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← What Is Time? One Physicist Hunts for the Ultimate Theory

What Is Time? One Physicist Hunts for the Ultimate Theory - Comments

Enlightenme..'s Avatar Comment 1 by Enlightenme..

So at max entropy, time ceases to exist?

Sun, 28 Feb 2010 16:39:00 UTC | #444860

Haymoon's Avatar Comment 2 by Haymoon

Time is, was and will be

Sun, 28 Feb 2010 16:55:00 UTC | #444866

Cook@Tahiti's Avatar Comment 3 by Cook@Tahiti

That was one big exercise in hand-waving. Good round-the-campfire talk with a bottle of port.

" expostulate what majesty should be
what duty is, why day is day, night night, and time is time
were nothing but to waste night day and time"

- Polonius, Hamlet

Sun, 28 Feb 2010 16:57:00 UTC | #444867

Jay Cee's Avatar Comment 4 by Jay Cee

There are things that happen, like you turn an egg into an omelet, but you can’t turn an omelet into an egg.

Not strictly correct.

Feed only omelettes to a chicken and the chicken will still lay eggs. Dan Dennett.

Sun, 28 Feb 2010 17:13:00 UTC | #444871

andrew.trapp's Avatar Comment 5 by andrew.trapp

Many people have pointed out the entropy relationship to time. For me though, the most striking feature of time is simply that the past is determined, while the future is undetermined.

While we live in a universe governed by natural laws, it is not a deterministic universe. The Heisenberg uncertainty principle shows us that real, genuine randomness exists, thus making a strictly deterministic future impossible. And yet, our past seemingly IS determined.

This makes sense with a multiverse theory. If you imagine time constantly branching off at every point, then looking to the future there are many paths you could take, but looking to the past there is only one path. This also neatly solves the "grandfather paradox" of time travel: if you were to travel back in time to kill your grandfather before you (or your parents) were born, from the point you arrived in the past you would simply follow a new forward timepath. You (as a time-traveling adult) would continue to exist after you killed your grandfather, because your having been born is already in your local, determined past. So there are (at least) two universes now, one in which you exist up to the point where you travel back in time, and one in which you don't until you pop into it from the future. When you travel back in time, you are not only leaving your time, but your universe as well. Different timeline, different universe.

So time travel also has implications for the 2nd law of thermodynamics. If it is possible (and yes, that is a big "if"), then it is possible to violate the 2nd law in a universe, but it would still apply to the multiverse as a whole. This is much the same as saying that entropy can decrease locally if it still increases overall universally. When you time travel to the past, you are transferring some matter (yourself) from one universe (timeline) to another. I imagine this also means that you are transferring some entropy from one universe to another.

This would be easier to explain with pictures, I know. Hopefully this is not too confusing....

Sun, 28 Feb 2010 18:02:00 UTC | #444880

JoeT's Avatar Comment 6 by JoeT

Can you explain your theory of time in layman’s terms?


Sun, 28 Feb 2010 18:34:00 UTC | #444891

Richard Dawkins's Avatar Comment 7 by Richard Dawkins

test comment

Sun, 28 Feb 2010 19:11:00 UTC | #444897

bnightm's Avatar Comment 8 by bnightm

Why would the entire universe have to reach maximum entropy for it to produce new universes? Because if he's just postulating that random quantum fluctuations are able to spawn these universes why would he need a neutral baseline in the first place?

Sounds very "just so".

Sun, 28 Feb 2010 19:43:00 UTC | #444904

InYourFaceNewYorker's Avatar Comment 9 by InYourFaceNewYorker

There is a really interesting book by physicist Ronald L. Mallet called Time Traveler. It's a memoir about how his father's sudden and untimely death in 1955 inspired him to study physics and try to build a time machine. The memoir chronicles his research, and ultimately what he learns about the possibilities-- and certain impossibilities-- about time travel into the past. Very interesting read; I highly recommend it.


Sun, 28 Feb 2010 20:17:00 UTC | #444910

Donald's Avatar Comment 10 by Donald

Sean Carroll also gave a TED talk:

Interesting and thought-provoking talk.

Sun, 28 Feb 2010 20:22:00 UTC | #444912

GregfromCanada's Avatar Comment 11 by GregfromCanada

Yah, I'm not seeing it. This idea really doesn't explain anything: where does this quiet, empty, parent universe come from?
It also seems pretty convenient that this QEPU is just active enough to occasionally spawn new, non-quiet and empty universes, yet isn't anything like these new universes.
What about this idea that these universes can bump into one another? Doesn't that imply there is some greater area that all these universes inhabit? Whats that place?

I don't know, but my understanding, admittedly limited, is that The Universe is the penultimate place. There is no outside, just inside. Now whether or not our universe is this place is unknown, but even if we find it is not and there are many universes, there still needs to be such a place for these universes' to be in.

As far as time goes, it is, as someone said earlier, a product of motion. Without time there is no motion, without motion there is no need for time. Our universe is not static, therefor there is time.

Sun, 28 Feb 2010 20:34:00 UTC | #444929

BigJohn's Avatar Comment 12 by BigJohn

Time flies like an arrow: fruit flies like a banana.

Sorry, I couldn't resist.

Sun, 28 Feb 2010 21:27:00 UTC | #444992

vega's Avatar Comment 13 by vega

andrew trapp - I concur with your travelling back into the past creating a new timeline. And the problem also that you cannot travel back to your own time/universe. Travelling back or forward in time creates new timelines. The question is, could we navigate these timelines? Perhaps each timeline/universe has a unique frequency or such like that can be used as a beacon...but of course this creates further paradoxes...

Sun, 28 Feb 2010 23:07:00 UTC | #445070

Friend Giskard's Avatar Comment 14 by Friend Giskard

Time present and time past
Are both perhaps present in time future,
And time future contained in time past.
If all time is eternally present
All time is unredeemable.
What might have been is an abstraction
Remaining a perpetual possibility
Only in a world of speculation.
What might have been and what has been
Point to one end, which is always present.
Footfalls echo in the memory
Down the passage which we did not take
Towards the door we never opened
Into the rose-garden. My words echo
Thus, in your mind. LOL

Sun, 28 Feb 2010 23:21:00 UTC | #445079

SaintStephen's Avatar Comment 15 by SaintStephen

What is time?

Tough question. In this entertaining and educational video, rockstar physicist Brian Cox is still trying to figure out what time it IS:

And in other news...

Sean Carroll: Great post, huh?
Sean B. Carroll: Not really. I've done better on
Sean Carroll: You? Mr. Evo-Devo Hippie? I thought Devo's best song was "Whip It Good".
Sean B. Carroll: Like you know music. You're still figuring out what time is.
Sean Carroll: Google contest?
Sean B. Carroll: Okay, Google contest: *Google search with keywords "sean carroll*

Sean Carroll: Yes! Check it out. Top billing.
Sean B. Carroll: That's an ad for your overpriced DVD, dude. That 1st real hit is me.
Sean Carroll: But... wait a minute. That search is specific to Check the entire web: *Google search with keywords "sean carroll*

Sean Carroll: Badda bing!
Sean B. Carroll: You're just insecure.
Sean Carroll: Sore loser.
Sean B. Carroll: Dude, I'm all over the web. Been on TV recently, too.
Sean Carroll: Big deal.


Mon, 01 Mar 2010 02:44:00 UTC | #445176

Cartomancer's Avatar Comment 16 by Cartomancer

I wish to complain in the strongest possible terms about the shrillness and stridency of comment number seven of this thread.

Mon, 01 Mar 2010 04:57:00 UTC | #445223

SaintStephen's Avatar Comment 17 by SaintStephen

I completely agree, Cartomancer.

I marked #7 as "Below Average".

Mon, 01 Mar 2010 06:25:00 UTC | #445244

Ubiquitous Che's Avatar Comment 18 by Ubiquitous Che

Reminds me a bit of a thought I had a few years back.

I always thought that at what we might currently call 'maximal entropy' something weird might happen: Could thermodynamics and expansion energy together somehow break apart a plank-sphere's worth of space.

From there I wondered if it could be possible that space itself is a form of energy. It's already weird to me to consider that matter has some kind of funky, dense relationship to energy.

So I had to wonder - what if space itself is a form of very dense, very stable energy?

If it was very dense, and if in a 'maximally entropic' state of the universe, if thermodynamics is true, eventually the only way for local entropy to increase would be to pick apart a plank-sphere of space and break it into energy.

Could this be a possible explanation for the big bang?

Obviously, I haven't a clue. But I enjoy navel-gazing out loud, and it would make for good fiction to have intergalactic space-ships that use space as fuel.

Mon, 01 Mar 2010 07:15:00 UTC | #445250

wice's Avatar Comment 19 by wice

@andrew.trapp (#5)

For me though, the most striking feature of time is simply that the past is determined, while the future is undetermined.

1. are you sure that the future is undetermined?
2. are you sure that the past is determined?

it may seem like a silly question, but it's really not obvious, at least not for me. as far as i know, the first question is entirely open, and although the answer to the second was thought to be evidently 'yes', some physicists (e.g. stephen hawking) currently suggest that it may be an open question as well.

consider a simple cellular automation. its 'future' states are determined by its current state (and by its rules of generation), but its past states are not. you can reach the same state from several different past states.

let's suppose that it's possible to build a consciousness with a deterministic turing machine. i wonder if we built a neural network from a cellular automaton, that could act as a 'conscious observer' of the world of the automaton, would it see its world as something that has a determined or undetermined future and that has a determined or undetermined past?

Mon, 01 Mar 2010 10:11:00 UTC | #445274

SaintStephen's Avatar Comment 20 by SaintStephen

A "Lawrence Krauss" commented recently on Jerry Coyne's blog, regarding Chris Mooney's decision to teach American children about the three R's: Rejecting Reason and Rationality:

Could this person be Arizona State University Professor of Physics Lawrence Krauss? If so, maybe he would like to stop by and briefly weigh-in on Sean Carroll's intriguing ideas. Carroll's concepts would seem to be slightly at odds with Krauss's "Universe began from nothing" lecture presented a while back on

Mon, 01 Mar 2010 10:14:00 UTC | #445276

Roland_F's Avatar Comment 21 by Roland_F

The topic from Sean Carroll is also available as Quantum to Cosmos (Q2C) festival video :
"the origin of the universe and the arrow of time":

Mon, 01 Mar 2010 10:33:00 UTC | #445278

AMreasonedthinker's Avatar Comment 22 by AMreasonedthinker

Ahh Quantum Fluctuations!!!

Since I saw the Krauss lecture 'A universe from nothing' (Brilliant BTW - thanks Richard) These little critters seem to have been rearing their heads much more frequently - through all the chaos and confusion, I can't help but get the feeling we might be on the verge of something remarkable RE insights into true nature of reality. Great time to be here.

P.S I have just been made aware of the 'debacle' occuring over past couple of days. Whilst I have no wish to 'get into' what happened (Really do not know enough of the facts) I would just like to state my thanks to Richard and Josh for creating and maintaining a fantastic site!!! Many thanks.

Mon, 01 Mar 2010 10:51:00 UTC | #445285

Severus's Avatar Comment 23 by Severus

Time compresses History
History compresses Time

That's why we count today in the minutes and hours
but count the past in years, Milleniw and era's.

Mon, 01 Mar 2010 12:54:00 UTC | #445336

Eyerish's Avatar Comment 24 by Eyerish

I'm sure that Sean Carroll is highly intelligent person. But why not just admit that he was running late for his anniversary dinner with his wife because of a calculus problem he was working on, instead of coming up with this brilliant, bamboozaling and difficult to understand excuse to avoid the obligatory "WELL!! Why are you late for our most important night - who is she?".

Ah the lengths that some people go to; to come up with away avoiding an argument with their wives. LOL...

Mon, 01 Mar 2010 13:57:00 UTC | #445372

Szymanowski's Avatar Comment 25 by Szymanowski


The Heisenberg uncertainty principle shows us that real, genuine randomness exists, thus making a strictly deterministic future impossible. And yet, our past seemingly IS determined.

What aspect of the uncertainty principle depends on time? I thought that, like most of what we know about physics apart from entropy, the uncertainty principle did not refer to time.

Mon, 01 Mar 2010 14:07:00 UTC | #445379

aquilacane's Avatar Comment 26 by aquilacane

What time? No such thing from what I can tell. Only the observance of cause and effect. Can't have one without the other.

Mon, 01 Mar 2010 15:01:00 UTC | #445410

Isaksson's Avatar Comment 27 by Isaksson


That is one of the things that, when I start to think about it, makes my head spin. The only thing my mind screams to me at the end is, Time=Movement!

Then my mind shuts down for a reboot.

Fri, 05 Mar 2010 06:17:00 UTC | #446695