When a group of Jews gets together for prayer, they are paying public respect to Hashem. Sitting with them while occupied with something else shows a lack of respect for what they are doing. For this reason, someone who finds himself in a place where Shema or Aleinu is being said should recite them along with the congregation, even if he is not part of that minyan (Mishna Berura 65,9).
If a person is learning Torah in shul and he hears these prayers, must he interrupt his study and say them with the congregation? Since Shema is said sitting down, he is not showing lack of respect if he continues to sit and learn at that time. However, since Aleinu is said standing, if he would not get up and say it this would be a conspicuous deviation from the congregation. Therefore, one should interrupt Torah study to recite it (Rav Shlomo Zalman Auerbach, as cited in Halichos Shlomo 6,27).
Halachic authorities note that in most places, the individual does not recite every tefilla in unison with the congregation. Aleinu and the first line of Shema, however, should be said together. Since these prayers express the fundamental tenets of the Jewish faith, neglecting to say them with the group is tantamount to a denial (Aruch Hashulchan 65,6).
If a person is in the middle of Shema or Pesukei D’zimra and the rest of the congregation is saying Aleinu, he should not say it with them (Responsa Minchas Yitzchak 9,8). However, he should bow down together with the rest of the congregation when they say “anachnu korim” (Aruch Hashulchan 85,6).
Text Copyright © 2011 by Rabbi Daniel Travis and Torah.org