well.

First of all, Hawking describes this *exact* idea in 1988's Brief History of Time - and I doubt he came up with it.

Secondly, the statement that we never directly measure time is absurd. Can we ever directly measure a anything? Mass is just the gravitational force something poses, charge is just how it moves in reference to a magnet, etc. etc. The intrinsic nature of the universe is measured by proxy.

So yes, time is the direction in which entropy increases. It's probably not a particle.
posted by Buckt at 2:37 PM on April 25, 2011 [2 favorites]

What's happening here, in this paper, is the total elimination of a fourth spacetime dimension with interesting results. Not only is there no absolute time, there's no dimension which can be considered time at all.
posted by mek at 2:38 PM on April 25, 2011 [1 favorite]

This guy might be a bit of a crank.
posted by empath at 2:38 PM on April 25, 2011 [3 favorites]

I think he is talking about time as well, not just space-time

“Minkowski space is not 3D + T, it is 4D,”

Well, I guess you can call it whatever you like.
posted by kuatto at 2:38 PM on April 25, 2011

Let me put it another way. They say that there are four spatial dimensions, and let's say I say that there are three spatial dimensions and a temporal one. How is that not just a terminological debate over how we are applying the words 'spatial' and 'temporal'?

Do they think the math works out differently or something, or is their discovery all conceptual? If it's the latter, I don't think there's really anything to this.
posted by painquale at 2:39 PM on April 25, 2011

EARTH HAS 1 CORNER
SIMULTANEOUS 1-DAY
TIME POINT
posted by oulipian at 2:39 PM on April 25, 2011 [14 favorites]

First!!!!!1
posted by hincandenza at 2:41 PM on April 25, 2011 [26 favorites]

Here's the paper itself. And yeah, I'm not convinced anything new or meaningful is in it.
posted by mek at 2:42 PM on April 25, 2011

Turns out all the primary author's articles are from Physics Essays, which a bit of Googling reveals to be not a reputable journal.
posted by painquale at 2:44 PM on April 25, 2011 [2 favorites]

Yeah, this seems really cranky. The guy has also been published here.
posted by empath at 2:45 PM on April 25, 2011

I've been saying the same shit for years, but people would rather fumble on using a simple convention that they can barely come to grips with, never mind master.

You want chaos and confusion? Cross the international date line and look at people's faces when they're alerted that shit's on fire, yo. Priceless.
posted by jsavimbi at 2:45 PM on April 25, 2011

Well, I think he is correct in that if we relabel the dimension of time it at least forces us to recognize our crude characterization and conceptions of time for what they are. Most people consider time be something like a timeline, which is not quite right. Let's discuss that!
posted by kuatto at 2:46 PM on April 25, 2011 [1 favorite]

Everything did not happen at once before the first Homo Sapiens. The world does not stop existing when I close my eyes.

Prove it. Go ahead, I'll wai...
posted by ChurchHatesTucker at 2:47 PM on April 25, 2011 [1 favorite]

posted by finite

I CANT BELIEVE NO ONE HAS SAID EPONYSTERICAL YET
posted by to sir with millipedes at 2:48 PM on April 25, 2011 [2 favorites]

This is garbledygook. Either it's terrible science reporting or new-agey bunk. I can't tell which right now because I'm on my phone.
posted by Salvor Hardin at 2:49 PM on April 25, 2011

Or both.
posted by Salvor Hardin at 2:53 PM on April 25, 2011

"Time may change me, but I can't change time."
posted by Sys Rq at 2:55 PM on April 25, 2011 [3 favorites]

This guy might be a bit of a crank.
posted by klangklangston at 2:58 PM on April 25, 2011

Well, what he is presenting makes sense, it's just that it is not particularly interesting. Let's not shift labels around, let's attack the labels directly. And by attack I mean intellectually investigate and seek to understand.
posted by kuatto at 2:58 PM on April 25, 2011 [1 favorite]

At least after all these years we've solved Zeno’s paradox of Achilles and the Tortoise!
posted by nikolayevich_ray at 3:01 PM on April 25, 2011

Where is physicsmatt when you need him!
posted by Hairy Lobster at 3:02 PM on April 25, 2011

Going to get a beer, that's where. Seriously people, two physics threads at once? I guess I've run out...

*takes off mirror sunglasses, squints*

... of TIME.

*yeaahhhhhhhh*
posted by physicsmatt at 3:05 PM on April 25, 2011 [40 favorites]

It's my humble opinion that any discussion of the nature of time without mentioning entropy is less than useless. This paper doesn't look like much more than mathematical trickery.

Not that mathematical trickery can't be very useful, but I can't really see the point of this one.
posted by auto-correct at 3:08 PM on April 25, 2011

They propose to replace these concepts of time with a view that corresponds more accurately to the physical world:

Onemyprettypony... Twomyprettypony... Threemyprettypony...
posted by quin at 3:09 PM on April 25, 2011 [1 favorite]

Scott Adams was right!
posted by peeedro at 3:12 PM on April 25, 2011

It's my humble opinion that any discussion of the nature of time without mentioning entropy is less than useless.

You can talk about time without mentioning entropy. Entropy doesn't have anything to do with a lot of physical systems.
posted by empath at 3:15 PM on April 25, 2011

Since I have nothing to say about this paper (which seems like a solid effort from the field of psychoceramics), I'll add to what auto-correct said by saying that, if you treat time as a 4th spatial dimension, you find that your equations for evolution in time become identical to those in thermodynamics, with the replacement of the temperature with (i*time). i being sqrt(-1).

What this has to do with entropy (a thermodynamic quantity) and its relation to time (which seems to be deeply intertwined with entropy, i.e. the arrow of time) are left as an exercise to the reader.

And now, beer.
posted by physicsmatt at 3:16 PM on April 25, 2011

You know how a piece of string can't not have length? That's like us, and time. We are too impressed by things that seem impressive to us. We are bound by the limits of what we are, and by the way our limited senses are forced to perceive those limits. Time is not an absolute thing, it is a way we limited beings perceive one part of reality.

Here's Tom with the weather.
posted by Decani at 3:18 PM on April 25, 2011 [6 favorites]

A friend just wanted to say that Physics Essays is shady.
posted by grobstein at 3:19 PM on April 25, 2011

Also check out co-author Sorli's description of some of his previous work: "Prana Has a Measurable Weight: Additional Weight And Unknown Vacuum Energies Of Living Organisms."

There's a vast tribe of physicists out there whose main strategy for visibility is to generate news-of-the-weird-type stories. You can read their papers about poltergeists on the Arxiv. Is this one of those? I don't have the technical background to say, and at least this story isn't built on dubious empirical results or theories. But it does seem like a confused reworking of well-known ideas about time.

Then again I could just be biased against Italians.
posted by grobstein at 3:26 PM on April 25, 2011

Minkowski spacetime is a question of how to measure distances between points when time is considered as a dimension. It's set up so that points (x1,x2,x3,xt) and (y1,y2,y3,yt) have their distances measured (perhaps with a bit of rescaling involving the speed of light) by the following quadratic form:
d^2 = (x1-y1)^2 + (x2-y2)^2 + (x3-y3)^2 - (xt-yt)^2

The interesting bit is the minus sign in front of the (xt-yt)^2 term. This lets points at different physical points, with different timestamps, have distance zero. These points at distance zero from one another form equivalence classes; you get big three-dimensional sheets. You get these sheets by setting d^2=0; then you can move over the term with the time difference and get:
(xt-yt)^2 = (x1-y1)^2 + (x2-y2)^2 + (x3-y3)^2.

In other words, two points have zero distance if their time-distance equals their space distance. Interestingly, if you pick a specific x=(x1,x2,x3,xt), and consider all the points at distance zero from x with a specific time difference t, you get:
t^2 = (x1-y1)^2 + (x2-y2)^2 + (x3-y3)^2,
which is the equation of a sphere in the 3d physical space. All of the points on that sphere are reachable by a flashlight being shone from the point x. Anything inside the sphere can be reached (and affected) by things moving slower than light. Barring shennanigans, anything outside of the sphere can't be affected by things going on at the point x at time xt.

This is the Minkowski view of the present: it's a big set of points in four dimensions that are connected by 'light lines,' or geodesics.

Now, the thing here is that Minkowski space-time is essentially four dimensional; it's just being measured in a clever way... This is apparently quite a useful way to approach tim0, contrary to the claims of the article. I think trying to move the discussion back to Euclidean 4D measurement is probably a bit wrongheaded.

(Math aside, I'm personally of the opinion that we live on a kind of membrane travelling in the time-like direction, and that the past no longer exists in any physical sense. After all, we only receive information about the past from presently existing phenomena; there is no direct observation of the past available to us. Even the distant supernovae that we see exploding billions of years ago, under the Minkowski metric, are just a very distant present. Back-to-the-Future style time travel would just land one in some empty void, or perhaps a wholly other universe traveling just behind our own...)
posted by kaibutsu at 3:29 PM on April 25, 2011 [8 favorites]

(And before I get torn apart too handily by the experts, let me throw out there that, dammit Jim, I'm a mathematician, not a physicist.)
posted by kaibutsu at 3:31 PM on April 25, 2011 [1 favorite]

As a time traveler, myself, I call bullshit. Time, however, is not the fourth dimension. Time is the path between dimensions. It is more like the forth dimension. But I've said too much. Again.
posted by It's Raining Florence Henderson at 3:34 PM on April 25, 2011 [1 favorite]

(And before I get torn apart too handily by the experts, let me throw out there that, dammit Jim, I'm a mathematician, not a physicist.)
posted by kaibutsu at 11:31 PM on April 25

It shows; you gorgeous, impenetrable bastard!
posted by Decani at 3:50 PM on April 25, 2011 [1 favorite]

Back-to-the-Future style time travel would just land one in some empty void, or perhaps a wholly other universe traveling just behind our own...)

Well, yeah. That would happen even if there was an absolute time dimension, because it would necessarily mean there were absolute space dimensions as well, so you would land on wherever the Earth was at that point in the past, which is virtually guaranteed to be the empty void of space.
posted by mek at 3:59 PM on April 25, 2011 [2 favorites]

First!
posted by Astro Zombie at 4:03 PM on April 25, 2011 [2 favorites]

Dammit.
posted by Astro Zombie at 4:03 PM on April 25, 2011 [1 favorite]

So it goes.
posted by Houyhnhnm at 4:04 PM on April 25, 2011 [1 favorite]

History is an illusion caused by the passage of time. Ttime is an illusion caused by the passage of history.
posted by Greg_Ace at 4:08 PM on April 25, 2011 [1 favorite]

I used to be a philosopher. I thought that philosophers already determined that the notion of "moment in time" was a complete fiction, even if it occasionally came in handy.
posted by yesster at 4:08 PM on April 25, 2011

B(t)=Bacon Time. It's always bacon time. Therefore, time is not an illusion. QEMotherfuckinD.
posted by It's Raining Florence Henderson at 4:10 PM on April 25, 2011 [1 favorite]

I can't see a way to have this and to not also have an absolutely deterministic universe. Which would be pointless, in the philosophical rather than the dimensional sense.
posted by aeschenkarnos at 4:10 PM on April 25, 2011

Time... It takes a licking and keeps on ticking.
posted by fairmettle at 4:16 PM on April 25, 2011

physicsmatt: And now, beer.

Time as a measure of the number of beers, huih? Me thinks you're onto something here...

There's a strong connection between my perception of time and the number of beers I consume and the level of entropy (impatience and annoyed-ness of wife) in my closed system (marriage). The gradually rising level of anger in the sound of my wife's voice during consecutive phone calls, asking when I'm gonna come home, clearly defines an inescapable arrow of time.

Just kidding... my wife's the greatest!
posted by Hairy Lobster at 4:20 PM on April 25, 2011

isitbeeroclock.com
posted by finite at 4:23 PM on April 25, 2011

Entropy is just the universe's way of saying it really wants to buy a sports car.
posted by It's Raining Florence Henderson at 4:31 PM on April 25, 2011

The concept of time as a way to measure the duration of events is not only deeply intuitive...

Uhhh..."but also, by definition, exactly what we mean by 'time'"?
posted by DU at 4:49 PM on April 25, 2011

You know how a piece of string can't not have length? That's like us, and time. We are too impressed by things that seem impressive to us. We are bound by the limits of what we are, and by the way our limited senses are forced to perceive those limits. Time is not an absolute thing, it is a way we limited beings perceive one part of reality.

-Kant
posted by skwt at 5:04 PM on April 25, 2011

-Kan2
posted by It's Raining Florence Henderson at 5:07 PM on April 25, 2011 [2 favorites]

Also "psychoceramics" is a hilarious word
posted by skwt at 5:08 PM on April 25, 2011

"If I could turn back time, I'd look at her butt again." -Garth
posted by drjimmy11 at 5:12 PM on April 25, 2011 [1 favorite]

If I could save time in a bottle, the first thing that I'd like to do is punch Jim Croce in the face.
posted by It's Raining Florence Henderson at 5:13 PM on April 25, 2011

DU: Uhhh..."but also, by definition, exactly what we mean by 'time'"?"

Maybe also what we mean by naive realism. There has been some serious thinking about the nature of what we mean by the word "time" recently, see Peter Lynds and Julian Barbour and my (SL) Time page. I don't think the 4th dimension is necessarily time in string theory, either. The guy does have a lot of prana in his eyes.
posted by psyche7 at 5:19 PM on April 25, 2011

I like to think one day linguists and computer scientists will work togehter, out of a common love of the Star Trek: TNG episode Darmok, and develop software to translate batshitinsanity into understandable phrases. Being an academic venture to improve discourse, they publish this project in peer reviewed journals and make it open source. Soon, people run the code over things they never quite understood.

Eventually, we find out that Timecube contains the Unified Theory of Everything and Alex Chiu (Kevin Trudeau approved! Sort of!) has basically solved the problems of medicine and biology forever. But we didn't believe them because of the batshitinsanity barrier.

Also, we'll finally be able to answer whether or not anyone has really been far even as decided to use even go want to do look more like, in the poster's native tongue.
posted by mccarty.tim at 6:02 PM on April 25, 2011 [1 favorite]

It is all one giant vector but it is being printed on God's ancient 4 column daisy wheel printer he picked up from Radio Shack back in the Tandy days. You perceive dimensions only as a result of an inconvenient forced line break. God says he's going to replace it but he doesn't want to give them his address. He has been stockpiling continuous feed tractor printer paper to make what will undoubtably be the longest banner ever made as soon as the new dot matrix gets ordered.....
posted by humanfont at 6:40 PM on April 25, 2011

speaking of our psychological experience of time, fs sez: "Burkhard Bilger's profile of David Eagleman is one of the great magazine articles of all time" (/em added ;)

cheers!
posted by kliuless at 6:54 PM on April 25, 2011 [1 favorite]

Wrote this 20 years ago: http://webpages.charter.net/stephenk1/Outlaw/time1.html

WHEREZ MY NOBEL?????
posted by LordSludge at 7:00 PM on April 25, 2011 [1 favorite]

Back near the end of 2008, Scott Aronson wrote a dazzling, breathtakingly good and quite accessible blog post asserting that time is essentially different from space but just as fundamental.

To give sense of the flavor of his arguments:

On the contrary, I’ll argue that the computer scientist’s view of the space/time distinction actually leads to something like a prediction, and that this prediction can be checked, not by experiment but mathematically. If reusability really is the key difference, then if we change the laws of physics so as to make time reusable—keeping everything else the same insofar as we can—polynomial time ought to collapse with polynomial space. In other words, the set of computational problems that are efficiently solvable ought to become PSPACE. By contrast, if reusability is not the key difference, then changing the laws of physics in this way might well give some complexity class other than PSPACE. ...

The bottom line is that, at least in the computational world, making time reusable (even while preserving its “directionality”) really does make it behave like space. To me, that lends some support to the contention that, in our world, the fact that space is reusable and time is not is at the core of what makes them different from each other.
posted by jamjam at 7:07 PM on April 25, 2011 [1 favorite]

But yeah, as an engineer, this idea that time is not a dimension, but rather a benchmark of change, really resonates with me. It seems much more intuitive to my simple brain to think of "stopping change" rather than "freezing time", or "accelerating change" rather than "speeding up time". Gravity time-dilation, I see as gravity squishing matter together and making change more difficult, rather than a "warping of space-time".

And of course time travel, as it's typically thought, is impossible. (Yeah, if you could reset every bit of matter in the universe back to its state in 1957, then you can "time travel to 1957". Good luck with that.)

It's all math. If it works, use it. But I do think this time-as-a-dimension thing is a dead end as far as understanding how the universe works.
posted by LordSludge at 7:12 PM on April 25, 2011 [3 favorites]

jamjam- how could time be reusable if it were directional? Could you provide a toy example of time being reused in computation while still preserving its directional character?
posted by Jpfed at 7:24 PM on April 25, 2011

LordSludge, but without time as a dimension, the whole theory of relativity makes no sense. With that dimension, we can understand the experimentally proven time dilation effect (and length contraction, etc), as simply maintaining a constant interval in space-time. Without that extra dimension, you'll have to go through a lot of extra contortions to get that, if you ever get there at all. Also, when in free-fall in a gravity well, nothing gets squished (it's that sudden stop at the end that gets you every time), but time slows regardless.

Anyway, I realize that these ideas are really counter-intuitive (just like quantum mechanics), so I'm not faulting anyone for not buying into the physics party line (well, unless you're a physics teacher in high school). Just thought I'd explain the problems with the classical interpretations that led physicists to the modern understanding. We don't make these things up just to see the look on your faces. That's just a side benefit.
posted by physicsmatt at 7:27 PM on April 25, 2011 [1 favorite]

Definition from my old dictionary:

"Time is a measured succession of events which can be quantified".
posted by ovvl at 7:29 PM on April 25, 2011

Aronson's example is Turing machines that can go back in time. Directionality is preserved by causality.
posted by jamjam at 7:30 PM on April 25, 2011

It's not at all clear to me what is even supposed to be meant by this. I'm having a difficult time understanding what is meant by "dimension" if not "independent variable". Are they claiming to be able to represent what we currently think of as spacetime with only three independent variables?

If that's not what they're claiming, then what are they claiming? Otherwise, it just seems like "for no apparent reason, let's use a different English word that really has no bearing on the underlying mathematics anyway".
posted by Flunkie at 7:34 PM on April 25, 2011 [1 favorite]

A-Changin', The Times They Are
posted by slogger at 7:43 PM on April 25, 2011

posted by finite

I CANT BELIEVE NO ONE HAS SAID EPONYSTERICAL YET
posted by to sir with millipedes at 5:48 PM on April 25 [2 favorites +] [!]

It's funny you should mention that, because you'll never believe what's going to show up in your mailbox tomorrow.
posted by A dead Quaker at 7:57 PM on April 25, 2011 [1 favorite]

boltzmann wept.
posted by ennui.bz at 8:03 PM on April 25, 2011

i have (but haven't read ;) sean carroll's from eternity to here; doesn't he posit a relationship between time and entropy? if not gravity...
posted by kliuless at 8:08 PM on April 25, 2011

From the article:

How it makes sense

In addition to providing a more accurate description of the nature of physical reality, the concept of time as a numerical order of change can also resolve Zeno’s paradox of Achilles and the Tortoise. In this paradox, the faster Achilles gives the Tortoise a head start in the race. But although Achilles can run 10 times faster than the Tortoise, he can never surpass the Tortoise because, for every distance unit that Achilles runs, the Tortoise also runs 1/10 that distance. So whenever Achilles reaches a point where the Tortoise has been, the Tortoise has also moved slightly ahead. Although the conclusion that Achilles can never surpass the Tortoise is obviously false, there are many different proposed explanations for why the argument is flawed.

Here, the researchers explain that the paradox can be resolved by redefining velocity, so that the velocity of both runners is derived from the numerical order of their motion, rather than their displacement and direction in time. From this perspective, Achilles and the Tortoise move through space only, and Achilles can surpass Tortoise in space, though not in absolute time.

Ok, maybe I'm missing something here, but I thought this was settled quite awhile ago, and far simpler than this explanation, which seems to be a non-explanation. (if I'm reading that right, their conclusion is that Achilles never catches the tortoise in their special flavor of time.)

The explanation I thought was fairly intuitively obvious is that Universe has a minimum distance, the Planck length. It's the resolution at which everything 'runs'. You can't halve the Planck length. Either you occupy a given volume or the adjacent given volume; there is no in-between. So you can touch things, and Achilles can catch the tortoise, because at the critical instant of touching/overtaking, the final smallest distance is indivisible, and you cross it. Achilles is behind the tortoise until he's precisely 1 Planck length away, and then the tortoise can't move 1/10th the distance, and Achilles catches up.

That's also the bit that really makes me wonder if we really might not be running in a simulation of some kind. There appears to be an absolute resolution to the universe, kind of like pixels, but unimaginably smaller. And a lot of the quantum weirdness we see appears at least vaguely consistent with some kind of 'lazy evaluation' algorithm, where objects and masses are abstracted and treated somewhat fuzzily until the actual fine result of a given interaction is important, at which point it's fully computed. And it appears that the Universe might be able to 'go back in time' and retroactively change earlier linked quantum events based on events that happen later. If we entangle photons, measure one, and then at a later time force the entangled partner into a given set of states before measuring it, we will find that the original partner also reflects the state change before it even happened.

There's certainly other explanations that fit, but all this weirdness sort of has the feel of an algorithm written by a godlike intelligence or intelligences to conserve computing power for the interesting stuff. Two icy asteroids colliding somewhere in deep space isn't especially interesting; a quantum physicist entangling photons and forcing weird behavior most certainly would be. If nothing else, it would be a big red flag on a massive simulation that Something Interesting Is Happening over on that small planet in that rather obscure galaxy.

Yes, I realize this is kind of a variant on 'brain vats on Jupiter', but I find it interesting to contemplate, and it might even eventually be falsifiable.
posted by Malor at 8:14 PM on April 25, 2011 [7 favorites]

Achilles and the Tortoise doesn't need a concept of minimum length any more than does the question of why two lines of different slopes intersect; the two questions really are the same.

The only issue, to the degree that there is an issue, is that it's an attempt to render mathematics into inexact English (or ancient Greek, or whatever) instead of letting the mathematics speak for itself.
posted by Flunkie at 8:57 PM on April 25, 2011

The explanation I thought was fairly intuitively obvious is that Universe has a minimum distance, the Planck length. It's the resolution at which everything 'runs'. You can't halve the Planck length.

This would solve the problem, but it should be an empirical fact that the universe is granular and has a smallest possible length, not an a priori one.
posted by painquale at 9:10 PM on April 25, 2011 [1 favorite]

doesn't he posit a relationship between time and entropy?

Well, yes and no. The fact that we can tell which way time is flowing has to do with entropy. But time itself exists, entropy or no.
posted by empath at 9:32 PM on April 25, 2011

Perhaps a better way of explaining that is that entropy orients us in time, so that we can tell the difference between 'past' and 'future' in a way similar to how gravity orients us in space, so that we can tell the difference between 'up' and 'down'.

The Z axis wouldn't cease to exist without gravity, and time doesn't cease to exist without entropy. It only makes it harder to tell which way is which.
posted by empath at 9:36 PM on April 25, 2011

And a lot of the quantum weirdness we see appears at least vaguely consistent with some kind of 'lazy evaluation' algorithm -Malor

I've thought the same thing, but the simulation argument seems too extravagant.
posted by bhnyc at 9:57 PM on April 25, 2011

"The explanation I thought was fairly intuitively obvious is that Universe has a minimum distance, the Planck length. It's the resolution at which everything 'runs'. You can't halve the Planck length. Either you occupy a given volume or the adjacent given volume; there is no in-between. So you can touch things, and Achilles can catch the tortoise, because at the critical instant of touching/overtaking, the final smallest distance is indivisible, and you cross it. Achilles is behind the tortoise until he's precisely 1 Planck length away, and then the tortoise can't move 1/10th the distance, and Achilles catches up."

I was always told this was the .999… = 1 problem.
posted by klangklangston at 10:11 PM on April 25, 2011

No dead Quakers in people's mailboxes, please. I don't care how many feet they have!
posted by Earthtopus at 12:12 AM on April 26, 2011

*shrugs*

Useful mathematical abstractions do not necessarily model the actual universe. Ptolemy had some wondrous systems for modeling the heavens; that they worked did not mean the heavens revolved around the Earth as his systems implied.

I personally assume time is just rate of change, a thing that can be altered as the message passing entities of forces are altered.
posted by effugas at 12:56 AM on April 26, 2011

This guy may or not be a crank, but there are serious players worrying about it. E.g.

Newsflash: Time May Not Exist
Julian Barbour & Craig Callender discuss (video)
Barbour: End of Time

And, of course, Einstein: "People like us, who believe in physics, know that the distinction between past, present and future is only a stubborn, persistant illusion."
posted by Twang at 2:28 AM on April 26, 2011

Maybe it's just me, but I can't help but feel that the concept of 'minimum length' (i.e. the Planck length) is a special kind of insanity. It feels too "clean" for this universe - too abstract and binary.

If there really is such a thing, rather than it being a clearly delineated divide ("thou canst be no smaller!"), perhaps its form would be more similar to an event horizon ("you can keep getting smaller and smaller but never actually get this small...")

Ah...yes, beer o'clock methinks.
posted by jet_manifesto at 6:25 AM on April 26, 2011 [1 favorite]

LordSludge, but without time as a dimension, the whole theory of relativity makes no sense. With that dimension, we can understand the experimentally proven time dilation effect (and length contraction, etc), as simply maintaining a constant interval in space-time. Without that extra dimension, you'll have to go through a lot of extra contortions to get that, if you ever get there at all. Also, when in free-fall in a gravity well, nothing gets squished (it's that sudden stop at the end that gets you every time), but time slows regardless.

I thought the theory of relativity said that these things only seem to happen because the observer is in a different frame of reference. The guy only appears to be getting stretched out and slowly, infinitely fading away in a gravity well because the gravity is affecting our ability to observe him. He is, or already has been, smushed to death.
posted by gjc at 7:10 AM on April 26, 2011 [1 favorite]

Maybe it's just me, but I can't help but feel that the concept of 'minimum length' (i.e. the Planck length) is a special kind of insanity.

The planck length isn't really a minimum, it's more that things get really weird when you get to that scale, because you start getting into quantum gravity effects. Strings are about the planck length, but they can be smaller.
posted by empath at 7:12 AM on April 26, 2011 [1 favorite]

I thought the theory of relativity said that these things only seem to happen because the observer is in a different frame of reference. The guy only appears to be getting stretched out and slowly, infinitely fading away in a gravity well because the gravity is affecting our ability to observe him. He is, or already has been, smushed to death.

The whole point of relativity is that there is no privileged frame of reference. Both are correct.
posted by empath at 7:17 AM on April 26, 2011 [1 favorite]

So my big beef with time travel actually comes from entropy: If one can go "back to the past", that would mean the universe is constantly accumulating information about previous states, at a mind-boggling rate.

That would seem to violate thermodynamics.

This wouldn't hold at the extremely small scale, because at that layer you're arguably faster than the very state changes that would be irreversably destroying information.
posted by effugas at 9:35 AM on April 26, 2011 [1 favorite]

If one can go "back to the past", that would mean the universe is constantly accumulating information about previous states, at a mind-boggling rate.

There are a lot of things that are mind-boggling in scope about the universe.

But anyway, I don't really understand how this is necessarily any different from someone who his whole life has been on a train moving to the east saying If one can go "back to the west", that would mean the universe is constantly accumulating information about previous states, at a mind-boggling rate.
posted by Flunkie at 10:15 AM on April 26, 2011

Flunkie,

Look at it this way: A 1TB hard drive stores 1TB of data. I can ship it to New York, I can ship it back to San Francisco, it has the same data.

If time travel worked, I could just repeatedly write 1TB of random data to the drive, and then go back in time at any point to retrieve that data at that time. The universe would effectively be spawning new storage capacity with the arrow of time.

That fairly cleanly violates thermodynamics.
posted by effugas at 10:28 AM on April 26, 2011 [2 favorites]

My point is that I don't really see why we should necessarily assume that, just because we travel in one direction, that means that the things that we have past do not have real existence.

Your example about the hard drive is based on perception of a fundamental difference between going forward in time and going east on a train. The fact that we feel there is such a difference doesn't mean that someone who only traveled east would feel that there is one, nor does it mean that there actually is one.

And "violations of thermodynamics" suffer from the same issue; we've come up with the laws of thermodynamics based upon our perceptions, and our perceptions tell us that we can only go forward in time.
posted by Flunkie at 10:36 AM on April 26, 2011 [1 favorite]

Flunkie- look closer at your example and you will see that the cases you attempt to compare are not analogous.

Your example about the hard drive is based on perception of a fundamental difference between going forward in time and going east on a train.

There's a huge difference. "Going" forward in time need not involve traveling along any spatial dimensions at all. Going east on a train necessarily involves time, though, unless you are willing to posit that you can teleport east instantaneously; this contradicts what I believe to be the current understanding that information cannot be transmitted faster than light.
posted by Jpfed at 11:05 AM on April 26, 2011 [1 favorite]

"Going" forward in time need not involve traveling along any spatial dimensions at all

There's no such thing as 'only' going forward in time. And there's no such thing as motion on a train. It's all relative.

If you're standing still in my reference frame, you're speeding through time at the speed of light (in a way of thinking about it) from my point of view. As you move faster and faster in comparison to me through space, approaching the speed of light, your move slower and slower through time, from my point of view. From your own point of view, your speed through time has never changed, but you'll see me moving slower and slower through time as you speed away from me at a higher and higher velocity.
posted by empath at 11:14 AM on April 26, 2011 [1 favorite]

And a lot of the quantum weirdness we see appears at least vaguely consistent with some kind of 'lazy evaluation' algorithm -Malor

I've thought the same thing, but the simulation argument seems too extravagant. -bhnyc

You might be surprised. There's a wonderful book that explores some of these issues called "Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos". I learned a lot of nuts-and-bolts stuff about quantum interactions, and some of the hypotheses really helped me clarify some my own intimations about the nature of reality. He's a really sharp dude and it's a great read, so instead of trying in vain to summarize it more, here's part of the Amazon blurb:

He contends that the universe itself is one big quantum computer producing what we see around us, and ourselves, as it runs a cosmic program. According to Lloyd, once we understand the laws of physics completely, we will be able to use small-scale quantum computing to understand the universe completely as well. In his scenario, the universe is processing information. The second law of thermodynamics (disorder increases) is all about information, and Lloyd spends much of the book explaining how quantum processes convey information.
posted by nTeleKy at 11:52 AM on April 26, 2011

LordSludge, but without time as a dimension, the whole theory of relativity makes no sense.

Sure it does -- you just need to look at it a different way.

With that dimension, we can understand the experimentally proven time dilation effect (and length contraction, etc), as simply maintaining a constant interval in space-time. Without that extra dimension, you'll have to go through a lot of extra contortions to get that, if you ever get there at all.

That's fine; it makes the math easier. But if it distorts the understand of what's actually happening, physically, and makes bogus predictions (time travel WTF), then there's a problem.

Also, when in free-fall in a gravity well, nothing gets squished (it's that sudden stop at the end that gets you every time), but time slows regardless.

Well, the idea I was positing (oversimplified as "squished") is that gravity makes movement more difficult -- even on a subatomic level -- which is equivalent to time dilation in modern physics. (Less movement = "time slowing")

Anyway, I realize that these ideas are really counter-intuitive (just like quantum mechanics), so I'm not faulting anyone for not buying into the physics party line (well, unless you're a physics teacher in high school). Just thought I'd explain the problems with the classical interpretations that led physicists to the modern understanding. We don't make these things up just to see the look on your faces. That's just a side benefit.

That's funny, because I actually did make up time-is-not-a-dimension just to see the look on physicists' faces. The side benefit is that it may actually yield some useful knowledge.
posted by LordSludge at 11:54 AM on April 26, 2011 [1 favorite]

Well, the idea I was positing (oversimplified as "squished") is that gravity makes movement more difficult

Please explain what you mean by this. Movement in free fall isn't constrained in any way.
posted by empath at 12:01 PM on April 26, 2011

empath- yes, yes. What I should've said in response to Flunkie was "there's no such thing as 'going only east' unless you plan on violating relativity.".
posted by Jpfed at 12:03 PM on April 26, 2011

I did time for violating relativity.
posted by It's Raining Florence Henderson at 12:16 PM on April 26, 2011 [1 favorite]

empath- just pick a reference frame and my post makes sense. You could recast my statement "an observer can see you standing still in space relative to them while still watching you 'move forward in time' (e.g. notice your clock ticking); but the converse will never happen- no observer will see you moving relative to them while your clock is stopped, assuming you have mass."
posted by Jpfed at 12:32 PM on April 26, 2011 [1 favorite]

"So my big beef with time travel actually comes from entropy: If one can go "back to the past", that would mean the universe is constantly accumulating information about previous states, at a mind-boggling rate.

That would seem to violate thermodynamics.

This wouldn't hold at the extremely small scale, because at that layer you're arguably faster than the very state changes that would be irreversably destroying information."

Well, yes and no. If you were to "go back in time," what would that look like? If you're talking about sending information back, then you'd want to locally reverse entropy. Ok, cool, it's possible to locally reverse entropy (at least in the sense that earth is "ordered" but at the cost of massive amounts of solar entropy). So let's say you could send some information back. As things are viewed from the forward direction, that flow of information would be reversed so that you'd be gradually forgetting things (or the data would be unwritten from your drives). You can't write the information and then go back, because you'd have to pass the point where the data was written, reversing the write process.

Still, I like to think it may eventually be possible to encode some backwards-flowing information—call it "artfully forgetting"—but purely as a premise for some kind of SF story.
posted by Eideteker at 12:33 PM on April 26, 2011 [1 favorite]

"assuming you have mass" is a big assumption, because that is precisely the case for the motion of massless particles like photons. Is there some reason you think that time should work differently for massless particles?
posted by empath at 12:34 PM on April 26, 2011 [1 favorite]

Eideteker,

Even reading the prior state of the hard drive is significant. That would basically mean the universe is growing in information content by one universe for each planck second going by.

Unlikely.
posted by effugas at 12:45 PM on April 26, 2011 [1 favorite]

"assuming you have mass" is a big assumption, because that is precisely the case for the motion of massless particles like photons.

Right. That's why I qualified my statement.

If you are a thing with mass, there are consequences in terms of what another mass-having observer can see you doing that appear to create a difference between time as a dimension on one hand and the spatial dimensions on the other. That's what my earlier posts were about.

Now let's pretend that, despite these apparent differences, time as a dimension were exactly the same as the spatial dimensions; after all, massless particles don't show the same pattern. It's just mass-having things with mass-having observers that pretend that time is special. Then how do mass-having things with their mass-having observer partners introduce this difference? How do they know (and seem to agree on) which dimension is different?
posted by Jpfed at 12:52 PM on April 26, 2011 [1 favorite]

You're still confused, I think. Time is treated exactly the same for massless and massive particles. The only difference is that particles with mass can't move at the speed of light, and so time doesn't dilate to the point of stopping.
posted by empath at 12:56 PM on April 26, 2011 [1 favorite]

Does this velocity make my mass look dilated?
posted by It's Raining Florence Henderson at 1:04 PM on April 26, 2011 [3 favorites]

That would basically mean the universe is growing in information content by one universe for each planck second going by.

Most interesting information-growth over a handful of Plancks so far today, IMO.
posted by herbplarfegan at 1:26 PM on April 26, 2011

I've been mentioning various hypothetical scenarios and in general being conversationally clumsy and it would be better for me to say what I mean more straightforwardly.

My main point is that I believe that time is a different kind of dimension than the spatial dimensions. I suspect we actually agree on this. However, I recognize that my understanding may be insufficiently general, and what I see as a difference may be spurious.

My reasoning for this is as follows:

1. At any point, the line L tangent to the world line of a massive particle falls within the light cone of that particle.
2. In flat space-time, that light cone's axis coincides with the time axis, and not "the" x, y, or z axes (in fact, the isotropy of flat space makes talking about "the" x axis ridiculous)
3. So, for flat space, you can't just swap the dimensions around at your whim: time is special.
posted by Jpfed at 1:27 PM on April 26, 2011

>Well, the idea I was positing (oversimplified as "squished") is that gravity makes movement more difficult

Please explain what you mean by this. Movement in free fall isn't constrained in any way.

I'm positing that gravity constrains movement on a subatomic level -- it's harder for change to occur within a gravity field. Or, you could say, "time slows" in a gravity field -- but to me this is ass-backwards, like saying "Winmarks dilated for this processor" when you really mean "this processor gets work done more quickly". I think time is just a benchmark of change. It doesn't exist as a real, physical dimension at all.
posted by LordSludge at 2:10 PM on April 26, 2011

3. So, for flat space, you can't just swap the dimensions around at your whim: time is special.

Oh, right, yeah you can't rotate space into a time dimension and vice versa. In fact usually time has the opposite sign as space in general relativity.

It's convenient to treat time as a dimension (in the geometric sense) like space because in general relativity you're working out geodesics, which are the shortest distances between two points on a curved surface -- but the curved surface is 4 dimensional space-time, not three-dimensional space. There are differences, though -- for example time in the space-time metric has the opposite sign as space.

I'm positing that gravity constrains movement on a subatomic level -- it's harder for change to occur within a gravity field. Or, you could say, "time slows" in a gravity field -- but to me this is ass-backwards, like saying "Winmarks dilated for this processor" when you really mean "this processor gets work done more quickly". I think time is just a benchmark of change. It doesn't exist as a real, physical dimension at all.

Time doesn't slow in a gravity field. Acceleration slows time, gravity merely causes acceleration. Imagine that there is a gigantic tower, with clocks on each floor. You jump off the top. From your point of view, you're in free fall, and in an inertial reference frame with no time dilation. However, from the planet and towers point of view, you're accelerating towards the earth at 9.8m/s^2. The first clock you pass will have some amount of time dilation from your point of view. As you pass the next, you'll be moving even faster, and it's clock will be moving even slower by comparison.

Now here is the part that gets confusing -- as you pass by these clocks -- they will be seeing YOUR time as going slower and slower as you pass each one.

Keep in mind, that at every point where you're comparing the passage of time, you're an equal distance from the distance from the center of gravity and just as affected by gravity. It can't be gravity that's causing time dilation.
posted by empath at 3:20 PM on April 26, 2011

(When I say that time doesn't slow in a gravity field, I mean that it doesn't NECESSARILY). If you're in free fall, it doesn't.
posted by empath at 3:26 PM on April 26, 2011

LordSludge, it's easy to say that there's another way to do GR without time as an extra dimension, it's a little harder to make the math work, and to make physical predictions (I have no doubt you could get the perihelion of Mercury right in some wacky theory without 4D spacetime, but would you get the time-dilation of GPS satellites without putting in by hand? I have no idea, but the theory of GR gets it right without any fudge factor. Now, the cosmological constant: that's a fudge factor). People in this thread are making the (understandable) mistake of using English words in a mathematical context where they have very different and precise meanings.

Also, gravity is entirely too weak to constrain particles "on a fundamental level," whatever that means. The point I was trying to make was that gravity doesn't "squish" a free-falling object, in the frame of the object. The atoms are not closer together, the electron orbitals are unchanged. Furthermore, your theory is postulating that EVERY conceivable physical process that measures time is affected by this "squishing" in the same exact way.

Also, empath, the last point you made is not correct. GPS satellites are in free fall (that is, they are in uncorrected orbits) and their internal clocks tick at a different rate than those on Earth for two reasons: their velocity (the Special Relativity effect), and the fact that they are higher in a gravity well than the surface of the Earth (General Relativity). The two effects are about the same order of magnitude - though if I remember right they have opposite sign- and we need to correct for both of them in order to get accurate GPS location service (since GPS works by asking at least two satellites what time it is).

The fact that GR predicts that you COULD have time travel seems like a sticking point for a lot of people. I should say that it's not at all clear that you can build a macroscopic closed time-like curve that positive energy density can travel along (i.e. a path in space that you follow and end up at the start "earlier" or "later" than you'd expect) in a Universe that doesn't already have one. The energy densities you need seem pretty exotic, it's not conclusively impossible, but it seems extremely hard. To go the "rubber sheet" analogy of spacetime, you need to tear the sheet apart and staple it together in a loop. While it's easy to SAY you can do that, it's not apparent that you physically CAN do that.

So, in my opinion (again, not proven), GR can describe universes with closed time-like curves - in which case yeah, entropy probably gets nutty in that universe. However, our Universe probably doesn't have one. Or, if it did, it always has, in which case, why worry? It had to happen that way, and entropy will figure itself out.

That's the best thing about time-travel, you get to stop worrying about things are going to come about, because they already have.

Finally, the reason I'm so resistant to the idea of time not as a dimension is (in addition to GR), the evidence from quantum field theory. QFT doesn't deal well with gravity, however, it assumes 4 space-time dimensions (you can of course consider other numbers of dimensions, and it works there too, but I'm talking about QFT for the particles we know and love). By assuming 4 dimensions, we limit what operators (interactions between particles) are relevant, which, amazingly enough, turn out to be the interactions we see around us. It didn't have to be this way, and it wasn't realized for some time early in the evolution of the theory. Additionally, many of the "constants of nature:" particle mass, charge, etc, are in fact energy dependent, and how those parameters change (the renormalization group evolution) critically depends on the number of dimensions. There can be other small dimensions, but there can't be less than 4 large ones.
posted by physicsmatt at 6:15 AM on April 27, 2011 [2 favorites]

That's the best thing about time-travel, you get to stop worrying about things are going to come about, because they already have.

Exactly. Thank you.
posted by yesster at 9:43 PM on May 5, 2011