Brian Cox explains why time travels in one direction – Wonders of the Universe – BBC Two

Brian Cox explains why time travels in one direction – Wonders of the Universe – BBC Two


Back in the 19th century engineers were concerned with
the efficiency of steam engines. How hot should the fire be? What substance should you boil in the steam engine? Should it be water? Should it be something else? These were profound questions and out of those questions arose the science of thermodynamics. It’s when concepts like heat
and temperature and energy enter the scientific vocabulary
for the first time. Along with that deeper understanding emerged what is probably the most important
law of physics for understanding heat, evolution of the universe and the passage of time. It’s called the Second Law of Thermodynamics The reason the second law
of thermodynamics was so profound, was because at its heart it
contained a radically new concept. Something physicists call entropy. Entropy explains why,
left to the mercy of the elements, mortar crumbles, glass shatters and buildings collapse. And a good way to understand how
is to think of objects not a single things, but as been made up of many constituent parts like the individual grains that make up this pile of sand. Now, entropy is a measure of how many ways
I can rearrange those grains and still keep the sand pile the same. And there are trillions and trillions
and trillions of ways of doing that. There’s pretty much anything I do to this sand pile. If I mess the sand around and move it around then it doesn’t change the shape or the structure at all. So, in the language of entropy this sand pile has high entropy because there are many many ways
that I can rearrange its constituents and not change it. Now, let me create some order in the universe. Now there are approximately as many sand grains in this sandcastle as there are in the sand pile. But now virtually anything I do to it will mess it up,
will remove the beautiful order from this structure. And because of that the sand castle has a low entropy. It’s a much more ordered state. So. Many ways of rearranging the sand grains
without changing the structure: high entropy. Very few ways of rearranging the sand grains
without changing the structure, without disordering it: low entropy. I imagine now to leave the castle in the desert all day. Then it’s obvious what’s going to happen. The desert winds are going to blow the sand around and this castle is going to disintegrate. It’s going to become less ordered.
It’s going to fall to bits. But think about what’s happening
on the fundamental level. The wind is taking the sand of the castle and blowing it over there
somewhere and making a sand pile. There’s nothing fundamental
in the laws of physics that says that the wind couldn’t pick up
some sand from over here, deposit it here and because of it
in precisely the shape of a sandcastle. So, in principle the wind could spontaneously build a sandcastle and the pile of sand. There’s no reason why that couldn’t happen. It’s just extremely, extremely unlikely because there are very few ways of organizing
this sand so that it looks like a castle. It’s overwhelmingly more likely that when the wind blows the sand around,
it will take the low entropy structure, the castle, and turn it into a high entropy structure, the sand castle. So, entropy always increases. Why is that? Because it’s overwhelmingly
more likely that it will. It seems incredible that a law that says that sandcastles don’t spontaneously form on the wind could solve one of the deepest mysteries in physics. But by saying entropy always increases the second law of thermodynamics is able to explain why time only runs in one direction.

100 thoughts on “Brian Cox explains why time travels in one direction – Wonders of the Universe – BBC Two”

  • Time travels! Bahahahahah wait…in one direction!!! Bahahahahahahahahahahahaahah Here, Bra I am Cocks, the shill defender of the orifficial standards of science. A well paid brit and time, nothing new, got too old. Wake up.

  • Seth Spurlock says:

    I gotta stop watchin this mf while I’m high I be wantin to quit my job and study stars and shit and also Brian wth you in the damn desert for anyway you know they got sand at the beach?

  • Sonof Osiris says:

    You do not understand something until you can explain it to someone so that they'll understand it! – A wise man said that.

  • Sorry mate you dont know what time is or if and how it travels, just like you dont know or understand Infinity, even if you do speak In a lovely soft voice 😁

  • I get it 🤔 the sandcastle that is not very well made is a metaphor for Brian Cox explanation on why time only moves forward.

  • I'm not a scientist but I believe time cant go back because of cause and effect. I think time is a mathematical equation that is constantly running and our time is set in stone somewhere.

  • If really disorder is increasing with time, then why do we have more human (population) and better living condition than it's in the past ?

  • crawford rae says:

    So if we did not exist then that sand castle would not exist so the words coming out his mouth would have no meaning and the sand would still be there?

  • Patrick Hannon says:

    I suppose it means given that the state of entropy always increases, then the likelihood of Time traveling backwards instead of forwards is a phenomenally less so, therefore only travels in one direction… just an uneducated guess, it is a poorly titles video though… On the bright side we all know how entropy works now  LOL

  • I still don't get why, but I now know that time moves in one direction, since I won't be getting back that 5 minutes and 32 seconds of my life.

  • "Entropy always increases" says part of the universe as it spontaneously lowers the entropy of a pile of sand.

    Does life itself not work directly against entropy? It uses energy to gather disordered high entropy matter into highly ordered, low entropy matter, and the living biosphere as a whole adds millions of tonnes of biomass to itself every year. Do physicists just forget that life is part of the universe?

  • But quantum mechanics shows that time is NOT unidirectional, but a continuum. Or does Brian prefer to disregard that for some reason?

  • Time apparently goes in one direction according to Cox, but he does not know that for sure….but cox only goes in one direction, to the totalitarian left, we know that for sure

  • Shame he got the theory of relativity completely wrong…I think he does it all by a script someone does for him and he tries his best …bless

  • Let me explain what I understood from it.
    Imagine you have a time machine and a piece of glass. You break the glass and go back 5 seconds at the moment before you break the glass. Entropy won’t allow the glass to un-break. Therefore, time travelling to past is not possible! It is only one directional.

  • N Tropy hey. "You're on my time now you little skunk, give me the crystals!" Boioiong =P

    When Brian was talking about the sand it reminded me of some lyrics to a song called Vampires by The Midnight "they say we come from nothing and to nothing we'll return". All things return to matter eventually. I have a theory that black holes are recycling engines of the Universe too. When one Universe gets to full, one comes along and balances things a bit by sending stuff through to another Universe.

    Lovely video. Very thought provoking too. And to the ones that are saying this doesn't explain how time only goes forwards, seriously. I'm quite dull when it comes to science but without time going in only one direction, Entropy wouldn't exist / happen. Silly buggers ; )

  • Enhbayr Bilegt says:

    Is it because future is way more complex than past? Im guessing he might have said past has low entropy but still i couldn't get to know why. My best guess is:complexity thing.

  • bikramjit singh says:

    Understood entropy but why time travels in one direction. May be because of particles accumulation and structural change.

  • I think the current definition and understanding of Entropy may be limited in the following situations
    1) our consciousness i.e our memory of the sandcastle is still there despite wind blows it high entropy, if we consider our memory also as part of the material world, unless one develop memory loss or death ( which is low entropy).

    2) At the edge of the universe where it is expanding into ….. we don't know for sure it is a high or low entropy

    3) Inside the black holes

    4) Also, it does not explain where is my tomorrow or my next minute or my next year is coming from? Is my future already made or created as I move forward?

    5) What had happened to my yesterday? vanished? still there? if still present then my past is with low entropy and still exists.

    at all the above philosophical arenas the entropy definition breaks down !!!!

  • So basically, time can in principal move backwards but, it's so unbelievably unlikely due to high entropy being fundamentally more common than low entropy (i.e the wind destroying a sand castle rather than the wind building a brand new sand castle).

  • "Entropy always increases. Why is that? Because it's overwhelmingly more likely that it will"

    That sounds a lot like "I don't know why. It just does".

  • Carlos Drummond de Andrade Sulmoneti says:

    Amazing explanation! I never could understand if time travels booth directions or not, now I can! I love the documentaries of BBC!

  • For anyone having difficulty with understanding why this explains the arrow of time, think of time as an illusion we experience as entropy keeps increasing.

  • Can you tell us what happened to that hole in the ozone layer we had crammed down our throats in the 80,s & 90,s…or was it just fake…😴

  • Time and the Universe were created by God just like your sand castle. Is that not an observable fact ! Everything in this Universe was set to work perfectly" In the Beginning" as per Genesis 1:1 of the Bible. Ever since then the Universe is ' ONE BIG UNWINDING CLOCK' . The world is a stage Time and Life is the gift of God to the Creature Man to Choose God and His plan for His life ! Though Man chose to rule out God from His own world ( and still does!) God is giving Him time to repent and turn to Him . 'The Heavens Declare the Glory of God and the Firmament His Handiwork' says the thundering words of the Bible in Psalm 14:1 . Time to wake up and think and return home to the Creator. One day everything will be folded up like a parchment on which man Earth is just a dot and Man shall stand before His Creator and see Him . But seek Him today for one day all science will prove beyond doubt that the Creator exists. But that day may not be given to the man of today . Today is your day !

  • I hope tv licence fees dont pay the fuckhead who titles these videos. Also take the ads off YouTube videos the licence is paid to have no adverts on bbc videos. What makes you think you can get away with it on here even though watching anything using a tablet requires you to have a licence ??

  • This is not an actual scientific way of explaining entropy, it is just an analogy for non scientists. Entropy is not a measure of the disorder, that is just incorrect

  • When you say that the sand castle could be blown to another part of the desert does it mean Imagine one day you are walking on a beach and you see a gigantic sand castle and you realise it was from a sandcastle competition from another side of the world that was blown there ? This is odd

  • the flawed and unbalanced laws of thermodynamics… incompletes, non natural, when you see nature you clearly see that it wont be posible if entropy were all that there is… and regrentopy is actual a military concept used and documented… once science complete this and leave time out of the equations human tech will make it to a whole new level.

  • I think his point is that because probability dictates entropy will most likely increase seeing something's entropy increase is most likely seeing something go forward in time whereas if entropy decreases it's most likely going back in time (like a rewound video)

  • alexandra charles says:

    If they make these episodes for young children, should they not merely be played for children in school instead of dumbing down people who are dumb enough anyway.

  • The first law of thermodynamics is you do not understand thermodynamics. The second law of thermodynamics is YOU DO NOT UNDERSTAND THERMODYNAMICS!

  • From: https://www.livescience.com/50941-second-law-thermodynamics.html

    "The arrow of time

    The Second Law indicates that thermodynamic processes, i.e., processes that involve the transfer or conversion of heat energy, are irreversible because they all result in an increase in entropy. Perhaps one of the most consequential implications of the Second Law, according to Mitra, is that it gives us the thermodynamic arrow of time.

    In theory, some interactions, such as collisions of rigid bodies or certain chemical
    reactions, look the same whether they are run forward or backward. In practice, however, all exchanges of energy are subject to inefficiencies, such as friction and radiative heat loss, which increase the entropy of the system being observed. Therefore, because there is no such thing as a perfectly reversible process, if someone asks what is the direction of time, we can answer with confidence that time always flows in the direction of increasing entropy."

  • pretty simple really.. our perception of "time" is based on physical phenomena that has been set in motion. thats all it is.. there is no such thing as "time", we invented it.. we based the idea on physical objects we can see and measure but "time" is our invention. problem is that stopping and reversing motion does not mean you go back to where you were before. it just means you stopped it and started it again in the opposite direction…(reversing in this analogy is a terrible description as the universe has no concept of reverse, only motion)… not that useful unless you need to get home at the end of the night and certainly not going to allow you to try the whole night again like groundhog day.. if you want to "time travel" you will have to find a way to break what we know currently as the laws of physics, which of course is possible. Though , seriously, how many people are trying? and no i am not talking about trying to get close to light speed and allowing time to pass at different rates as described by relativity. i am talking about tapping in a date to a machine and you pop up when you chose to go. not one person i can think is working on that… can anybody name anyone working on that? and if so how far have they got? and no i am not talking about atoms.. atomic level physics has been shown to be quite different to the nature of physics at the human level. there is no reason to think that if you can send atoms in one direction or another that you can do that with humans. is anybody working on a machine that can transport humans forward and backwards in time.. the smartest engineers on the planet, any of them?

  • probably time is constant. Waves from infinity past to infinity future, and from infinity future to infinity past. Human perception is not the same(and entropy). Energy is constant, energy is not like entropy.

  • This was obviously designed for kindergarten kids. But they, too, would probably been bored before it was over.
    The BBC has gone a long way downhill since days of yore.
    .

  • All physical processes (and the science that describes them) are probabalitistic. The chances that his random pile of sand will spontaneously rearrange by wind or other natural phenomena into a virtual copy of his castle are LOW but NOT ZERO. Given an infinite period of time, everything that is 'possible' must eventually happen. As to the 'time' argument, time is not an actual 'thing', it's just that not everything takes place simultaneously.

  • Ordered systems tend to move towards disorder which is why Time never travels backwards hence the entropy of an ordered system will tend to increase over time

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