Why does time have a direction?
The laws of physics barely distinguish past from future. We do. Where does the difference come from?
The question
An egg falls and breaks; we never see a broken egg jump back into a whole one. Coffee cools, milk mixes in and never unmixes. We remember the past and not the future. Our lives run in one direction.
Yet the basic laws of physics hardly care. Newton's equations, Maxwell's electromagnetism, Einstein's relativity and quantum mechanics all work just as well if time runs backwards. Film two billiard balls colliding and play it in reverse: nothing looks wrong. So where does the difference between past and future come from, if not from the laws themselves?
What we know
- The clearest one-way law is the second law of thermodynamics: in an isolated system, entropy — roughly, the amount of disorder, or the number of ways its parts can be arranged — doesn't decrease. Broken eggs and mixed coffee are high-entropy states.
- In 1872 Ludwig Boltzmann tried to derive this one-way law from the reversible motion of molecules. A few years later his critics, notably Josef Loschmidt in 1876, pointed out the problem: if every molecular motion can be reversed, irreversibility can't follow from the laws of motion alone. Boltzmann's answer was statistical: there are vastly more disordered states than ordered ones, so a system almost always moves towards disorder.
- But statistics alone work equally in both directions. They predict that entropy should increase towards the future, and equally that it should increase towards the past. To get the arrow we see, something more is needed: the past must have been in a very low-entropy state.
- The early universe really was. Shortly after the Big Bang, matter was spread almost perfectly evenly. For gravity, that is an extraordinarily ordered, low-entropy state, since gravity tends to clump matter together. Since then, entropy has been growing, and with it, it seems, every other arrow: memory, causation, the lives of stars and living things.
- There is one small exception in the laws themselves. Some decays of particles violate time symmetry slightly, as was confirmed directly in 1998 and 2012. The effect is tiny and seems unrelated to everyday irreversibility.
Main hypotheses
The past hypothesis
The arrow of time comes entirely from the special initial state of the universe. The philosopher David Albert called this assumption "the past hypothesis". It explains the arrow, but it moves the question: why did the universe begin in such an improbable state?
Inflation or new cosmology
Some cosmologists hope that a theory of the beginning of the universe, such as cosmic inflation, will explain why entropy started low. Critics, including Roger Penrose, argue that inflation itself requires special initial conditions and only pushes the question further back.
A larger, symmetric universe
Sean Carroll and Jennifer Chen proposed that the universe as a whole may have no preferred direction: from a low-entropy point, time arrows could point away in both directions, and our Big Bang is one such point.
Time itself is directional
Some physicists and philosophers think the arrow isn't only about entropy: time may genuinely flow, or the future may be truly open in a way the past isn't. This is hard to formulate in physics as we know it.
Where understanding ends
We understand why, given a low-entropy beginning, things become more disordered. We don't know why the beginning was low-entropy. That question lies at the edge of cosmology, where we have no accepted theory of the earliest moments and no way to observe what came before them.
We also don't fully understand how all the different arrows are connected. Why we remember the past and not the future, why causes come before effects, why we feel time passing — it is widely believed that all of these come from the growth of entropy, but showing it in detail, especially for memory and experience, is unfinished work. And whether the "flow" of time we experience is something physical at all, or a feature of how our brains organise information, remains a question for both physics and philosophy.
What would settle it
- A theory of the origin of the universe that explains its low initial entropy instead of assuming it.
- A detailed derivation of the psychological and causal arrows from thermodynamics, showing that no further ingredient is needed.
- An observation that distinguishes between cosmological models, for example signs of what came before the Big Bang or of a symmetric universe.
Sources
- Boltzmann, L. (1872). Weitere Studien über das Wärmegleichgewicht unter Gasmolekülen. Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Classe, 66, 275–370.
- Penrose, R. (1979). Singularities and time-asymmetry. In S. W. Hawking & W. Israel (Eds.), General Relativity: An Einstein Centenary Survey (pp. 581–638). Cambridge University Press.
- Penrose, R. (1989). Difficulties with inflationary cosmology. Annals of the New York Academy of Sciences, 571(1), 249–264.
- Angelopoulos, A., et al. (CPLEAR Collaboration) (1998). First direct observation of time-reversal non-invariance in the neutral-kaon system. Physics Letters B, 444(1–2), 43–51.
- Albert, D. Z. (2000). Time and Chance. Harvard University Press.
- Carroll, S. M., & Chen, J. (2004). Spontaneous inflation and the origin of the arrow of time. arXiv:hep-th/0410270.
- Lees, J. P., et al. (BaBar Collaboration) (2012). Observation of time-reversal violation in the B⁰ meson system. Physical Review Letters, 109(21), 211801.