0
$\begingroup$

I'm an amateur and this is my first question here, I'm trying to formulate question about a general representation I have in mind after trying to grasp the idea of relativity and the concept of space-time.

We always talk about the "speed of light", but it's a bit of a misuse of terminology, since we should really talk about "speed of electromagnetic radiations (EMRs)".

Moreover, we recently confirmed that gravitational waves, despite not being of the same nature as EMRs, travel at the same "speed of light".

Considering that, according to relativity, an object travelling at the speed of light would not experience time passing, and that it doesn't seem to make sense to envision travelling faster than the speed of light,

Would it not be natural to consider the "speed of light" as being the "speed of time", or, let's say the speed at which the present propagates through space ?

This way the speed of light and gravitational waves would be actually instantaneous, or at least as much instantaneous as the universe can be, and each point of the universe would "communicate" with the other points at the "speed of light".

Usually when trying to [in]validate this idea by reading other questions or other sources, I find complex explanations that do not really help me evaluate the correctness of the broad model I have in mind.

I do not know if this is something that seems quite obvious to everyone already, but I couldn't find it expressed this way, and this mental picture really makes sense to me. If it does not correspond to our current state of understanding, could you give me some clue of where my formulation does not fit reality ?

Improve this question
$\endgroup$
4
  • 3
    $\begingroup$ What you hint at is that the spacetime interval along a light-like geodesic is zero. So yes, it is obvious to someone that has studied relativity, but still a deep enough idea to be for example one of the conceptual basis of twistor theory. As for your own mental pictures, I would recommend you let them live and die, because building an intuition on these matters is very difficult, and can only be done after having thoroughly studied the subject, not before, and not from pop culture representations. $\endgroup$
    – Stéphane Rollandin
    Commented Jul 17, 2023 at 8:36
  • 3
    $\begingroup$ I don't agree with closing this question or the downvotes. It inquires after the fundamental nature of what $c$ is, which is a concept that underpins all of modern physics. $\endgroup$
    – RC_23
    Commented Jul 17, 2023 at 15:37
  • $\begingroup$ Related: physics.stackexchange.com/q/291316/123208 & links therein. $\endgroup$
    – PM 2Ring
    Commented Jul 17, 2023 at 18:45
  • $\begingroup$ You might be interested in youtu.be/vPi1lyAx4ws $\endgroup$
    – RC_23
    Commented Jul 17, 2023 at 20:44

6 Answers 6

Reset to default
8
$\begingroup$

If your question is simply about terminology, it's called "speed of light" because light was the first thing people knew of that travels at that speed. It is the speed of massless particles or field disturbances. It is also the speed of causality, i.e. the maximum speed at which any event can affect anything else. See the concept of "light cone."

We could name the speed of light anything, but that will not change what it is.

Improve this answer
$\endgroup$
1
  • 3
    $\begingroup$ A rose, called by any other name, would smell as sweet; but names do influence the mental models that beginners form when they are trying to learn a new subject. $\endgroup$
    – Solomon Slow
    Commented Jul 17, 2023 at 10:12
2
$\begingroup$

Moreover, we recently confirmed that gravitational waves, despite not being of the same nature as EMRs, travel at the same "speed of light".

Things like these makes some of us want to go beyond "speed of electromagnetic radiation in vacuum" to "universal speed limit", or better, (natural) "spacetime conversion factor". In reality, that we even speak of the speed of light at all, is a consequence of us having defined SI units before we knew about the Special Theory of Relativity. If we moved towards natural units, it would not be the concept to talk about.

and that it doesn't seem to make sense to envision travelling faster than the speed of light

Curiously, SR does not actually forbid this. For example, we speak of the expansion of the universe giving us the illusion that some stuff are exiting the finite observable range of our universe at faster than light. It is also somewhat common, in the rare case that time is allowed at the university level, to cover the consequences of tachyons going faster than light, and how that is connected to anti-particles. SR only says that you cannot accelerate a slow thing onto the universal speed limit, and you cannot decelerate a tachyon downwards onto the universal speed limit. We simply do not have a tachyon on our hands to play with.

Would it not be natural to consider the "speed of light" as being the "speed of time", or, let's say the speed at which the present propagates through space?

What you are thinking of is very clearly a time slice moving forward in time. This is to do with a frame of reference, and very little to do with spacetime (and of time) at all. As the other answers have already pointed out, the standard reference with which you measure out a standard thickness of this time slice propagating, is merely convention, and not at all selected by our universe.

This way the speed of light and gravitational waves would be actually instantaneous

No, that is not how any of these works. What you should be doing is to use a suitable observer's time and space measurements, and deduce that light and gravity waves would have a spacetime interval of zero, i.e. they would find an elapsed time in their own proper frame of reference, to be zero.

That is, the light from the Sun takes 8 minutes to come to Earth, and while other observers will disagree with this value, what they can all do and agree, is that in the Sun's frame of reference, the time it takes from the light to get from the Sun to Earth is 8 minutes. This is why we call it proper time. The proper time of the light itself is zero, but that just makes for a complication.

Under no circumstance can you consider the propagation of light waves or gravitational waves as instantaneous.

Improve this answer
$\endgroup$
4
  • $\begingroup$ Thank you for your answer, which, with the help of other's comments too, helped me to improve my understanding of this concept. The concept of "time slice moving forward in time" really put words on what I was trying to express, thank you! As for the "instantaneous" part, is was a miswording, my bad! Thank you for the correction, and for the precise phrasing and details about frames of reference, which are indeed the keypoint. $\endgroup$
    – mikaël
    Commented Jul 19, 2023 at 9:27
  • $\begingroup$ I think I used "instantaneous" because, during there 8 minutes we would have absolutely no way to tell the sun "already" exploded, and "light travels as fast as the time slice", in a way it could be considered as instantaneous because it moves as fast as what defines the time passing. If we imagine the sun exploding, for a duration of 8 minutes we would still see it ad we would have absolutely no way of telling wether it has already exploded or not, at least not without going faster than the speed of time slices, if that makes sense and to not use the term "speed of light". $\endgroup$
    – mikaël
    Commented Jul 19, 2023 at 9:31
  • 1
    $\begingroup$ I have many years of experience teaching physics, so I know where people get confused here. And it is still not called instantaenous; it is just that the information about the Sun exploding will take the time it takes for the light to arrive. It is important to keep the terminology sensible so that we are not confusing ourselves, confusing students, and so on. $\endgroup$
    – naturallyInconsistent
    Commented Jul 19, 2023 at 9:33
  • 1
    $\begingroup$ I do absolutely understand that my understanding is limited by my lack of technical knowledge of the subject, and that having a proper mathematical education would for example be of great help in conveing these types of ideas without the need of imaginary situations. Thank you very much for taking the time to answer my question by adapting your message and making it accessible to my understanding. It is truly a thrilling subject and I hopefully can continue educating myself =) $\endgroup$
    – mikaël
    Commented Jul 19, 2023 at 9:34
2
$\begingroup$

The word "speed" has a perfectly well established meaning: It is the distance an object travels divided by the time it takes to travel that distance.

It is always permissible, but rarely a good idea, to take a word with a perfectly well-established meaning and give it some other meaning as well.

So yes, you can choose to use the word "speed" this way. You can also choose to call a bluebird a giraffe. I do not recommend it.

Improve this answer
$\endgroup$
2
$\begingroup$

One of the fundamental features of relativity is that time is relative, just as space is relative. What "here" means differs between observers, and one observers "here" can appear from another observer's frame of reference to change over time (e.g. the "here" of a passenger sitting on a train is not a single place according to a person on the ground). Similarly "now" is relative, and different observers have different ideas of what things happen "at the same time". This is called the relativity of simultaneity and is very important in resolving many apparent paradoxes of relativity.

Given that time is relative, there really isn't any well defined notion of "the speed of time". Indeed, it's better not to think of time as passing at all. It's probably more useful to think of time as a dimension (like x, y, z) through which objects pass. Different observers may choose different directions in 4 dimensional spacetime for their t, x, y, and z axes.

Improve this answer
$\endgroup$
2
$\begingroup$

I want to add something to the different responses you already got. I want to mention that the term you tried to use, that $c$ is the "speed of time", actually has an interesting sense in which it can be said to be true:

Whereas in ordinary mechanics one works with a velocity vector, in relativity one works with something called a four-velocity. This is a four-vector in 4 dimensions, space and time. The magnitude of this four-velocity is always $c$. This means that if an object is at rest in space, it moves at speed c through time. If the object has some velocity in space, it moves slower through time (so in total the magnitude of the 4-velocity remains $c$), which is "time dilation". All of this is of course a gross over-simplification, but it provides a sense in which $c$ is a "speed of time" - the speed that time passes for an object at rest.

Improve this answer
$\endgroup$
2
  • 2
    $\begingroup$ I would disagree with this interpretation. The 4-velocity is just the tangent vector to the worldline; it could have any normalization and mean the same thing. To the extent that we normalize it to $c$, it's just because $c$ is the natural geometrical conversion factor between lengths and times, i.e. it is natural to say that $c=1$, and it is natural to normalize direction vectors to be $1$. "Movement through time" can also be a misleading concept; in relativity, an object simply exists at all points along its worldline. It doesn't move along the worldline in any sense that isn't tautological. $\endgroup$
    – Sten
    Commented Jul 17, 2023 at 21:51
  • $\begingroup$ Sten, I agree that maybe "speed of time" isn't the best way to understand things but I was trying to explain to the original poster in what limited sense his idea makes sense. Anyway, yes, the worldline should be thought as "eternal", just existing in space-time and not "moving" through time, but it still makes sense to define its derivative (the "tangent" as you said) which is some sense the "velocity" of this line and its time component is "sort of" how much this worldline is speeding through time vs. speeding through space. $\endgroup$
    – Nadav Har'El
    Commented Jul 18, 2023 at 8:37
0
$\begingroup$

A speed through time should measure the rate of the passage of time with respect to some parameter, so it better have dimensions of 'unit time per unit something else'. $c$ does not have those dimensions.

The notion of 'speed through time' of some object that makes the most sense to me is $\frac{\text{d}\tau}{\text{d}t} = \frac1{\gamma}$, which expresses the rate of change of its proper time $\tau$ with respect to the time $t$ of an observer. In other words, it measures the relative rate of the same type of physical process (such as the decay of a muon) between the 2 reference frames. This is of course relative and whichever observer you ask will say the other one is slower. Also an observer will always move at 1 second per second in their own frame.

I should point out that this only makes sense for an object on a timelike trajectory; and a lightspeed object does not have a concept of proper time since no physical processes take place on its trajectory to use as a clock. You can't associate a speed through time to light.

Improve this answer
$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.