When Sheldon kicks Amy out to work solo, she and Leonard bond during a series of science experiments. Also, Bert the geologist replaces Wolowitz band Footprints on the Moon, and Sheldon finds Penny a surprising source of scientific inspiration.
While the gang is having dinner in Leonard and Penny’s place, Raj walks in triumphant with copies of that day’s newspapers. Leonard accuses him of slipping back into the 1990’s. Raj tells them that the paper reviewed his planetarium show, though it was just in the column under things to do that weekend. Penny just makes a boat that she turns it into a hat. Amy adds that her and Howard’s project has been written up in a journal. Leonard and Bert are also making progress on their meteorite project. Sheldon gets defensive they ask what he was working on.Returning to their apartment, Amy wants to know what is going on. Sheldon was upset that he was currently not working on anything. Amy suggests that he take time and try to refocus and find something that excites him. He replies that he doesn’t know what he would do without him, and then he kicks her out of the apartment.
Howard and Raj are getting ready to have band practice. They start playing “Sherlock Around the Clock”, when Bernadette calls down and tells them to keep the noise down. Starting to play quieter, they still wake up Halley and Howard is told to go get some diapers by his wife. He still motions to Raj that he thought they were rocking.
Sheldon is getting organized to work when the telephone rings greatly disturbing him. It’s his mother who wants him to guess who she met at the barbecue festival. Sheldon complains that he is busy, she retorts as to why he answered the phone. Sheldon tries to start working at his whiteboard with no results so he calls his mother back. She tells him who she met and he thinks about his work for a minute and then calls her back starts to discuss the gentleman she met.
The next day Raj is having lunch with Leonard who had fun with Amy when he found out that she was also a spelling bee champ. They tried to stump each other until Penny when to bed at only nine o’clock. Howard shows up and Raj asks him about band practice. He is awfully busy taking Halley over to Bernadette’s folks. Raj is thinking about canceling the bar mitzvah gig. Howard doesn’t want to break up the band, but he thinks Raj should find someone else. Raj jumps up quickly to go ask Bert who plays the guitar.Penny returns home and find Amy and Leonard reenacting a science fair experiment they both did in the ninth grade. Penny remembers getting included in a science project by showing Jenny Runyon how to flirt with guys. Penny got an A and Jenny got pregnant. Next they decide to build a cobra wave like they did in the seventh grade. Penny quips that she didn’t think they could top last night’s spelling bee, but they did with math. Amy wonders if they should watch that show she likes where people who want to buy a house, then do that. Instead, Penny goes to get dinner.
Sheldon is excited about completing some equations so he calls his mother and tells her that the odds of him running into Mr. Watkins was one in eighteen million. Mary has bad news that he passed away this morning and wonders out loud what the odds of that are. Sheldon goes to figure it out.
Penny returns with a pizza and then hears how excited Leonard and Amy are constructing their wave she knocks on Sheldon’s door. “Sheldon”; knock...knock...knock. “Sheldon”; knock...knock…knock. “Sheldon”; knock...knock...knock. He finds her using his knock annoying. She claims to have brought him dinner, to find Sheldon is not really working since he is drawing parallelograms and listing disasters. He wants to find a new approach to dark matter, but keeps getting interrupted. He asks Penny to sit down and not say anything. She starts mocking him in sign language.
Raj and Bert and getting ready to practice at Raj’s apartment. Bert tells Raj about his boulder song about the boulder that chases Indiana Jones. Raj likes the idea and Bert plays his geology song. “Six tons of granite and some micaceous schist.”
Amy and Leonard have built a wave front out of Popsicle sticks and want to predict its wave height. Amy feels is’ like being back in preschool. She also asks Leonard whether being married feels any different. No. It was the answer Amy was hoping for since she and Sheldon are in a real great place now. She doesn’t want to mess it up. Leonard reminds her that he did kick her out. Amy explains that his work is real important to him and she finds that really sexy. That and…Leonard doesn’t want to know as he sets off the Popsicle sticks. It’s Sheldon’s butt.Sheldon is describing to Penny is next dark matter approach which he finds too pedestrian. Amy agrees and the wonders about the cheese in the pizza crust. Penny then tries to get him to think about what excited him about dark matter. Penny describe it as a rebound science that which he goes after to make himself still feel pretty. Sheldon still has not forgotten the wonder and simplicity of string theory. He begins to describe it to Penny who tries to understand it.
Back in Bernadette’s bed, she can’t sleep because Howard who without his band is trying to write an astronaut musical. Bernadette thought his time was going to be spent helping her with the new baby. Howard starts to play the opening song as Bernie looks at him like he is crazy. Next Howard finds out he get to go back with the band. Howard is amazed, but it was his wife’s idea.
Back in string theory, Penny describes string theory like a guitar string that vibrates which he vibration is a different particle. Penny wonders if they just figured out string theory, but it was not just one night’s project scoffs Sheldon. Science has been working on it for decades which baffles Penny since it is only a string. String can be straight, in loops, get tied up in knots. Sheldon explains that there are no knots in more than four dimensions; unless one considers them as sheets. Sheldon starts with a new idea which has a lot of possibilities. Penny quips that that only took her a minute.
Penny returns home and tells them that he stopped at Sheldon and helped him solve string theory. “What” asks Amy? She tells them that the answer is knots. Leonard mentions that there are no knots in more than four dimensions. Penny tells him that he should consider the knots as sheets and then tells them good night. Both Amy and Leonard what was happening.
Finally “Footprints on the Moon” are playing at Toby’s bar mitzvah. Raj says that he will experience many obstacles in life. “Some that feel like boulders”, adds Bert. Cue his Indiana Jones boulder song.
- Guest starring:
- Teleplay: Eric Kaplan, Maria Ferrari & Jeremy Howe
- Story: Chuck Lorre, Steve Holland & Anthony Del Broccolo
- Title Reference: Sheldon starts to work alone and then is working with Penny.
- Taping date: December 12, 2017
- This episode was watched by 15.93 million people with a rating of 3.1 (adults 18-49).
- Total viewers including DVR users xx.xx million.
- The Big Bang Theory was ranked #x for the week ending xx January 2018.
- This episode aired in Canada on January 11, 2018.
- Chuck Lorre's vanity card 
- Caroline Preece at Den of Geek! - We may all be agreed that The Big Bang Theory is long past its best at this point, but it does still have some tools in its arsenal and The Solo Oscillation proves it...Good episodes of this show almost always come down to which pairings the writers decide to feature...Magical unicorn that she is, not only does Penny help Sheldon understand why he’s been so unenthused by Dark Matter - it’s his rebound science to make him feel pretty again - but she gladly acts as a blank surface for him to bounce ideas against. As ever, Penny is smarter than she lets on, and the different way her mind works allows Sheldon to find a new way of looking at string theory. 
- IMDb user reviews 
- Amy thinks that Sheldon's dedication to his work and his butt are the sexiest points about him.
- Penny actually gives Sheldon a new idea on how he can approach explaining string theory. "Knots".
- This is the second episode after "The Spoiler Alert Segmentation" (S6E15) in which Bernadette doesn't wear her glasses at all.
- Sheldon's mother Mary Cooper and Bert return.
- For the first time Leonard and Amy bond over something, something that hasn't been seen during the past seasons after they had become good friends with each other.
- Sheldon: I'm trying to come up with a new approach to dark matter, but people keep distracting me. First, my mother kept answering the phone when I called, even though she knew I was busy. And now you show up with my favorite shape of food-- a circle made of triangles served in a square box.
- Penny: [knock knock knock] Sheldon? [knock knock knock] Sheldon? [knock knock knock] Sheldon?
- Sheldon: [opens door] It's annoying when you do it.
- Leonard: What are you doing?
- Penny: Making a boat. When I was a kid, my dad showed me how.
- Leonard: Boy, you'll do anything to avoid reading.
- Raj: Hey! Look what I got everybody.
- Leonard: Newspapers? Did you find a portal back to the 1990s?
- Penny: No. If he had that, he'd be trying to prevent *NSYNC from breaking up.
- Raj: Oh, please, I'm glad they broke up. Otherwise, Justin would never have brought sexy back.
- Sheldon: One thing you can't get on an iPad, the smell of ink and paper. One more reason iPads are better.
- Raj: Hey, uh, what do you think we should open our show with? Uh, "Thor and Dr. Jones" or "Let's Get Astrophysical"?
- Howard: I think we should start with something that gets them up on their feet. Maybe "Sherlock Around the Clock".
- Amy: Do you remember any of your high school projects?
- Penny: Uh... well, I remember telling Jenny Runyon that I would teach her how to flirt with boys if she put my name on her project. I got an "A", she got pregnant.
- Amy: Girls like you are why I had to come straight home after school.
- Bert: Hey, you want to hear one of my geology songs?
- Raj: So it's about rocks?
- Bert: Better. It's about a boulder.
- Raj: Isn't that the same thing?
- Bert: Far from it. A boulder has a diameter greater than 25.6 centimeters.
- Raj: Is that fact in the song?
- Bert: No. Yes. It's sung from the viewpoint of the boulder that chases Indiana Jones.
- Amy: I was gonna ask if being married felt any different.
- Leonard: Oh. Uh... not really. Sorry. That probably wasn't the answer you were looking for.
- Amy: No, actually it is. I mean, Sheldon and I are in a really great place right now, and I just... I don't want anything to mess that up.
- Leonard: Mm-hmm. You do remember you're here because he kicked you out of your apartment?
- Sheldon: And then I was thinking about inventing a new dark matter particle to evade the omega baryon constraints, but that just seems like something anyone could come up with.
- Penny: Mm. Agreed. You know what's blowing my mind? Somebody thought about putting cheese in this crust.
- Sheldon: I just wish I could find something that excites me.
- Penny: You...you do understand that crust doesn't normally come with cheese in it?
- Penny: What got you excited about dark matter in the first place?
- Sheldon: Well, I left string theory, which I'd been working on for a long time, and everyone was talking about how cool dark matter was, and I thought "Well, sure, I'll give that a whirl."
- Penny: So it's your rebound science?
- Sheldon: What's that?
- Penny: Well, not the science you spend the rest of your life with, but the one you use to make yourself feel pretty again.
- Penny: So it's sort of like a guitar string, but instead of making an actual sound, each vibration is a different particle.
- Sheldon: Precisely. And when you express it in eleven dimensions, Einstein's relativity equations pop out. Does that sound like a coincidence?
- Penny: It does not.
- Sheldon: Yup. That's what I think.
- Penny: So... so, did we do it? Did we just solve string theory?
- Sheldon: [chuckling] Oh. I appreciate your enthusiasm, but this is not the sort of thing we can figure out in a night. People have been stuck on this for decades.
- Penny: What, decades? Really? It's... it's a string. How hard can it be? It-it... it's straight, it's in a loop, it gets knotted up with other strings. Uh...
- Sheldon: Well, actually, there are no knots in anything greater than four dimensions. Ooh, unless we get around that by considering them as sheets. You know, topologically speaking, that has a lot of interesting possibilities.
- Penny: See? How long did that take me, like a minute?
- Leonard: Thought you were getting us dinner.
- Penny: Sorry. I had to stop at Sheldon's and help him solve string theory.
- Amy: [befuddled] What?
- Penny: Yeah, turns out the answer's knots.
- Leonard: That's cute, but you can't have knots in more than four dimensions.
- Penny: [heading towards their bedroom] Mm... You can if you consider them sheets. Good night.