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The portal question - Finally answered by a physicist

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The portal question - Finally answered by a physicist

Old 06-12-19, 02:38 PM
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Dan
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The portal question - Finally answered by a physicist

Note the position of the orange portal and the blue portal.

edit: More details on the question itself:
In Portal, something goes in one portal, and comes out the other end. There is one orange portal and one blue portal. If an object travels through the orange portal, it will come out of the blue portal, and vice versa.
In the below scenario, a cube is on a platform. An orange portal is on a press above it. The blue portal is on a different 45-degree platform.
If the press - which has the orange portal on it - is moving at a given speed in which it will touch the cube's platform, encompassing the cube, what is the outcome? A or B? Will the cube simply drop off of the platform with the blue portal, or will the cube travel at the same speed that the press was traveling when the orange portal encompassed it?



Physicist Sean M. Carroll answered this during an interview, and he sounded pretty confident, kids.

Before you click on the spoiler, answer the poll, cowards.

Spoiler:
Did you answer the poll? I hope you did.
Spoiler:

Steven: I have to ask you, this is the most important question I'm going to ask you all day. I'm not going to argue with it, I just want to hear what you think the answer is and then your rationale for it, okay?
Sean: Yeah.
Steven: Have you ever - I don't know what your background is with video games - but have you ever heard of a game called Portal?
Sean: I have heard of Portal, yes.
Steven: Okay. We have spent an inordinate amount of time discussing a very stupid picture. So in Portal, something goes in one portal, and comes out the other end, right. That's how it essentially works, right.
Sean: Yes.
Steven: Okay. You are the most highly-qualified physicist we've ever asked this question to. I've linked you a picture in the Skype call.
Sean: Here it is.
Steven: Given that portal orange comes down at some given speed and then stops and encompasses the little box on the platform, will A or B result from this and why? I'm so curious.
Sean: Do you want to guess, or should I te-- I know the right answer.
Steven: There's a right answer?
Sean: There is a right answer, yes. Absolutely.
Steven: Okay, tell us, go.

Sean: It's B.

Steven: Why is that?
Sean: Because, really, the portal is a wormhole. This is something that we have in the theoretical physicist toolbox. We don't have them in the real world, don't get too excited. But we can talk about what is going on. And the way to answer it is this: Imagine that, just to the other side of the orange portal, right, someone is standing there. From their point of view, they're stationary with respect to the orange portal, right. And they see the little box rushing toward them when they look through the portal. So when the box crosses the threshold from one side to the other, it doesn't stop rushing toward them, it continues to rush toward them, so they would see it on their side, rushing toward their face.
Steven: That's a perfect answer, I 100% agree. We've spent so much time arguing about this.
This part of the interview starts at 44:17
PHP Code:
https://www.youtube.com/watch?v=531FKCzTX40 

Last edited by Dan; 06-12-19 at 04:39 PM.
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Old 06-12-19, 03:12 PM
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Re: The portal question - Finally answered by a physicist

I got it wrong but totally get why it's the other one..

Though it does raise another question
Spoiler:
Assuming there is gravity in the room, how is it sitting on a portal? It should already be flying through the hole just like if you lifted it three feet up and let go, it would accelerate like any falling object. Though of course that doesn't change the answer in either case .
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Old 06-12-19, 03:20 PM
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Re: The portal question - Finally answered by a physicist

It's sitting on a platform, and the portal is above it on what appears to be some sort of press.
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Old 06-12-19, 03:53 PM
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Re: The portal question - Finally answered by a physicist

Yeah I see that now. My eyes are crapping out on me. I now need readers full time apparently.
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Old 06-12-19, 03:57 PM
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Re: The portal question - Finally answered by a physicist

Spoiler:
Yep, the relative velocity will be preserved.

Unless you're using portals like those in the Expanse novels, then the "speed limit effect" might lead to A.
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Old 06-12-19, 04:29 PM
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Re: The portal question - Finally answered by a physicist

Where's the poll option for "I'm stupid and have no idea what I'm looking at"?
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Old 06-12-19, 04:31 PM
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Re: The portal question - Finally answered by a physicist

Is this instructions from IKEA??
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Old 06-12-19, 04:33 PM
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Re: The portal question - Finally answered by a physicist

Originally Posted by cultshock View Post
Where's the poll option for "I'm stupid and have no idea what I'm looking at"?
I realize that the question itself is hidden in the spoiler tags. I'll copy it so it makes more sense.
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Old 06-12-19, 05:35 PM
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Re: The portal question - Finally answered by a physicist

The Lensmen books are some of my favorites. In them, momentum is conserved. The cube has no momentum in relation to the Earth because it is moving the same speed and direction. So using that logic, the answer is A.
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Old 06-12-19, 05:37 PM
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Re: The portal question - Finally answered by a physicist

Originally Posted by IDrinkMolson View Post
Is this instructions from IKEA??
Ok, what tools will I need...a portal?? Where the fuck am I going to get that???
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Old 06-12-19, 05:39 PM
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Re: The portal question - Finally answered by a physicist

Spoiler:
The momentum relative to the portal is what is conserved.
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Old 06-12-19, 05:42 PM
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Re: The portal question - Finally answered by a physicist

B doesn't make sense to me.

The diagram, as it's supplied, shows the portal being on the bottom of the press. That means the box itself has zero inertia - it is simply being enveloped by the portal. When it comes out the other side it wouldn't suddenly have inertia.

Think of the portal on the bottom of the press as a big bubble wand. If you slam that wand down on top of the box it will simply pass through the bubble in the same way it would pass through the portal. There would be no movement.
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Old 06-12-19, 06:34 PM
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Re: The portal question - Finally answered by a physicist

The orange portal is not at rest in relation to the box, therefore the box is not at rest in relation to the orange portal. The blue portal is merely the other side of the orange portal, so the box is not at rest in relation to the blue portal either. Hence B.
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Old 06-12-19, 06:41 PM
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Re: The portal question - Finally answered by a physicist

What if the portal is an airplane taking off on a conveyer belt?
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Old 06-12-19, 06:45 PM
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Re: The portal question - Finally answered by a physicist

Then the portal won't be able to take off because it won't achieve adequate airspeed.
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Old 06-12-19, 07:58 PM
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Re: The portal question - Finally answered by a physicist

That answer doesn’t make sense to me, but I just work in a dairy plant. I’m not splitting the atom.
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Old 06-12-19, 08:48 PM
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Re: The portal question - Finally answered by a physicist

Upon further reflection, I think B is right for a different reason...

If something is going through the portal on one side, at a particular speed, it has to come out the other side at the same speed. If it doesn't, it (or rather a portion of it) is going to effectively be in 'nowhere' at some point. As an example if the speed through the orange portal is 10m/s, the speed at which the cube is entering the portal is 10m/s. In order for it NOT to have momentum (or at least very little momentum), coming out of the blue portal, its speed would have to be slower. Say it's 1m/s. Then the cube is going into the orange portal more quickly than it is exiting the blue portal. 100% of the cube would be into the orange portal, while only 10% was out of the blue portal. Where would the rest be?
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Old 06-12-19, 08:49 PM
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Re: The portal question - Finally answered by a physicist

I disagree with the physicist. Just because the view of someone in the portal makes it seem as if the cube is coming rapidly towards it, doesn't mean that it is in relation to the table. The cube will just pass through the portal.

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Old 06-13-19, 12:02 AM
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Re: The portal question - Finally answered by a physicist

Originally Posted by Nick Danger View Post
I disagree with the physicist. Just because the view of someone in the portal makes it seem as if the cube is coming rapidly towards it, doesn't mean that it is in relation to the table. The cube will just pass through the portal.
I agree. Think of the portal like a hula hoop being passed over the cube. The cube never moves. Doesn't matter that each opening of the hula hoop is in a different location. The cube would just "appear" out of the blue one really fast, then plop.
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Old 06-13-19, 12:30 AM
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Re: The portal question - Finally answered by a physicist

In the game, it would've worked like A, so that's my answer.

Mostly though, I just want Portal 3, goddammit

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Old 06-13-19, 12:39 AM
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Re: The portal question - Finally answered by a physicist

I got it right

I was going to go with A but thought about it again and went with B.

Think of it this way. If the portal were standing still and the platform was moving up to the portal you would think that it is choice B. Well the net effect is the same no matter which is moving the portal or the platform with the cube they are still moving in relation to the portal.
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Old 06-13-19, 12:57 AM
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Re: The portal question - Finally answered by a physicist

Originally Posted by General Zod View Post
B doesn't make sense to me.

The diagram, as it's supplied, shows the portal being on the bottom of the press. That means the box itself has zero inertia - it is simply being enveloped by the portal. When it comes out the other side it wouldn't suddenly have inertia.

Think of the portal on the bottom of the press as a big bubble wand. If you slam that wand down on top of the box it will simply pass through the bubble in the same way it would pass through the portal. There would be no movement.
But one thing you're forgetting is that the other side, back side, of the bubble wand is still moving with the front side so the momentum is conserved. In the portal scenario the blue portal is standing still so the conservation of movement would apply to the box.

Here's another example that should be much easier to visualize. Say the orange portal is on something that is moving really fast like say attached to the front of a bullet train traveling at 100 mph. If you were to look into the blue portal that was stationary do you think you would feel 100 mph winds? Or do you think that somehow you wouldn't feel anything. The air itself in front of the train is stationary. It's the train and the portal that's moving yet the movement would have be be conserved by the wind rushing in at 100 mph on the other side of the blue portal.

Heck you don't even have to have a portal. Just imagine being in the cockpit of said bullet train and you open the windshield. The air in front of the train isn't moving but relative to you moving at 100 mph on the train it is moving 100 mph into your face.

It's choice B
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Old 06-13-19, 09:04 AM
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Re: The portal question - Finally answered by a physicist

Originally Posted by ultimaton View Post
Mostly though, I just want Portal 3, goddammit

Don't we all. Don't we all...

Originally Posted by tanman View Post
Say the orange portal is on something that is moving really fast like say attached to the front of a bullet train traveling at 100 mph. If you were to look into the blue portal that was stationary do you think you would feel 100 mph winds? Or do you think that somehow you wouldn't feel anything. The air itself in front of the train is stationary. It's the train and the portal that's moving yet the movement would have be be conserved by the wind rushing in at 100 mph on the other side of the blue portal.
Excellent analogy! I love it!
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Old 06-13-19, 09:36 AM
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Re: The portal question - Finally answered by a physicist

I chose B and did not cheat. It seems to me that if the portal is striking the object with a high velocity it only makes sense that the journey continues with some velocity. Something along the lines of an 'equal and opposite reaction' the reaction being that the object is moved with a comparable velocity in the new direction through and out of the portal.

Science bitches!
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Old 06-13-19, 09:43 AM
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Re: The portal question - Finally answered by a physicist

Portals don’t exist.
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