The Nature of Space and Time AMA

TL;DR

Relativity merges space and time into spacetime and makes simultaneity observer-dependent, undermining the idea of a single universal “now.”

Briefing Cornell Notes

Briefing

Space and time remain fundamentally mysterious, but the most concrete thread running through the discussion is that modern physics treats them as a unified, flexible geometry—then runs into a breakdown at the smallest scales where quantum effects and gravity can’t both be described smoothly. Newtonian physics frames space as a fixed stage (a coordinate grid) and time as a separate sequence of updates. Einstein’s relativity merges them into spacetime, where what counts as “now” depends on the observer because simultaneity is relative. That shift supports the “block universe” picture: past and future coexist in a single timeless geometry, while observers trace worldlines through it.

Even that geometric picture hits limits. Relativity and quantum mechanics disagree when spacetime is pushed below the Planck length and Planck time, suggesting there are no indefinitely smooth building blocks. The central question becomes what spacetime is made of—whether it emerges from deeper, non-spacetime entities. Several quantum-gravity approaches were contrasted at a high level: string theory assumes extra dimensions exist from the start; loop quantum gravity replaces spacetime with abstract quantum connections from which geometry emerges; and Stephen Wolfram’s cellular-automaton-inspired ideas treat space and physical laws as emergent from interaction rules on a graph, with time behaving more like an updating process than a dimension.

From there, the Q&A zooms into how “space” and “time” behave in known physics. On cosmic expansion, the answer is that “expansion,” “stretching,” and “creation of space” are essentially the same description: the Friedmann equations track a growing scale factor that enlarges the effective grid of space. Because the grid can always be re-gridded after stretching, the universe’s expansion can be pictured as generating new space while keeping the same vacuum energy density and dark-energy behavior. Observationally, astronomers avoid saying galaxies move through space away from each other into empty surroundings; instead, the whole universe’s expansion is built into the geometry.

That geometry also makes relativistic effects look the same. Time dilation and length contraction for galaxies receding due to expansion match what special relativity predicts for objects moving at an equivalent apparent velocity. A key example is cosmological redshift: light’s wavelength stretches as it propagates through expanding space, lowering photon energy in a way that parallels Doppler shift.

Gravity, too, is treated as geometry with multiple “stories” that reproduce the same math. The classic rubber-sheet analogy is criticized because it can mislead about dimensions: gravity isn’t just space “dipping” in a literal extra direction. A more accurate intuition is that gravity corresponds to changes in the spacetime grid—especially the stretching of time in a gravitational field—while visual metaphors remain secondary to the equations.

On the arrow of time, the discussion ties time’s direction to entropy and quantum entanglement. The laws of physics run symmetrically forward and backward, but the universe’s boundary condition—low entropy early on—breaks that symmetry. As systems interact, entanglement grows, effectively increasing entropy and creating the statistical direction in which memory and correlations accumulate. Local entropy decreases can occur, but the overall statistical trend keeps the arrow pointing forward.

Finally, the speed of light question tests whether fundamental constants might vary. Relativity treats the speed of light as invariant, and it works across the universe’s history. Still, alternative models can mimic expansion by changing the speed of light, a line of work associated with Robert Dicke. The discussion emphasizes that “changing c” isn’t a simple re-labeling: it would generally affect unitless constants like the fine-structure constant, and searches for time variation in such constants (including the fine-structure constant) have so far found no convincing evidence.

Cornell Notes

The discussion frames spacetime as a geometric entity that works extremely well in relativity, but becomes unreliable at the smallest scales where quantum mechanics and gravity conflict. Einstein’s relativity merges space and time into spacetime and makes simultaneity observer-dependent, supporting the “block universe” view where past and future coexist. Quantum-gravity candidates aim to explain spacetime as emergent from deeper structures—string theory via extra dimensions, loop quantum gravity via quantum connections, and Wolfram-style cellular automata via graph rules. In known physics, cosmic expansion is described by a growing scale factor (often pictured as “creating new space”), and gravitational effects can be visualized in multiple ways but must match the same equations. The arrow of time is linked to low-entropy initial conditions and increasing entanglement, while the possibility of varying the speed of light is constrained by the success of relativity and the expectation that unitless constants would change too.

Why does relativity force a rethink of what “now” means?

Relativity of simultaneity means different observers slice spacetime into different “time slices.” What one observer calls “now” can correspond to another observer’s past or future. That undermines the idea of a single universal moment across space and motivates the block-universe picture, where all events exist in a single spacetime geometry and observers move along worldlines through it.

How does the universe’s expansion relate to “stretching” and “creating new space”?

In the Friedmann-equation description, expansion is encoded in a growing scale factor: the effective grid of space increases in size over time. Stretching and expansion are the same mathematical operation—points on the grid separate as the scale factor grows. Because the grid can be re-drawn after stretching, the process can be pictured as generating new space, while keeping the same vacuum energy density and dark-energy behavior (dark energy density treated as constant).

Why do cosmological redshift and Doppler shift end up looking similar?

Light traveling through expanding space gets stretched: the wavelength increases and photon energy decreases. The discussion notes that this produces the same observable effect as if the source were receding through static space at an equivalent apparent velocity, matching the Doppler-shift logic of wave peak separation changing due to relative motion. The key point is that the two descriptions are observationally indistinguishable in how they affect measured wavelengths and energies.

What’s the best way to interpret gravity in the “rubber sheet” analogy?

The rubber-sheet picture can mislead because it treats gravity like a literal dip in an extra dimension. The discussion stresses that the more important physical content is how spacetime geometry changes—especially time dilation in a gravitational field. While space can be described as “stretched” in a gravitational field (e.g., more volume inside a surrounding sphere than in empty space), the analogy should be treated as a visualization of the math rather than a literal mechanism.

What determines the arrow of time if the fundamental equations are time-symmetric?

The direction comes from boundary conditions: the universe began in a very low-entropy, highly ordered state. The microscopic laws don’t prefer forward over backward time, but entropy’s statistical behavior does. The discussion links entropy growth to increasing quantum entanglement: interactions spread correlations, raising entanglement and thus entropy. Memory and record-keeping then naturally accumulate in the forward direction, making the arrow robust even if small local entropy fluctuations occur.

Could the speed of light have varied over cosmic history?

Relativity treats c as invariant and uses it successfully from nearby experiments to deep gravitational wells and distant past observations. Still, some models can mimic expansion and redshift by letting c change (work associated with Robert Dicke), effectively reproducing the same observational outcomes as a changing scale factor. The discussion highlights that changing c alone is not a trivial redefinition: it would typically alter unitless constants such as the fine-structure constant, and searches for time variation in such constants have not found clear evidence.

Review Questions

What features of relativity make a single universal “present” impossible?
Explain how the scale factor in the Friedmann equations connects to the idea of “creating new space.”
How does increasing quantum entanglement relate to entropy and the arrow of time?

Key Points

1
Relativity merges space and time into spacetime and makes simultaneity observer-dependent, undermining the idea of a single universal “now.”
2
The block-universe view treats past and future as coexisting events in a single spacetime geometry, with observers distinguished by their worldlines.
3
Planck-scale limits (Planck length and Planck time) signal where smooth spacetime descriptions likely fail, motivating quantum-gravity theories that build spacetime from deeper elements.
4
Cosmic expansion is captured by a growing scale factor in the Friedmann equations; “expansion,” “stretching,” and “creating new space” are equivalent ways to describe the same geometry.
5
Cosmological redshift arises because expanding space stretches light waves, producing effects observationally similar to Doppler shift.
6
Gravity can be visualized with multiple metaphors, but the reliable content is how spacetime geometry changes—especially time dilation in gravitational fields.
7
The arrow of time aligns with low-entropy initial conditions and increasing entanglement; time-symmetric laws still yield a statistical time direction through boundary conditions.

Highlights

Simultaneity is relative: different observers can slice spacetime so that what one calls “now” may correspond to another’s past or future.

Expansion is modeled by a scale factor that grows the effective “grid” of space; re-gridding after stretching supports a “creating new space” intuition.

Entropy’s arrow is tied to quantum entanglement growth: interactions spread correlations, making memory and statistical order accumulate in one direction.

Rubber-sheet gravity analogies can mislead about dimensions; the core physics is spacetime geometry, particularly gravitational time dilation.

Varying the speed of light is constrained because unitless constants (like the fine-structure constant) would likely change too, and searches have not confirmed such variation.