What is Length Contraction? (Thought Experiment)

[@FortheLoveofPhysics]

Derivation of Lorentz Transormations ► kzbin.info/www/bejne/bKHNYplurZh0hsk

[@AngelAlita84]

hello thanks for your videos, would you be so kind to explain the at 9.34 when you cancel the denominators, one is in root the other is not how can they cancel each other?

[@MrMarkgyuro]

these was the sharpest and the cleanest explanation regarding length contraction! thank you!

[@mandystarr404]

This man is the only reason why im passing physics at this point

[@abhipriyeshukla5431]

What an explanation I am speechless!!

[@aliciaduartesilva4266]

The greatest explanation i´ve found!!

[@Mnemonic-X]

But it's bullshit

[@manaoharsam4211]

Love the derivation with a great illustration.

[@parthees]

Beautiful & simple explanation bro… I have been searching a lot, nothing was so simple and mathematically proven.

[@Mnemonic-X]

But I have a really simple disproof of the STR. 😁

[@pinkighosh2192]

​@@Mnemonic-Xplease share with us

[@pandit7130]

I was looking for this explanation. Thank you.

[@uzairhussain4856]

Loved the way you derived the equation with explaining each and every step.

[@Mnemonic-X]

But I love the way I disproved the special theory of relativity. That's a really simple disproof😁

[@SivaKumar-xl9bd]

Easy to understand thank u sir

[@jrrahulsingh]

This is what I imagined in today's class . Explaining length contradiction using time dilation.

[@sunnypala9694]

Sr you explained awesomely

[@shamzanasim3749]

Excellent explanation

[@FTFEOfficial]

Brilliant. Thank you.

[@bibhutibhusansahoo6019]

Bro it's really help me a lot Thank u

[@adwait_x1]

Are sir sir ....so much thanks sir

[@abhisheknath5723]

Thank you sir... Very simple explanation

[@nthieu229]

If i only consider half of the experiment, from the event which the light starts to the event which the light hits the opposite mirror, is it tru now? It means that now i have dx0= cdt0 and dx=(c-v)dt, if i use the relation of dt and dt0, i will get a very different version of length contraction. Where was i wrong?

[@jacobgonzalez731]

Wondering the same thing. Why does this only work for the round trip and not looking at just the first half?

[@pavanajsridhar939]

very nice video professor , hats off!!

[@Mnemonic-X]

I have a simple disproof of the special theory of relativity.

[@SHIVANIYADAV-vq4qu]

Thanks so much sir

[@dibyendurana7935]

Respected sir , instead of observing incident path and reflected path , if one observe only incident path , then what will happen ... ?

[@zakirhussain-js9ku]

Your videos have clarity and simplicity.I like them. I think we are not comparing like with like. For Observer inside the train nothing is moving with respect to space except light photon. For Observer outside the train both the train and photon are moving relative to space. Photon in both cases is moving in space but this is not the case with stationary and moving train. Moving objects produce an illusion of space contraction experienced by Observer outside the train. Distance or length between two points is a conserved quality which cannot be changed.

[@alwaysdisputin9930]

Yes there are like 2 hidden premises: 1) Everyone sees themselves as stationary. Even when we run it's like we are on a treadmill & the world is the thing that is moving past us 2) Everyone's point of view is valid. Therefore when we run it's valid for you to say the world IS moving. But it also valid for the world to say we're moving. It's like if I have a dream & you have a dream & our dreams interconnect so we can see each other. Then in your dream you of course are the centre of the universe because you're basically like god in your dream. So when you dream that you're stationary, *it is true* that you are stationary because you're god. *But* I am the god of my dream so when I dream you're moving, it's true that you're moving. This is not a contradiction because there are 2 dreams i.e. 2 versions of reality Therefore, when the person on the train looks out of the window he sees the person at the station moving past him & he's perfectly entitled to say 'she's moving not me'

[@zakirhussain-js9ku]

Observer cannot change reality. Reality is that train is moving and person on platform is stationary in space ignoring earth's motion. Points of view of both observers are valid in relativity but not in reality.

[@alwaysdisputin9930]

​@@zakirhussain-js9ku We strongly feel & believe that the platform at our local train station is stationary but this is wrong. The platform is moving around our sun at 30 km per second. You can say 'yeah but we are ignoring that'. OK but then we can no longer say "in reality" because the reality is the platform's moving with respect to our sun. If we are both in spaceships with no windows & 1 is moving at constant velocity & 1 is stationary, there's no experiment that we can do to tell which 1 of us is moving eg if we juggle cats it looks exactly the same in both spaceships.

[@zakirhussain-js9ku]

In my view observer in a spaceship without windows cannot make valid observations. Motion in space has to be viewed individually and not relatively. Assume two observers one in each stationary trains. Trains have glass windows through which both observers can only look at each other. When one of the trains starts moving both observers will think they are at rest and opposite train is moving. If you chech status of both trains individually you will come to know that both observers are wrong. Moving frame does not affect rest frame, there is no mutual interaction.The problem is not with the observers but they just do not know about a hidden variable i.e. the earth on which trains are sitting. Every object has its own unique motion and associated time. Moving observer travels in two time frames present and future while observer at rest is in the present. Both cannot be equated. For simplicity I am taking earth as rest frame. Observer in inertial frame cannot determine status of his motion since the information to make correct judgment is not inside but outside of his frame of reference.

[@alwaysdisputin9930]

​@@zakirhussain-js9ku Sorry but I disagree with everything you say. _"In my view observer in a spaceship without windows cannot make valid observations."_ Wrong. He can perform experiments & make valid observations. _"Motion in space has to be viewed individually and not relatively."_ Wrong. All motion is relative. There is no such thing as absolute motion. Nobel prize winner Kip Thorne says that it's better to use the word 'personal' instead of 'relative'. However, he would still say there is no such thing as absolute motion. This is no absolute frame of reference. _"Assume two observers one in each stationary trains. Trains have glass windows through which both observers can only look at each other. When one of the trains starts moving both observers will think they are at rest and opposite train is moving."_ Wrong. They might decide they are moving or they might decide the other train is moving or they might think 'i dunno'. There's often no way of telling until you can see the platform. If they see the platform moving away, then most people think 'my train is moving'. Nobody ever thinks 'I'm stationary'. However, our basic perception is always that we are stationary at the centre of the universe. On a train we see trees fly past us as if they are running quickly. _" If you chech status of both trains individually you will come to know that both observers are wrong."_ Wrong. If you're on a moving train then the train is moving & not moving at the same time. It entirely depends on what you are comparing yourself to. If you say you're moving compared to the trees then that's true. If you say you're not moving compared to a table inside the train, then that's also true. _"Moving frame does not affect rest frame, there is no mutual interaction."_ Both frames are moving & at rest. I don't know what you mean by 1 frame affecting or interacting with the other frame. _"The problem is not with the observers but they just do not know about a hidden variable i.e. the earth on which trains are sitting."_ I don't know what you mean. _"Moving observer travels in two time frames present and future while observer at rest is in the present."_ Wrong. Both exist in 1 point of time which they call the present. However, if 2 events are simultaneous for 1 person, then they are not simultaneous for a 2nd person, if there is a difference in velocity between them. Therefore they have different ideas about what the present moment actually is. _" Observer in inertial frame cannot determine status of his motion since the information to make correct judgment is not inside but outside of his frame of reference."_ Wrong. All anyone can ever do is compare themselves to some other object & say compared to that object I am moving. Therefore they can determine the status of their motion. However, cannot determine their absolute motion because such a thing does not exist.

[@yashshah5727]

I think there could be a simpler proof in which we start with the fact that speed of light is constant for all inertial frames and speed is distance/time so if time is reduced by a gamma factor then the length should also be reduced by the same gamma factor to keep the ratio equal. I'm not sure though if this is a valid proof or not. Please Let me know if it's a valid proof or not

[@rafbi4874]

Thank you.

[@victoryiu1481]

Thank you!

[@baraklevy3344]

nice ! i liked it

[@shivangithakur1292]

Great sir

[@SuleymanBulutistatis]

Thanks a lot!

[@kingdomofknowledge5960]

How did you cancel "1 - v2/c2" in the last ?

[@nicholasmalambo6941]

Yeah how? I got confused too anyway?

[@kayrstar8965]

He didn't cancell it ...it tends to 1 for v

[@alwaysdisputin9930]

​@@nicholasmalambo6941 @Kingdom of Knowledge Here's the derivation in a slightly less condensed form: 8:01 Δt = 2 Δx c = the time it takes to go from the light bulb to the mirror & back from the point of view of an observer on the platform ̅( ̅c ̅² ̅- ̅v ̅² ̅) 8:16 We now use the time dilation equation derived in a previous video: ________________________ | | | Δt = ̲ ̲ ̲ ̲ ̲Δ ̲t ̲₀ ̲ ̲ ̲ ̲ | | _____________ | | √ 1 - v² | | ̅ ̅c ̅² ̅ | |_____________________| 9:04 => ̲ ̲ ̲ ̲ ̲Δ ̲t ̲₀ ̲ ̲ ̲ ̲ = 2 Δx c ______________ ̅( ̅c ̅² ̅- ̅v ̅² ̅) √ 1 - v² ̅c ̅² ̅ 2:10 but we know: Δt₀ = 2 Δx₀ ̅ ̅ ̅c ̅ ̅ ̅ 9:25 => 2 Δx₀ = 2 Δx c ̅ ̅ ̅c ̅ ̅ ̅ ̅( ̅c ̅² ̅- ̅v ̅² ̅) ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ √ ̅ ̅1 ̅ ̅- ̅ ̅v ̅² ̅ ̅ ̅c ̅² ̅ 2 Δx c = 2 Δx c & 1 = c² ̅( ̅c ̅² ̅- ̅v ̅² ̅) ‒‒‒‒‒‒‒‒‒‒‒‒‒ ̅c ̅² ̅ 1 times ( c² - v²) => 2 Δx c = 2 Δx c = 2 Δx c ̅( ̅c ̅² ̅- ̅v ̅² ̅) ‒‒‒‒‒‒‒‒‒‒‒‒‒ ‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒ c² ( c² - v²) c² ⎧ c² - v² ⎫ ̅c ̅² ̅ ⎩ ̅c ̅² ̅c ̅² ̅ ⎭ => 2 Δx₀ = 2 Δx c ̅ ̅ ̅c ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ c² ⎧ 1 - v² ⎫ √ ̅ ̅1 ̅ ̅- ̅ ̅v ̅² ̅ ̅ ⎩ ̅c ̅² ̅ ⎭ ̅c ̅² ̅ If we multiply both sides by c we get: => 2 Δx₀ = 2 Δx c² = 2 Δx ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ c² ⎧ 1 - v² ⎫ ⎧ 1 - v² ⎫ ​̅c ̅² ̅ ⎩ ̅c ̅² ̅ ⎭ ⎩ ̅c ̅² ̅ ⎭ Divide both sides by 2: => Δx₀ = Δx ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎧ 1 - v² ⎫ ​̅c ̅² ̅ ⎩ ̅c ̅² ̅ ⎭ Let B = ⎧ 1 - v² ⎫ ⎩ ̅c ̅² ̅ ⎭ 4 = √4 times √4 => B = √B times √B => B = ⎧ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎫⎧ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎫ ⎩ ̅c ̅² ̅ ⎭⎩ ̅c ̅² ̅ ⎭ => Δx₀ = Δx = Δx ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ ̅ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎧ 1 - v² ⎫ ⎧ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎫⎧ √ ̅ ̅ ̅ 1 ̅ ̅ ̅- ̅ ̅ ̅v ̅² ̅ ̅ ̅ ⎫ ​̅c ̅² ̅ ⎩ ̅c ̅² ̅ ⎭ ⎩ ̅c ̅² ̅ ⎭⎩ ̅c ̅² ̅ ⎭ 9:39 _____________ Finally, we multiply both sides by √ 1 - v²/c² => Δx₀ = ̲ ̲ ̲ ̲ ̲Δ ̲x ̲ ̲ ̲ ̲ ___________ √ 1 - v² ̅c ̅² ̅ ___________________________ | ____________ | => | Δx = √ 1 - v² Δx₀ | | ̅c ̅² ̅ | |_______________________|

[@zakirhussain-js9ku]

While motions can be added as vector, how is time added for concurrent motions.

[@FortheLoveofPhysics]

What is Time Dilation? kzbin.info/www/bejne/onPMcnhqgd2sbsk

[@the_sophile]

thank you so much!

[@nicholasmalambo6941]

Well explained...I love your emotions when explaining physics and the modulation....thanks!

[@theadvancelearning]

How length is contracted since term in braket shall be always less than 1 as v must be less than c as per postulate of relativity so length for observor in train frame must have greater value vis-a-vis the rest observor. Any explanation

[@criticalthinking575]

Sir I have a doubt, that the distance get reduced in moving frame for ever Or it's just observed

[@merajhossainpromit6152]

What if the line of velocity is not in the x axis only, I mean it can be in between X & Y too..!! How to solve then!?

[@dfershgdd]

thank you bro..

[@JohnWick-to8dq]

Thankyou srr

[@zakirhussain-js9ku]

I think length contraction is an illusion. For person in the train everything is in same frame (rest) while for person on platform light is in rest frame and train is in moving frame which creats the illusion. It is like moving on treadmill which creats illusion of being stationary when you are actually moving. Whatever distance you walked it appeared to have contracted to zero since you appeared stationary to everyone including yourself.

[@noone-bp2iv]

SIR,isnt the the longitudinal distance X*(x not) IN BOTH CASES [5.15 timestamp]. PLS reply sir.I am confused.

[@CodewizardSumit]

Hey sir, is there is any length contraction due to gravity? Actually I formulated it i.e L=L°√1-2GM/r c^2 Where L°= length of object in gravitational field M=mass of the star r=distance of the object from the star And other symbols have their usual meanings M

[@sayanjitb]

how did you do this?

[@lowersaxon]

Suppose there is n o rest frame of ref. Then?

[@rashmikhamandana5498]

How do you say Del t2 #Del t1 they are equal

[@AngelAlita84]

the part of cancellation in 9.34 is not really clear, one is in root the other isnt

[@FortheLoveofPhysics]

One is sqrt, the other is power 1, so after cancellation the other's power becomes 1-1/2 i.e it becomes a sqrt

[@AngelAlita84]

@@FortheLoveofPhysics thank you, i don't have strong background so sometimes i have to see details. I appreciate it a lot sir.

[@AngelAlita84]

@@FortheLoveofPhysics can you please be so kind to explain why dx' is in numerator in the end? Shouldnt it be in denominator?

[@europhotographic]

Roy Wilson 2 seconds ago Very Clear, and well explained. I have a question Ive been looking to answer and wondered if anybody can help. Whats it called in maths when part of the calculation or working in a formula produces a result that is counter intuitive, but is necessary and logical to produce the final equation . For example in this working when delta t2 = delta x over c+v. Where C+V cant actually exist. Or in other calculations where the square root of -1 is used to derive correct formulas. I can understand why it works, i just wanted to know what its called, does it have a name in maths. ?

[@sanalnath1772]

Nice question, as far as I know it's a third person(person standing on the floor) expressing relative velocity of wall and light, it can be greater than c. For both photon and wall relative velocity is c itself. So it's a third person's expression for relative velocity between two particle so postulate is not violated. In short if two photons are approaching in opposite direction each will feel relative velocity as c but you(looking from ground) can express that relative velocity as 2c. That's 'RELATIVE VELOCITY' in your frame, not velocity... Hope it helps

[@zzzoldik8749]

How about for the point view of photon it self. Length become zero because of length contraction. This is redicilious, we know photon need time to travel long distance, like from sun to earth they need 8 minute.

[@FortheLoveofPhysics]

Let me boggle your mind a little bit more. Time stops for a photon.

[@irishguy200007]

You mean Fitzgerald contraction??

[@banglavibes5251]

Really good sir,but as a Bangladeshi HSC student,it is little bit difficult to understand the whole explanation.

[@irishguy200007]

Since he proposed it in 1889 three years before Lorentz.

[@sbkarajan]

I am amazed how simplistic and rudimentary the SR concept is.... Just coordinate transformation to fix the seemingly constant speed of LIGHT.... Well, well, well.... I don't think it can be that simple... For one these coordinate transformation does not assume any fixed reference point, everything is relative... Thus, twin paradox is real.... And then, the GPS satellite slowing down... never verified, plus, I am told that they reset the GPS clock every 12 hours anyway, because there are so many variables that goes wrong anyway... And the "proof" of black holes and so on... Man, these are never verifiable in million years.... So, SR is I think BS....

[@jrrahulsingh]

I also used second postulate in my imagination .

[@thespotlightkid4138]

Ah, length contraction? ...wasn't it a faith-based concept (in as much as that it cannot be proved by demonstration or observation) & dreamed-up just after & especially for debunking how the Michelson-Morley experiment results showed that the celestial bodies revolved around a stationary earth? (Leaving the slight & arguable fringe effect" discrepancies that Potier grumbled about aside as negligible in light of what *Lorentz would later muster up) Didn't Lorenz invoke Einstein's then brand-new "special relativity" to say that the experiment was flawed because the apparatus used had changed length during a speed-change as the earth sped around the sun? (tho true scientists don't place assumptions into their equations) Didn't Lorenz's Transform Equation then become a Cornerstone Equation in Astrophysics to be used like a "scientific Haribo pick 'n mix" to prove or disprove any old cosmological slop? ....because this is what is recorded as having happened. (Lorentz or Lorenz ? ~ for many years i've been seeing both spellings used)

[@sushiladevi9799]

3 Saal ho gaye aur 10k views bas....in topics ko koi shauk se v chhoota v nahin hai samajh me aaye ya na aaye...baat hai ki exam ka bolbala hai ,padhaai baad me aur padhai ka aim pahle select Kar lete hain log.