one of the biggest mysteries if not the biggest mystery of our existence is the passage of time the question of why the past is different from the present and the future is such an obvious fact of our existence yesterday there was cheese in the fridge today there isn't why is that okay I can think of a reason for that but if we look at the fundamental laws of nature and the behavior of tiny particles we don't see this difference between forward in time and backward in time so where does it come from a team of
physicists from the UK said out to track down the origin of this time asymmetry and found quite a surprise they say there's no asymmetry time can run into two directions I find this fascinating because it might mean that our universe has a twin in which time runs the other way let's have a look physicists usually attribute the directionality of time to the second law of Thermodynamics According to which entropy can't decrease physicists then say that at the Big Bang the universe for whatever reasons started out in a state of very low entropy it was a
very ordered State and almost perfectly smooth SpaceTime this is the past hypothesis but ever since the Big Bang entropy has been increasing this increase of entropy gives rise to the direct it of time the eror of time as it's called even though the fundamental laws on the level of individual particles are time symmetric this is how this story usually goes and indeed I've told it myself but it isn't quite right because you see the second law of Thermodynamics doesn't say that entropy increases it says that entropy can't decrease it can very well stay the same
and whether it does or doesn't stay the same is a matter of arbitrary mathematical choices to get entropy to increase you need to disregard some details you need to average over them this means that mathematically it's not true that the error of time comes from just the initial state it comes from both the initial State and the averaging at least this was the status so far the new paper's now a mathematically sophisticated way to show that the direction of time in indeed just comes from the initial State the Big Bang they looked at exactly how
the time symmetric gloss of quantum mechanics turn into the time asymmetric loss for larger objects this calculation has been done many times and it works by using some sort of random motion for the particles it's called a markof process and then you average over this if you do this you get a law that has a direction of time this is very perplexing because it seems to not depend on an initial State the direction seems to come from the averaging however the researchers point out that the Assumption about the initial state was hidden in the random
motion of the particles they started out without a memory and then they show that if you remove this hidden assumption if you make the random process truly symmetric in time then the the law for the macroscopic objects also remains symmetric in time this doesn't mean that the time symmetric law actually applies to us that cheese is still gone this is just a mathematical way to demonstrate that the era of time is really entirely due to the initial state of low entropy the Big Bang mathematically the Big Bang is a singularity and infinity in Einstein's theory
however most physicists and that includes me think that their Singularity isn't physically real it's just a sign that our mouths Goes Up in Smoke we need a better Theory to describe it a theory of quantum gravity we don't have this Theory all right but we expect that it somehow does a away with the singularity which means that there could have been something before whatever replaces the Big Bang Singularity and this is where things get interesting because if the Big Bang is the state of lowest entropy then whatever came before it must have had higher entropy
and that means that time there is running the other way so the Big Bang didn't give rise to just one universe but two with time running into two opposite directions in case you've already forgotten half of what I said this video comes with a quiz that will help you remember what we talked about this idea has been proposed several times previously Julian Barber for example has called it the Janus point that the Big Bang of our universe was a double start he arrived at this conclusion by assuming that time is an entirely relational construct that
is time is not itself fundamental but we derive it from the way that the relations between particles change Neil Turok and his collaborators too have a double big bang with a mirror universe as they call it in their case they tie the idea to the symmetries that we already observe among particles in the standup model and since time is money physicists now also understand that money might flow both into a bank account and out of it you see physics makes progress after all did you know there's a free and easy way to learn more about
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