What is time? Where does time come from? Is there really a flow of time?
In what way is time really something objective, something actually out there?
In what ways is time not real, but just a way that humans use to describe our perception of the universe?
Understanding these questions begins with an understanding the laws of thermodynamics especially the second law of thermodyamics.
We’ve also learned that a deeper understanding of the nature of time may rely on understanding modern physics especially quantum mechanics .
Quote
“In summary, time is not an illusion, but the flow of time is. So is change. In spacetime, the future exists and the past doesn’t disappear. When we combine Einstein’s classical spacetime with quantum mechanics, we get quantum parallel universes… This means that there are many pasts and futures that are all real-but this in no way diminishes the unchanging mathematical nature of the full physical reality.
This is how I see it. However, although this idea of an unchanging reality is venerable and dates back to Einstein, it remains controversial and subject to vibrant scientific debate, with scientists I greatly respect expressing a spectrum of views. For example, in his book The Hidden Reality, Brian Greene expresses unease toward letting go of the notions that change and creation are fundamental, writing, “I’m partial to there being a process, however tentative…that we can imagine generating the multiverse.”
Lee Smolin goes further in his book Time Reborn, arguing that not only is change real, but that indeed time may be the only thing that’s real. At the other end of the spectrum, Julian Barbour argues in his book The End of Time not only that change is illusory, but that one can even describe physical reality without introducing the time concept at all.”
― Max Tegmark, Our Mathematical Universe: My Quest for the Ultimate Nature of Reality
What is entropy?
entropy is the amount of disorder (in a gas, a liquid, objects in a room, etc)
Entropy really measures the dispersal of energy: how much energy is spread out in a particular process
We know from everyday experience how energy spreads out – and how it doesn’t. This ice melting looks normal.
commons.wikimedia.org/wiki/File:Melting_icecubes.gif
Yet Newton’s laws of motion do allow the below event to occur, as well:
Water molecules move randomly in water – so by sheer chance, couldn’t they all come together to spontaneously form ice cubes? Yet this obviously never happens.
The following is from: Entropy Explained, With Sheep: From Melting Ice Cubes to a Mystery About Time, by Aatish Bhatia, https://aatishb.github.io/entropy/
Imagine you could zoom in and see the atoms and molecules in a melting cube of ice. If you could film the motion of any particle, and then play that film back in reverse, what you’d see would still be perfectly consistent with the laws of physics. It wouldn’t look unusual at all.
The movements of the atoms and molecules in the first gif are every bit as ‘legal’ (in the court of physical law) as those in the second gif.
So why is the first gif an everyday occurrence, while the reverse one impossible?
This isn’t just about ice cubes. Imagine you dropped an egg on the floor. Every atomic motion taking place in this messy event could have happened in reverse. The pieces of the egg could theoretically start on the floor, hurtle towards each other, reforming into an egg as it lifts off the ground, travel up through the air, and arrive gently in your hand. The movement of every atom in this time-reversed egg would still be perfectly consistent with the laws of physics. And yet, this never happens.
So there’s a deep mystery lurking behind our seemingly simple ice-melting puzzle. At the level of microscopic particles, nature doesn’t have a preference for doing things in one direction versus doing them in reverse. The atomic world is a two-way street.
And yet, for some reason, when we get to large collections of atoms, a one-way street emerges for the direction in which events take place, even though this wasn’t present at the microscopic level. An arrow of time emerges.
The arrow of time
Why does time never go backward? The answer apparently lies not in the laws of nature, which hardly distinguish between past and future, but in the conditions prevailing in the early universe.
The Arrow of Time, Scientific American article. David Layzer
Is the Universe Leaking Energy? Total energy must be conserved. Every student of physics learns this fundamental law. The trouble is, it does not apply to the universe as a whole. By Tamara M. Davis
Is The Universe Leaking Energy? Scientific American article
Does Time Really Flow?
Does Time Really Flow? New Clues Come From a Century-Old Approach to Math.
The laws of physics imply that the passage of time is an illusion. To avoid this conclusion, we might have to rethink the reality of infinitely precise numbers. Dave Whyte, Quanta magazine, 4,7,2020
In Albert Einstein’s theory of relativity time is woven together with the three dimensions of space, forming a bendy, four-dimensional space-time continuum — a “block universe” encompassing the entire past, present and future.
Image from Scientific American
Einstein’s equations portray everything in the block universe as decided from the beginning; the initial conditions of the cosmos determine what comes later, and surprises do not occur — they only seem to.
“For us believing physicists,” Einstein wrote in 1955, weeks before his death, “the distinction between past, present and future is only a stubbornly persistent illusion.”
The timeless, pre-determined view of reality held by Einstein remains popular today…. [But] Physicists who think carefully about time point to troubles posed by quantum mechanics, the laws describing the probabilistic behavior of particles.
At the quantum scale, irreversible changes occur that distinguish the past from the future: A particle maintains simultaneous quantum states until you measure it, at which point the particle adopts one of the states.
Mysteriously, individual measurement outcomes are random and unpredictable, even as particle behavior collectively follows statistical patterns.
This apparent inconsistency between the nature of time in quantum mechanics and the way it functions in relativity has created uncertainty and confusion.
Over the past year, the Swiss physicist Nicolas Gisin has published four papers that attempt to dispel the fog surrounding time in physics. As Gisin sees it, the problem all along has been mathematical.
Gisin argues that time in general and the time we call the present are easily expressed in a century-old mathematical language called intuitionist mathematics, which rejects the existence of numbers with infinitely many digits.
When intuitionist math is used to describe the evolution of physical systems, it makes clear, according to Gisin, that “time really passes and new information is created.”
Moreover, with this formalism, the strict determinism implied by Einstein’s equations gives way to a quantum-like unpredictability. If numbers are finite and limited in their precision, then nature itself is inherently imprecise, and thus unpredictable….
Physicists are still digesting Gisin’s work — it’s not often that someone tries to reformulate the laws of physics in a new mathematical language — but many of those who have engaged with his arguments think they could potentially bridge the conceptual divide between the determinism of general relativity and the inherent randomness at the quantum scale.
… It was on a Sunday about two and a half years ago that he realized that the deterministic picture of time in Einstein’s theory and the rest of “classical” physics implicitly assumes the existence of infinite information.
Consider the weather. Because it’s chaotic, or highly sensitive to small differences, we can’t predict exactly what the weather will be a week from now. But because it’s a classical system, textbooks tell us that we could, in principle, predict the weather a week on, if only we could measure every cloud, gust of wind and butterfly’s wing precisely enough…
Now expand this idea to the entire universe. In a predetermined world in which time only seems to unfold, exactly what will happen for all time actually had to be set from the start, with the initial state of every single particle encoded with infinitely many digits of precision. Otherwise there would be a time in the far future when the clockwork universe itself would break down.
But information is physical. Modern research shows it requires energy and occupies space. Any volume of space is known to have a finite information capacity (with the densest possible information storage happening inside black holes).
The universe’s initial conditions would, Gisin realized, require far too much information crammed into too little space. “A real number with infinite digits can’t be physically relevant,” he said. The block universe, which implicitly assumes the existence of infinite information, must fall apart.
He sought a new way of describing time in physics that didn’t presume infinitely precise knowledge of the initial conditions.
See Mathematical languages shape our understanding of time in physics, Nicolas Gisin, Nature Physics, volume 16, p.114–116 (2020) https://doi.org/10.1038/s41567-019-0748-5
Is time travel actually possible
new section TBA
New math theory suggests time travel is impossible, Tristan Greene. Relates to the new Gisin paper.
Presentism vs. eternalism
The relativity of simultaneity in modern physics favors the philosophical view known as eternalism or four dimensionalism (Sider, 2001), in which physical objects are either temporally extended space-time worms, or space-time worm stages, and this view would be favored further by the possibility of time travel (Sider, 2001).
Eternalism, also sometimes known as “block universe theory”, builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space (Sider, 2001). This would mean that time is just another dimension, that future events are “already there”, and that there is no objective flow of time. This view is disputed by Tim Maudlin in his The Metaphysics Within Physics.
Image from Scientific American
Presentism is a school of philosophy that holds that neither the future nor the past exist, and there are no non-present objects. In this view, time travel is impossible because there is no future or past to travel to. However, some 21st century presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to appear in the present could explain the time traveler’s actual existence in the present.
Related articles
The Quantum Thermodynamics Revolution
Time’s Arrow Traced to Quantum Source
Learning Standards
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields.
Clarification Statements: Examples of phenomena at the macroscopic scale could include evaporation and condensation, the conversion of kinetic energy to thermal energy,
HS-PS3-4a. Provide evidence that when two objects of different temperature are in thermal contact within a closed system, the transfer of thermal energy from higher temperature objects to lower-temperature objects results in thermal equilibrium, or a more uniform energy distribution among the objects and that temperature changes
necessary to achieve thermal equilibrium depend on the specific heat values of the two substances. Energy changes should be described both quantitatively in a single phase (Q =m·c·∆T) and conceptually either in a single phase or during a phase change.
Next Generation Science Standards
HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
Influence of Science, Engineering and Technology on Society and the Natural World: Modern civilization depends on major technological systems. Engineers continuously modify these technological systems by applying scientific knowledge and engineering design practices to increase benefits while decreasing costs and risks. (HS-PS3-3)
Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. (HS-PS3-3)
Energy cannot be created or destroyed—only moves between one place and another place, between objects and/or fields, or between systems. (HS-PS3-2)
AP Physics
7.B.2.1: The student is able to connect qualitatively the second law of thermodynamics in terms of the state function called entropy and how it (entropy) behaves in reversible and irreversible processes. [SP 7.1]
– AP Physics Course and Exam Description
Further notes
“About Time: Einstein’s Unfinished Revolution” Paul Davies VERY GOOD.
“The End of Certainty” Ilya Prigogine
“World in Process” John A. Jungerman Description of the ideas of modern physics and cosmology; also connects those ideas to process thought.
“Travels in Four Dimensions: The Enigmas of Space and Time” Robin Le Poidevin
“Time’s Arrows Today: Recent Physical and Philosophical Work on the Direction of Time”
Steven Frederick Savitt, Cambridge University Press, 1995
“Time and History: Proceeding of the 28 International Ludwig Wittgenstein Symposium” by Friedrich Stadler
Introducing Time, Third Edition, Totem Books; New Ed edition (June 25, 2005) Craig Callender
“The Ontology of Time” L. Nathan Oaklander, Studies in Analytic Philosophy, Prometheus Books
http://www.prometheusbooks.com/catalog/book_1532.html
http://en.wikipedia.org/wiki/Arrow_of_time
http://en.wikipedia.org/wiki/Block_time
http://en.wikipedia.org/wiki/J._M._E._McTaggart
http://en.wikipedia.org/wiki/The_Unreality_of_Time
http://en.wikipedia.org/wiki/B-Theory_of_time
http://en.wikipedia.org/wiki/Presentism_%28philosophy_of_time%29
http://en.wikipedia.org/wiki/Eternalism_%28philosophy_of_time%29
http://en.wikipedia.org/wiki/Endurantism
http://en.wikipedia.org/wiki/Perdurantism
http://plato.stanford.edu/entries/change/
http://plato.stanford.edu/entries/spacetime-bebecome/
http://plato.stanford.edu/entries/causation-backwards/
http://plato.stanford.edu/entries/time-experience/
See post by Moving Finger, http://www.physicsforums.com/showthread.php?t=124119&highlight=time
http://www.physicsforums.com/showthread.php?t=124119&page=2&highlight=time
See post by Moving Finger 06-30-2006, 05:00 AM
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