Why gravity bends light even without mass?
Vložit
- čas přidán 7. 03. 2024
- Head to squarespace.com/floatheadphysics to save 10% off your first purchase of a website or domain using code FLOATHEADPHYSICS
In this video, we rediscover Einstein's equivalence principle. It solves one of the biggest mysteries of Newtonian physics - why gravitational mass = inertial mass. But in doing so, Einstein completely reinvents the ideas of gravity. Starting with gravity being an illusion. A mere side effect of the 'ground' accelerating 'upwards'.
Basket Ball Feather Video
• Brian Cox visits the w...
Hammer Feather Video
• David Scott does the f...
This video is sponsored by squarespace
Head to squarespace.com/floatheadphysics to save 10% off your first purchase of a website or domain using code FLOATHEADPHYSICS
PS: There are quite a few comments about how gravity doesn’t bend light, but it bends spacetime and light just follows it. Well, If we didn’t know this already, how would we go from special relativity + Newtonian gravity to rediscover space time curvature (and in the process, gain a deeper insight into it)? That’s the question this video series is trying to answer!
PPS: Yes, I should have used limit m->0. If not show it, at least mention it.
If our sun is bending the light then it is accelerated upwards and light come from every direction so, if sun is accelerating upwards at every direction then why is it not expanding?
Please clear it...
This is not conceptually correct. If you are an elevator and you start moving upwards and you shine the light, the elevator does not drag the space with it, so the photon remains in that space. Only heavy objects drag space behing them as they move through space. Also, there is a big misunderstanding of deep space and floating in it with no gravity. There is plenty of gravity in space and it is all over the Universe. All Universe's space is permeated with gravity.
Absolute horse shit.
@@petervankas1352 A good fertilizer then. Ha Ha.
I kind of agree in that the earth can't go upwards in all directions.
There are more ads than teaching in your videos. I literally had to go through 4 ads in a 20 minute video(excluding your square space thing). I'm not saying that there must be no ads, in just saying don't make your videos "only ads". Have clarity in your mind whether you're here as a teacher or a businessman. Imagine how a teacher feels when he's fully involved in his teaching and someone for no reason disturbs the flow, how bad will be feel? You allowing ads on between the teaching is like admitting that the your work is not so important that you cannot put an advertisement in between!
Gravity doesn't change light, it changes space, light travels in a straight line through space, if space is curved, the light curves with it. In a straight line.
Yes
I usually say it as, gravity doesn't bend light, it bends the universe and takes light on for the ride.
One of the biggest problems in physics isn't a problem in physics, it's a problem in miseducation initially that gravity is a force.
@@davidmudry5622the very description of a progressive collapse.
Easily prevented by a spacetime straightener. ;)
I guess that the easiest way to explain gravity is that mass loves to tell spacetime to get bent.
I'll just get my coat...
Exactly. The answer is geodesics. Mass curves spacetime and light traverses space in a straight line on a curved surface, which is basically a curved line because you cannot trace an straight line in a curved surface.
Wrong. DENSITY converts the amplitudes into propagation of mass or not.
Please do a video where you show off all of your joke t-shirts and explain the jokes.
In this particular case, it looks like the shirt is schematic for a circuit with a diode in it, maybe. A diode only lets current flow one way (so only positive feedback). It may be something else, I can't get a clear look at it.
@@fieryweasel this one is an operational amplifier (the triangle symbol) with a positive feedback reaction circuit that's why it has the text
@@fieryweasel in the way a circuit flows, electrons are sent from the ground (the negative plug) to the voltage (the posititve plug) so if there is a negative comment or phrase sent, it is recieved as positive.
Yesss I wanna order some those are great
@@thebusdriver_gaming7573 In ideal Op-Amp case there is nothing to do with -ve plug, because there is open circuit between + and - ones. So whatever singnal you are giving at positive side, will be given to the output side as feedback and you will see no input inverting blocks. Positive amplitude will increase and vice versa according to the i/p voltage at the + side.
work's done
Yeah when I saw it, I thought it was the old video but I saw that it was uploaded 1 hour ago
I was wondering why this video is not marked as watched since I have for sure seen it
Yes bro
This is true
Fax
That rollover to the sponsor message...
Infinite pricelessness achieved.
Zooming out - there is another person on the other side of the planet, where ground accelerates "up" (which is other direction for our first elevator guy). So planet accelerates in all directions at once. And since it is "impossible" it means that it is not a planet moving in all direction but it's space moving into planet from all directions :D
The explanation with the ground going up has a limit and you are right. Except that spaceTIME is curved.The earth has a mass which is energy, big enough to curve the space. From all directions.
Yes i think thats where he is going to take us in the next video :)
The earth is exploding!
The surface speed will very soon reach the speed of light so some thing is wrong.
Space time bending may be able to explain it but I think there could be other explanations too. That is a good project for you to find out if you are a physicist.
@@leonhardtkristensen4093 Well no , see in general relativity we redefine what the acceleration means , in flat spacetime i.e no concentration of large amounts of energy/mass , the second derivative of your position is defined as acceleration but in curved spacetime things go a lot different the second derivative of position is now = acceleration- a new term which represents curvature in , this part of the equation is named the Ricci tensor , so if things go well the acceleration and the new term cancel leaving second derivative of position= 0
yeeeeei finally someone who really underdstands gravity. bin waiting a long time for this moment. pleased to make you acquaintance
Gravity doesn't bend light. It bends space and time. Light simply follows that curvature.
I finally understand the equivalence principle!
I still have some questions but like you said, I'll wait till the next episode of dragon ba... I mean of float head physics...
:(
RIP Akira Toriyama 😢
Its like listening to the story. It does not seems like watching the educational videos. This is because of mahesh sir incredible talent. Who all agrees ?
I’ll second that. 👍
Indians always make the best educational content on CZcams. You have no idea how much I learned from Indian CZcamsrs during my bachelor's degree in Computer science
Newton pours water into a glass. Einstein moves the glass up to meet the water.
😂good one
Einstein *accelerates* the glass upwards, without moving it, in a curved space time to meet the inertial water.
Seems like almost everyone has drunk the Kool aid
...drank ?
No crazy terminology, no textbook explanation…. Just comprehensive enough for a child to understand. The best so far
I don't understand it either. Is it legitimate to randomly replace objects and ground as the curve in the cone and hence they're accelerating upwards now. Huh?
Nice start… was hoping you get to the bend space part that counter the acceleration, but I think that’s the topic of the next part… 😊
Your videos are great and some of the best explanations of complex topics made easy to understand. Keep up the good work. 👍
Space does not bend what on earth are you talking about
Beautiful exposition. Thank you so much!
Gravity bends space not light. So what we are observing is light traveling trough bended space
So when you do Coriolis/Centrifugal forces in Newtonian mechanics, all that matters is inertial mass: there is no gravitational mass in the problem, its your choice of "moving" spatial coordinates... All of gravitation is the same...it's a choice of "moving" spacetime coordinates.
Yes but don't mix Newtonian mechanics with Relativity things become too complicated with the math of General relativity of tensor
ffs, there is no inertial mass. Mass is produced by inertia/EM waves without protonic mass.
SPACE/TIME was Einstein's way of dealing with the differentia of wavelength and wavecycle. C^2 is only relevant to dimensional analysis. It creates a holdable point.
@@Reaction1s yeah in most in the cases we generally take c = 1
Nobody is looking over the magnifician Cavendish' experiment that proves that gravity is only an interaction between masses!
Oh man that transition from independent prop to website add in between was good.
I'm so happy I stumbled upon your channel. You do such a great job of explaining things in a way in which it's easily - about as easy as physics can be anyway :) - digestible.
Great stuff, thanks!
Edit: and entertaining!!
@4:12 a/a = 1 is only defined if a is not equal to 0
Limit m tends to 0 would be a better way I guess
Correct
And no, you can not just cross out the m at the top and bottom. That would be dividing by 0 twice. You need to actually do something else, like looking a the limit when m goes towards 0.
@@goswinvonbrederlow6602 the limit when m-> 0 = 1
4:03
You can't just score throught both m from mg/m, if m equals 0, cause than you divide by 0. You have to take the limit for m approaching 0.
For m→0:
m≠0
a=lim F/m=lim m*g/m=g
Zero is a Logical NOT. It can be approached in mass, though never achieved by it. Once the"density" is enough mass is never achieved by light.
Love your explanations. And the follow on questions to be answered.
thank you so much this is one of the things ive been trying to understand so long. cant wait for that part 2
Einstein’s explanation is nice when we only focus on this small elevator, but the Earth is a sphere, so all objects on Earth are accelerating upward toward the sky as if the Earth were exploding? That is weird😅
There is no frame of reference where Earth is accelerating in more than one direction. But there are many different frames, and relative to them Earth is accelerating in different directions, one direction per frame.
In a centrifuge, all parts of the wall are accelerating inwards, but it is not shrinking :P
Einstein is right, and can be proved with a simple accelerometer. Hold one in hand, and it shows you are accelerating up, even if you don't move at all.
I quite like the reference to good old Newtonian physics.. the mass or its absence not making a difference to gravitational fall is a good thought-provoking beginning to this video.
I also recall another video of yours where it was the Newtonian concept of relativity (of uniform motion) that Einstein used to figure out the constancy of c (speed of light or causality).
I was in college before computers were born.. I am slightly partial to things classical - physics, art or music!
Replace mass with charges and you would really "thought provoke" Relativists. Maybe there's electric space time 😂
They seem to be obsessed with gravity only.
Light gets bent by water. Refraction explains light bending.
The limit of c is a philosophical dogma of Einstein that everything is energy and do not have a place in relativity of a world under unstopped motion!
You win my subscription.
Awesome explanation. Thanks
Wonderful series Mahesh. I'm very excited for the next video
Yes, even medium to large molecules fall at 9.8 m/s. We have many experiments about it! Brilliant explanation!
Mahesh is the only person in the world, who “speaks” with dead people and I’m sure he’s totally fine and adequate.
I have no idea who is Mahesh (at least for now), but the way he shows us the theoretical conversations between him and greatest/smartest people from the past, and the way how such conversations are built, what questions are asked… personally for me - I feel like I’m participating in the science debates…
Just amazing. I have no interest in science, but Mahesh, oh my lord, I can’t skip your videos in my suggestion tab. And I decided to subscribe.
For me, an adult guy, the Mahesh is the perfect teacher. Instead of “that is a law, now remember it”, we have this brilliant theoretical “discussions”. For younger generations this is a perfect approach to build interests I believe. This approach should be patented and named as “Mahesh’s approach in teaching” or something like that. Can be applied to any science subject, even to astrophysics.
Daaaamn, just imagine such conversations with still alive great people. For example discussing some topic with Mahesh, Neil DeGrasse Tyson and “Einstein” for example. It will be interesting, full of great questions, with a little touch of fun. Maybe that or similar things were done already… Great idea to check the whole channel!
Thanks Mahesh ☺️
Wow, that means a lot 🥲. Thanks for sharing thsi
I also had a discussion with Newton in my head when I learned about him in highschool: so you just multiplied kg’s by 10 and called it “Newtons” instead and got famous for that?? But also: how does a rock “know” how to fall down to earth? How does the earth communicate to the rock “iam this way over here”
He’s right, Mahesh… this is a wonderful Socratic dialogue way of investigating these thought experiments that pull the rug out from under our intuitive assumptions. You’re actually going to help people develop an embodied sense of the strangeness of what’s really going on with space-time…. That can have huge implications for our societal evolution!
The path will be curved even if the elevator is moving up with constant velocity, in which case there is no g. What am I missing?
@@tapashnandy3594I had the same question. Here is how I solved it. Imagine the setup where the speed of light is ‘c’, speed of elevator is ‘v’ and width of elevator is ‘d’. The amount of deviation x at a distance d comes out to be -vc/d. That is a straight line in x-d coordinates. Hence no curve. In case of acceleration, there is a curve.
Nice presentation! The enthusiasm is great.
Thank you Mahesh nobody has the explanatory power like you. Truly gifted teacher. Can't wait for the next episode to find out.
Mahesh is not pregnant,but he never fails to deliver (his insights)😂
\*epic facepalm*
Edit: My reasoning is wrong, and this comment isn't true. If you want to see why, go to replies.
6:05
The reason why inertial mass (in `F = ma`) and gravitational mass (in `F = mg`) are the same:
***This comment is edited, if you are confused by replies the orginal comment is at the bottom***
0. Assume that gravity accelerates everything, but not necesary at the same rate.
1. Imagine an apple with mass `M` close to some source of gravity.
2. Becouse of assumption 0 apple has some accelaration `A`.
3. Now imagine we split the apple into `X` **identical** parts (this is not possibile with a real apple).
4. This is a theoretical split, not actual cutting, the apple is still whole, we just think of it as `X` parts.
5. All these parts will fall with the same acceleration `a` (because they are identical).
6. The apple doesn't care if we think of it as one part or `X` parts, and will still accelerates at the same rate `A`.
7. Therefore all parts should also accelerate at the same rate.
8. So `A` (acceleration of the apple) and `a` (acclereation of each part) are the same! (Let's call it `g`).
9. But `M` (mass of the apple) and `m` (mass of each part) are different!
10. From `F = ma` we get that `F ~ Mg` and `f ~ mg` (`F` is force acting on the apple, `f` is force acting on each part and ~ means "is directly proportional to")
11. As you can see `g` doesn't depend on the mass of an object (apple or it's part), but it may depend on other factors (distance from earth or earths mass).
12. `F ~ mg` is just a less specific version of `F = mg` or `F = GmM/r²`!
13. All the lowercase `m`s are the the same thing (inertial mass).
14. But lowecase `m`s in `F = mg` and `F = GmMr²` are gravitational mass!
15. Therefore inertial mass = gravitational mass!
Capital `M` in `F = GmM/r²` is also inertial mass because of newtons 3rd law (if something is affected by force proportional to it's mass, then it should also inflict a force proportional to its mass).
***Orginal comment:***
Imagine an apple with mass 2.
The apple is falling with some acceleration g.
Now imagine we cut the apple in half.
The mass off both halves is 1.
The laws of physics don't care whenever the apple is whole or cut in half so both halves still fall with the same acceleration g.
Therefore acceleration doesn't depend on mass and it's always g.
From F = ma follows that gravitational force must be = mg.
That's a special case.
You could divide into unequal portions, then each mass will pull on the Earth slightly differently, as F=GMm/r².
The larger mass will receive and give out slightly more force than the smaller mass.
The gravity strength is g= ↓GM/r² for the Earth but it's g'= ↑Gm/r² for the apple 🍎 portions.
Earth 🌎 will fall up↑ to the apple at |g'|(«g).
The net relativistic effect is the apple falling to the Earth at g↓-g'↑=(G/r²)(M+m)↓ but as m«M, this is ~g↓ and g' can be ignored.
Can't you apply the same logic to magnets or charges moving in horizontal direction? And if you can, then it's not about gravity, so it doesn't tell us anything about gravitational mass and its connection to inertial mass.
@@thedeemon there is an assumption that gravity works on everything, while electric force only works on things that have charge. If one half of an apple has charge, and the other doesn't, they will fall differently.
@@The_Green_Man_OAP i am not sure if I understand your comment, but my reasoning is true for unequal portions: laws of physics don't care if you considier an apple to be one object, two halves, or bilion atoms.
If you go for F= GMm/rr I assume you mean M is mass1 and m is mass2.
If you have those 2 masses as the only influencing masses then G should be a force between them I believe.
If say mass1 is much bigger than mass 2 then wouldn't inertia decide which mass would move the fastest?
Thinking about the earth and the apple wouldn't that mean that the apple should move towards the earth?
Using this logic light with no mass should not bend towards the earth and why should the earth move towards the light?
Bending of space time around any mass might explain it but my brain isn't good enough to see that.
I think many explanations are made without thinking it all through.
Love your videos, Mahesh!
Nice technique in the open there... the "I used to think... but then I learned..." thing is a very nice way to open someone's mind to new information even if they currently have some misconceptions about it. Nicely done.
Are you the Khan Academy guy?
💀
1:46 Sir, i am unable to find the links to the videos mentioned just before this timestamp.
It is in discription
@@Master-zf5um hey ssup. Umm .. it initially wasn't
That blew my mind. Thanks for explaining it the way you did!
Thanks for mentioning at the end of the video about the paradox that the earth accelerates upwards but doesn’t expand. I never understood that so standing by for its resolution 😊
Well, I hope you're into maths, because that answer is general relativity.
For everyone asking: The force you are currently feeling on the surface of Earth is the lithoststic pressure of 3000 miles of molten rock and metal. The human body is too small to directly feel the Earth's gravity directly.
You "fall" through air and water on Earth, but not rocks unless you can apply more than 15000PSI to the rocks under your feet. You sink into mud because you can apply enough pressure to the mud, and you can sink in snow, but not ice for the same reason. The iron-nickel core of the Earth is at about 1,000,000PSI
The "force of gravity" is what keeps all this rock pressurized. The surface of the Earth could freeze solid about four billion years ago after it reached an equilibrium between how much pressure makes it accelerate "up", and how much gravity shrinks it "down".
But what is the movement of the crust- the acceleration? Some places it is very slowly sinking and some places it is very slowly rising, but overall it is not moving- it is not accelerating!
@@windwardproGravity is shrinking the volume of space occupied by the Earth, which is what is pressurizing it. Like a compressed spring, it gets harder and harder to compress the material, because the force keeping the molecules of the material from occupying the same location in space will push them apart. That's what you feel on the surface of the Earth. The repulsive electrostatic force that is pushing molecules apart from each other at least as fast as gravity is pushing them together.
If the rocky surface of the Earth was not pushing you up, you would fall down, through the center of the Earth, and probably go into orbit around the Earth's center of mass.
You are actually in orbit around the center of the Earth right now, but the ground keeps pushing you up into a higher orbit, so you never get any closer to the Earth's center of mass. (Just like you are on a rocket continously accelerating away from Earth's center of mass at 9.8m/s² to maintain the same distance away from the Earth's center of mass. The geometry of spacetime itself is curved, and this just looks like motion in 3D space.)
(Also, I looked it up, and the radius of the Earth is actually between 3,950 and 3,963 miles depending on latitude.)
@@windwardproBasically, imagine holding a pumb bob, a weight on a string. The string will be pointing directly at the Earth's center of mass (assuming a perfectly spherical non-rotating homogeneous Earth). Nearby, use a plumb bob to draw a line that points directly at the center of mass of the Earth from that location. Now, to the naked eye, these vertical lines will appear to be perfectly parallel lines... but... when you extend these lines 4000 miles down into the Earth, these lines will intersect! They are not parallel, they are two sides of a very long thin triangle with one vertex located at the Earth's center of mass...
These straight lines... *_ARE_* straight lines... it's _the space between the lines that shrinks_ the rocks below our feet are getting squished on the side closest to the Earth's center.
If you imagine using four plumb bob lines to be the corners of a square, the area of the square will shrink as you go down towards the center of the Earth, and would shrink all the way to zero area at the Earth's center of mass if all of Earth's mass was concentrated there at a point.
Because the Earth's mass is spread out over 260 billion cubic miles (one trillion cubic kilometers) the amount of gravity _decreases_ below the surface. Yes, you weigh less inside of a cave. The core of the Earth is experiencing weightlessness, and is floating in orbit around the Sun. The mass of all the rocks and metal and stuff in and on Earth is essentially "pulling up" evenly on the center of the Earth. The 4000 miles column of rock on one side of the core, pulls by the same amount as the 4000 miles column of rocks on the opposite side of the core, and the two sides cancel out to zero. (Repeat for every direction.)
So, yeah, the Earth's core is incredibly pressurized, and mostly weightless. Gravity keeps it pressurized, gravity does not give it weight.
Save yourself 17 minutes: gravity bends the fabric of space not light itself. You’re welcome! 😉
It’s not just about the destination but the journey.
Light follow the shortest path
Imagine being this insufferably obnoxious and getting it wrong anyway.
We care about the delivery of the video, not just the answer. So no tnx.
@@thomasshelby1922 100%
Great video as always. Can't wait for part 2 - I cannot "see" why / how someone is pushed up by the ground, and I still cannot get how the mass of a large body really "bends" space(time) to apparently "deflect" the light to begin with 🙂
It's not pushed up his explanation is bit ortodox if we are pushed up on this side of planet how in same time object are pushed up on other side of planet
Very much appreciate your fantastic tutorials.
Why can't people make a video without showing their face all the way through it?
more engaging with a face and will get more views
I think it's fine. Don't like it? Don't watch😂
What are you asking?
Mind 🤯 and I could actually understand it. Thank you so much! Instant subscription.
I'm excited for your video on double slit experiment.
Please start a quantum physics series and also explain that what is photoelectric effect of Einstein
please continue the series it much fun with impeccable knowledge
Love your style Manesh!
Very nice video and explanation!
Love your physics videos!
Not a clue what you're referring to, Gohan, but I love every episode of this series a LOT!
A video that left me on the edge of my seat the whole way through. And he ends it on a cliffhanger. Brilliant.
Best science explanation vids on the internet. Tk u.
Bro i love your enthusiasm for physics. Easy sub
Mahesh, in the example at 14:45, if the lift is moving upward at a constant velocity (instead of accelerating), will the light still seem bending towards the lift floor?
That's what I thought
i don't think so.as it would be at a state of vertical rest in relation to the lift.
No
This works for accelerating frames
The issue is this is a thought experiment and for it light normally has to be slowed down drastically to normal everyday speeds
At the speed of light it happens but the effect is just too insignificant to measure or observe
Sir it can happen that in the conversation of energy if suppose K.E to sound energy. When a ball just reaching the ground and height becomes almost zero, speed tends to zero and the conversation not yet started i.e K.E to S.E therefore at a moment the K.E and the S.E becomes zero as the conversation not yet started simultaneously. If then where the energy gone? and if not what is happening?
The velocity does not tend to zero as the ball approaches the ground. What have you that idea?
The ball accelerates until it touches the ground. Now, you can apply either elastic or inelastic collision calculations.
The resolution to the question in the cliffhanger is one of or a combination of: time dilation, length contraction or relativety of simultaneity)
What do you use for animations??
I can comprehend about 1/3 of the things you share, but am jazzed by 100% of your enthusiasm.
Love this channel... where can I donate?!
Not a stientist here.Thanks for these, Brother. Love these deep dives made for designers like me. I have a feeling I'm going to learn more about the Higgs field and space-time just so I can understand mass and time. Down the rabbit hole we go. Cheers mate.
Waiting for part 2. When it will be live??
Continue please I have so many questions.
Hey, I have a question, its weird tho. If the gravity bends the space-time and creates like a hollow bended downwards space, like we see in Einstein's theory. Wouldn't be the object ultimately fall into its own bended space Time. And if the object doesn't fall, wouldn't be the poles of earth have a greater radius than the equator?
This is mind blowing
What a intuitive way to explain the concept i think i am now intesrested to go into deep concept behind it
Both Force and Energy are abstract constructs to make calculations easier
Wow!! Im astonished!!
That transition to Squarespace was so clean im not even upset lol.
Bro, I too think the same thing every day. What if I am in a static position because if I look, everything around me is moving, our planet Earth is moving, our entire solar system is moving and our entire galaxy is also moving... That's really mind-blowing, brother. Thanks for making these types of videos for us. And sorry for the bad English..😶😶
If you're in a static position (relative to what?)
If it's the Earth that you're stationary relative to, then the Earth is not moving relative to you.
I've always pictured the reason that a hammer and feather fall at the same rate was due to inertia. The greater mass of the hammer simply takes longer to accelerate. I've also always thought it was weird that they just happened to be EXACTLY inverse, and cancel each other out.
Great video, thanks!
Can't wait for the next video
I need more explanation
It is a mindblowing discovery until I imagine a person on North Pole and a person on South Pole. Both jump at the same time. How is it possible for the Earth to simultaneously accelerate in opposite directions?
ty
I smell calculus around that m/m cancellation. Maybe that's where a more satisfying justification lies. As always, great stuff Mahesh!
Ah noooo! I was anxious to see how this all works in 3 dimensions in all directions on a spherical mass! I knew you would get there but we just got a tease at the end lol. Cant wait for the next video and have a better understanding how this all works together! Youre a great teacher! Even if we dont all have the time, dedication, or ability to get into all the actual math. It is very exciting to understand conceptually how the universe works.
ngl you are really smooth with the promos
Where can I start to learn about these things more formally?
That depends on what your goal is, and what level you're at right now.
For the _most_ formal way to learn about these things, and a way that gives you something you can show people to _prove_ you've learned it and potentially acquire gainful employment because of that knowledge, go to your local university (or whatever non-local university you prefer) and spend anywhere from several years to about a decade there. (And about $100,000)
If you're fairly well-versed in popular science and wanting to learn more detail in a (mostly) non-mathy way for your own personal growth and expansion, I highly recommend Sean Carroll's CZcams series "The Biggest Ideas In the Universe" for a sort of "one level up from typical science communication". czcams.com/play/PLrxfgDEc2NxZJcWcrxH3jyjUUrJlnoyzX.html
If you don't mind math but don't want or need the absurdly expensive sheet of paper that proves you learned this stuff, MIT's OpenCourseware has basically a full-on top-tier university education in most topics in science and technology, completely for free. How much you get out of it is entirely up to how much you put into it, but nobody is likely to hire you for having done it.
Can I get some help?15:55 why does light bend down?
I mean, if a photon is emitted when the elevator hits the wall, why would it’s path appear bent from the wall’s perspective?
We deal with light, while, I drop a pencil, and it bounces 4 ways and is in the weirdest place.
Lemme see if i can help here
Provided there are no other sources of acceleration, for an observer in an accelerating frame light doesn't actually bend "down" per say, it actually just bends away from the direction of motion of the accelerating frame
The equivalence principle tells that barring tidal forces it's impossible for an observer to tell the difference between an accelerating frame of reference far in space and a gravitational one here (say being here on earth, provided the accelerating frame accelerates at 9.8m/s²)
More or less things the physical behaviour of objects in a gravitational frame and an acceletating one will always be the same and unless using other forces you can tell which is which
Now if you imagine yourself in an accelerating frame (say a rocket) in space you'd find out that whilst the rocket accelerated in one direction, inertia would cause you to sort of move in the opposite direction (very loose description of inertia and in case you don't understand inertia a simple example would be that backward jerk you feel in a car when it starts moving or it accelerates or the forward jerk you feel when you apply the brakes, note too that inertia is not a force just an opposition to it)
Gravity envisioned as a force or not always acts inwards towards the mass generating it, this is what we've come to call "down"
Gravity causes things to fall "down"
So in a gravitational field the source of the field will always be your "downwards" direction
In space that is not necessarily the same thing
"Down" would be more or less where the direction in which inertia causes you to move towards
So if we apply the equivalence principle to light in a rocket or accelerating frame, we'll notice that since light in an accelerating frame would seem to bend away from the direction of motion, and towards the "direction of inertia" which would be our "down" then light in a gravitational field would seem to bend towards the source of gravitation which is the massive object giving the illusion that light bends "down"
I have a question.. only related to this video due to talking about light, more specifically, photons. We often talk about the size of photons.. do we know their actual size? What do I mean? With length contraction and the other things that happens as you near or reach the speed of light how does that effect the photon itself? Does it not care about these effects due to it being mass less? Or are they larger than a planet if you were able to stop one?
Einstein is right, and can be proved with a simple accelerometer. Hold one in hand, and it shows you are accelerating up, even if you don't move at all.
I have heard these facts all my life. Mahesh explains it in a way that is helpful to me. I am starting to 'get it' (slightly). We each learn in our own way.
"Your results may vary."
4:04 in maths you can only cancle them when assuming m≠0, but in case of light it is zero, so in Newtonian physics acceleration of light towards mass is just not defined
You’re the best teacher ever 🔥🔥
Sir please another question I have that is light is in perpetual motion and it also doesn't violate the first law of Newton i.e any object either remains at rest or in uniform motion at a constant speed moves forever until it is effected by an external force?
What exactly is your question?
Any object that is not affected by any force will move perpetually relative to some reference frame.
light cant be at rest, so drop that. but why is there a question? CMB for instance.
Haven't you answered your own question? Object in motion stays in motion, light is in motion so it stays in motion. What is it specifically that you're having trouble grasping?
I don't understand what you are telling but clarify my question that is perpetual motion means the violate of the law of conservation of energy and also the second law of thermodynamics i.e if we put certain amount of energy let says 10 kg and then the conversation of energy happen let says water energy to electricity then we can't get more than the energy we given.
@@narfwhals7843the speed of light is always same for all the observer regardless of their reference frame???
Suppose we have a black hole and we insert sufficient amount of positive charge in it and put a proton on it's event horizon. The positive charge inserted is sufficient to counterbalance the gravitational pull of the black hole , then will the proton on the event horizon be pulled inside the black hole??
If no, then is it possible to continue this process and reach the centre of the black hole???
Good question!
There are some _practical difficulties_ with accomplishing this... but let's pretend that we can overcome those difficulties.
So....
Let's also assume that the black hole is not rotating, and your test charge is exactly lined up with the black hole's center of mass, so we don't need to worry about magnetic fields.
Hmm... assuming a "Classical" Swarzschild eternal vacuum solution black hole without any messy QM stuff...
Hmm... hmmm... what exactly do you mean by "put a proton on it's event horizon"? Because the answer depends on the exact details of this. Assuming that by "event horizon" you mean the location in space, a certain constant radius from the black hole's center of mass, where a very distant observer will never receive any escaping light (or anything else) originating from beyond that horizon.
So... "no", but... there are a lot of details I skipped over.
So, assume that this takes place in an otherwise empty universe with just you and your positive electric charges.
Because, presumably you are charging up this black hole by dropping protons (or whatever stuff with intrinsic positive electric charge) into it... and as the black hole's positive electric charge increases, you are not going to be able to get new additional positive electric charges anywhere near your black hole. (In fact, if you have any neutral atoms anywhere near the black hole, just the electric charge will shreed the electrons from the atoms, repell the positive ions, and neutralize the electric charge of your black hole.)
The strength of the electromagnetic force is orders of magnitude greater than the "force" of gravity.
There will be a limit, some distance outside the black hole's event horizon, when you can't get a positive electric charge to "fall straight down" past the event horizon.
If the falling proton can cross the event horizon, it's stuck forever, if it can't quite reach it, it's either going to be at equilibrium (and remain at a constant radius from the center of the black hole) or be repelled away from the black hole and escape to infinite distance.
I forgot to mention, I'm treating the test charge "proton" in your question as though it is a microscopic classical sphere with mass and charge, and not an elementary particle with intrinsic magnetic moment and gluon binding energy.
There's more... there's a lot more details to consider before I even get to using the EM tensor and tidal effects.
So... classically, beyond the event horizon of a black hole, spacetime is still locally continuous. That "event horizon" only exists in the coordinate system of a _very distant observer_ . Like a mirage, you don't "see" that "event horizon" when you are actually there at that location. (You will see an event horizon in the direction of the center of the black hole, and you will never see yourself cross it.)
Blah blah blah... you can build a pile of positively charged matter outside of the black hole's event horizon which will never fall in... but if you _could_ build such a structure within the black hole's "event horizon for a distant observer", you still can't escape from the black hole by climbing up it... because to climb up using the electromagnetic force... because atoms are held together and repelled with the electric force... you can't push yourself "up" faster than you can push something down... and the fastest that two electric charges can push on each other is the "speed of light" (litterally the definition of an electromagnetic wave).
The _coordinate system_ of anything beyond the event horizon of a black hole is moving away from a distant observer faster than the speed of light (as they say). The "force" of electric charge propagates at the speed of light, and can't catch up with the difference in movement between a coordinate system "inside the event horizon of a black hole" and the coordinate system of a "very distant observer far away from the event horizon of the black hole". If you're familiar with how proper acceleration works in Special Relativity, you get a Rindler "event" horizon far behind you while you're experiencing proper acceleration, because light can't catch up with you as you're running away from it with enough head start.
I hope CZcams doesn't loose this reply, I don't want to rewrite this. CZcams's comment system is broken on the back end. The comment database isn't replicating between all of Google's data centers.
I remember when I realized the difference between gravitational mass and inertial mass. I asked why they were equal, and hardly anyone understood the question much less could answer it. The best I got was "they have to be the same thing."
My first thought was that mass wasn't actually the fundamental property causing gravity. Instead, I thought what if gravity acted on momentum, and mass was just the result of mathematical simplification. It certainly could explain why massless particles (light) are affected by gravity because they still have momentum. But then I actually learned about Newton & general relativity.
I don't particularly like the description of space being curved. Yes, the math works--not arguing that. But the mechanism of curvature causing movement seems to be a problem for particles of zero size. With zero size, the curvature of space seems like it would have no effect on them because they have no size to interact with that curvature.
Instead, I choose to think of space like a fluid. That fluid can move. It can expand or contract. Or, it can be thought of as graph paper that moves, expands/contracts like a fluid.
The difference is that mass acts as a 'drain' for the fluid of space. Space flows into mass like water flowing down a drain. Instead of thinking of a 'gravity well' as a location where space has curvature causing light to bend because it's traveling in the a straight light in bent space, think of it as light being a boat moving in a straight line in a moving stream that pulls it 'off-course.' For a black hole, the space is flowing/accelerating into the drain faster than light can move through space to escape.
Now, the elevator thought experiment is intuitive:
- 'Inertial effects' occur whenever you accelerate with respect to space. You feel it as a force because to move where you are in space, a force is needed.
- In deep space, you are accelerating through the fluid of space, and you feel the inertial effects.
- In gravity, space accelerates through you, and you feel the inertial effects.
- The delta-acceleration of you & space are identical, so all math, sensations, & physics are identical.
- No, the ground is not accelerating up and pushing on you. Space is flowing through both you and the ground. You want to move with space (aka, free fall) but the ground stops you, hence you feel inertial effects.
Also, the effect of gravity on particles of zero size is also intuitive. All particles, regardless of size move with space, or they require a force to have an acceleration with respect to space.
The idea of space being a fluid could also explain the universe's expansion. Space itself could be self-replicating. Wherever there is space, it creates more of itself, meaning if you have two points on that graph paper that are adjacent at time = 0, then at time = 1, there are now points on the graph paper between the two initially adjacent points. How? Don't know, just like how space disappears down the drain caused by mass. Just go with it.
In a volume like the solar system, there isn't enough empty space between the sun & planets to generate enough new space to throw off the balance, and it behaves what we could call 'normally.' But with enough empty space between masses, such as over an entire galaxy, then things don't act normally, which could eliminate the need for dark matter. Again, there is even more space between galaxies, and they accelerate apart, because as expected, the increase in space from its self-duplication would do that. It isn't that anything pushes them apart (dark energy), it's just that there's an ever increasing amount of space between them.
I do not see the gravity demonstration links in the description? :/
Sorry, I just added. Check again!
got em @@Mahesh_Shenoy, man that was fast! I just finished the video and boooomm the links are there.
The lightest particle you can put on an obvious ballistic trajectory is the neutron. See: "Ultra Cold Neutrons (UCN)"..which are around 7 m/s in the lab (2 mK).
Nice video! I have a persistent question. In a vaccum, both ball and feather will fall at the same time. But isn't gravity (weight) mg? In the video, you had shown that a=g under free fall. But gravity will still act on the ball? And it is mg. It's somewhat confusing. Please clarify and explain.
Thank you
This is my new fav channel
In Newtonian physics, the force g is calculated using the formula M1*M2*G/R^2 so if the mass of light is zero shouldn’t g be 0?
Yes, in Newtonian mechanics it should (the force should be 0).
Yes that seems to be correct.
But Newton's 2nd law also says that force is rate of change of momentum.
Photons have momentum, and it does change due to gravity, which itself as we will see is due to curvature of space time.
Relativity of simultaneity saves the day again!
I think I saw you with Aanand Srinivas sir, both of you are really a true master of physics..❤👑❤
Thanks for the video, keep up the great work! I have a question about the seeming incompatibility of general relativity and quantum mechanics. This thought came to me when reading about theories of the universe as a computer or mathematical universe. If we take the analogy of a computer directly and think about the fundamental features of any universal computer, as laid out by the greats like Turing, it seems like a computer must consist of at least two general components, namely, hardware and software. The hardware consists of memory and the processor/logic gates, while the software consists of information encoded in the memory. In modern digital computers we have CPUs and GPUs as processors, RAM and solid state drives as memory devices to store information, and then software which is a specific state of the memory (i.e. a specific configuration of bits/ones and zeros). We all know that software, i.e. the information component of a universal computer, has very different properties and constraints compared to hardware. Software is like a mathematical object, exists outside of space and time in a sense, while the processor is not information but a physical object, extended and operating in space and time. So this got me thinking, are general relativity and quantum mechanics in some way equivalent or analogous to the hardware and software, respectively, of a universal computer that IS the universe (and we are a part of that computer/computational process observing it from inside)? Quantum mechanics (the software) is precise, defines all of the measurable events that occur in the universe (since it governs particles and we can only measure particles, we could never measure things like gravity in a universe without particles), but is only valid on small scales, while general relativity (the hardware) plays a background role (like the hardware in a computer, it doesn’t change the inputs/outputs determined by the software but determines things like lag/frame rate, thermodynamic efficiency). So perhaps gravity bending space time is like a core in a CPU getting overloaded as the number of processes/computations/particle interactions in a certain volume of space-time, causing a “lag” in that local region of the universe. My doctorate is in molecular biology so I don’t have sufficient formal training in math and physics to explore these insights in further detail but intuitively it seems like there could be something there worth exploring.
Very conversational/easy to listen to. A few technical problems, including that Newton never cold have made any statement about how fast a BASKETBALL would fall...
Tremendous teaching energy from our friend here that’s roughly equal to maths x cosmology²
Einstein never said the ground is accelerating up. That’s a common misconception. Happy for someone to please show me a quote from Einstein about this.
I AM VERY EXCITED FOR YOUR EXPLANATION OF SPACETIME CURVATURE
This ended my sleepless nights. And makes one ask new questions that make even more time dilated nights,
What if you drop heavier items. At what mass does the 9.8 stop working?
Okay, so the question that the photon travels more distance in the upper curve than the lower curve; I think that, the photon is also a transverse wave so it is going up and down in its path, so the distance would be the same, I guess....
the wiggling is not in physical space.
in the diagram the path looks curved, but wouldn't the radius of this curve be similar to the radius of the earth, and over a short distance look like a diagonal/ straight line?
how does the acceleration work if on the other side of the globe? how is everything accerating outwards? doesn't acceleration have direction/ require movement?
There's two notions of acceleration in physics. Relative acceleration, the second derivative of position. And absolute acceleration: whether you experience a force.
The radius would be much larger than the radius of the earth. Light passed the earth in much less than a second, so it feels the earth acceleration much shorter than a second. So, in 1 second the light goes straight for 300000km but bends much less than 9.81 m.... That's huge radius. Light only noticely bends near big stars.