this post was submitted on 18 Aug 2023
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Yeah definitely A. The momentum of the object going through the portal matters not the objective that has the portal on it.
Here's how I always phase it. Imagine you have a shovel and you are using that shovel to flick some dog shit into your neighbours garden.
With no portal the shit hits the shovel and you flick it, transfering the speed of the shovel into the turds. You stop the shovel and the turds fly away.
Now imagine the shovel has a big rusty hole in it. So it's like a n shape. No portal yet. You go to flick the dog dumps but you just pass straight over them with the hole and the dumps go nowhere. The dumps have gained no momentum because nothing touched them and transferred that to them.
Now put a portal on the end of the shovel. As you sweep it over the cack has anything touched them? Has any object transferred it's momentum to the dog eggs? No, so the dumps just gently tumble out of the other side of the portal.
I think that's a very strong argument and a great metaphor, but you forget relativity.
All reference frames are valid - you could say the Earth and the people are moving and the train is stationary, you could say the train is moving and the earth and people are stationary, or you could say they each have a vector moving around the sun or anything else
But when you travel through a portal, the only valid reference frames are you and the entry portal. Your momentum relative to the Earth doesn't matter - why would it? You can open a portal to the moon and jump through, and we see momentum is preserved. The Earth isn't a special reference frame, it's just the most noticeable one.
So let's pick the reference frame of someone on the track. Let's look through the portal and say there's a sign on the other side - as it approaches, you'd see a sign approaching you through the portal. Relative to you, through the portal the sign is moving at 30mph. The portal passes over you - you haven't moved, but you enter a new reference frame, a frame in which the Earth and everything on it is moving at 30mph