Do the Past and Future Exist?

TL;DR

Special relativity makes simultaneity relative, so “now” cannot be a single shared instant across all observers.

Briefing Cornell Notes

Briefing

The past and future may not be “gone” and “blank” in the way everyday language suggests. Instead, physics—especially relativity—undermines the idea that there is a single, universal “now,” pushing time toward a four-dimensional “block” where past, present, and future all coexist. The practical payoff is philosophical but also physical: once “present” becomes observer-dependent, it becomes hard to justify why only one thin slice of reality should count as real.

Newton’s deterministic universe offered a clean picture. If every particle’s position, velocity, and forces are known, then the entire past and future can be calculated. That vision also relied on an absolute, shared clock: everyone would agree on what counts as the present instant, and therefore on what is past and what is future. The transcript then points out that this universal “now” is exactly what Einstein’s special relativity breaks. In relativity, simultaneity is not absolute—events that are simultaneous for one observer may not be simultaneous for another.

The argument is built using the light-cone. Because nothing travels faster than light, an observer can only receive information from within their past light cone. Mapping the “present” slice is therefore impossible in real time: the full set of events that belong to a given time-slice only becomes observable after light from those events reaches the observer. Even then, different observers slice the same spacetime differently because motion tilts the spacetime grid. The result is that the “present” is not a single shared surface; it is a tilted slice whose orientation depends on the observer’s velocity. In extreme cases—near the edge of the observable universe—the shift can amount to centuries when an observer changes direction.

From there, the transcript tests two competing views: presentism (only the current instant exists) and the growing-block idea (the present “creates” the block as it advances). Both struggle because “the present” cannot be uniquely defined across observers. If someone else shares your spacetime slice, their “present” could fall anywhere along your future light-cone. That makes it difficult to claim that the future is nonexistent in any observer-independent sense.

The remaining options are stark. Either the entire block universe has meaningful existence (eternalism), or reality outside one’s immediate past-light-cone is denied until interaction occurs—an approach that edges toward solipsism. The transcript then suggests quantum mechanics as a potential bridge. Depending on interpretation, quantum theory can treat unobserved possibilities as indeterminate until measurement, but it can also preserve materialism and determinism if all possible realities exist simultaneously and persist into the future—an idea tied to Copenhagen and Many Worlds.

The episode closes by shifting to a related astro-biology discussion: whether Venus might host life, and what that would imply for the “great filter” that may prevent civilizations from emerging. If life is easy to start, the filter may lie ahead; if life is rare, the filter may lie behind. Either way, the conversation keeps returning to the same theme: what seems obvious about “now” and “elsewhere” becomes less certain once physics and interpretation are taken seriously.

Cornell Notes

Relativity removes the idea of a single, universal “now.” Because simultaneity depends on the observer’s motion, different observers slice the same four-dimensional spacetime at different angles, and what counts as “present” can shift dramatically across the observable universe. That makes presentism and the growing-block view hard to defend, since “the present” cannot be uniquely defined for everyone. The transcript argues that the remaining coherent options are eternalism (past, present, and future coexist in a block universe) or a restricted reality limited to what lies within one’s past light cone. Quantum mechanics is then introduced as a way to reconcile external reality with indeterminacy or with a framework where multiple possibilities persist into the future.

Why does special relativity make “the present” hard to define?

Special relativity says simultaneity is relative: two events that are simultaneous for one observer may not be simultaneous for another. The transcript links this to the fact that nothing can travel faster than light, so an observer’s observable region is constrained by a past light cone. As observers move, Lorentz transformations tilt the spacetime grid, so each observer constructs a different “now-slice” through the same block spacetime. That means there is no single shared instant everyone agrees on.

How does the light-cone argument affect what an observer can know about their own present?

The transcript emphasizes that the entire time-slice an observer wants to map is never fully observable in the present moment. Signals from different parts of that slice arrive only after light has had time to propagate. Successive “shells” of light reach the observer from more distant regions, gradually expanding the observable mapping of the time-slice. In effect, the “present” becomes complete only after waiting for information to arrive.

What does the block-universe (eternalism) picture claim, and why does it follow from relativity’s slicing?

The block-universe view treats spacetime as a fixed four-dimensional structure where past, present, and future all exist together. The transcript explains this using a flip-book analogy: the sequence of cause and effect and the experience of flow emerge only when the slices are played in a particular order. Since relativity makes the “present” observer-dependent and prevents a unique global slicing, eternalism becomes a natural fit: the whole spacetime block can be real even if “now” is not universal.

Why do presentism and the growing-block view run into trouble with observer dependence?

If only the current instant exists, the theory needs a uniquely meaningful “current” slice. But relativity prevents that: another observer on your spacetime slice can have a different “present” that corresponds to points you would call your future. The transcript argues that once you populate your slice with all possible observers, you can’t consistently say that parts of the block are “not real yet” for everyone. That undermines both presentism (only one instant exists) and growing-block (the present creates the block) as observer-independent claims.

How does quantum mechanics enter the discussion of determinism and external reality?

Quantum mechanics is introduced as a way to handle what lies outside an observer’s past light cone and what is unobserved within it. Depending on interpretation, unobserved aspects can be indeterminate until measurement—potentially supporting a solipsism-like restriction. Alternatively, quantum mechanics can preserve materialism and determinism if all possible realities exist simultaneously and persist into the future, tying the discussion to Copenhagen and Many Worlds and to a “block multiverse” style of spacetime.

What does the Venus-life discussion add to the episode’s themes?

The transcript pivots to whether Venus might host life and what that would imply for the “great filter,” the hypothesized factor that prevents many planets from producing spacefaring civilizations. If life is found on Venus and is likely seeded between Earth and Venus via panspermia, it could suggest life starts relatively easily—making the great filter more likely ahead of us (e.g., nuclear war or environmental collapse). If life is hard to start, the filter might be behind us. Either way, the stakes depend on how often life emerges, not on what feels intuitively “present” or “possible.”

Review Questions

What specific feature of special relativity prevents a universal definition of simultaneity, and how does that translate into different “now” slices for different observers?
How does the past light cone constrain what an observer can fully reconstruct about their own present time-slice?
Which two broad options remain after rejecting both presentism and the growing-block view, and how does quantum mechanics complicate the choice?

Key Points

1
Special relativity makes simultaneity relative, so “now” cannot be a single shared instant across all observers.
2
Because nothing travels faster than light, an observer’s full mapping of their own present requires waiting for signals to arrive from across their past light cone.
3
Different observers slice the same four-dimensional spacetime at different angles due to motion, so the “present” surface depends on velocity.
4
Observer dependence makes presentism and the growing-block view difficult to defend as observer-independent claims about what exists.
5
The transcript frames the remaining coherent positions as eternalism (the entire block universe exists) versus a reality restricted to what lies within one’s past light cone.
6
Quantum mechanics is introduced as a potential bridge: it can treat unobserved possibilities as indeterminate or, under some interpretations, as simultaneously real possibilities persisting into the future.
7
The Venus-life discussion connects to the “great filter,” where finding life on Venus would shift the likely location of the filter ahead of or behind humanity.

Highlights

Einstein’s relativity replaces Newton’s universal clock with observer-dependent slicing: the “present” tilts depending on motion.

An observer can’t fully observe their own present time-slice immediately; light-cone limits mean parts of it only become knowable later.

If “present” differs across observers, presentism and the growing-block idea lose their clean, global meaning.

Quantum mechanics is used to keep materialism and determinism possible—either via indeterminacy until measurement or via simultaneous persistence of possibilities.

Finding life on Venus would feed directly into great-filter reasoning about whether the bottleneck lies ahead or behind us.