Arrow of Time - Sixty Symbols

what is the arrow of time it's the simple fact that the past is different from the future right if you were an astronaut in your spacesuit far up in orbit far away from everything else there would be no arrow of space all the directions you could point in would be equally good you wouldn't even notice you wouldn't be able to say I was facing up or facing down but not obviously that's not true for time everyone agrees on which direction of time is yesterday in which direction is tomorrow for example we can remember things that happened yesterday none of us can remember things that happen tomorrow even though things will happen so there's a million ways in which one direction of time moving toward the past looks different to us than the other direction moving to the future and that directedness from past to future is the arrow of time this is a great example of a problem that only exists because you learn a little bit you didn't think that it was a problem you know you don't see Aristotle or Descartes worrying about the arrow of time right well what happens is we developed some notion of the fundamental laws of physics starting with Galileo and Newton and working up through Maxwell and Einstein and others there's a feature of Newton's laws for example which is that they don't have an arrow of time they treat directionality toward the future and directional a toward the past exactly the same one way of saying it is if you if you make a movie of an egg breaking and being scrambled and you play it backwards it's obvious that you're playing it backwards right you notice it that's the arrow of time you can notice when someone reverses it but when you make a movie of a pendulum swinging back and forth very simple system or you made a movie of the earth going around the Sun another very simple system if you played those backward you wouldn't be able to tell that they were reversed and that's because we need a small number of pieces fundamental physics without dissipation and friction and noise and all that stuff there isn't any arrow of time so the the puzzle that we need to explain is how to reconcile the absence of an arrow at the microscopic fundamental level with the obvious presence of the arrow in our everyday lives but with that pendulum in the earth you say there wasn't an arrow there was an arrow just it just wasn't obvious it was just harder to say because of the visual properties of the system no no if you if you look at the equations look at the equations for the earth going around the Sun they're reversible right if you take the earth put it in the same position but reverse its velocity it will exactly retrace the path that it took so the fundamental laws of physics have this property that for every way that you can evolve forward in time there is an exactly corresponding way that you can both evolve backward in time the macroscopic world is irreversible you can take the AG and you can scramble it but you can't unscramble the egg very easily or to put it this way if you come across like a cool glass of water it might have been the 10 minutes ago it was a cool glass water or it might have been a warm glass of water with an ice cube in it you can't tell the difference from the present state of the system whereas Newton's laws say that if you know the present state of the system you can always uniquely tell us where it came from but with your glass of ice water or your scrambled egg why is it not the case that it is reversible just our feeble Minds couldn't possibly come up with the complex equations well it is at some level so if when you looked at the glass of water instead of seeing water you saw with your eyeballs the position and velocity of every water molecule and your brain was good enough to do the calculation then you could reverse it and that is the secret to reconciling the microscopic reversibility to the macroscopic irreversibility namely that in the macroscopic world we don't see everything we only see some coarse-grained features of reality when we have the air in this room we don't see the position and velocity of every air molecule and it turns out that in that coarse-grained description of reality the past is very very different from the future and the way that we characterized this is by entropy of course entropy is the way the physicists talk about the disorderliness the messiness the chaoticness of the macroscopic universe and there's a law of thermodynamics the second law of thermodynamics very famous says that entropy increases toward the future that's easy to understand actually getting Messier is a very natural thing the hard thing to understand is that entropy was lower in the past that's the same statement as entropy increasing toward the future but there's no easy dynamical principle of physics that would explain why the entropy was ever low in particular in the real world it was low 13.7 billion years ago near the Big Bang our our observable universe started in a state of exquisite order and then ever since it's been expanding and cooling and disorderliness has come into being so really it becomes a cosmology question we understand the equations behind the arrow of time what we don't understand is why was ever put into the universe at the Big Bang does time flow just in wonder like a river then or are you saying it is going both ways well time flowing has always been a somewhat dicey kind of simile or metaphor because flowing means changing with time and time doesn't change with time we we speak about time we don't use very precise language very often but I think you know an accurate way of saying the same question is does time truly have a directionality or at the fundamental level or both directions of time on an equal footing I would say that at the fundamental level both directions of time are equal that when we observe the arrow of time in our real world it's just like the arrow of space here on earth remember we did the analogy with the astronaut we had to go up into orbit far away from the earth here on the surface there's a difference between up and down I should say down and up right why is there that difference well because we are in the vicinity of a very influential object the earth that's what gives space its arrow locally what I'm saying is that the reason why there's an arrow of time is precisely analogous there's an arrow of time in our environment because we are in the vicinity of a very influential event the Big Bang there's two kinds of questions that arise here one is a good kind of question one is it not so good kind of question but not so good kind of question is there are many many people out there who just deny the fact that the origin of the arrow of time is the fact that our early universe had a low entropy there were debates that went on it from the 19th century from the you know 1870s with Boltzmann and Maxwell and Gibbs and Lowe Schmidt and Zermelo and others trying to figure out how to reconcile microscopic reversibility with macroscopic irreversibility and they sort of solved the problem halfway they said if you have a low entropy condition long ago then entropy increases and everything follows what they didn't do is explain to us why we had a low entropy boundary condition very long ago modern cosmology says you know we can blame it on the Big Bang it doesn't solve the problem right it just puts it off to the Big Bang but we don't mention the Big Bang when we teach people thermodynamics or statistical mechanics because that's all about predicting the future it's not about retro dick ting the past so many people including people who know a lot of physics don't get that you need to put a boundary condition at the Big Bang to understand the arrow of time those people are just incorrect the other thing of course is well if you do want to explain why the early universe had a low entropy what's the right explanation does inflationary cosmology already explain it does Stephen Hawking's wave function of the universe already explain it you need a multiverse to explain it she just accept it as a brute fact and not look for explanations these are very good questions these are all viable alternatives on the table it's part and parcel of what we discuss in modern cosmology you live on earth as a human you know you had to be here at 3 o'clock you've got to be on your plane tomorrow time rules your life as a human when you think about time as much as you do on this higher plane does it affect your everyday life do you distill all these times that somehow seem irrelevant or other two very separate parts of your personality well I think that the way that we human beings very correctly perceive the world is a multi-leveled way and especially if you're a working physicist you better think this way and you know you don't need to use the standard model of particle physics to scramble your egg that would make life very very difficult indeed so we have different kinds of vocabularies we have this fundamental vocabulary of particle physics and quantum mechanics and relativity and then this higher level vocabulary of thermodynamics and fluids and and objects and solids and an even higher level vocabulary human beings and their wants and desires and aspirations and thoughts and rational reasons and so forth so they all fit together there are connections between the different levels and it's all one consistent story I believe we haven't figured out all the details of that consistent story but I get just as anxious when I'm running late as anyone who doesn't know anything whatsoever about the arrow of time the Higgs boson is a tiny little vibration in a field called the Higgs field just

Channel: Sixty Symbols

Views: 410,932

Rating: 4.9464865 out of 5

Keywords: time, sean carroll, arrow of time

Id: 9VFGuupXwng

Length: 9min 36sec (576 seconds)

Published: Mon May 20 2013

Reddit Comments

[u/HamThief]

Sean Carroll doesn't get enough dap for being such a fantastic ambassador for physics. Great video.

[u/BlazeOrangeDeer]

The analogy between the earth and the big bang is awesome

[u/Stormdancer]

Predicting the future, retrodicting the past. What a fantastic word.

[u/bawki]

One of my favourite channels on youtube.

[u/Shelleen]

The thing I can't wrap my head around regarding entropy: If the big bang started as a pretty homogenous disordered mass with very very small variations and then sort of crystallized into stars, planets and a pretty stable geometry - how is that not entropy in reverse?

[u/Enum1]

i like the other post more!

[u/comport]

I don't like muddying the comments of this subreddit with my non-physicist comments, but since this is a general interest physics video anyway -

I don't think Sean Carroll convincingly answers the question, or acknowledges the question Brady Haran? asked, that couldn't entropy just be something that seems to emerge because of human limitations in seeing the details of very complicated systems.

As a video intended to educate non-scientists, I think that's something he should have talked about. He seems to just sidestep it by bringing out the second law of thermodynamics, like "hey, it's a law of nature okay!". I wish they'd paused the recording and thought of a way to phrase an answer.

[u/AD-Edge]

The way he talks about being aware of the big bang and taking it into account for the arrow of time makes me think he's hinting that time 'started' at that point. Which then makes me think that in order for something to happen, time needs to exist right? (because without time nothing would happen)

Feels like another of those 'which came first' questions when you start to think about it - The big bang or time?

[u/ffwiffo]

Here's a fantastic paper summarizing the difficulties many prominent physicists have had regarding the big bang boundary point of the arrow of time:

Huw Price: 'Cosmology, Time's Arrow, and That Old Double Standard'

http://arxiv.org/abs/gr-qc/9310022