The Claim

Krish Ramkumar

Claim:

What if the universe was to contract, or goes in the direction of the big crunch, is the arrow of time then reversed for the inhabitants in that universe?

Stephen Hawking argues that the inhabitants in this particular universe will have their psychological arrow of time (How we perceive time) reversed and that it points in the same direction as the thermodynamic arrow of time as the thermodynamic arrow of time essentially determines the psychological arrow of time.

(Note : some parts of this blog are repeated from my previous post for the sake of completeness)

According to the 2^nd law of thermodynamics, entropy (randomness) increases with time and the arrow of time points in the direction of higher entropy. If the arrow of time in the present universe is in the direction of higher entropy (unordered state), then the formation of stars, star clusters and galaxies seem to be a paradox, if we consider stars as closed systems (since the law is valid only in closed systems). The law does not lay any restrictions on how one must choose a “system”. In that case I could choose a star as a closed system and that would be an anomaly from the 2^nd law. These anomalies are small fluctuations in the ever expanding universe which are compensated by very high entropy at various other parts of the universe. But the important question is even if the net entropy of the universe is increasing, why are stars formed in the first place? If randomness were to increase with time in the universe, even the formation of molecules and atoms at the quantum level seem like a contradiction to the 2^nd law. Energy dissipated by a closed system is probably the answer to this paradox. Perhaps energy dissipated into the environment by a star (The star could be considered as the closed system) during its formation is so high that this increases the entropy in the universe and overshadows the increase in order (lower entropy) within a star. In other words, if I break a solid plate (low entropy) and try to fix it with glue, I will be exerting a lot of energy into the environment thus increasing the net entropy of the universe. As per the 2^nd law, entropy of the universe as a whole can only increase but the entropy of the system as such can increase or decrease (system + environment= Universe).

The arrow of time could be defined as the direction in which the entity called “time” progresses. The arrow of time of our universe points in the direction of increasing randomness as stated earlier. This also means that “future” points towards expansion.

Since the claim is very abstract, one cannot accept or disregard this claim immediately. Science is always subject to change or addition in theories. Take for instance the period Copernicus. People in that era thought the earth was flat. This is just to show how far science has come in the past 500 years. So when asked if the source is reliable or not, in my opinion ,even though the claim might not be proved anytime soon the person making the claim, Prof. Stephen hawking is considered as one of the most brilliant minds since Einstein. Since I am an ardent believer of science I would only want to explore this further and not have a bias in my opinion as weather it is right or wrong.

Theoretical physics is mostly based on sensationalist claims that are backed up by mathematical evidence, thereby making such claims rather common. Prof Stephen Hawking being a savant in this field has made several claims that make our minds reel and many that have been backed up mathematically.

However Roger Penrose's Emperor’s New Mind discusses the big crunch and why the arrow of time would not turn backwards, even after the universe was collapsing on itself.  Since I am no expert in this field I don’t quite know how the idea was discredited partially.

Hawking in the Brief History of time argues that the thermodynamic arrow of time will always point in the same direction as the psychological arrow of time which brings us back to the basis of his claim. This means even during the contraction of the universe, the thermodynamic arrow of time points in the direction of the psychological arrow of time. Claim seems to be intuitive as to where the arrow of time must point but certainly not obvious. However if there were to be inhabitants in the contracting universe they will perceive the world in an entirely different way which probes me to say that it does not fit the way we look at the present universe but  the claim certainly has theoretical backup.

As stated earlier it is not very easy to disprove or prove this claim completely but several scientists have come up with ways to perceive this idea and I think that the majority of them have not paid much importance to the “Big –Crunch” phenomena itself because as per the current state of the universe, the speculation is that the universe is accelerating. But we still don’t know the cause that drives this phenomenon (Dark matter).  This goes to say that there is no observational evidence as of now for the “Big Crunch”.

Hawking certainly approaches the problem with strong logic. Take our present universe for example, according to the 2’nd law the arrow of time points in the direction of increasing entropy. This means that we don’t normally see broken cups or plates coming back together on their own.  The innate property of everything in the universe is to go to an unordered state.  If the universe were to contract, the thermodynamic arrow of time is reversed and hence reversing the psychological arrow of time. In other words, this means that the inhabitants in this universe will not remember the broken cup as an effect but as a cause, perhaps.

This claim however is a thought experiment rather than something that can be backed up with observational evidence.  If the net entropy of the universe were to decrease (Big Crunch--> High Order) the arrow of time is reversed but the Net Entropy of the universe cannot decrease according to the 2nd law. The question here really is why do we need the universe to obey the 2^nd’d law? The law has been derived from a series of observational evidence’. This in no way restricts the universe to behave in a particular way. This means that not obeying the law does not necessarily make this a paradox; it only means that from the point of view of how our present universe works, it is a paradox.

Hawking’s claim on the arrow of time is in fact the perfect example of a scientific claim as it has no confirmation bias what so ever. It goes against the normal traditional way of thinking about the universe and perceiving something that might or might not be true. But on the contrary I would not say that this is a good theory from the scientific reasoning as there is no observational evidence as of now to prove it wrong. Nevertheless it could be proven wrong or right through mathematics. Then again the proof offered by mathematics takes into account the 2^nd law of thermodynamics which as stated earlier does not have to agree with the idea.

Arrow of Time

Krish  Ramkumar

                                                                 Arrow of time

Is the arrow of time a consequence of the 2^nd law of thermodynamics or the law is due to the defined arrow of time? This question is a little hazy to me as the entity “time” is undefined in the law. If the law does in fact define a particular arrow of time in the universe, we need to define “time”.

According to the 2^nd law of thermodynamics, entropy (randomness) increases with time and the arrow of time points in the direction of higher entropy. If the arrow of time in the present universe is in the direction of higher entropy (unordered state), then the formation of stars, star clusters and galaxies seem to be a paradox, if we consider stars as closed systems (since the law is valid only in closed systems). The law does not lay any restrictions on how one must choose a “system”. In that case I could choose a star as a closed system and that would be an anomaly from the 2^nd law. Maybe these anomalies are small fluctuations in the ever expanding universe which are compensated by very high entropy at various other parts of the universe. But the important question is even if the net entropy of the universe is increasing, why are stars formed in the first place? If randomness were to increase with time in the universe, even the formation of molecules and atoms at the quantum level seem like a contradiction to the 2^nd law. Energy dissipated by a closed system is probably the answer to this paradox. Perhaps energy dissipated into the universe by a star (The star could be considered as the closed system) during its formation is so high that this increases the entropy in the universe and overshadows the increase in order (lower entropy) within a star. In my understanding, the same reason could be applied to star clusters as well.  To me this question is still hazy. If the universe did have some kind of innate property of moving towards randomness (higher entropy), this brings me back to my previous question- why are stars formed in the first place?

The arrow of time could be defined as the direction in which the entity called “time” progresses. The arrow of time of our universe points in the direction of increasing randomness as stated earlier. This also means that “future” points towards expansion. This is a way of understanding what happened before, what we call the past. The arrow of time has most certainly enabled man to trace back to his celestial ancestors. After all we are all star dust, remains of the Big Bang.

What if the universe were to contract, or go in the direction of the big crunch? The arrow of time then is reversed for the inhabitants in that universe. This simply means that what we call as “future” would be their past. This illustrates that the contracting universe is moving in the direction of lower entropy or high order. Perhaps the human brain is developed only to realize and perceive the arrow of time we are experiencing, which points towards higher entropy. We are probably trained to analyze only those phenomena that are directed toward an unordered state. Will man be able to perceive the universe if the arrow of time were different than what it is in our present universe? Will man be able to understand the universe if the universe were moving toward a state of lower order? Imaginably, our senses that have enabled us to understand the universe so far become an obstacle in understanding the universe after a certain point.

What happens to the arrow of time in the vicinity of a highly ordered structure? Is this how gravity is related to time at the most fundamental level? Possibly! Highly ordered structures have lower entropy. These objects also curve the space-time around them depending upon how massive they are. The curvature in space-time is considered gravity and this entity called gravity drags time (Einstein’s General -Relativity). In other words, if I were to go close to a black hole, just close enough that I’m in the orbit but not too close that I might get pulled into the singularity and then go around the orbit for a while and return back to earth, I would have actually travelled to the future. Time in my perspective slowed down when I was orbiting around the black hole (assumption is that a black is a highly ordered system as the density is concentrated at a single point). Does this mean highly ordered systems typically drag time? What I seem to infer from all this is that the arrow of time in the vicinity of a highly ordered system is not the same as what it is elsewhere in the universe.

References-

 Brief History of time - Stephen Hawking.

Einstein’s Universe- Nigel Calder