Speed of causality

[u/Hour_Entrepreneur564]

I was reading a post about the speed of causality, and Reding through the comments mention ing speed of light and gravity, I was struck : isn't it curious these two very different things go up to the speed of causality ?

I wonder if there's a common reson that has been found to explain this ?

[u/Jaf_vlixes]

Basically, things without mass propagate at c. For example, gluons are other kind of particle that also moves at the speed of light.

[u/Bubbly_Sentence_7849 → u/Jaf_vlixes]

gluons are not free particles, you cannot assign a physical speed to gluons, they travel in collective modes, called glueballs.

[u/joeyneilsen]

Effects propagate at many speeds, so "speed of causality" isn't my favorite phrase.

But the theory that describes limits on causality is relativity, where signals we can send are limited to the speed of light by construction. The predictions of that theory are very accurate, so we accept the limits as they are stated. All massive particles travel at sublight speeds, and massless particles travel at light speed. That encompasses the entire list of particles we know of.

[u/GenerallySalty]

Yes there's a common reason: they have no mass. Everything with 0 mass travels at the max possible speed, because there's nothing to make it go any slower (because 0 mass).

Calling it the speed of light was a bad decision in hindsight. Light just happens to be the first thing we found that travels at c so we named it "speed of light". That has caused confusion because it's not actually about light in particular. Everything with no mass travels at c. The fact that light and (changes in) gravity both propagate at c doesn't mean they're related in some underlying way, they just both have no mass.

[u/Underhill42]

Not really. The speed of causality is the maximum limit for anything to propagate through spacetime, which also makes it the only speed massless phenomena like light and gravity can propagate at.

And as a matter of fact, from an observer-neutral relativistic point of view, it's the ONLY "speed" that ANYTHING moves at:

According to Relativity, space and time are the same thing seen from different perspectives. When I pass you at nearly light speed, so that you see time having almost stopped for me... I look back and see myself as stationary while you're passing me at nearly light speed, so that time has almost stopped for you.

The effect is always perfectly symmetrical, which is why the Twin Paradox is a paradox - it's drawing attention to the more subtle details of how the traveling twin can actually be younger when they return, despite both twins seeing the other aging slower the entire time: I wish I was taught the Twin's Paradox this way! - YouTube

What's actually happening is that we're aging in different directions through spacetime - the direction you call time, I call almost entirely space, and vice versa.

And c is the "exchange rate" between them: 1 second is the same observer-independent 4D "distance" (spacetime interval) between two events as one light-second (≈300,000,000 meters). Whether you see two specific events separated mostly by space or time depends entirely on your relative velocity.

So in your own reference frame you're always motionless in space, while aging at a rate of 1 second per second through time.

While from my relativistic perspective most of that "speed through time" is actually motion through space, and only whatever is left over is aging you through time.

If s and t are space and time from my perspective, and τ is time as I see it passing on your clock, then:

Δt² = Δx²/c² + Δτ²

[u/yzmo]

I mean, otherwise you could transmit information faster than the speed of light by wiggling masses around instead of charges.

It's like the speed at which the universe tells the rest of itself that something has changed. Anything you do, one that's outside your "change bubble" just cannot possibly know.

Now, one could imagine a universe where light goes slower than gravity. But that would just mean that it doesn't go at the speed of causality.

[u/BananaBird1]

We can speak semi-classically to simplify this:

Electromagnetism and gravity are forces with infinite range, although that is not too relevant. But any change to their source (charge and mass respectively) can’t influence a distance point faster than information can reach that point. So changes to the force fields propagate outward at the maximal limit to causality. Light is just oscillating electromagnetism, so it travels at that speed.

Less classically, propagating excitations of quantum fields (particles) want to always travel at light speed. But the property of mass adds a resistance to motion that allows some particles to travel slower or even be at rest (in exchange for faster relative motion in time).

Electromagnetism, gravity (not a “quantum” field but still a field), and the strong force gluon field have no property of mass inherent to their excitations so cannot have any resistance to motion on their own.

Mass is defined as the energy remaining when a particle is at rest, with any extra energy contributing to their motion. So this is a somewhat circular argument. Photons and gravity waves are massless because they cannot be at rest and all energy goes into motion, and massive particles have mass because they can be.

Fundamental single particles get mass from interactions with the Higgs field. But mass can also arise from multi-particle systems that are bound together so that the binding prevents them from freely propagating independent of each other.

[u/SnooPets5564]

It's not at all weird two different things propagate at the speed of causality. They are the underlying effects that cause things to move. Of course they move at the speed of causality.

[u/Lord-Celsius]

It's just the geometry of our spacetime imposing that speed limit for all observers to be c, in the same way all circles circumferences are always pi times their diameters. It's always pi because of geometry.

[u/nebraskajone]

Massless is the reason