Why can it not stay put. What is the force that makes light travel? Such vast distances in vacuum.
It isn’t just light. Everything travels. And everything travels at the same speed. That is the speed of causality. The only trick is that things with mass travel in both space and time, and things without mass only travel in space. Photons have no mass. So light travels at the speed of causality. As would anything else with no mass. Like gravity.
There isn’t a force that makes things travel in space time. It is the nature of space time. You can apply force to make things with mass change the balance of how much they travel in space and how much in time, but the only way to stop light from travelling is to extinguish it.
Your theory is… um… unusual. :rolleyes:
When did you come up with it?
1905?
It’s the geometric point of view that a particle, without forces acting on it, travels in a “straight line” (also considering the space-time may be curved as in general relativity). (Mutatis mutandis for calculations involving free particles in quantum field theory, which simultaneously take many paths…)
Cf one can derive equations of motions from an action principle
No… I don’t think Francis Vaughan is Einstein. 
And that is most certainly NOT what Einstein’s theories say.
Well, then, what DO his theories say? The O.P. asked a good question, I thought, and I was kinda curious if anyone might come in to post a lay-level answer that I could parse.
Can you provide anything further besides, “No, that’s not it” here, please?
Fascinating.
So light and sound travel at the same speed, as does my truck.
Fascinating.
Well, through space-time, anyway.
GreenWyvern, can you explain what’s wrong with Francis Vaughan’s explanation? This is GQ after all.
I’ve never seen relatively explained that way, but I really like it. If it’s wrong, I’d like to know why.
It’s a common way to look at special relativity. This neatly explains things like time dilation—at rest, you travel only through time, so to speak, while if you move through space, and your total speed remains invariant, the movement through space reduces the movement through time: a moving clock runs slower.
Mathematically, this just says that the magnitude of the four-velocity—the rate of change of your four-position (ct, x, y, z), your spacetime coordinate, with respect to your proper time—is always constantly equal to c, for everything. So indeed, for anything that moves only through space, it follows that its movement must occur at the speed of light.
But of course, that doesn’t answer the question. Why does light ‘only move through space’?
I can’t really think of a good intuitive explanation at the moment. But mathematically, it’s simple. Take the familiar Einstein mass/energy relation
E = mc[sup]2[/sup]
Here, the mass ‘m’ is what’s known as the ‘relativistic mass’, related to the rest mass m[sub]0[/sub] by
m = γm[sub]0[/sub],
where γ is the Lorentz factor given by
γ = 1/√(1 - (v/c)[sup]2[/sup]).
Now, ‘relativistic mass’ is a bit of a dodgy concept, and widely depreciated; Einstein himself argued against using it. But it will serve us well in this context.
The mass-energy relation can be written in a different way:
E[sup]2[/sup] = m[sub]0[/sub][sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup]
Here, ‘p’ is the momentum of the system, and for a photon (m[sub]0[/sub] = 0), we thus get
E = pc,
a relation already familiar from classical electrodynamics.
Both definitions of relativistic energy come out to be the same:
m[sup]2[/sup]c[sup]4[/sup] = E[sup]2[/sup] = m[sub]0[/sub][sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup].
Now we replace the momentum with its definition
p = mv,
v being the velocity, obtaining:
m[sup]2[/sup]c[sup]4[/sup] = m[sub]0[/sub][sup]2[/sup]c[sup]4[/sup] + m[sup]2[/sup]v[sup]2[/sup]c[sup]2[/sup]
We divide through by m[sup]2[/sup]c[sup]4[/sup] and rearrange to get
(m[sub]0[/sub]/m)[sup]2[/sup] = 1 - (v/c)[sup]2[/sup].
Then, for m[sub]0[/sub] = 0, we obtain
(v/c)[sup]2[/sup] = 1,
or equivalently,
v = c.
Hence, photons always move at c.
If you stipulate that light rays have zero mass (this requires some understanding of the nature of light), it should then follow from that fact that they have to travel at the “speed of light” (that at least must follow from a relativistic picture of space-time)
Maxwell’s Equations explain EM propagation really nicely. It does take some Math background to full understand it. But note the end of this section of the Wikipedia article on it.
The “E” part of the wave generates the “M” part which in turn generates the “E” part, etc. Perpendicular to each other (and to the direction of travel) and in perfect sync. This back and forth takes time. The particular time, which gives the speed of light is one of those “it is what is” things. It’s going to have a finite value and this happens to be it. It’s a large finite value given the near-ghostlike entities involved. But it’s also not infinite given there is a cause-and-effect process going on and that can’t happen instantaneously.
To have light “stand still” messes up the dimensions. If there is no direction of travel then what are the axes of the E and M part of the wave perpendicular to? It’s a 3-d Universe and you need a 3rd axis to orient things around.
Please elaborate. It certainly jibes with the physics I learned back in college.
Can you simplify it intuitively by using Newton’s First law?
Say that a photon is produced by an electron moving to a lower orbital and kicking out a bit of energy in the form of a photon. The photon is given a “push” outward to put it into motion. Once created it then moves at a constant velocity, in this case c.
There is no way of making any photon “stay put” without having an outside force acting on it. And there is no reason for any photon to stop unless it interacts with another particle, even if that means traveling billions of light years.
It’s not uncommon - this site http://abyss.uoregon.edu/~js/cosmo/lectures/lec06.html has this quote, which is pretty similar to Francis Vaughn’s
" The ultimate result from a spacetime view for the Universe is the realization that all objects move at one ‘speed’, the speed of light through space and time. Consider an object with mass at rest versus an object with zero mass (a photon). The object with mass moves through time, but not space (it is at rest). This ‘motion’ through spacetime is all time, no space. If you move an object from rest, then special relativity states that its clock slows down. In other words, it has given up some of its time ‘velocity’ to move through space. The sum of spatial and time velocity always equals the speed of light. You either spent this velocity all in the temporal direction (at rest) or some mixture of time and space.
Likewise, a photon has no time. It uses all of its spacetime velocity in the spatial direction and has none leftover for time. There is no time for energy. Zero time has passed for a photon created at the beginning of the Universe."
This is helpful.
Just to be clear, what you quoted is the quote from the website I cited, not by me.
Quibble: “Why” assumes volition. “How” deals with mechanics. How does a hawk fly? With certain sophisticated wing motions. Why does a hawk fly? To look for food because hungry. How does a machine work? By certain physical principles. Why does a machine work? Because somebody wanted it to, and switched it ON. How does a photon move? According to certain forces. Why does a photon move? I don’t know, I haven’t asked any for their emotional drive. Why does light travel? Bored with its old x,y,z location, I guess. Or frightened by gnomes.
I brought this up with Joe Nickell who dismissed my concern with, “Why means how.” The poet within me disagrees. Should I start an IMHO thread for answers?
Just chiming in here to add another voice of agreement to Francis Vaughan. The way he phrased it may make it sound strange, but there’s nothing inaccurate about it.
“Things without mass only travel in space”? So light travels instantaneously, since it travels only through space, not through time?
NO. Please read what it says in that link by Andy L about light cones. It’s something very different. Light travels at the maximum angle through BOTH space and time.
NO. Everything, with mass and without, travels with the same ‘speed’ through spacetime, but with different ‘angles’.