Special Relativity Part 1: From Galileo to Einstein

We talked a little bit about relative motion in the classical physics course, with Galileo dropping stuff in boats. But once Einstein got his hands on relativity, he went a little nutty with it! The inertial reference frames are back but now we are talking about objects moving near the speed of light, which is where things start to get a little... weird.
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[@LunaticBiker11]

This is so well explained. I have no reason to be watching these videos during summer break at 2:30am other than my curiosity left over from ap physics and your explanations are 10000x better than the textbook we used.

[@JVerde853]

I’m currently watching this at 2:10 am during spring break…

[@prapannmalavia2574]

@@JVerde853 I'm currently watching this at 1:50 am during spring break...

[@yakububenjamin8086]

2:30am spring break too

[@solounomas0]

1:50 am summer here

[@cjg8763]

Special Relativity is one of my favorite subjects (as well General Relativity) can never get enough no matter how many videos I've seen or articles I've read. Looking forward to the rest of this series.

[@luwen77777]

i like relativity too :D

[@everythingisalllies2141]

but SR and GR are just nonsense... you know that right?

[@CameronBFunny]

It's incredible how calmly you stand while projecting your voice so well. You were born to be a teacher and public speaker!

[@chrismersits3504]

Wow. Incredible job explaining this in roughly 5 minutes!

[@freshely9309]

Well, relative to you ;)

[@elnuraliyev6603]

Would have never understood without you, Professor Dave, thanks a lot!

[@wytototas4000]

I only found, subscribed your channel yesterday - and I already LOVE it. Keep up the good work and keep on posting dank science :)

[@Avisha_Jain]

This is the easiest explainantion I've seen of relativity, you are an amazing teacher

[@raanaejaz200]

What an awesome explanation. I love it. Too good !

[@nithinc.p.5599]

sounds simple. Thanks for explaining.

[@vincentjakeescolar2346]

This is the best explanation of Special Theory of Relativity here in YT.

[@marksimpson2321]

Ive been interested in popular science and relativity for 20 years and this is a very clear explanation!

[@marksimpson2321]

This is a very clear explanation of the amazing consequences of special relativity: the fixed speed of light.

[@urminanani4]

Saw many vds of special relativity but, would have never underatood w/o you . Thank u professor ur great explainer👍

[@kiarachero9691]

WAH thank you so much for this explanation! Made it much clearer

[@jamesfrench7299]

Time resolves the paradox! Great lightbulb moment for me.

[@sydneyhunt6681]

Well explained buddy great video 😎

[@shirinalali759]

Thank you..

[@ashekshanto8537]

Wow. Amazing. Never been so clear.

[@quedin9990]

bro u are very good at explaining things. bravo!

[@alobeeboo]

Hi Dave, im reading a book right now that explains physics concepts in layman’s terms and was looking up a video that explained special relativity and was surprised when your channel popped up. I realized that I’ve been watching way too many of your flat earth and pseudo science exposed videos that I forgot you were an actual science channel lol. I’ll be sure to dig deeper for good content like this.

[@siddharthtewari58]

I'm in love with your videos

[@dejavudummy6947]

Well explained.

[@thomasalberto613]

I can finally not look like an idiot when my friends start talking about physics!

[@asura-2467]

ur comment remind me of penny from big bang theory

[@srinivasans1819]

@@asura-2467 I was about to say that 😂

[@jay1232000]

Thanks for sharing. Liked and subscribed.

[@tinalgh4636]

Thank you 🎉

[@Fanatik4Ever]

Thank you so much ! ..

[@EpicGifted]

4:40 was an absolutely mind shattering moment for me

[@angeliemaebonaobra4448]

Thank you!!!!

[@helifynoe9930]

Within the 4D environment known as space-time, every reference frame is in motion, and is always in motion, and every reference frame is in motion with one specific magnitude of motion. They all share that one specific magnitude of motion. Therefore, what differs from one frame to another, is the direction the motion is pointing within the 4D space-time environment. Now if you create a simple geometric representation of this, you can use it to derive the SR mathematical equations. It is the easiest way possible to derive the equations. The job is complete in mere minutes.

[@abdixakiinosman8725]

Thank you.

[@angeliemaebonaobra4448]

Thank you

[@SaquibFaisal]

can you please make video about spin (quantum state)???

[@leonardocastano]

Explain please how was tested this theory, many thanks for your effort

[@keithmccann6601]

brilliant bite sized little videos - i just watched six back to back - very addictive - just one question so far - what is the 'h' that crops up in so many equations?

[@ProfessorDaveExplains]

Planck's constant! Watch the modern physics playlist from the beginning for explanation.

[@prashanthhiremath2597]

fentastic channel👍

[@beshylle2032]

before : wtf is this topic 😵 after : a piece of cake 🍰 thank you so much!!

[@satellite964]

Finally someone explains it

[@ma.ysabelleung7739]

You are amazing

[@urdad1725]

just.... wow!!

[@farhanurmiah2635]

thanks

[@tommydv]

Thanks, Professor Dave. I would like to see Kent Hovind try to explain this stuff!

[@charlieghague]

"He knows a lot about the science stuff, Professor Dave explains." Bah buh. Lol Your facial expression during the intro cracks me up everytime! Anyways, I was trying to find a good beginner explanation on special relativity and couldn't find one. Then I thought "Oh yea.. Professor Dave!!". Boom! Problem solved! Thanks for being awesome and continuing to "know a lot about the science stuff". 😉

[@bst129_nitro2]

lol

[@franklinelel]

Nice animation.

[@csipus133]

Hi Professor, can u make ur intro 1-2 sec longer, im watching it via phone, and the 10 sec forward "skip intro" barely cut into ur starting sentence. Thanks, im enjoying ur videos

[@bzzzvzzze]

If its that short, might as well watch it ;)

[@kimsahl8555]

In the physical nature, rest/motion is not absolute. In the physical nature rest/motion is not relative. - but - In the physical nature rest is just rest. In the physical nature motion is just motion (unaccelerated or accelerated).

[@matthewjackson337]

with inertial reference frames, is it speed or velocity constant, the earth surely is accelerating due to it undergoing circular motion orbiting the sun?

[@jukestv4873]

if the earth is rotating on its own axis . is it possible if the plane will fly up and get steady above for 24 hrs without moving in any direction, will the plane land in a different place on earth?, or will the plane land on the same place? pls answer. and justify yuor answer. TIA

[@adityashankar5267]

Hey prof! Let's say you run at the speed of light and I see you standing outside the frame in which you are running, and your frame itself moves at some velocity v. So according to me your velocity is c+v>c !!! How can this be possible?!

[@ProfessorDaveExplains]

It's not possible because nothing with mass can go the speed of light!

[@adityashankar5267]

@@ProfessorDaveExplains ok but what about velocity comparable to c? Say v1 = 2.9 × 10^8 m/s v2= 1×10^8m/s Relative velocity=v1 + v2 = 3.9 × 10^8 m/s > 3×10^8

[@ProfessorDaveExplains]

Keep watching through this series, the next one is on time dilation and it will explain why the kind of situations you are describing are impossible.

[@SolomonUcko]

In SR, velocities don't add normally.

[@yathinraj3964]

hope this helps kzbin.info/www/bejne/jaKkY2mjZ7JmgNU

[@OnumLCT]

I like your humour

[@CraftCrazy69]

Which experiments have proved these statements? Can someone provide links? Its all so interesting

[@ProfessorDaveExplains]

just look up "tests of special relativity" on wikipedia

[@sird135]

It always makes me laugh thinking that the speed of light trolls you in whatever reference frame you are. That cheeky c

[@rameezabid356]

Professor Dave, would we then say that the earth and the planets are moving around the sun relative to the space around it?

[@jasmines4993]

Rameez Abid we can’t accurately say that since the space itself is expanding

[@chasing_horizons]

Please tell, In Special theory of relativity, Einstein said that when he threw a stone on embankment without dropping it; By his view ( Frame of refrence ) stone travels only vertical distance amd stone reaches its hand by travelling vertical distance. But by the refrence frame of the person who is Passing through the embank ment notice this. By his view, stone travels in parabolic curve (parabola). Sir so my question is ;;:: Why; for the passer ; stone travels in parabolic curve instead of any randomly roughly curve?

[@ProfessorDaveExplains]

So this is actually not special relativity, it's just plain old Galilean relativity. Check out my tutorial on relative motion in the classical physics playlist, it should clarify everything!

[@chasing_horizons]

@@ProfessorDaveExplains thank you sir. THANKYOU SO MUCH FOR REPLYING.

[@chasing_horizons]

@@ProfessorDaveExplains I shall watch.

[@sadlyaritari6238]

Wait I have a question, if the faster we go, the faster time passes, and the slower you move the slower time is, so what if there is a point where if you go slower than that, you will be able to technically go back in time? I’ve had this question in while for some toner. It might be stupid but, I want to comfirm.

[@ProfessorDaveExplains]

No, check part 2 about time dilation, it should make this more clear.

[@nijilpk2856]

When two observer one is at rest and other is at spaceship then how c will be constant?

[@ProfessorDaveExplains]

check out part 2 on time dilation to find out!

[@niksi_fpv2313]

Cool

[@ahmadbaydoun707]

Could you please explain the second postulate more please? I understood that if i am traveling at 299M m/s and light is traveling near me at v=c. Then after one sec the distance between me and the light will be 1M m or 599M meters? M:million m:meter There is another thing that i dont get it If we have a ship moving at c/2 and that fires a beam. Then after 12s the crew saw that the light went 6 light secs within 12 secs, how is that possible? Or is it wrong and what should actually happen?

[@ProfessorDaveExplains]

hmm, i don't really understand the question, but all issues of this nature are solved by time dilation! time flows at different rates for different observers so that the speed of light will remain constant for all observers.

[@ahmadbaydoun707]

I ll try to make it easy for you , there is a video on youtube called Time Dilation | Einstein’s theory. The video was uploaded by Science Tv . On approximately 3:50min there is an example of 2 starships, one at rest and the other moving, when the moving one come right under the one at rest ( the moving one started behind the one at rest) they both fire a beam. Now the crew of the ship at rest will notice that the beam traveled 12Ls in 12s and this is something that I understand. But what i dont is that for the ship moving : why after 12s they measured the distance travelled by the beam between the ship and the beam’s final postion and concluded that light travelled 6Ls in 12s.. i mean light travelled 12ls just like the ship at rest because we should measure the distance travelled from light’s final postion to light’s initial position( from where it was fired. Thats my prob , amd thx for explaining professor

[@ProfessorDaveExplains]

still not understanding, but if you watch part two of this series, regarding time dilation, it discusses precisely this kind of scenario.

[@ahmadbaydoun707]

I actually watched it before asking but I didn’t understand. The point is that i dont understand what it means that the speed of light is constant for all observers. I mean if someone is standing still he will see the light with a speed of c. But for someone moving at 40m/s. Will he see the light traveling at c+40 or c-40? Or at c? What does it mean that the light is always traveling at a constant speed for all observers??

[@ProfessorDaveExplains]

constant means never changing, so it's always c! no matter what you do.

[@kadhm]

thank you physics jesus

[@gretawilliams8799]

What's the point of relative velocity if there is no fixed reference frame??

[@ProfessorDaveExplains]

well if there is no fixed reference frame, then all velocities are relative! so the point is that it's literally all we can do.

[@phlurim]

A person on a space vehicle, moving at 299x10^6 km/s will always measure the speed of a beam of light as 300x10^6 km/s, regardless of his own speed. That is, he WILL NOT observe the light beam moving only 1x10^6 km/s faster than himself. And an observer on Earth will also measure that same beam of light moving at the same 300x10^6 km/s. This means the speed of light is an absolute universal constant, and results in the flow of time being different in each individual's inertial reference frame. Also, the person moving at 299x10^6 km/s will not perceive that he's aging considerably slower than (i.e. relative to) the Earth-bound observer. So, please let me know if I've got this right, or not? And is there a readily available simple version of the derivation of Einstein's Special Relativity equation that lead to e=mc^2? Thx.

[@utkarshraj3272]

👍👍👍

[@thedude4795]

this is the best video for explaining light speed, relativity, for anyone. I'm slightly embarrassed to say that I never understood it until now, and am so disappointed in how poor Western society has been in actually communicating this stuff to young people, and it seems like families have to do the heavy lifting them selves, since the earlier kids learn about c being constant, no matter the speed of matter, the better!

[@San_Dude]

Other question we see stars lights year away.. Somehow we look into them with a ultra zoom telescope then what we would watching it... Incident happening 1 year before... Or the current incident of that planet

[@ProfessorDaveExplains]

We see objects as they were when the light we are collecting left that object.

[@San_Dude]

@@ProfessorDaveExplains that means the Galaxy light years far we observe them help of our telescope.... They are or that footage or natural data or that incident is not of current but years old ... If I get it right

[@ProfessorDaveExplains]

Yes, if we look at a galaxy a million light years away, we see it as it was a million years ago.

[@San_Dude]

@@ProfessorDaveExplains that means we can see the past.. Of other but cant do anything about it.. And it would be soo hard to predict and calculate in what condition now or what is going on other part of creation cause we limited by the speed of light ... But what if somehow we can figure out control of nuclear fusion and can focus it energy as booster so the craft can attain . 8c around speed time would dilate like muon particle we are other side of creation. Different galaxies . Telling alien that we know about them more than them .... Coming thousands years and we would be young if I'm talking according GRT or special relativity

[@fatimahayssam120]

But How Time flows in different rates ? is that because of differ in speed ?

[@ProfessorDaveExplains]

just wait for part 2!

[@fatimahayssam120]

+Professor Dave Explains will wait for it ..Thank you 😊

[@kethavathhemanth2103]

Plz launch the part2....

[@weathergad5358]

Ok so if both observers getting the same speed doesn't it mean the rocket ship and the earth is experiencing the speed at the same time.. even if not at the precise time but in the same time signature other than different times? But then again speed of light as infinite power source and time doesn't affect it because everything would be slow down if not stop but if everything is stop it doesn't matter what different time it operates on cause time doesn't apply to it.. E.g a piece of 6 inches of steal as electricity all around it so much that it's even visible but it doesn't matter how much time the current craves and pass through that steal it doesn't matter how many times it happens the steal will remind the same meaning it won't bend, crave or even get flexible and flow with the current just because of the current around it. it's still going to stay straight regardless of what's happening around it... so if we are the current and speed of light is the steal no matter how much we try to measure it in any time it would still be the same cause time doesn't affect it... just of if the current wont affect the steal shape..am I wrong of what i analyse? If so and you got the time feel free to correct me if I didnt get it right..

[@aeon3261]

Loveu lifesaver

[@Sanjana-sq9dr]

What if we travel at speed of light?

[@San_Dude]

Someone said to me if you travel as the speed of light.. Time will stop for you.. As I take it time ll stop for me forever.. Means I would see nothing changing everything would stop.. But I don't understand one thing suppose we sent a signal of light to a celestial object 1 light year far.. Then why it takes 1 year to reach... If Im in that signal would it take me to reach 1 year. Or if time is nothing for light... Then why it took one year to reach that planet.. Or galaxy

[@ProfessorDaveExplains]

Different reference frames. Things moving the speed of light do not experience time. We experience time. We are not moving the speed of light.

[@tada7009]

Wouldn't it break the conservation of momentum if it observe the same

[@michaelcole506]

Nothing changes the way light behaves for any one of the observers. It is time that is moving differently for each of them. Space time is altered within their own frames of reference due to their relative speeds. If space time did not change due to motion, this would then require that light travel at different speeds in order for everyone in different reference frames to view its behavior at the same absolute time. This would then beg one to question; how does light (from the same source) know how fast to travel for different viewers. Do you see now? Look up Einstein's lightning thought experiment.

[@amedeofilippi6336]

It has no meaning to say “ what happens when you travel at a speed close to that of light” if you don’ clarify what frame of reference you are referring to. How can we know what is our real speed? We could well be travelling close to speed of light with respect to a certain planet located in a distant galaxy and despite this we don’t feel anything strange occurring here.

[@ProfessorDaveExplains]

I think you'd better watch this whole four-part series.

[@michaelgonzalez9058]

It's not exact because the fast ness is immaculat

[@pavankumar-ff9bo8zc5y]

if an object is travelling near ~ speed of light say 299,792,457 m/s then speed of light relative to this object still clocks at 299,792,458 m/s?

[@ProfessorDaveExplains]

yep

[@pavankumar-ff9bo8zc5y]

@@ProfessorDaveExplains That's crazy and really difficult to comprehend for a normal human being or at least for me. Don't know how Einstein got hold of it.

[@pavankumar-ff9bo8zc5y]

​@@ProfessorDaveExplains Okay, lets's consider a particle with negligible mass (or no mass) similar to that of a photon and if this particle travels anywhere near ~ speed of light. What happens then? Same results?

[@pavankumar-ff9bo8zc5y]

Sorry if I am asking too many questions.....I am just baffled at this mind bending revelation by Einstein. He definitely had some different level IQ to arrive at this.....

[@ProfessorDaveExplains]

anything massless travels exclusively at the speed of light, and anything with mass travels exclusively below the speed of light.

[@RolanRoyce]

Prof Dave is a known smart pants, let's see him outsmart this little thought experiment I recently designed. I call this the Triplets Paradox. This ain't flat earth, Dave, you'll have to do some serious thinking to explain this. Will he be the first person in existence to resolve this paradox, other than by admitting that SR is bogus? There's no planets involved, it's in empty space, and we'll say they're all traveling in the same direction in rockets but at different speeds, I'll refer to them as R1, R2 and R3. R2 is traveling toward R1 at 0.25 c and R3 is traveling toward R1 at 0.5 c, behind R2, which means it's closing in on R2 at 0.25 c. and their paths are parallel and slightly separated, so they won't collide but just pass closely by each other. The rockets were also traveling at the same velocity the whole time, there's no acceleration involved. We now assume that the pilot of each rocket is the same age when they're at a certain distance apart, whatever distance is required for the thought experiment to work out with the age differences I'll describe. We'll say they all started out at 20 years old. At a certain point, all 3 rockets will be at the same place, in a straight line side by side perpendicular to the paths of travel, like 3 side by side train tracks with train cars parked side by side, and they can see each other holding up a sign with their current age on it. We'll say that the speed differences work out to a time dilation of 10 years between each rocket. What ages would each of the pilots be holding up relative to each other, meaning which ? Here's the tricky part, what happens when we switch from one rocket's frame to another? R1 would see the other 2 rockets coming toward it at different speeds, the far one at twice that of the near one. R2 would see both R1 and R3 coming toward it at the same speed in opposite directions with himself in the middle. R3 would see the same thing as R1, but reversed, he would see both coming toward him from the opposite direction to what R1 saw. Since, in the R2 frame, R1 and R3 are seen to be traveling at the same velocity, R2 would have to see both of them as being 10 years younger than him. R1 would see R2 as being 10 years younger than him and R3 as being 20 years younger than him. R3 would see R2 as 10 years younger and R1 as 20 years younger. Do you see the conundrum here? Are R1 and R3 the same age when they meet up with R2 or not? They can't both be 40 while R2 is 50 and also one be 40 and the other 30 while R1 is 50. How do you fix that using relativity theory?

[@heathercollazo5203]

Wait a sec... So, technically, as an observer on Earth, aren't we hurling through time space orbiting our sun within our galaxy within our universe? Which means we are actually traveling in an oscillating motion, and thus have an ever-changing acceleration... so how can an observer on Earth be able to be the inertial reference frame?

[@ProfessorDaveExplains]

We approximate the earth as one, the math ends up solid.

[@nutanvardam8818]

If and only if I m travelling with the speed of light in spacecraft then wht will be the speed of light outside my vehicle....ummmmm zero or 3*10 raise to 8???

[@ProfessorDaveExplains]

if you were traveling the speed of light you wouldn't be experiencing any time at all and you couldn't even measure anything! one of the many reasons you can never go the speed of light.

[@InDepthCreation]

Professor Dave Explains Experiencing zero time means ..we couldn't do any work??????

[@willk9847]

Please list the experiments you refer to when you say 'this has been verified experimentally'. Thanks

[@ProfessorDaveExplains]

just look at "tests of special relativity" on wikipedia, there are hundreds.

[@michaelgonzalez9058]

I made the watch as a time increase of every military second to see the judgement Of my making

[@monopolyman3410]

I'm confused :/

[@scarlettulippss]

Why ain't anyone talking about the opening?😂

[@sunjayroy312]

Show me one experiment where a moving body measures c at 3x10^8

[@declanwk1]

all experiments measuring the speed of light on earth give the same answer, and the earth is a moving body.

[@San_Dude]

Last thing we are limited to moon and Mars and solar system then how we predict shapes of galaxies and which is millions of light year away... How we know they look like that or work like that or move like that etc..

[@ProfessorDaveExplains]

We have powerful telescopes.

[@cedrigodarwin6973]

if there's a light, and there's a ship with 0.999999999c velocity beside the light, and there's also observer from the earth (let's say he can see the light and ship moving), what will he sees??

[@cedrigodarwin6973]

i mean, how will the ship be seen by him?

[@ProfessorDaveExplains]

well these thought experiments are not so much about how we would literally "see" these things, humans can't really perceive a beam of light as it passes. it's more about how we would measure these velocities if we were hypothetically measuring them.

[@cristig243]

The Sagnac experiment indeed proves that the speed of light is not invariant c. The relativistic derivation produced to show that the Sagnac effect is in fact relativistic is nothing but a crafty mathematical trick. Houdini style.

[@athought3252]

But how

[@bushbob5074]

I don't know how, I don't when, I don't know where... but Einstein is going to be proved wrong.

[@tnt_grieferz9176]

oh yea, so how is the speed of light wrong? u didnt even dive deep into the fundamental law, how do u know? Srly, u? challenging einstein?

[@hunglikeahorse120]

Time kept on slipping slipping slipping, into the fuuuutttrreeee! Sorry, I watched Space Jam the other day.

[@craigfordyce4645]

What do you suppose is God's reference frame? "A man can imagine many things, but he can know only that which is true." I.N.

[@thomasc7526]

My honda is faster

[@tada7009]

If I have a rocket traveling 299999996m /s and I throw the ball at 10 m/s at the same direction, then the ball should be faster than speed of light at 300000006 m/s

[@ProfessorDaveExplains]

It won't, for the reasons outlined in this series on special relativity. You should watch them.

[@tada7009]

@rent a shill but the energy it needed is just 50*ball mass

[@tada7009]

@rent a shill we have the kinetic energy formula, E=1/2mv^2

[@SAMACAG]

Please watch ... Einstein's three biggest Blunders ...

[@SumantKumar-qt1gi]

change ur shirt type

[@thedisintegrador]

he records all his videos in a series in one day :)

[@SumantKumar-qt1gi]

cool I have learned a lot from his video

[@redpilljudahite7402]

But we all know the earth is flat..