# Speed of Causality

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I’ve only just learnt that the speed of light is really the speed of causality and I’m trying to understand it. I’m sorry if this is a stupid question but imagine I had a seesaw a lightyear long. I sit at one end and the other end lifts instantaneously. If someone was sat at the other end I would be lifting them faster than light could reach them. Their being lifted would be faster than the speed of causality. Is this wrong? Does one end of a seesaw dropping and the other end lifting not happen instantaneously with one another?

EDIT: Thank you to the people who have all responded so far. I can see that my thought that the two ends of the sawsee moving simultaneously with one another was the error in my thinking, and that the reason I thought this was due to scale and in reality a seesaw seems instantaneous but it’s not. Thanks again to those who took the time to reply and I’m grateful for your kindness.

In: Physics

It is wrong, because there is nothing that can exist in our universe that’s rigid enough for the seesaw to lift up instantaneously like you’re describing.

In reality if you pushed down on one end of the seesaw the atoms where you pushed would interact with the atoms next them, and that interaction would travel along the length of the seesaw until you eventually get to the end. But that “wave” of interaction that travels along the seesaw would never be faster than the speed of light – the speed limit of our universe.

With questions like this, it helps to think about “edge cases”. Change the scenario into something you understand better. What happens if you have a really really long, thin “see saw”. Maybe a ten foot long tape measure. When you lift one end, it bends. If you *snap* it up and down violently, the it’ll send a sort of wave from one end to the next.

Any seesaw a light year long will be much like this tape measure. Anything you do to one end will travel down the length at a speed significantly lower than the speed of light.

A seesaw will push up the other side at the speed of sound in the medium. That’s likely to be pretty fast – a lot faster than the speed of sound in air – but way slower than the speed of light.

Practically, though, the light-year long seesaw will snap.

> I sit at one end and the other end lifts instantaneously.

Nope, that’s not how it happens.

When you sit on a seesaw, your weight pushes down on the atoms in the seat. Those atoms push down on the atoms at your end of the beam, which push on the atoms next to them, and so on and so forth until the other end of the beam pushes up on the other person.

This seems instantaneous on the playground, and then the speed of light is really fast. But, it’s neither. In a substance with normal stiffness, the wave moves closer to the speed of sound than the speed of light. Still really fast on the playground. The speed of light is the fastest speed that causality can spread, most real systems not using light are much slower.

>Imagine I had a seesaw a lightyear long. I sit at one end and the other end lifts instantaneously.

Your seesaw is made out of atoms. Those atoms do not transfer the momentum of your butt instantaneously between each other, there’s always some wiggle room. This wiggle room on a large scale translates to the speed of sound of a material. So depending on the material of your seesaw, it’ll only lift at the other end after one light year divided by the speed of sound of that material (likely tens of thousands of years).

the seasaw is actually subatancially slower than the speed of light. Motion on “rigid” objects propogates at the objects speed of sound

Yeah the others are right the key is the seesaw isn’t actually instantaneous. Everything has *some* flex, and the flex moves at the speed of sound in that material. Even if you just pushed straight on the end of a steel rod, the far end doesn’t move ahead instantly, it compresses and the compression moves along the rod at the speed of sound in steel and the far end extends when the compression wave gets that far.

Gravity and light don’t have a physical material so the “flex” moves through them at speech c.

You know how light from the sun takes 8 mins to get here right? Well that means if the sun instantly disappeared from existence, not only would it still be bright in Earth for 8 minutes, but we’d also continue orbiting the spot where the sun used to be for those 8 minutes as well! Then it would simultaneously go dark and Earth would fly off in a straight line as the lack of light and gravity both reached us.

The motion would not occur instantly. It would actually propogate through at the speed of sound through the material it is made of. Believing it would happen instantly is a common thought because we usually never deal with object large enough or the speed of sound slow enough to actually notice this.

That speed will always be less than c for any medium.

Light and gravity don’t have mediums, so they travel at c

I’ve seen this question in here a lot and what it comes down to is that there’s no perfectly ridged material that would allow for that kind of transfer of energy from one side to another instantly. Even steel, though it seems like it’s hard and inflexible, is still very bendy at those size scales.

Even if we pretend like it exists, this material would have to have a ton of weird and paradoxical properties. For example, there’s nothing you could do to move it because it would require infinite energy due to the atoms needing to accelerate to the speed of light the moment you try to move it.

If we assume we can move it, then the moment it moves, you and everything around you would instantly burst into a ball of fire and death because you just had an object accelerate to the speed of light right next to your hand.