Shabbos 100 Relative versus objective movement
On this daf, we have a number of interesting discussions that touch on physics and make an impact on the laws of shabbos.
One of the recurring themes in this Masechta is that in order to transgress the biblical prohibitions of transferring an item from one domain to another, one has to both lift it up from one domain and put it down at rest in the other.
If the item never comes to rest in the forbidden domain, one will generally only have transgressed a rabbinical prohibition at a maximum. (One exception is the rule of קלוטה כמי שהונחה דמי according to some opinions in the Gemara.)
We are told that if one draws water from one domain and puts it down in another on top of water (such as in a river or pond) , even though the water actually mixes and flows with the other water and never “rests”, its is considered to have rested, as this is the way of water.
Rashi adds that if one were to pickup or splash water from a body of water, it would similarly be considered uprooting it, even though it was never really at rest.
Water by its nature is constantly moving (unless absorbed by a solid) and that is halachically considered its natural state of rest!
In contrast, if one picks up a solid item such as a nut from one domain and puts it down in another domain on top of water, so that it flows on the surface of the water, and never rests, one is exempt from the biblical prohibition, as the solid never comes to its natural state of rest, which is a state of stillness.
Rava then asks an interesting related question:
What would happen if our famous nut is picked up from one domain and placed inside a container floating down a stream of water in another domain?
The item is at rest in the vessel, but the basket is moving with the water.
Is this considered to be an act of הנחה (putting to rest) the nut, seeing as it is stationary relative to the container it is in, or is it considered not to be at rest, seeing as it is inside a moving container?
By Rashi’s extension, we could then also ask whether lifting up the item is considered an act of uprooting, seeing as the item is in its natural state of stillness within the vessel, or whether it considered as if it always was moving, seeing as it was inside a moving vessel!
The Gemara leaves this question unresolved.
What exactly is the uncertainly of the Gemora?
It seems clear that the doubt concerns whether an item’s halachik state of rest or movement is defined in absolute terms, or relative to the surface it is dependent on for support.
On the one hand, it seems that this status must be relative- after all, all items and beings at “rest” on the surface of the Earth, are essentially only at rest relative to the Earth- in more “objective” terms, they are all moving at an incredible speed around the Earth’s axis as well as around the sun!
Yet halacha considers such items or people to be fully at rest.
However, if one takes a closer look, there is another possible reason why this is so.
Perhaps in general, halacha defines “at rest” as objectively “at rest”, unless the items’ natural state is to be in constant movement, like water.
Seeing as the natural state of anything on the surface of this planet is to be moving with the planet, that too is considered its natural state of rest.
However, it is not the natural state of a nut to be inside a vessel floating down the river- perhaps in such a case, we go by an item’s objective state, and thus do not consider it to be still but rather moving!
To formulate this in more formal Brisker format:
Is the reason why an item at rest on the surface of the Earth is halachically considered to be “at rest” because
i. Halacha goes by relative state, not objective state, and relative to the Earth, it is indeed at rest, just like a nut inside a container floating down a river is at rest relative to container it is in.
ii. Halacha usually goes by objective state, but just like water’s natural state of rest is a state of movement, so to any terrestrial item’s natural state of rest is one of moving with the sun.
The Nafka Minah (practical difference) would be that now infamous nut inside the container:
If option 1 is correct, then the nut will indeed be considered to be at rest, even though the container it is in is moving.
If, on the other hand, option 2 is correct, the nut will be considered to be moving and NOT at rest, unlike a terrestrial item that moves with the earth.
The next question of the Gemara is about two liquids with different densities on top of one another, such as oil and wine.
Is this considered like a solid on top of a liquid, or a liquid on top of a liquid.
There is no time left for this fascinating issue today, but it raises a very interesting question:
What happens when water of one density is transferred to a body of water of different density in a different domain?
Sound Impossible? Then you obviously haven’t been to the “Meeting of the waters” in Brazil!
But I leave that for further discussion- hint: color….
One other curveball- I have assumed in this analysis that Chazal were aware/believed that the Earth revolves around its orbit and/or around the sun. Is this a fair assumption, or way off track?