This spawns from part of an answer to a question I asked.

All sorts of things go to 0 and/or ∞ if you start boosting at c, and so you cannot boost into and out of a photon's frame.

It somewhat relates to a video from the discovery channel where they talk about space, time, and reality. They describe the speed of light and its implications specifically they describe the speed of light in a counter intuitive definition.

(This is what I remember its not a quote exactly, something along the lines of)

The speed of light means there must be so much space for every moment of time

What they mean by this is that the speed of light is constant so there must be so much space for every moment in time. This is because something moving at light speed covers so much distance but never more in one unit of time.

Though I don't think that covers the whole idea, I feel I am missing a part about a particle moving at a fraction of the speed of light and how much space it is allotted?

I think this is valuable to phrase physical constants like this. Are there any other units, constants, ideas in physics that can be stated this way?


  • Planck scales
  • Time
  • Relativity, or parts of it
  • golden ratio, e, pi

closed as not a real question by Emilio Pisanty, user1504, Qmechanic May 14 '13 at 15:12

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ The "miles of space per second" really just comes from the units of $\mathrm{length}/\mathrm{time}$ so there is some $\mathrm{length}$ per units of $\mathrm{time}$. Acceleration ($\mathrm{m}/\mathrm{s^2}$) and other measurements make less intuitive sense when phrased in the $\mathrm{numerator}$ per $\mathrm{denominator}$ way. $\endgroup$ – Brandon Enright May 13 '13 at 21:44
  • $\begingroup$ Since this question is mostly about wording, it is way too soft and too localized in my opinion. $\endgroup$ – Dilaton May 14 '13 at 14:40
  • $\begingroup$ Well get the mods in here to haul it off, vote for delete please. $\endgroup$ – KDecker May 14 '13 at 14:46
  • $\begingroup$ You're probably talking about the Minkowski metric? $\endgroup$ – Abhimanyu Pallavi Sudhir Jul 22 '13 at 11:12
  • $\begingroup$ Whatever I was saying was a no-no, thus it was closed. $\endgroup$ – KDecker Jul 22 '13 at 11:54

In the context of Special Relativity, the speed c is both an invariant and a constant.

As in invariant speed, some object with a speed c in an inertial frame of reference has the same speed c in any inertial frame of reference.

As a constant, the speed c is the conversion factor from temporal units to spatial units, i.e., one second (a measure of time) to one light-second (a measure of distance).

So, in this second sense, there is indeed a measure of distance associated with a measure of time.

Remarkably, the result of all this is that every entity "travels" at c through spacetime; all entities have a "four-speed" equal to c.

In a particle's rest frame, the "travel" is entirely through time - the particle's four-velocity vector points along the time axis - with speed c; in one second, the particle travels a "distance" through time of one light-second.

In a frame in which the particle is moving through space, the four-velocity points off of the time axis but, and importantly, the length of the four-velocity vector is still c.


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