I’m exploring a model where the present moment (“now”) is the actual boundary of the universe. The universe is not a fixed block of past, present, and future, but something that is continuously being created at this boundary.

In this view, the universe expands at the speed of light, not just in space but in spacetime itself. The speed of light is therefore not just a property of light, but a fundamental limit that defines how fast reality can grow.

Light is interpreted differently here: rather than “moving through space,” it can be seen as a trace or trail that connects the present to the past, marking the structure of what has already happened.

Mass, on the other hand, represents the actual realized path of events, while light defines the possible causal structure.

So the picture is:

• The present = the edge of reality

• The future = not yet existing

• The past = built structure

• The speed of light = rate at which the universe unfolds

I’m curious how this idea fits (or conflicts) with modern physics, especially relativity.

1 hour ago, Rudolf said:

So the picture is:

• The present = the edge of reality

• The future = not yet existing

• The past = built structure

• The speed of light = rate at which the universe unfolds

I’m curious how this idea fits (or conflicts) with modern physics, especially relativity.

If by "present" one means the present defined by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, while the idea probably doesn't conflict with known physics, it is probably quite meaningless in the sense of having no observable consequences contrasting with the notion of a block universe in which the future pre-exists. In other words, the idea violates Occam's razor.

Edited April 19Apr 19 by KJW

One aspect which causes concern is the fact that this surface of the present, that is expanding at the speed of light, is the surface of a 4D hypersphere, in effect, a 3D volume.
Does this imply a volume of 'now' that is essentially fixed ?
A universal 'now' would cause issues.

6 hours ago, MigL said:

One aspect which causes concern is the fact that this surface of the present, that is expanding at the speed of light, is the surface of a 4D hypersphere, in effect, a 3D volume.
Does this imply a volume of 'now' that is essentially fixed ?
A universal 'now' would cause issues.

I agree that a universal “now” would be problematic. In this model, there is no single global present.

Instead, the “now” is local and observer-dependent. Each observer has their own present, defined by their position and motion, and by the causal structure around them. What counts as “now” for one observer does not have to match the “now” of another.

So rather than a single expanding surface (like a hypersphere), it’s better to think of reality as having many overlapping local “nows”, each defined by what can be causally connected at that moment.

In that sense, the “edge of the universe” is not a fixed global boundary, but a local boundary of causal interaction that exists for every observer individually.

13 hours ago, Rudolf said:

In this view, the universe expands at the speed of light, not just in space but in spacetime itself. The speed of light is therefore not just a property of light, but a fundamental limit that defines how fast reality can grow.

What do you mean by this ?

Please define (mathematically) the speed of light in spacetime ?

One such model would go against relativity on a number of levels. For starters, there's no "present" in relativity. Not even in special relativity. In the De Sitter model, eg, every observer carries its past, present, and future.

Observer-dependent are the key words.

3 hours ago, studiot said:

Please define (mathematically) the speed of light in spacetime ?

The speed of light defines the maximum rate at which causal connections can form between events. In this sense, it sets the rate at which physical reality can emerge.

1 hour ago, joigus said:

One such model would go against relativity on a number of levels. For starters, there's no "present" in relativity. Not even in special relativity. In the De Sitter model, eg, every observer carries its past, present, and future.

Observer-dependent are the key words.

I agree that in special relativity there is no universal “present.” That’s actually consistent with my idea.

When I talk about the “present,” I don’t mean one global moment for the whole universe. I mean a local, observer-dependent “now.” Each observer has their own present, depending on their motion and position.

So I’m not trying to introduce a preferred frame or a universal time, which would indeed conflict with relativity. Instead, I’m trying to reinterpret the existing causal structure (defined by light) in a different way.

In my view, the “present” is simply the local boundary of what is becoming causally connected for an observer.

So rather than saying there is one present, I would say:

There are many local “nows,” one for each observer, defined by causality.

So in what way is this different from GR, except for a re-wording of the usual concepts?

Can you offer any new insights provided by this re-wording?

I mean, you can define sets of coordinate vector fields, and introduce a smooth metric on your manifold. Then obviously the points where g(dx,dx)=0 are the boundary separating those for which g(dx,dx)>0 from those for which g(dx,dx)<0

This is tautological.

1 hour ago, Rudolf said:

The speed of light defines the maximum rate at which causal connections can form between events. In this sense, it sets the rate at which physical reality can emerge.

I am sorry that goes no way at all to answering my question.

You specified speed in space and in spacetime.

Are you aware that these represent different physics quantities in space and spacetime, as the time rate of change of a 3 vector in space and a 4vector in spacetime ?

6 hours ago, Rudolf said:

In that sense, the “edge of the universe” is not a fixed global boundary, but a local boundary of causal interaction that exists for every observer individually.

So you are saying that this 'surface of the present' is a foliation of spacetime, and where this foliation represents the present, it is 'advancing' at the SoL. Further, this foliation, representing the present, is not smooth and constant, but 'dimpled' and smoothly varying, with the dimples being local variances due to observer position/motion. And with 'wells', going as deep as their origin, representing the stagnant space-time approaching Black Hole event horizons.

These are well established 'interpretations' of relativity, and as such, bring no new insights to Physics.

But maybe I'm missing something; can you point out any new science from this approach ?

Edited April 20Apr 20 by MigL

This hypersurface, the 'snapshot' that represents the 'present' is called a Cauchy Surface.
( wanted to add that, but could no longer edit )

Edited April 20Apr 20 by MigL

2 hours ago, MigL said:

So you are saying that this 'surface of the present' is a foliation of spacetime, and where this foliation represents the present, it is 'advancing' at the SoL. Further, this foliation, representing the present, is not smooth and constant, but 'dimpled' and smoothly varying, with the dimples being local variances due to observer position/motion. And with 'wells', going as deep as their origin, representing the stagnant space-time approaching Black Hole event horizons.

These are well established 'interpretations' of relativity, and as such, bring no new insights to Physics.

But maybe I'm missing something; can you point out any new science from this approach ?

Okay, this is not exactly going the way I was thinking of.

I don't think that standard relativity theory, especially when interpreted in terms of foliations, lightcones, and gravitational time dilation etc.. is wrong.

Where my perspective differs slightly is not in the structure itself, but in the emphasis: instead of treating foliation as just a mathematical slicing of a pre-given spacetime, I am asking whether it can be interpreted as a description of how spacetime is locally realized through causal accessibility. In that reading, the “present” is not a global surface, but an observer-dependent causal boundary.

I agree with you that at this stage this does not produce new empirical predictions or should not be presented as new physics. It is an interpretational reframing rather than a new theory.

So I don’t think you are missing anything essential. The only “extra” is a different interpretation of spacetime as a dimension, not new science yet.

Hopefully you are actually going to answer my question.

You need to understand the difference between 'c' which is both a constant and an invariant, which happens to have the dimensions M⁰LT^-1, regardless of the number of spatial and temporal dimensions you employ. This would also include time independant solutions to motion equations, such as standing waves (light can exhibit standing waves).

The'speed of light' is a different animal entirely which has different dimensions in different models.

Unfortunately too many pass glibly over this distinction and I don't think any popsci authors have any inkling of it altogether.

Edited April 20Apr 20 by studiot

Thanks so far for all the thoughtful replies—I really appreciate the discussion. I’ll take some time to think this through and come back later with a clearer answer. I don’t think we actually disagree completely; I just need to find the right words to better explain how I see a possibility where the “now” (here and time) could be the point from which future spacetime is created.

15 hours ago, Rudolf said:

Thanks so far for all the thoughtful replies—I really appreciate the discussion. I’ll take some time to think this through and come back later with a clearer answer. I don’t think we actually disagree completely; I just need to find the right words to better explain how I see a possibility where the “now” (here and time) could be the point from which future spacetime is created.

It's good that you are talking the time to think about what has been said. I look forward to your thoughts when they arrive.

+1

Another point to add into your cogitation.

Simultaneity, what do we mean by it ?

If we mean that the temporal distance between two points in space or spacetime is exactly zero then there is a problem to overcome.

Never mind foliations, branes and other exotic maths.

The issue is simply that of the metric, since the question implies a coordinate system

The first and most fundamental axiom of any distance function (metric) is that

If D(a, b) is a function defining the distance between two points in a coordinate system then

D(a,b) = 0 if and only if a = b

Okay, I´m back here and was actually waiting for a few more opinions for readers with the concept idea that the current spacetime we are living is the boundary of the expanding universe.

I’m playing with a picture of spacetime where the present is not just an observer-dependent slice of a pre-existing block universe, but the actual leading edge of reality. In that picture, the universe is a growing spacetime: the past is the already-formed structure, the future does not yet exist, and the present is the frontier where space and time are generated together.

I’m wondering whether the invariant speed c should be thought of not only as the speed of light, but as the fundamental rate at which this spacetime frontier advances. Then photons would not be just particles moving through a static spacetime, but traces of the causal structure laid down immediately behind that advancing edge — almost like trails into the newly formed past.

I’m not claiming this is standard relativity, and I know it would need a much sharper mathematical formulation to count as physics. At this stage I’m treating it as a conceptual alternative to the block-universe picture: a “growing spacetime” ontology in which becoming is physically real rather than just perspectival.

Welcome back.

Did you come up with any answers to my questions whilst you were thinking ?

1 hour ago, studiot said:

Welcome back.

Did you come up with any answers to my questions whilst you were thinking ?

On 4/20/2026 at 11:56 AM, studiot said:

What do you mean by this ?

Please define (mathematically) the speed of light in spacetime ?

First, I would argue that every material object is in constant motion through spacetime, and that this motion is governed by the same fundamental limit we know as the speed of light. From this perspective, what we call motion through space is only one expression of a more basic movement, the other being motion through time. The two are not independent, but complementary aspects of a single process: the more an object moves through space, the less it moves through time, while its total motion through spacetime remains unchanged. My purpose here is not to focus on the mathematical structure of relativity, but on its philosophical significance. It suggests that matter should not be understood as something simply at rest in the world, but rather as something whose very existence consists in a continuous movement through the fabric of spacetime.

9 minutes ago, Rudolf said:

First, I would argue that every material object is in constant motion through spacetime, and that this motion is governed by the same fundamental limit we know as the speed of light. From this perspective, what we call motion through space is only one expression of a more basic movement, the other being motion through time. The two are not independent, but complementary aspects of a single process: the more an object moves through space, the less it moves through time, while its total motion through spacetime remains unchanged. My purpose here is not to focus on the mathematical structure of relativity, but on its philosophical significance. It suggests that matter should not be understood as something simply at rest in the world, but rather as something whose very existence consists in a continuous movement through the fabric of spacetime.

Thank you for replying but I'm afraid that adds nothing new to your explanation.

On 4/19/2026 at 9:37 PM, Rudolf said:

I’m curious how this idea fits (or conflicts) with modern physics, especially relativity.

You asked and I am trying to tell you it conflicts.

Speed is distance divided by time.

You cannot get away from defining how you are measuring distance (and time) to specify what you mean by speed in a four dimensional continuum.

This inevitably means you have to define how space and time are connected to satisfy your proposals.

2 hours ago, Rudolf said:

the more an object moves through space, the less it moves through time

As much as I can visualize what you are talking about, it seems to me you are saying objects that have changed the most are the farthest advanced in time, which actually makes some sense even if not strictly true. However, (ignoring speed effects on time) doesn't that mean said object would have moved more through time rather than less?

24 minutes ago, npts2020 said:

As much as I can visualize what you are talking about, it seems to me you are saying objects that have changed the most are the farthest advanced in time

No.

2 hours ago, Rudolf said:

The two are not independent, but complementary aspects of a single process: the more an object moves through space, the less it moves through time, while its total motion through spacetime remains unchanged.

A simplistic, but accurate, description of relativity.