How do you expect them to explain gravity at a fundamental level? Quantum gravity and spin foam?
I love that analogy. When I first learned it years ago, it made the relationship between the color force and the strong force crystal clear to me. Likely because I was teaching chemistry at the time, which included talking about van der Walls forces a lot.
I’d like to ask a question. Does gravity travel at the speed of light, or is it instantaneous across all distances?
So for example if the sun somehow winked out of existence, would the earth continue in its orbit for around 8 minutes (the time it takes light to travel from the sun to the earth IIRC) before it “noticed”, or would it leave orbit instantly and start sailing outwards at the same moment the sun vanished?
The OP somewhat disparages gravity because the Earth is pulling at us from an effective distance of 4000 miles away- average of matter directly under our feet versus far side of the Earth. Gravity is still the weakest force on an absolute scale but overcoming the pull of “only” a billion tonnes of neutronium at close range would be a harder proposition.
Do particle physicists still insist on considering pions to be the “boson” of the strong force holding nuclii together?
Speed of light
This is one of those questions where the only answer is “That’s magic, so what happens is whatever the laws of magic state happens”. Things can’t wink out of existence, so we can’t describe what happens when they do that.
We can, however, have things like two masses colliding, or a mass ejecting another mass, or any of a variety of other things that would change a gravitational field, albeit in a more subtle way. And those changes in gravitational fields propagate at c.
That said, however, if you have a system of objects that are only interacting via gravity, there are some other weird effects that kick in with the gravitational fields, such that the gravitational force vector on each object is the sum of vectors that point directly to the other objects’ current position, not their position whatever time ago. Since large-scale structures (like a solar system or a galaxy) are usually governed almost entirely by gravity, this can produce the illusion that gravity acts instantaneously, even though it doesn’t.
Thanks.
Just to put a slightly different spin on it, one of the fundamental conclusions of Special Relativity is that there’s no such thing as simultaneity at points separated in space. No causal phenomenon at point “A” can cause an effect at point “B” any sooner than the transit time of light, or else basic cause and effect itself would be violated. Another way of looking at it is that the speed of light is not limited by the properties of any particular propagation medium; it’s essentially infinite; its apparent speed c just reflects this fundamental property of spacetime. Photons, after all, in their frame of reference can traverse the entire universe in precisely zero time.
Don’t thought experiments posit “magical” events sometimes?
This brings to mind a question I’ve long been tempted to ask here, but haven’t out of fear of showing my ignorance but…
With respect to escape velocity, why does there need to be velocity? Couldn’t something with enough torque or strong enough thrust just power out of earths gravity at a sub-escape velocity speed? Or does escape velocity not mean what I think it does?
It means from the Earth’s surface (disallowing the atmosphere), how fast would something have to start at traveling up for its velocity away from Earth to never fall to zero?
That’s exactly the point I was making. Realistically, however, no one knows how to build a rocket engine that produces continuous thrust for an indefinite period of time sufficient to lift itself against gravity and propel itself far enough away that the influence of gravity is no longer a factor. In practice, we depend on accurately attained ballistic trajectories. The Apollo spacecraft were propelled towards the moon at an initial speed of just under 11 km/s, a little less than escape velocity but just enough to get them grabbed by the moon’s gravitational field.
I just watched an excellent video featuring Neil DeGrasse Tyson explaining the “three body problem” in terms that even I could understand.
“Escape velocity” means “the speed at which you can coast unpowered outwards forevermore, slowing all the time, and yet never be pulled back in”.
It says exactly zero about the behavior of powered vehicles; only coasting vehicles.
As such, “escape velocity” is really only meaningful as to a particular body at a particular distance. Which distance is usually left implicit in various publications about escape velocity.
The escape velocity from Earth at the surface (ignoring atmosphere) vs. from 200 miles up are slightly different. The escape velocity from Earth out at Saturn’s orbit is a different and much smaller number.
Yup.
If there were a staircase from earth to the moon anyone could walk up it and get to the moon at a very slow speed. It’d take a while though.
(please skip nitpicks on reasons why we could not have a staircase to the moon)
But we can have a Earth-Moon Fire Pole (xkcd.com).
You (any you) will learn a lot about gravity from that cite.
Right, hence my thought experiment about why you couldn’t get out of a black hole even if you had an infinitely durable arbitrarily powerful thruster – the spacetime geometry at the event horizon makes it impossible. The term “event horizon” is meant to describe an absolute boundary beyond which no events can affect an outside observer. Things going on within the event horizon are no longer of this universe – nothing but the body’s overall mass, spin, and charge can be observed, but absolutely nothing else.
Thank you, that helps. And thanks to everyone else who answered.
Fascinating. Admittedly, I read through it kinda fast, but did he ever answer the question? (How long [factoring in all his variables] would it lake to get from the moon to the Earth?)
Not the moon, but the space elevator that used to be talked about almost as much as people talk about AI today is a device that could slowly move objects into orbit. Here’s another “sooner than you think” article from 2018.
Somehow the lack of magic materials to build it has proved a mild deterrence.
He did not because it is a very variable equation. For some time you have to climb and he posits that cannot be much more than a meter per second. Eventually you get to a point where you can (maybe) move yourself at around 45 m/s. Then you start falling and will be going 11 km/s at the end and will burn up in earth’s atmosphere unless you slow yourself down.
Maybe someone more mathematically adventurous will take a shot at that math (with a lot of simplifying assumptions). Not me though.
If you climb the hypothetical stairs and manage 20 miles per day (~32km/day…which would be a very good pace) it’d take you about 32-33 years to get to the moon.