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Comment by bell-cot | Hacker News Reader

If you have a sufficiently tall* first stage, and use hot staging, then you can make it work on even on an extremely large Earth.**

*First stage may need to extend well above the atmosphere.

**No, that's for-sure not a Randall Munroe book in my hand.

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This is the same way you make a train capable of traveling from Los Angeles to New York in 1 second. A sufficiently long train.

I think the speed of sound in steel (or whatever the train is made out of) is slower than that.

But I guess you could cheat by having multiple engines along the way that all accelerate in lockstep.

Or you could just put the engine in the front. There's no rule a train can't arrive before leaving after all.

Related theoretical question for those who are of the physics mindset - if I had a long (very long like 1 light minute long) bar of metal and I pushed on one end, I'm assuming the other end would not move instantaneously because that would imply some part somewhere inside the bar was moving faster than the speed of light. So I'm assuming that the bar would just compress slightly and for a period of time in between when I pushed on one end and when the other end moved the bar would be slightly shorter. That's fine if that's the case.

But what if the thing I push on is a quantum particle? Does this same thing happen at the smallest scales? If one end of a quark is pushed on does the other end move instantaneously or is there a small(!) delay?

Probably the answer is just "that's not how quarks work" but I've always been curious.

There's no such thing as instantenously "pushing" on a particle. E.g. electrons can be accelerated by electromagnetic fields. If the field changes, the electron feels a force and is accelerated according to a = F/m (handwaving away relativity). When you macroscopically push against a rigid body, what happens at the particle level is your constituent atoms' electrons (and protons) interact with each other through the electromagnetic field.

The other answers are correct that it's an acoustic wave, but sometimes it helps to see a demo "proving" it:

https://youtu.be/DqhXsEgLMJ0

When you tap on the bar it creates an acoustic wave that will propagate at the speed of sound in the material.

For subatomic particles, the most intuitive way to think about things is to adopt the "fields are real" mindset. Here fields are the underlying reality, and particles are just a pattern of waves excited in the fields. Disturbances in all fundamental fields we've discovered propagate at the speed of light, and we have pretty solid reasons for believing no future discovery will contradict that, as it would break causality in a fundamental way.

> If one end of a quark is pushed on does the other end move instantaneously

Quarks don't have an "other end." To the best of our knowledge, particles are points.

Well, at that level, you are talking about clouds of probability or structures in the wave function or something like that.

So an object of zero volume will arrive in New York.

On your long bar, the push propagates at the speed of sound in the material. Look up Slinky drops on YouTube.

Interactions between particles such as quarks are mediated by fields filling the space between them (such as electromagnetic field and gluon field). Ripples in these fields propagate at speed less than or equal to c.

This is a classical picture, but the quantum picture is similar: evolution is generated by a local Hamiltonian constructed out of field operators attached to every point of space.

So, both classically and quantumly, relativity demands the existence of fields filling space to propagate causal influences at finite speed.

"Speed of light" makes more intuitive sense when you think of it more as the speed of causality i.e. the fastest physical speed a cause can have an effect

The speed of light in a vacuum happens to be the best representation of the maximum speed of causality

Which also makes more sense why you can't do things like travel faster than light (your effect would precede the cause), and why two protons going past each other in opposite directions don't violate this law

Surely for a train to "travel" from LA to NY, it "starts" when the front of the vehicle passes a line in LA, and "finishes" when the front of the vehicle crosses a line in NY.

Just build two fronts. The Germans did it with their ICE trains.

I’m trying to decide whether you’re describing a rocket or a space elevator. If you build a tower that extends to somewhere near geostationary orbit, you can pretend it’s a rocket stage delivering a delta V of zero, and you can “hot stage” a tiny little stage off the top, and voila, you’re in orbit.

Of course, once you’ve managed to build this, the rockets are basically optional. :)

You’re going to have a hell of a difficult time trying to construct anything that tall on a high-g planet. The taper ratio between the base and the top would have to be enormous – likely a sizeable fraction of the radius of the planet! Though I guess it would have to be anyway so you have somewhere to attach all those first-stage engines…

If you're allowed to build it tall enough, just don't even light the first stages. Launch the final stage directly from a high enough altitude that it can escape on its own.

Well, yeah, but building something tall enough to reach the synchronous orbit is impossible even on Earth, there’s no material with even a thousandth of the compressive strength required. Space elevators are only possible because they’re tensile structures and the “bottom” that supports the weight of the entire structure is up there in a low gee environment.

Remember that just getting outside the atmosphere is the almost trivial part of rocketry compared to the problem of having to then accelerate to >= orbital speed fast enough to not fall down!

And anyway you’d have to dismantle the planet to build your launch tower, which I guess would solve your problem, in a fashion. Though – whatever you turned your planet into would just have an annoying tendency to rapidly collapse back into a ball.

If you really have a lot of time for the project (starting early in the star's burn?), you might try using photovoltaics to move a lot of mass across the surface to ahead of where tides would accumulate, slowly speeding up the day/night cycle. The faster you spin, the flatter your geoid and you should probably stop accelerating before your entire equator region goes interplanetary.

On earth, you could use active support to build your tall structures, then you don't need exotic super-materials. See eg https://en.wikipedia.org/wiki/Space_fountain

You just need to launch off of the highest point on the planet! (Kerbal Space Program approved).

We don't talk about ground logistics though.

** And assuming sufficient propellant exists on your planet :)

You could just perch the first stage at the top of a sufficiently tall mountain.

Could, but at least on Earth the difficulties outweigh the gains enough to make it too expensive. https://youtu.be/4m75t4x1V2o?si=6FzbQYrLtl7Zfe0J&t=157

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