The Gemara says that Rav Yosef son of Rebbi Yehoshua ben Levi saw "an upside down world."
I found a Perush on Torah from Tosafosist Rivah on Chaya Sarah that remarks when Yitzhak comes and Rivkah sees him that he was walking upside down. What is the source for Rivah's comment, and is it related to this Gemara?
Chaim Schild, ny usa
This is an interesting question. I am in doubt about whether there is a direct connection between the two sources.
1. I found the source you cited about Yitzchak and Rivka in Moshav Zekeinim, and in Pa'ane'ach Raza on Bereishis 24:65. (I think this is the same source that you cite.) These works cite the Midrash as the origin of this account. (I did not manage to find the actual Midrash, and in fact the footnotes to both Pa'ane'ach Raza and the Ba'alei ha'Tosfos on the Torah (by Rav Yaakov Gelis) do not make any note of this, so it appears that the location of this Midrash is unknown.)
2. The full quote is that Rivka "saw that Yitzchak was walking with his head down and his legs up, which is the way those who come from Gan Eden walk. This is what we also find in Shmuel I 28:13, where the Ba'alas Ov said, 'I have seen Elokim coming up from the ground.' The same applies to those coming from Gan Eden."
3. I found that the Midrash Rabah (Vayikra 26:7) elaborates on the incident involving Shmuel. The Midrash says when summoned by a Ba'alas Ov, ordinarily the dead rise up from the ground with their heads downwards and their feet upwards (but to honor King Shaul, Shmuel's spirit rose with his face upwards). It appears, therefore, that this Midrash and the Midrash concerning Yitzchak refer to the physical way that people who come from Gan Eden rise up.
4. In contrast, the Gemara in Pesachim (50a) appears to relate more to the spiritual state of people in the World to Come. One sees from the Gemara in Pesachim and Rashi there (DH Bnei Adam) that it refers to people who are wealthy and distinguished in this world but will be lowly in the next World. Rashi in Bava Basra (10b, DH v'Tachtonim) writes that those who are poor in this World and are in a lowly position here will be distinguished in Olam ha'Ba. This is what the upside-down world in Pesachim is about.
Chaim - The connection you made between the Midrash cited by the Riva and the Gemara in Pesachim is fascinating indeed! It is very possible that this is what the Gemara is alluding to, and that this Gemara is one of the sources for the Riva.
The reason that the dead in Gan Eden appear to us upside-down is because often those whom we honor in this world are not worthy of honor in the next, and vice versa. That is what the Gemara means when it learns from "Elyonim l'Matah" that when someone seems worthy of honor in this world, the opposite may be true in the World to Come.
By the way - R' Pincus in one of his Purim Maamarim Darshins on this Midrash about Yitzchak being upside down. Ayin Sham.