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speed of light means moving through space with the same speed you move through time?
or to move "only" through space and not through time?
Does this mean that you exist in two (or more) places at the same time?

Qmechanic
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MpH81679
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    Welcome to SE. I suggest to read about the basics of special relativity. In general, "Speed of light" means the magnitude of the velocity with which light moves in vacuum. However, this speed is an important quantity in physics, coinciding with the upper bound on speed of anything (which light does not define, but simply has). Moving through time at the same speed as through space is not a meaningful question, since space and time have different dimensions. Your third question may refer to this: https://physics.stackexchange.com/q/54162/. – Koschi Jan 27 '23 at 15:38
  • Well, yes time and space ARE different dimensions. but my question is if we graph space/time of a point moving through space with speed of light, "Moving through time at the same speed as through space" means graph has a slope of 45 degrees or infinite? – MpH81679 Jan 27 '23 at 15:53
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    There is a good question here. I suspect the OP is reading about the basics of special relativity as has encountered some of the counter intuitive ideas. For example, you cannot travel at the speed of light. And yet light does. If you go fast, space contracts and light slows. It is often said (wrongly) that from the point of view of light, space has contracted to $0$ length and time has stopped. In this context, it is very reasonable to ask what the speed of light means. – mmesser314 Jan 27 '23 at 16:12
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    If you look up space-time diagram, you will see that light travels at $45^o$. – mmesser314 Jan 27 '23 at 16:16
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    @MpH81679 We often draw space-time diagrams with light moving at 45 degrees from the axes. This can be insightful, but also misleading, as the paper represents an absolute frame of reference, and the spacetime geometry we're attempting to represent is not Euclidean. We've looked for an absolute frame of reference in reality and failed to find it, and our understanding of how that works involves non-Euclidean geometry. – John Doty Jan 27 '23 at 16:18
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    This may help - How does a photon experience space and time? – mmesser314 Jan 27 '23 at 16:18
  • While meters and seconds are different units, physicists also work in units where time and distance have the same units. E.G. seconds and light-seconds. It is like working with horizontal distances in kilometers and altitude in meters. Distances calculations need a conversion constant. $d^2 = x^2 + y^2 +(cz)^2$. Life gets simpler if you work with everything in meters. – mmesser314 Jan 27 '23 at 16:29
  • Refering to your 45 degree angle (which light has an $x,t$ diagram if you normalize $t$ with $c$)... think back to high school physics: If I draw the position of an object moving with constant speed in one direction ($x$) into an $x,t$ diagram, what does the slope tell you? Does it tell you the speed? Or does it tell you "how fast the object moves through space and time" (whatever the second phrase means)? (At this stage it does not matter if we talk about cars, bikes or light here)... – Koschi Jan 27 '23 at 16:30
  • ... you may be confused by/referring to the notions of three-velocity and the special relativity (SR) concept of four-velocity, where the first one is the usual notion of velocity, while the second one is a concept from SR incorporating the time component, a quantity that always gives $c$ for ANY object. This could be interpreted as saying "Since my four velocity is always $c$, if I stand I move only through time (with $c$), but if I move, I move "less fast" through time, while moving through space also." I find this interpretation at least confusing. – Koschi Jan 27 '23 at 16:34
  • @mmesser314 No, measuring everything in meters is crazy in the real world. I don't set the exposure time for a camera or the baking time for bread in meters. – John Doty Jan 27 '23 at 16:38
  • @JohnDoty - True. Life stays complicated. But formulas get simpler. – mmesser314 Jan 27 '23 at 16:44
  • @mmesser314 The sort of formulas used by a certain kind of theoretician, yes. The rest of us will keep using our clocks. – John Doty Jan 27 '23 at 16:47
  • @mmesser314 Life stays physical. Formulae are mathematics, very useful, but not fundamental. – John Doty Jan 27 '23 at 16:49
  • @JohnDoty - Don't forget astronomers. They use years and light-years. – mmesser314 Jan 27 '23 at 16:55
  • @mmesser314 I don't know of an astronomer who uses light-years: astronomical units, parsecs, or just dimensionless redshift are all common. – John Doty Jan 27 '23 at 16:59
  • @mmesser314 I recall a talk by Roger Blandford where he was modeling the jet-producing dynamos in hypernova cores as electrical machinery. Naturally he used SI units: volts, amps, ohms, etc. This was baffling to the theorists in the audience, but illuminating to more practical physicists. ツ – John Doty Jan 27 '23 at 17:09

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The meaning of physical concepts comes from experiments and observations. The earliest observation demonstrating that light isn't an instantaneous phenomenon was Romer's discovery that the timing of observed eclipses of Jupiter's moon Io depends on how far Jupiter is from Earth.

Later experiments sent light pulses on round trips of known distance and measured the time it took. Light behaves as a thing that moves with a definite speed: we call that the "speed of light".

But speed relative to what? All measurements involved observers on Earth, in a similar state of motion. Attempts to figure this out led to Einstein's Special Relativity, and then Minkowski showed that theory could be interpreted geometrically, with every object "moving" through spacetime at the speed of light, but in different directions.

Light remains a phenomenon that is associated with a definite speed in the laboratory. As measurements of that speed improved, the speed of light became known as well as it could be, given the accuracy of distance measurements based in the international length standard, a piece of metal in Paris. So, taking into account relativity, we redefined distance based on an adopted speed of light. Thus, for the last 50 years, our fundamental measurements of both space and time have been based on our best clocks, with space and time thus abstractly the same thing.

It's a lot like mariners measuring location in nautical miles (by measuring star locations in the sky) and depth in fathoms (using ropes or poles). They understood that in theory they were both spatial measurements, but in practice they were different, requiring different tools and concepts.

So, to answer your questions, Minkowski's geometry does not change your normal qualitative experience. You move through time as you age, and through space as you travel. You exist where you exist, not elsewhere. Physical theory does not change the way the world works.

John Doty
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