Why in the world would Einstein suggest....

@layman,

Or in other words, you don't have an argument. All you have is two more names to drop.

1 Reply

@Thomas,

Quote:

Downtown is moving.

OK. How do you reconcile that claim with this one?

Quote:

...no amount of gas will ever move planet Earth, as observed in the car's frame of reference.

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@Thomas,

Quote:

Or in other words, you don't have an argument. All you have is two more names to drop.

Heh, OK. I cite experts, while you just make one unsupported assertion after another. I guess you don't have to "drop" your own name in such cases, eh? The source of the "authority" is self-evident.

I added a little to my original post (on this particular subject), btw, in case you didn't notice.

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@layman,

layman wrote:

Quote:
Downtown is moving.

OK. How do you reconcile that claim with this one?

Quote:
...no amount of gas will ever move planet Earth, as observed in the car's frame of reference.

You want to look up the difference between angular momentum and momentum. The motor changes the angular momentum between the Earth and the car, but not the momentum.

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@Thomas,

Quote:

You want to look up the difference between angular momentum and momentum. The motor changes the angular momentum between the Earth and the car, but not the momentum.

OK, I'll take your word for that. How does that relate to the amount of gas it takes to make the earth start moving under your car while you car remains motionless?

1 Reply

@layman,

It doesn't. It relates to your earlier assertion that describing the physics from the car's frame of reference would violate the law of the conservation of momentum.

1 Reply

@Thomas,

And, since we're still on this topic, how about another question I raised in this example? Let me flesh it out a little.

Two guys, A & B. A is 10 miles south of town, B is 10 miles north of town. Both guys decide they want downtown to come to them, so both start their cars, etc.

Both travel at the same speed, relative to the earth, but neither one of them is really "travelling" (at least not when they're on the freeway, maintaining a steady speed). Nonetheless they both happen to arrive downtown at the same time.

How would that work? How would "downtown" simultaneously move in opposite directions?

1 Reply

@Thomas,

Quote:

It relates to your earlier assertion that describing the physics from the car's frame of reference would violate the law of the conservation of momentum

Would it make any difference if I had said "angular momentum" instead?

1 Reply

@layman,

layman wrote:

Both travel at the same speed, relative to the earth,

Velocity is a vector, not a scalar. The cars may travel at the same speed. But they are travelling in opposite directions. So their velocity relative to the Earth is NOT the same.

layman wrote:

but neither one of them is really "travelling" (at least not when they're on the freeway, maintaining a steady speed). Nonetheless they both happen to arrive downtown at the same time.

Neither is moving relative to their own frame of reference. But relative to the other's frame of reference, the each is moving at twice the speed of Chicago's.

1 Reply

@layman,

layman wrote:

Would it make any difference if I had said "angular momentum" instead?

I think so. It would throw me back to my follow-up point. The conservation of angular momentum only applies to closed systems. And the Earth is not a closed system because of the motor's energy.

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@Thomas,

Quote:

Neither is moving relative to their own frame of reference. But relative to the other's frame of reference, the each is moving at twice the speed of Chicago's.

That's great to know. But the question was:

Quote:

How would that work? How would "downtown" simultaneously move in opposite directions?

Care to respond to that question?

1 Reply

@layman,

layman wrote:

How would that work? How would "downtown" simultaneously move in opposite directions?

There is nothing to explain, because there's no frame of reference in which downtown would move in opposite directions simultaneously.

In downtown's frame of reference, downtown rests, and the cars move in opposite directions simultaneously, which is unremarkable.

In the first car's frame of reference, the first car rests, downtown moves towards it, and the second car also moves towards it at twice the speed of downtown.

In the second car's frame of reference, the second car rests, downtown moves towards it, and the first car moves towards it at twice the speed of downtown.

To repeat: There is no frame of reference according to which downtown moves in two different directions simultaneously.

2 Replies

@Thomas,

Quote:

To repeat: There is no frame of reference according to which downtown moves in two different directions simultaneously.

I hope to hell there isn't. But, that said, there is surely a frame of reference where the relative motion of all three can be observed from, isn't there. Suppose, for example that I'm observing from a point 5 miles east of town (with hi-powered binoculars). What would I see then?

1 Reply

@Thomas,

According to SR, all three (A. B, and downtown) would all consider themselves to be at rest.

Is it logically possible for all three to be "correct" in their assumptions about their lack of motion, and STILL have all three end up at the same place at the same time.

How would that work?

For the purpose of this question, let's change the hypothetical a little.

A. headed north from New Orleans, is cruising at a steady clip of 60 mph, 10 miles south of Memphis.

B, heading south from St. Louis is also cruising at the same steady speed, but is 10 miles north of Memphis.

10 minutes later, all three (Memphis, A & B) are at the same place. All three claim they have not moved a single inch during the last 10 minutes.

Can they all be correct?

1 Reply

@layman,

layman wrote:

Suppose, for example that I'm observing from a point 5 miles east of town (with hi-powered binoculars). What would I see then?

You mean a point on the ground 5 miles East of town? You would see a stationary city and two cars moving towards the city from opposite directions.

Or do you mean a point inside a moving car 5 miles East of town? Then it would depend on the speed and direction in which that car is moving.

1 Reply

@layman,

layman wrote:

Is it logically possible for all three to be "correct" in their assumptions about their lack of motion, and STILL have all three end up at the same place at the same time.

Sure, because the "lack of motion" isn't a universal. It is relative to each frame of reference. There is nothing to explain because nothing needs to work.

1 Reply

@Thomas,

Quote:

Sure, because the "lack of motion" isn't a universal.

I don't follow your logic here, Thomas. Is anyone "really" moving, or is all motion just an illusion?

1 Reply

@Thomas,

Quote:

You mean a point on the ground 5 miles East of town? You would see a stationary city and two cars moving towards the city from opposite directions.

OK, that's what I meant. And I agree, that's what he would see.

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@layman,

layman wrote:

I don't follow your logic here, Thomas. Is anyone "really" moving, or is all motion just an illusion?

Neither. Motion is defined relative to a frame of reference. You get to pick your frame of reference in the beginning. But once you have, you need to be consequent about it. Once you picked the city's frame of reference, the cars are really moving. Once you have picked one of the car's frame of reference the city and the other car really move toward it. You can't not have a frame of reference to talk sensibly about movement.

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@Thomas,

Well, you certainly assert that with an air of authority, Thomas, but it really doesn't make sense at all to anyone who is not a total solipsist.

You would give the exact same response to this question, I presume:

Quote:

10 minutes later, all three (Memphis, A & B) are at the same place. All three claim they have not moved a single inch during the last 10 minutes.

Can they all be correct?

1 Reply

Why in the world would Einstein suggest....

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