# Fascinating intro to the physics behind the Arrow of Time.

I like thinking about the Arrow of Time because it is surprising to me that something as simple as the notion that time has a direction, and the past is different from the future is not intrinsic to the laws of physics.

Note by Grace Doughty
5 years, 11 months ago

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Since we're discussing the arrow of time in cosmology, the study of how the universe evolves, I'll throw a provocative wrench into everyone's belief that we as scientists know exactly what we're talking about even about very basic questions. As Carroll says, the notion of an arrow of time relies on the low entropy initial conditions for the universe. So, on one hand, we have that a "thermodynamic" quantity, the entropy of the universe, is changing and we can define a time by this behavior. On the other hand, the dynamics of the universe are governed by general relativity, Einstein's theory of gravity. Hence the increasing entropy and the arrow of time is a result of the dynamics of gravity (and a couple assumptions about the allowed types of matter we have in the universe) and the low entropy initial conditions. In a sense, thermodynamics comes as a consequence of gravity and initial conditions. Amazingly, one can actually reverse the argument and say that gravity itself is simply a result of entropy and thermodynamics. This is a fun modern discussion in theoretical physics and you can read a synopsis here.

Staff - 5 years, 11 months ago

@Johnson I too often wonder how the universe arrived at laws that it must abide by or even if they are laws at all or merely our understanding of what we see.

@David M I read the article you linked to. I know almost nothing about string theory or general relativity :( But, if I understand the article correctly, I can't intuitively see why gravity should not be a consequence of thermodynamics and as opposed to the the other way around. It's good to know that the pro's are just as lost as me.

- 5 years, 11 months ago

I love Minute Physics! He does a good job of explaining concepts in a clear concise manner.

@Raoul, In order for us to make things orderly, we exert a lot of 'chaos' in doing so. For example, we need to eat about 10J worth of energy in food, for us to exert 1J worth of energy. A lot of energy is wasted keeping our body temperatures high, maintaining equilibrium etc. So, the next time screams at you to clean up the mess in your room, tell her that you're doing your best to reduce chaos in the universe!

Staff - 5 years, 11 months ago

Yeah, I always thought the universe went from chaos to order as well, not the other way around. Cool youtube channel.

- 5 years, 11 months ago

@Jacob Yes there are possibilities. If gravity really is a "thermodynamic limit" of some underlying theory, then it emerges at our experimentally testable length scales from some microscopic physics. This is similar to how thermodynamic quantities like temperature only emerge when you average over macroscopic systems. Consider a bunch of gas in a box. If you looked microscopically at each individual molecule, there would be no notion of a temperature of the gas, only the kinetic energy of each molecule. Only by averaging macroscopically over all the molecules do you get the notion of temperature.

There are mathematical theorems that pin down how this emergence can and cannot happen if the macroscopic limit is our gravity. If emergence does happen, then there must be things like an extra force around, a little bit of nonlocality in physics, a small breakdown of special relativity, etc. What exactly happens depends on the model in question. But...we can go looking for all the observational effects of this new physics.

So far, we haven't seen anything out of the ordinary :)

Staff - 5 years, 11 months ago

@Johnson, You are correct. In this view there are many, many 'universes'. In some atoms form, in others the universe collapses almost as soon as it's made, in still others everything is blown apart before atoms can form etc. The question we ask is "why are the initial conditions of our universe so special?". Anthropism says that the specialness of the initial conditions is related to the specialness of us actually existing. So there's no fundamental reason why the initial conditions are special, we as humans just happen to only be possible when the initial conditions are like this. So the same random chance that generated those initial conditions and physical laws also generated us. It looks to us like something special happened from our viewpoint, but not really.

Staff - 5 years, 11 months ago

@Grace, Be assured that many others find a version of this principle to be adequate and possible, but very unsatisfying. Anthropism is taken by some to be the 'end of physics' in terms of progress, because eventually we get to no fundamental deterministic rule, but instead wind up with a 'this universe was just one particular roll of the dice'.

Staff - 5 years, 11 months ago

@ David Do you know if they have any theories or conjectures on what could possibly be observed that could rule on whether gravity-->thermo vs. thermo-->gravity. Or is the theory still too controversial?

- 5 years, 11 months ago

@Johnson I am confused as well, but am also not sure why Davids anthropic principle is not good enough for me.

- 5 years, 11 months ago

@Johnson and David, Johnson, I think what David is saying is that there could be/have been a universe with our same laws with atoms, but in a universe with different, or random laws, an atom likely wouldn't exist (or at least not be atoms as we see them). So the anthropic principle as you have explained it to me makes surprisingly good but not satisfying sense as a justification for why the universe is governed by laws.

- 5 years, 11 months ago

I still don't really understand why the universe is in such a way. David the point you made was clear I understand but don't as well because I imagine if the universe was formed another way then an atom could be defined with a a random law and then would just follow the rules that could make another universe (completely different) from our own. It's confusing.

- 5 years, 11 months ago

@Johnson and @Grace So, let's talk about why the universe has the physical laws that we see. By physical laws we mean, for example, that gravity has a certain strength relative to E&M, forces fall off as 1/r^2, etc. The traditional viewpoint is that there must be a reason why our laws are the way they are - they couldn't be any different. This is a reductionist philosophy as we as scientists really want to explain why from fundamental principles. A more recent idea is that our laws may be simply selected randomly as the universe gets created again and again. Sometimes they are very different from what we see now and the universe doesn't live very long, or doesn't do much of anything interesting. We see our laws because if the laws were different we couldn't form atoms, or stars, or galaxies, or...us! Namely, the existence of people and observers is only possible with a certain set of laws, so out of all the possibilities we see ours simply because we are here. This is what's called the anthropic principle.

Staff - 5 years, 11 months ago

This is fascinating! I do really wonder why the universe was possibly at a state of perfect order. I also wonder if anyone here understands me how did the universe arrive at the frames of laws that it must abide by? i.e. Why things are structured the way the are and hence why is the universe so orderly to a degree ?

- 5 years, 11 months ago

Yeah I just discovered it Calvin. It makes me wish all classroom instruction came in two minute videos.

- 5 years, 11 months ago

- 5 years, 11 months ago

One cool thing I realized from the video is that the present universe is actually 'more disorderly' than the past universe. I have been used to thinking that everything started as chaos and becomes orderly through time..

- 5 years, 11 months ago

Thanks for posting this, Grace! After watching this video, I tried other interesting videos on topics like the 10th dimension and how to turn a sphere inside out ;)

- 5 years, 11 months ago

Fun video! I searched "arrow of time" and found this more detailed FAQ about the arrow of time by the same scientist. My intuition was always that irreversible process are what gave time a direction. I was wrong.

sean carroll in the link above says:

"The observed macroscopic irreversibility is not a consequence of the fundamental laws of physics, it's a consequence of the particular configuration in which the universe finds itself. In particular, the unusual low-entropy conditions in the very early universe, near the Big Bang."

- 5 years, 11 months ago