Frequent question replies and corrections! Sup everybody! I'll edit this comment when common questions show up or people find errors I want to correct! (I know there will be many, but I don't know what they are yet, or I would have put them in the video!) #0: the "model" I use to describe interatomic bonds is ludicrously oversimplified, but it's kinda close enough to make it look like a spring. #1: at 20:02 said "poisson ratio" when the graphic clearly said "E" for Young's modulus. It's almost like filming brian can't even see the things editing Brian pastes on the screen... #2: lots of people asked about the delay in the wires or sensor squish. if there was a noticeable delay, the plot I show at 17:26 would have had a nonzero intercept (if the spark between the hammer and bar sparked early, the intercept would be negative, and if the sensor or circuitry added a serious delay, the intercept would be positive. If both are happening and cancel out, they happen every time the same way, apparently! #3 yes everything from water, to steel, to neutronium is somewhat compressible (the nutronium comments were great - thanks! Apparently in such a structure held up by Pauli pressure, Vs approaches the speed of light. Now I’m wondering what the refractive index of nutronium is and if it’s crystalline or not…) #4 I've had enough people ask about hitting the bar witha hammer moving faster than the speed of sound that I actually looked into it. I'd need an ultralight projectile like railguned into the end of the bar in a big vacuum chamber - there's no reasonable way I can think of to make something go that fast but I if can think of one I'll make a video. bottom line though, as long as that impactor isn't actually penetrating the bar of steel, the wave will still pass through the steel at the speed of sound.

That feeling when you apply a small theoretical correction and the model snaps precisely to the empirical data is probably the single best feeling in the entire world. You don’t get it often.

This has application in structural pile testing, because a hammer impulse happens too quickly to load the entire member at once. Static tests are expensive because you usually have to build two reaction piles just to apply the force. There's a cool method called statnamic testing that uses explosives to create a slower impulse on the pile as a load test.

I never really considered how soft, flexible, and noodly steel is until I became a machinist. Now it's a constant thought and struggle in my daily life.

15:18 This is the best part of the video by far. I learned an interesting concept with your demonstration, but so much more practically about your thought process when reviewing concept to fruition. You learn so much context just troubleshooting your setup, questioning your sensors and methodology. I love that you share this part with every project Guess it just goes back to the motto that "Plan A always goes up in flames" :)

"The more correct a physics model is the more painful it is to use" That's why experimental physics is so great - The universe takes care of all that figuring work for you!

The end explaining the methods and failures was even more interesting than the initial question, great video 👍

This is so much more interesting then the majority of shorts and quick videos you can find online. I wish more people took the time explaining and testing nature. Not for the views but to actually learn stuff. Great channel!

When I was a kid I used to always ponder this question as well as whether or not two objects really could “touch” each other. Fun to see a video like this

I find the fact that you ALSO explained the "failures" in the experiment, as valuable as the results themselves.

As a 66 yo electrical engineer, I've found myself going back to learn physics over the past 20 years. Your demonstrations of physics are REALLY well done... really informative. Nice job.

I love your explanations of the physics. Brilliant job on the experiment, so interesting!

This exact concept blew my mind about 15 years ago, but I never saw it actually demonstrated until you did it here.

Really really interesting video (like all of yours)! I had no idea about the 1d and 3d difference of speed of sound. Thanks so much for sharing your curiosity and experiments with us!

Really great video!

What an amazing educator you are. Knowledge, along with the personality. As well as the straightforwardness of your presentation. Amazing!

I can’t believe i’ve never heard of your channel before! You’ve earned my subscription, and i’m excited you have an assortment of videos in my queue.

I'm glad somebody else has had this exact question before. Always thought about how faster than light communication could come down to just having a really long stick, knew that it couldn't be right, and now I know exactly why it couldn't.

I love this video. As a machinist, we have a saying, " Everything is rubber." It's an anecdote about the difficulty of measuring things to extreme precision. There are literally calculations for the deformation of ruby on tungsten carbide . Sure you don't need it until you are measuring tens of millionths of an inch and by that time you need a climate controlled room and can't touch what your measuring for days before taking the measurement because the thermal expansion will throw it off more then the compression. They still exist

This video was VERY interesting to watch. I didn’t learn much of anything new as such, but how it all came together is fantastic!!

Impressive! Love hearing the behind the scenes design and testing of the experiment as well! Very cool.

I loved the "electrode clipped to hammer" solution - I didn't think that was messy at all. Great solution!

When I was young information was less accessible and it has been a mystery for years, I was pretty sure it was possible to transfer data faster than the speed of light this way. Nice video.

Nice video! great work and explanations!

Rule #1: Everything's a spring

There are at least three solid wave speeds involved: (A) rod or bar, (B) plate, (C) infinitely-sized solid. Each of these involves the vibrations of the molecules (which is the same for all three cases) but the effects of the boundaries (the visible surface of the steel) allows the molecules to move transversely rather than along the length of the bar. This has to do with the spectrum, or frequency content, of the source --- which is a hammer impulse. At low frequencies (related to how long the duration of the hammer face acts on the rod), the bar or rod appears to be thin relative the sound wavelength, so you observe the rod speed. The bar gets progressively fatter and thinner as the wave moves. For steel this is around 5100 m/s. If you did the experiment with a thin plate, you would measure around 5400 m/s, because it can only feel the boundaries through the thickness direction rather than all the way around the plate. In an infinite solid, where the waves never feel the boundaries, you would measure around 5900 m/s. The wave has no where to go. A lot of this behavior is related to the Poisson effect for static loads on solids, and the math to show the 3 waves speeds involves the Poisson ratio. The speeds are approximate depending on the chemical composition of the steel. I don't know if anyone who commented already posted this explanation so I apologize if they already did. There are also shear and interface waves that are beyond what I wrote here, hence I said that there are at least three mechanical wave types. Great work BTW.

The only experiment I need to see is “Can one make a Fing-Longer long enough?” Great video as always. Thank you for putting these concepts we often see as theoreticals into practicals!

This is one of the coolest video ideas I’ve seen and a pretty nice vid 👍🏻

Thankyou for showing the accurate experiment first and then showing the process of elimination you used to figure out the correct setup. It's nice (as a viewer) to get to the point quickly rather than dragging it out. But it's also important (for scientific literacy) to communicate how many sources of error you had to eliminate to get the setup right. Bravo 🎉

At first while watching the video I said to myself “I wish I could see his trial and error process for this experiment” and was pleasantly surprised when he actually did. Well done.

Love the setup. Very simple and clever. Great demo!

Great experimental setup, well done.

Excellent video. You nicely take us through your thought process as you iterate between your experiments and theoretical calculations. This is one of the better videos to help highlight the experimenter’s burden and will be worth watching for budding experimenters in all science and engineering, not just material science. I’d like to point out here (and likely others have too - I haven’t gone through enough of the comments) that though you started out seeking to use experiments to get the right answer, you (likely inadvertently) switched your objective to making your experiments agree with theory (some theoretical formula; any theoretical formula) because that is what would convince you that your experiments are precise and accurate. I’ll certainly agree that the fact that the final experimental numbers did agree with the most seemingly appropriate theoretical formula (longitudinal or 1D rods formula for speed of sound) suggests you may well have gotten to the accurate enough and precise enough answer to your question. But this approach (of ‘fixing’ your experiment until it agrees with one theoretical formula) only works when others have already done the experiments and have reached a consensus and you are trying to replicate that as an amateur (no offense, I mean it in the most respectful sense) for UKposts viewers. This approach is not adequate for actual real world experiments in the scientific world where we seek to truly ‘test’ theory. Often yours would be step 1 - make your experiments as precise and reliable as possible by testing against previously established theory and THEN start acquiring truly new data to test new theoretical ‘formulae’ or ‘models’ that extend into unchartered territory. Anyway, kudos for an excellent video. It reminded me of the saying, ‘No one believes theoretical results except the person who performed the theoretical calculations; everyone believes the experimental results except the person who performed the experiments’.

Yes! Fellow engineer here. Thanks so much for explaining how you learned the real final result and why it affected your test. I learned something new today! It's such a good feeling as an engineer to see the theory match exactly with physical objects. It's like learning how to predict the future or something.

I was so happy to see that you validated your test set-up by testing a range of rod lengths, and then graphed the results to make sure it made sense. As an engineer, I've had to learn the hard way to not just trust that your assumptions are correct. 🙂

It’s really cool that you were able to measure it with such a short bar

Nice job. I was thoroughly engrossed the entire time. I also appreciate your honesty and integrity in trying to find the error in your thinking, and telling us what you found. Can I theorize that the denser the material the fewer free electrons?

I am a metal worker and I often ponder this when striking a large piece of metal with a hammer and how the noise is generated. When I saw a 1000fps camera watching a drumstick hit a symbol I then realized that metal is kinda like a jello ! this further grants me a deeper insight into the nature of the materials that I craft with. Thanks a ton! Now I have questions of material hardness like mild steel vs machine steel or hardened? Answers always leed to more questions, I am grateful for people with such a wealth of knowledge and perspective that can break this stuff down for simple minds like mine.

I wanted to comment and say I'm really glad you added the section at the end where you went over how you were getting the "wrong" results back during testing and why they occurred. It's a part of science that doesn't always get talked about often, but is easily one of the most important.

This really deserves a very definite like! Good science and enthusiasm

First truly informative video on youtube. I appreciate the care of the presentor and his humility in approaching these experiments, versus the typical rhetoric usually dumped upon the observer as if all truth was encased and questioning the results was heresy. Thank you.

As a blacksmith i must say i'm always very interested to know how energy travels trough the steel, the hammer, the anvil etc. Nice video!

12:00 it's interesting to thing that real springs are made of atomic electromagnetic springs

Bro! I had this tought experiment recently! To find your video is awesome.

Thanks for the rundown. I looked, while watching your video, up quantum physics, the law of attraction, what time is and wave stuff... Thanks 😂❤ now i think i know more.

Going to give you my guess here: C, speed of sound. I'm no physicist (graduated in computer science) but my gut feeling says "somewhere much lower than the speed of light" -- atoms have to propagate repulsion along the entire length of the bar, which is something mostly "one after the other" -- and I had completely forgotten that a term, "speed of sound", already exists for what is pretty much the same phenomenon. I do wonder if the strength of the impact affects it, because pushing those atoms closer together would generate more repulsive force in response, which would mean greater initial acceleration for the next atom in the chain. However that would have the consequence that louder sounds -- or even just higher-amplitude components of a sound -- travel faster than quieter ones, so that would be a surprising result to me. I'll watch the video and see what happens :)

Everything is a spring. In this case, the bar has a really low spring constant. Instead of compressing the spring, you move the spring entirely. The force travels from one end of the rod to the other as a (fast) speed determined by subatomic particles, the rod compresses, then it transfers its force from the end of the rod to it's target.

im new to your channel and the intro is the coolest idea i've seen

This was fascinating thinking, thank u so much!

Some interesting thoughts I had about a 300,000 km long steel bar in space: If you were floating there with it, it would look like a small diameter steel bar but you wound not be able to move it by hand due to the mass of it's length. It would feel like an immovable object anchored on nothing. I guess it might bend on a large radius but snap back when let go. That would be a very weird thing to see. Also, a bar that long of the diameter shown would drift around like a soggy spaghetti noodle if other forces like gravity were there acting on it. It's too bad we can't have a very long steel bar in space to see how it would behave first hand. edit: also want to add that the effect of heat and cold on a steel bar that long would make for massive changes in it's length. Railway operators sometimes need to heat up rails in cold weather because they shrink so much and create a dangerous gap between rail lengths. If my math is right (please correct me if I'm wrong) a 1 degree c increase in the entire length of a 300,000 km long bar would increase it's length by over 3000 km. Insane.

WoW1!!! that was mind blowing but also made perfect sense at the end haha! Thank you for such a well explained video! :D

Omg. I’ve had this question bouncing around in my head for YEARS and had simply just never bothered to find an answer for myself. This is fantastic!!

I really like the inclusion of troubleshooting at the end. It humanises the experiment and the work behind it. It's inspiring in a way.

One of my favorite things about this channel is that it understands that the vast majority of the time we use shortcuts so that we can actually do something with the data, so many people seem to forget about this when they scale things up

This Video was highly entertaining and very easy to understand. I am actually really impressed!

This is the most exciting thing ive seen since the last thing. It also made me forget the last thing. This is so cool and so well done. So much good stuff. Thank you.

This is easily the most underrated channel on yt. The explanations are, as always, very on point and easy to understand. Keep up the great work

I TA'd intro geology back in grad school and a lot of people would have no reaction to hearing about waves travelling through rock, but when you talk about it in terms of displacement people's reaction and intuition were very different!

What a great video. Great storytelling and explanations. Very refreshing.

I've wondered about this for YEARS. Ever since Gavin Free (from Slow Mo Guys/Rooster Teeth) asked "what is the speed of push?" on a Rooster Teeth podcast. They all made fun of him, but I was thinking "..thats a good question." I dont have to wonder anymore! Great video

Man, this channel is one of the best channels there are. Great video.

This was so great ! Thank you !

Love it, especially the addenda on the messy reality of getting this to work!

I really respect your ability to explain such concepts easily. That's a trait not everyone has and it's really valuable.

More physics stuff!! :D I love your very solid explanations

I’ve had this question since I was a kid but never knew the words to use to ask it and didn’t really pursue it. Excellent work. Thanks!

Beautifully-presented! Thank you.

This is exactly the kind of content I love. Questions that seem so simple, but are complex to answer and the answer is not commonly known. The kind of questions a curios child would ask, but no one knows the answer.

This is one thing I think about a lot relative to high velocity explosives. The way they instantly shred very dense materials into tiny fragments. I assume this is due to out-running the speed of sound in the material.

A complete tour de force of pedagogy - and meta pedagogy. Seldom has a science demonstration more directly connected with me. Superb work.

Good job brother i really appreciate whatever you do to increase our science and practical knowledge

This was without question The Best science video I've ever seen on YT. ( I'm a retired chemist / materials scientist from the thermoset composites industry.) And it's the process you recount that makes it so, and, of course, the joy of discovery that we finally see happen. I think it should inspire young people. Well done!!!

The question is, what would happen if both ends are pushed at the same time? Both ends would travel "inwards" then the 2 waves would collide in the middle and a few hours later both ends would snap back?

18:16 you took my comment idea! I was about to suggest it could be something where you had a different alloy than the website was using since I imagine that (since so many properties of steel vary based on the amount of carbon) the speed of sound must vary a good bit depending on the type of steel. That's really cool how the speed of sound varies based on the "dimension" of the object that you're compressing and that a really thin bar will have a different speed of sound than one that would squish outwards a little bit.

have been wondering for years about this. now I got the answer in a concise and exciting way

Wow this is great. I can remember the day when my thinking finally made the switch from thinking about science/engineering and it's equations as a form of truth to rather a set of useful models that we've made for our universe. The earlier you make the transition, the better, so I really think this is how science should be pushed to students (probably no earlier than high school.)

Re: bar speed: I remember Veritasium talking about it taking the speed of sound in the object. It was how shining a(n idealized) laser at one edge of the moon, and then flicking the laser point across the moon to the other makes the dot go faster than the speed of light. Nothing is actually going FTL (the dot is just an image), and if you tried it with a solid rod (I believe it was a wooden plank in his version), he said it can only travel as fast as the speed of sound in the object

Physics models masterclass. Would have been cool to add also mini animations of the different models you were talking about. Great video

It really took me most of a-level physics (16-18 yrs) to understand the concept of what was really happening when two solids collided. Like I vaguely understood that atoms had bonds forming molecules, and molecules couldn’t occupy the same quantum state as each-other, but learning about how it’s photons that transfer kinetic energy and not just a particle bumping into another particle seemed to make so much sense when I first heard it. That’s probably one of the reasons I’m know trying to complete a degree in physics, because I finally understood a concept intuitively that really seemed convoluted at first. I think it’s very similar to the reason I first became interested in learning anything whatsoever. I will never forget my year 2 teacher miss goody (when i was 8-9 years old) who left the school but left me a note saying to never stop asking such great questions.

I can't say I'd ever thought of this before. On some level I understood that every solid object was just individual atoms bonded tightly together, but I never really considered how applying force to one side meant that the force had to propagate through the material. Edit: Coming back to this a year later having forgotten about this video was interesting. I've definitely internalized what I learned from this. The model I had for this situation in my head was a lot more accurate and I more or less thought it out step for step with how it's explained here. Very satisfying.

I was always under the impression that the speed-of-sound is the intrinsic "speed of motion". If you tap the end of a pole with a hammer, that will dictate when you can first hear the tapping on the other end of the pole. Although that can't answer the question fully, as it is obviously possible to move stuff faster around than the speed of sound. Also: DSOs are really a marvel of economies of scale. You can basically buy scientific equipment for only a few hundred bucks that will show events in the ns range. Maybe I should clarify: a high-explosive for example isn't limited by the sound of speed. In fact, any shock wave travels faster than the local speed of sound of the medium. But that's something for AlphaPhoenix to explain.

That was enthralling, really enjoyed that

Well done. This was a standard experiment in the UK many years ago with simple timers. I found it always gave an answer that seemed right but after many years it appeared to me that whatever you did the kit 'happened' to give an answer of the right order of magnitude! All good fun though!

either c or d. since the speed of sound is based on the rate at which a wave travels through particles, that makes the most sense to me as having relation to this problem. though the amount of force applied and other constraints may affect it as well. I'm not as well versed there

Thank you for going over your methodology *and* the problems you had in setting up your test apparatus to eliminate error *and* how you realized you were getting the wrong-right answer!

You regularly prove that Physics are fun when yo have the right approach to a question. And you also demonstrate that physical models very far from the underlying exact quantum reality do allow to solve our problems very well at least at our everyday worldly scale. Well done, it is a pleasure to follow this eye opener channel.

I love the exlanation and the break down of how you figured it out!

Your videos are always so well researched, well thought out and your demos are top notch. All the while the editing is there, too. Thanks for all the work you put in and not cutting corners! It really makes a difference, I love learning about the basics again as too many times we think we are too smart for our own good. I just last week had to re-learn bernoullis equation to understand again how energies are distributed in fluids within closed loop systems; something I always thought of as an easy basic - living is truly constant learning.

Even though I came into this video knowing the answer was the speed of sound, it was still fascinating to see it experimentally demonstrated and be shown the theory behind it. It's one thing to _know_ the answer, it's another thing to *understand* why that answer is correct. I love videos that elucidate concepts like this. Great job!

Hi! new subscriber here, loved this video and I binged some of your others. great stuff! keep it up, and would love to see what happens to materials cut with a saw on the molecular level haha😅

Amazing work!! Keep going!

Nice! My fave thing, philosophically, that you said: "I love it when...the underlying mathematical machinery can be recycled." A close second, "Almost every physics model...is technically wrong." That our understanding of the world is really just a collection of imperfect models, but that the models of the different phenomena share mathematical pattern, is the basis of successful engineering, and kind of astonishing. My fave practical thing is the illustration here of the impact of ratios on the validity of models. In this case the small radius to length ratio allowed for the 1-dimensionalization of the model, simplifying it and the math, and it was accurate!

This was the most fun I've had watching a scientific video of any kind. I'm a mechanical engineering student and have always been very interested in macro level material science. You explain your thought process and methods in a way that is very pleasant to listen to. Earned yourself a new sub!

Wow. I've always thought about this. You finally answered the question I've had in my head for years.

Great video. I'm currently doing my phd in organic chemistry, i did my masters in pure old school synthesis, but my phd is in a more materials chemistry side of things, more on the edge of solid state physics, i find these demonstrations to be very good, vibration in a solid, phonon , its just another thing like a electron hole pair, an exciton

this guy deserves way more subs. Kudos for the content you are making.

Fantastic video! It was great hearing about the problems you encountered in measuring this - that really helped.

Very good. That was a lot of work to do. Thanks for creating the video.

Thank you for this video! I've been looking for an explanation. I've been imagining objects colliding in a void to try and visualize this.

Just discovered this series. I really enjoy them. You obviously have a passion for this, and that combined with your really clear explanations make these a joy to watch. Thank you!