Rudolf

You raise a fair question. I do think the distinction is meaningful, but only if we first question the assumption that spacetime is a pre-existing four-dimensional structure. My point is not about whether the fourth coordinate should be written as ct or ict. I understand why relativity uses that notation. My point is more fundamental. I question whether spacetime exists independently of matter in the first place. If spacetime is an existing four-dimensional block, then the future already exists and the distinction indeed becomes difficult to make. But if spacetime is continually generated by the evolution of matter and energy, then the future does not yet exist as part of reality. It exists only as a set of possibilities until physical processes bring it into existence. So, to me, there is a genuine difference between an already existing future and a continually created future. The former treats the future as something that already exists and is merely waiting to be encountered. The latter sees the future as something that comes into being through the ongoing evolution of the universe. That is the assumption I am questioning. The issue is not how we describe spacetime mathematically, but whether spacetime is fundamental or emergent?

Before we end up in a simple "yes/no" argument, I think it would be more productive to establish some common ground first. Can we at least agree that a light-year is a unit of distance? If so, then we can move on to discussing what "velocity" actually means and whether that concept necessarily applies to light. For example, I think we can agree that, according to relativity, moving clocks are observed to run slower than clocks at rest in the chosen reference frame. That's an experimentally verified prediction of the theory. If we can agree on that point, then we can discuss what that implies for concepts like velocity and the role of time in defining it. It's better to build from shared premises than to argue over conclusions. You're right that a reference frame has to be specified. Let me state it more carefully. I think we can agree that relativity predicts that a clock moving relative to a given inertial observer is measured by that observer to accumulate less proper time than the observer's own clock between two comparable events. That is an experimentally verified result. Can we agree on that statement?

I will argue that distances are differences in time. We experience reality with a delay of 1sec per 300.000km. Your own moment in time "now" is the exact point from where reality is created.

"A light-year is usually interpreted as the distance light travels in one year. I propose an alternative interpretation: what we call a light-year is actually the distance that our own frame advances relative to the propagation of light. In that view, we are measuring our own motion through spacetime rather than the motion of light through space."

I do agree, so that´s why I prefer to use a rocket as an example traveling at 99.999%c traveling between 2 (A - B ) stars with a distance of 5 lightyears between each other. While the time at star A and B will be more than 5 years, the rocket will travel from A to B in less than 5 years. We only can imagine that the time at light speed it would be 0 secondes to travel this distance. In a very extreme situation you might argue that it would take only 1 second if you travel near light speed over a distance of 5 lightyears. All matter always travel with the velocity of light. By changing our speed we only change the velocity of time. But this never will be zero.

That is not really an answer to the question, I think it should be more like this: To answer how long this trip takes, we have to look at it from two different perspectives due to Einstein's theory of special relativity: from Earth, the trip takes just over 5 years, but for the astronauts inside the rocket, it takes only about 8.17 days.

How long does a rocket take to travel a distance of 5 lightyears with 99.999%c?

Let´s try this in a different way then.. How much distance and time does photon travels (on his own clock) to travel a distance over 5 lightyears between 2 stars? I do know that it would take for us 5 years to receive a photon which travels 5 lightyear distance. But how much time would the photo need for this?

At this moment I do believe that time and distance do not exist at the speed of light. Only matter can measure time and space. I would argue that in the twin paradox the younger brother (the one which will travel from earth and back) would have taken a shorter path through space time than the persons who haverstayed on earth. However he seems to travel from earth with a certain speed, I will argue that he put on the break in space and time and will travel with a lower speed that the person(s) on earth.

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.

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.

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.

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.

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 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:

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.