How Does Gravity Affect Light?

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We know that gravity exerts its pull on light, and we have an explanation for why. Actually, we have multiple explanations that all predict the same thing. And at first glance, these explanations seem to describe completely different causes. So what is the true connection between light and gravity, or is truth, in fact, entirely relative?
Sabine Hossenfelder's episode "Do we travel through time at the speed of light?"
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Hosted by Matt O'Dowd
Written by Matt O'Dowd
Graphics by Leonardo Scholzer, Yago Ballarini, Pedro Osinski, Adriano Leal & Stephanie Faria
GFX Visualizations: Katherine Kornei
Directed by Andrew Kornhaber
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End Credits Music by J.R.S. Schattenberg: / @jrsschattenberg
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[@inlegivel]

I love how my brain is now conditioned to predict when he's going to end the video. It can sense the "space time" coming.

[@Woffenhorst]

He does change tone and cadence, so it's quite apparent when you know what to listen for.

[@SodiumInteresting]

yes you can tell hes wrapping things up before he says it

[@SodiumInteresting]

no answering of questions this time :/

[@RWZiggy]

Yes but that relativistic model broke down because we now know the comment discussions exist in a superposition of being not there and there. We need a quantum spacetime model for the comments.; and don't bother to ask the string comment modelers, they've been working on it for years and have nothing.

[@shaunyman]

Space time make brain go brrr

[@realBeltalowda]

My life has been made immeasurably better due to the very fact that PBS Space Time exists.

[@marcussmart3275]

It would be even better if I were aware of their new videos. I never recieved notifications from them.

[@damienhunt4264]

YES

[@chrisstevens-xq2vb]

Ha! This is pure nonsense. How can mass bend an imaginary mathematical grid?

[@AdilKhan-gd2sc]

Gravity, “I met space and time , and couldn’t tell one from the other.”

[@Pope_iShovel]

The best part about this channel is that some of us can watch it endlessly due to not understanding much. Me for example - I could rewatch any episode, and it would be fairly close to the same as the first time watching.

[@Sayuri-cr8cy]

Props to the graphic designers on this show lol without them I’d never understand anything

[@danielmurphy1982]

stepped it up a level in this video hey!?

[@SolaceEasy]

But they had a big mess in this episode: The eclipse of the sun behind Sir Eddington was wobbling.

[@natanielbumppo6961]

Ah c'mon even with them...

[@fernandogajo8800]

True story

[@natanielbumppo6961]

@COVID 19 noone does

[@gracekerns7534]

when you use the wrong assumptions to get the right answer: "task failed successfully."

[@Greitone]

An everyday thing in computing, funnily enough

[@jawbone83]

#engineering

[@chrisray1567]

The foster fail of science.

[@kamikeserpentail3778]

It's like being lost, deciding to turn around and retrace your steps, and finding that you end up at your destination instead of your origin.

[@liwoszarchaeologist]

story of my life

[@r-pupz7032]

That was a particularly good episode from my frame of reference, just been binging old episodes & finally got my head round relativity enough to really appreciate the new info. I only understand it relative to my prior ignorance, but that is more than enough for me. Thanks Matt & PBS!

[@frankdimeglio8216]

@@hyperduality2838 Consider the man who IS standing on what is the Earth/ground. Touch AND feeling BLEND, AS ELECTROMAGNETISM/energy is gravity. The Earth is blue, AND the sky is blue. SO, overlay THE EYE in BALANCED RELATION to/WITH what is the Earth ! (The BLUE SKY is ALSO translucent !) GREAT. Objects fall at the SAME RATE (neglecting air resistance, of course), AS E=MC2 IS F=ma; AS ELECTROMAGNETISM/ENERGY IS GRAVITY. NOW, we proceed to the next step. The stars AND PLANETS are POINTS in the night sky. Invisible AND VISIBLE SPACE in fundamental equilibrium and BALANCE IS the MIDDLE DISTANCE in/of SPACE in fundamental RELATION to the universal fact that E=mc2 is F=ma. The Earth (A PLANET) involves BALANCED electromagnetic/gravitational force/ENERGY, AS E=MC2 IS F=MA. The Earth AND the Sun are CLEARLY E=mc2 AND F=ma IN BALANCE pursuant to what is the BALANCED MIDDLE DISTANCE in/of SPACE !!! Consider what is the speed of light (c), AND consider WHAT IS THE SUN. NOW, think about what is the Earth. Importantly, outer "space" involves full inertia; AND it is fully invisible AND black ! Great. Accordingly, the Earth is in BALANCE with what is the Sun, AS the orange Sun represents what is LAVA ON BALANCE !!! Great. (Notice the role and relation of what is the EYElid.) The MIDDLE DISTANCE in/of SPACE is universally balanced to/with what is THE SUN AND the speed of light (c), AS E=MC2 IS F=MA !!! Gravity IS ELECTROMAGNETISM/energy. Notice the black space of WHAT IS THE EYE, as the stars AND PLANETS are POINTS in the night sky IN BALANCED RELATION to/WITH what is the MIDDLE DISTANCE in/of SPACE; AS E=MC2 IS F=MA !!! The FULL DISTANCE in/of SPACE is truly linked AND BALANCED to/with what is THE MIDDLE DISTANCE in/of SPACE, AS E=mc2 IS F=ma IN BALANCE !!!; AS ELECTROMAGNETISM/energy is gravity !!!!!! GREAT !!!! Notice that the viscosity of lava IS BETWEEN that of WATER AND the Earth/ground ! The orange Sun is the same size as THE EYE. Outstanding. E=mc2 IS F=ma as what is the BALANCED MIDDLE DISTANCE in/of SPACE !!!! GREAT !!! LOOK at the progression AND THE BALANCE regarding FULL DISTANCE in/of SPACE, MIDDLE DISTANCE in/of SPACE !!!, AND A POINT (ALSO c) !!! E=mc2 IS F=ma !!!, AS ELECTROMAGNETISM/energy is gravity ON BALANCE !!! GREAT !!! It ALL CLEARLY makes perfect sense. BALANCE AND completeness go hand in hand. E=mc2 is F=ma. This NECESSARILY represents, INVOLVES, AND DESCRIBES what is possible/potential AND actual IN BALANCE, AS ELECTROMAGNETISM/energy is gravity !!! Gravity IS ELECTROMAGNETISM/energy ON BALANCE !!! TIME is NECESSARILY possible/potential AND actual IN BALANCE, AS E=mc2 is F=ma; AS ELECTROMAGNETISM/energy is gravity. INDEED, TIME DILATION ultimately proves ON BALANCE that ELECTROMAGNETISM/energy is gravity; AS E=MC2 IS F=ma !!!! INSTANTANEITY is thus FUNDAMENTAL to what is the FULL and proper UNDERSTANDING of physics/physical experience, AS E=MC2 IS F=MA ON BALANCE !!!; AS ELECTROMAGNETISM/ENERGY IS GRAVITY !!! GRAVITATIONAL force/ENERGY IS proportional to (or BALANCED with/as) inertia/INERTIAL RESISTANCE, AS E=MC2 IS F=ma; AS ELECTROMAGNETISM/energy is gravity. Gravity/acceleration involves BALANCED inertia/INERTIAL RESISTANCE, AS E=mc2 IS F=MA; AS ELECTROMAGNETISM/energy is gravity. Accordingly, the rotation of WHAT IS THE MOON matches it's revolution !!! Notice what is the fully illuminated (AND setting) Moon in DIRECT comparison with what is the orange Sun !!! Again, the stars AND PLANETS are POINTS in the night sky. A given PLANET (including WHAT IS THE EARTH) sweeps out EQUAL AREAS in equal times consistent WITH/AS F=ma, E=mc2, AND what is perpetual motion, AS ELECTROMAGNETISM/energy is gravity; AS E=MC2 IS F=MA ON BALANCE !!!!!! GREAT. Gravity IS ELECTROMAGNETISM/energy, AS E=MC2 IS F=ma !!! By Frank DiMeglio

[@612Tiberius]

I have a slightly different definition of the Theory of Relativity: *"That temporal phenomenon whereby time seems to slow down whenever relatives come to visit".*

[@unitedspacepirates9075]

Einstein married his first cousin to study relative tity.

[@meltedyakkystick3891]

The effect is greatest with in-laws specifically

[@ccelik97]

@@meltedyakkystick3891 greatest with the black in-laws

[@TrixieWolf]

Counterexample: this pandemic keeps us from seeing relatives, yet appears to last forever.

[@HakunaMatata-os1og]

You can counter this apparent "relative-induced" time dilation with a liberal application of alcohol.

[@MargoMB19]

I love that whenever there isn't a comment section at the end of the video, the explanation always has something to do with the video's topic. It's such a little thing that really adds to the quality of the whole thing, imo.

[@delusionalluy]

PBS Space Time: * post a video * Me: Cool! it's time to melt a few neurons

[@crowsnest8295]

What worries me is when I Actually understand something

[@Partho_The_King]

My worry is that when i understand something and get completely lost in the other second

[@itarry4]

Slaps head to make it accept what it thinks it might understand possibly maybe.... Nahhh missed it again dam.

[@maxmusterman3371]

Casting involves melting

[@dexterrity]

The closing statement is pure Space Time poetic brilliance

[@pinfarmer]

Thank you for doing these! I feel spoiled that I've had 4 years of this consistently excellent content.

[@Eyerleth]

"These giant alien spiders are no joke." You got your FTL in my Space Time! You got your Space Time in my FTL!

[@dddd6606]

Marvel: "Infinity War is the most ambitious crossover in history" Matt & FTL: "hold my beer"

[@Izerion]

I giggled when I heard that reference

[@randywoodworth4022]

2 great theories that taste great together? Sounds like "peanut butter" physics to me.

[@ebigunso]

I noticed the reference too. Nice Easter egg.

[@T3sl4]

Good thing I've got a quantum cloning bay for just this eventuality!

[@paulperkins1615]

Some physicists think that the idea of time flowing at different rates is more confusing than helpful in explaining relativity. They prefer to compare time to the length of a path, but a path through space-time, not just space. Different paths that connect the same two points in space-time can be different length-analogues.

[@DJHastingsFeverPitch]

Your point at the end makes an interesting point about the function of beliefs in general. That is, that from the perspective of human beings, beliefs only need to be accurate enough to successfully predict future events. In that sense, beliefs don't necessarily need to give a comprehensive account of that aspect of reality, but merely need to give an accurate enough account to be comprehensively pragmatic.

[@kurtweinstein8450]

Been doing a history of science and technology graduate class this semester. It focuses on Latin America from 1500 to 1900. As you can imagine it isn't the standard history of science. One recurring theme is inescapable bias and lack of truly objective facts. Your conclusion about how opposite interpretations can be valid and the universe being relative is a wonderful example of how science is about explanatory power rather than a set of ideal, objective facts to be "discovered'.

[@devinfaux6987]

Wait, shouldn't the spider see the laser light *blueshifted* in the first scenario, since the laser gun is moving towards it? Or is this not the case because of the accelerating reference frame?

[@kristynicole6201]

The spider is holding onto the spacecraft, so the craft is carrying the spider away from the point at which the laser fired from, and accelerating.

[@joepgeuskens527]

Note that the spider has grabbed onto the spaceship, so it accelerates along with it. If it was at rest, it would indeed observe blueshifted light

[@WilliamDye-willdye]

At 2:40 the spider moves from the front of the ship towards the laser (as the rewind at 3:03 makes clear), but in Hollywood at 3:40 the spider doesn't move. I hate to nitpick such excellent graphics, but the discrepancy does seem a bit confusing when the question involves redshift vs blueshift. And yes, they got Earth rotation backwards, but that one doesn't matter.

[@Biskawow]

Depends if it was beam laser or pulse laser

[@BBCCheese]

Yeah I was confused too because I thought the spider was supposed to be an inertial observer. Apparently not though

[@riverground]

I believe this is the first time Space Time has made me go "Hmm" in a way that is both simultaneously satisfied with the explanation yet dissatisfied with our incomplete understanding of the universe.

[@Fortune090]

That "Hmm" is actually what got me into the field, hah. Not just from the curiosity in science in itself, but also being dissatisfied with how we don't know or how seemingly close we are in figuring out so many things about the universe. Just compels me to contribute in any way possible!

[@theobvious1958]

humans are dumb

[@stormnova9757]

@@theobvious1958 Yet simultaneously smart.

[@RyuuKageDesu]

I am relatively uncertain about our shifting times, but I can easily observe the gravity of how enlightening these episodes are.

[@gvidasbrilius2385]

Ok, I watch quite a lot of your videos, but this one is weird because i actually feel like i understood it all. Props to you guys for making such worth-while entertainment, I bet it's a hassle to make it understandable.

[@andyroye5622]

I’m going to have to watch this again, it ain’t heavy it’s just the slowing of time.

[@jaredf6205]

There's that word again! Heavy. Is there a problem with Earth's gravitational field in the future?

[@SahilP2648]

@@jaredf6205 Earth's poles could flip and that would be a complete reset for the human civilization as we know it lol

[@Canadian_Ry]

Spaceballs reference?! Yes please, and thank you!!!!

[@moosemaimer]

He neglected to explain how alien spiders react to plaid-shifted energy weapons though.

[@T3sl4]

"We're not just doing this KZbin video for the views. We're doing it for a s***load of views!"

[@jimmym3352]

Glad I wasn't the only one to catch that. Had to scroll a fair ways down to see someone get this reference. Now we need an episode of him explaining how the power of Swartz works.

[@lordcirth]

I thought it was an FTL reference. Was FTL's a reference to Spaceballs?

[@WWLinkMasterX]

@@lordcirth The spiders are FTL, the spaceship made out of a Winnebago is Spaceballs.

[@swbaxter13]

"What I told you was the truth... from a certain point of view." -- Obi-Wan explaining cosmology to Luke.

[@winstonknowitall4181]

John Mitchell predicting bending of light in 1783 was like this giant supercomputer in a Douglas Adams' novel that gave the correct answer to a not-yet-asked question.

[@jeffpkamp]

Light has no time to lose? Sounds like someone has or is going to have kids because that is a solid dad joke right there.

[@DrZedDrZedDrZed]

I wanna be Matts kid.

[@BiswarupRay]

Next 007 movie, "No time to lose"😎

[@altrag]

Or an infinite amount of time to lose, depending on how you look at it.

[@-slt]

"Travel light, you have no time to lose"

[@LeopoldoGhielmetti]

It's the reason why light travels at the fastest possible speed.

[@leokovacic707]

In gravitational fields Time slows and space flows. This was one of the more intellectually beautiful episodes.

[@thephantomeagle2]

Great video and very informative. Thanks. I love how you used Eagle 5 as your spaceship. Easily one of my all time favorites and recognized it instantly.

[@danieljensen2626]

As a physics PhD student who doesn't do GR, these past couple videos on this subject have been eye opening. Now instead of acceleration I'm just thinking about rotating 4- vectors.

[@nelutumohan2833]

“This alien spiders are no Joke..” even tho all they do is being a target.. hilarious xDD

[@Vix2066]

Oh no not the aliens spiders again😣😖 worrying lol

[@dernthehermit3541]

It's all right, I have a Clone Bay so I won't lose any crew

[@greyduck4965]

it's a reference to the game FTL

[@chadgessner4412]

I think its a Faster Than Light reference.

[@DashRantic]

they're not targets if you can't hit them!

[@rodrigodamatamelodasilva911]

Great episode. Just one thing: actually this famous solar eclipse was observed from Sobral, a city in Brazil. The group (including Eddington) that went to Príncipe was not as lucky because of bad weather conditions.

[@raizdesamauma8607]

I went to comments section looking for this. Thanks for clarifying it!

[@jamesb1856]

Relativity did much more than I think initially intended. It opened a door to a whole new level of understanding

[@leonhardtkristensen4093]

Or possibly it made everything more confused.

[@earthbound9381]

As I live in NZ I just cannot understand how an Aussie can have such an impressive intellect that Matt demonstrates so fluently. I'm sure that if it were possible for him to explain that the world will end tomorrow then I would believe him but nevertheless be unafraid.

[@RolfWRasmussen]

5:11 made me go off on a tangent to try to figure out if the "Wilhelm Scream" is in the public domain or not.

[@punkdigerati]

Effectively yes. Many factors point that way, such as when it was recorded, it's length, and if it constitutes an original work. The biggest is that it's never been defended as a copyright.

[@evilotto9200]

i think warner music made 14 million off the birthday song's copyright before the world collectively agreed "this is dumb" i now wanna make the wilhelm scream mine and mine alone

[@shroomedup]

I really love that closing remark you made. It is so damn true and it makes me respect you and your channel content a lot.

[@DanielZajic]

This one was awesome, nice job. You know it's good stuff when it's almost all narration and still just as captivating.

[@ilkoderez601]

Still my favorite channel. Bravo! This has been my favorite channel for _years_, I mean, there were some slow points but you guys have been KILLING IT lately!!!

[@eku154708]

That 🤩 face you make, when you refresh to see a new SpaceTime episode!

[@SterbenCyrodill]

Had just opened KZbin after Diner. What a pleasant desert!

[@loganstrong5426]

That's it, I've decided I will no longer be learning about relativity in any context that does not include giant alien spiders.

[@FredPlanatia]

brilliant, not just the explanation of how light paths are bent around massive objects, but how different descriptions (or conceptions) of reality can predict the same phenomena. There are many examples of this in physics and i think it shows how physics and mathematics are used as a means to describe nature. Our conception gives one component of the truth but a different way of viewing the same phenomenon can be equivalent and give a different view point. Different vantage points same phenomenon.

[@RuhrRedArmy]

The time it takes to get from one star to another star is insane. You need to travel at the speed of light. And us humans can't even... fathom the concept of that kind of time because it's really really really really really really really really fun, to think about taking a speed of light ride.

[@mikejohnstonbob935]

so that's why stormtroopers miss all the time

[@SolaceEasy]

Gotta use a spray pattern.

[@johnshepherd6925]

That's funny I just watched the Mandalorian episode that makes fun of that 😂🤣😂🤣

[@tomkerruish2982]

When you apply Huygen's Principle to quantum mechanics, you get Feynman's path integral formulation.

[@OuroborosVengeance]

I thought about it. Not sure if its quiet true that it follows the same principle, but it sure looks similar

[@ornessarhithfaeron3576]

@@hyperduality2838 Too long; didn't read

[@pjweiner8700]

@@hyperduality2838 you should check out dialectics, dialectical materialism, and Marxism

[@Vares65]

Love this channel SOOOOO much! Thanks for all the hard work you do to give me a free education. You are all appreciated more than you can ever know. 🥰

[@Volamek]

I've fallen in love with this channel. When I first started watching, the concepts went way over my head. However, after watching and re-watching much of your library, I find myself commenting on equations and theories before you tie them into your main topic. You have provided an interesting and exciting medium for learning the most complex concepts in the universe. Thank you for your hard work and dedication in bringing us such complete content. 42.

[@SoundzAlive1]

I agree, instead of being way over my head they are just a little over my head now. André

[@WackoMcGoose]

"Giant alien spiders are no joke." See, that's why you need to get an Anti-Bio Beam as soon as you can.

[@SashaGarcia]

Clone bay is more up my alley

[@hamentaschen]

"Just remember Jerry... It's not a lie, if you believe it."

[@cristianmoss7828]

beLIEve

[@Partho_The_King]

Lies lies everywhere

[@jrobinson1215]

Costanza!

[@lavidaescorta3220]

This is a very quick paced episode. I could not understand half of the explanations the first couple of times I watched the video. Yeah, I may be a little "slow" but I usually understand 80% to 90% of the PBS Space Time videos the first or second time.

[@SointexJambis]

Ok, I can't kid myself anymore. I sit thru these like i can keep up, but i have no freakin idea what just happened. But i still sat here staring at my phone for 13mins. Keep up the great work, even if I can't understand what you are doing. LOL

[@YYYValentine]

I am wondering how many comments will be written about the backwards rotation of Earth at 10:09

[@michaelsommers2356]

They do it deliberately to trigger comments, which helps with The Algorithm.

[@devinfaux6987]

It's not rotating backwards; we're just orbiting it below geosynchronous orbit, so it appears to be spinning the other direction.

[@timjohnson979]

That's because the camera is passing the earth faster than the earth's rotational speed. It's an optical illusion. :-)

[@JohnDoe-dj3lw]

doesn't Earth spin clockwise..? it's right

[@cavalrycome]

Devin and Tim, You can tell that it's rotating the wrong way based on the day/night boundary continually moving eastwards so it doesn't matter how the camera is moving.

[@En_theo]

Great episode as always. Btw, I remind you your promise in the previous one ; that you would explain further the equivalence principle and the apparent contradiction that, in a free fall, our feet are more attracted than our head (and therefore we can tell if we are in a gravitational field).

[@DeeFeeCee]

Oh, yes!

[@JeroenMW2]

The equivalence principle only holds at a small local level. If you have a giant human with his head reaching satellite orbit, he'll notice that clocks at his feet run slower than clocks at his head. However, time dilation should work at infinitesimal small distances so to me it also seems that the equivalence principle should fail at the local level if you have a really accurate clock.

[@gerardolebron8838]

Light does travel more distance in a dense space but Time does not dilate. Space is compressed were mass is present make a black hole with a weaker pull. Compress space does requires more distant travel.

[@juancuelloespinosa]

dude I've been waiting for this video. Nice work leaving me wanting more PBS spacetime!

[@davidnewbaum6346]

I swear, the more I learn, the less I know about inner workings of the universe.

[@vesawuoristo4162]

Well. He just said two opposite ideas are both true

[@DrZedDrZedDrZed]

Ipse se nihil scire id unum sciat

[@techman2553]

And the logical reverse of that is....The less I know, the more I think I understand the workings of the universe. This says a lot about modern society.

[@michaelsommers2356]

@@techman2553 It says nothing about society; it's just the Dunning-Kruger effect.

[@livinlicious]

TBH many of those references he made, are some form of circular argument. He starts at the end, and makes the explanation work with what he is trying to convince you of. Thats highly problematic, since an explanation should not be relative to the framework you are trying to prove, that it is in fact true. So its not you who doesnt understand, its a form of over-complicating things to the point of loosing a simple explanation. Its like asking a question what is 2+2, and you get the answer "well it depends, what is even 2?" Which might sound like a viable question, but it misses the point.

[@Yora21]

The question how photons are affected by gravity even though they have no mass ("masses attract each other"), was what let me down to discover what this whole space-time thing is really about.

[@charlesjmouse]

Mitchell & Cavendish provide a good example as to how one can be completely wrong and yet come up with the right answer. I wonder in what ways our correct answers are based on wholly incorrect 'understanding'? I wonder what unexpected insights we may gain in the process of discovering our latest mistakes?

[@tommywhite3545]

Hopefully Erik Verlinde will get his right, because in all fairness "we're" kinda stuck at this point.

[@adriana.7111]

Great episode! I am now one slice closer to understanding the very nature of spacetime.

[@MindfulAttraction]

Nice! Time to learn

[@dustinswatsons9150]

Oh is it actually time to learn or hasn't been relative or has or has been f*** it

[@dustinswatsons9150]

Oh I just spoke to what organization

[@dustinswatsons9150]

The good penis joke is arriving soon with these very video I look forward to seeing it in Instagram

[@davep8221]

@COVID 19 But you just taught me to do that! I guess I'm stuck at my current knowledge level :-(

[@P-G-77]

NiceSpaceTime

[@1urie1]

Me: Invents a time machine and goes back in Newton's time. Finds Newton and Huygens and tells them that actually light behaves both as a particle and a wave. Also me: Now kiss

[@NiffirgkcaJ]

ಠ_ʖಠ

[@wren7195]

THAT'S SO HOTTTTTT!!! *KISSES PASSIONATELY*......... Oh oh, oh... oh, you.... you meant, you... meant them kiss, I'm SO SORRY, I'm... well... *looks up tentatively...* ...it's ... maybe only a particle when you look at it, but without any relative moment/space it's a wave.....? *smiles looking up through her lashes* You ah.... ah have any "spare time?"

[@NiffirgkcaJ]

@@wren7195 what's wrong with you? ಠ_ʖಠ

[@wren7195]

@@NiffirgkcaJ Which part? But Crohn's disease, it means I can't stand myself to the point where my own immune system is allergic to me. Unless you mean my personality in which case eh... Maybe let's just say I really like wave functions and oscillations? On a platonic level. Yeah definitely not on a platonic level.

[@NiffirgkcaJ]

@@wren7195 *_slow claps_*

[@bennettfoley3018]

This episode, especially the final sentence, is incredible. I don't have a smart thing to say about it, but the yt gods should promote it overall!

[@waffles3987]

I love this topic and this channel, I get soo excited when new vids come out! thank you for the great content!

[@slimnagirac3393]

Alternate title- "Two times scientists were right... even when they were wrong "

[@Rhuind]

Speaking as a neuroscientist, it happens more often than you might think.

[@EditioCastigata]

“Alien spider” is known as red beet and hunted for dishes in Russia.

[@Oberon4278]

Russians use beets as dinnerware?

[@unitedspacepirates9075]

In Russia, flying object identify YOU.

[@Partho_The_King]

And what about Australia? I bet they munch them as snacks!

[@newpix7555]

The only video that actually explains why and how this happens.

[@KonekoEalain]

Light bending in a gravitational well makes sense to me now, and I didn't even put any points into int when I leveled up, great video!

[@frun]

Dark energy stars: "As a column of superfluid grows taller, at some point, density increases, slowing down the speed of sound, so that it approaches zero. However, at that point, quantum physics makes sound waves dissipate their energy into the superfluid, so that the zero sound speed condition is never encountered."

[@hairohukosu433]

Hold up, doesnt higher densities increase the speed of sound?

[@johnshepherd6925]

Omg they used the ship from Spaceballs!!! Yes! 🤣

[@MrJailbreakdude]

This episode is truly a masterpiece, great work.

[@OpticalVirus]

Just shows that any observable data or complex theory always ends with more questions than answers...but also that we have discovered so much about our universe, most of all how incredible and complex and beautiful it all is....

[@melekhine]

I've never heard that explanation for light refraction before. Thanks for the good mental model. But I wonder how light can be seen as an expanding frontier of new ripples if there is no medium for it to vibrate through from the sun.

[@shmunkyman33]

The "medium" light travels through is the electromagnetic field, which exists at every point in spacetime

[@chaz000006]

@@shmunkyman33 Or do light particles generate a wave in the medium we call space time, or the ether, or subspace, or something else...

[@shmunkyman33]

@@chaz000006 The "luminiferous aether" hypothesis was refuted by the Michelson-Morley experiment in 1887. The electromagnetic field exists IN spacetime, but it is not the "medium" of spacetime itself. "Subspace" is a made-up word from Star Trek. There's answers to these questions if you'd actually look for them.

[@chaz000006]

@@shmunkyman33 I am aware the ether based model of the universe has been refuted, as I am aware that the relativity model of the universe is incomplete, as evidenced by quantum entanglement. The point being, that whatever you call the medium, it exists and fast moving particles create a disturbance in it, generating a wave of it. The only other example I know of this is with electrons orbiting a nucleus at a high rate of speed. I just wonder if the medium slows or limits the speed of light, and without it, would light travel instantaneously?

[@shmunkyman33]

@@chaz000006 No, the electromagnetic field is the only thing that allows light to travel, so it does not "slow it down". There is no real distinction between light particles and the electromagnetic field, so if there were no field light would not exist.

[@jenniperkins4260]

New PBS Space time ? Click !

[@vincenzo7597]

Each video is a piece of art on its own. Thanks.

[@pauldickinson1075]

Best utilization of the Willhelm Scream ever. EVER.

[@pauldickinson1075]

5:12

[@onebylandtwoifbysearunifby5475]

1783, Or: When you're smart enough to figure out the correct answer, even though the physics won't be discovered for 120 years.

[@DominikJaniec]

2:39 love ludicrous speed!

[@volbla]

Hah! I didn't notice that. Nice catch.

[@KnightsWithoutATable]

This was a really interesting one, especially how you point out that for all that we know about our universe, we are just scratching the surface and peering into some really complex and connected truths.

[@jimmycricket5366]

What I got out of it is that what one can see and when one sees it depends on your vantage point. Perhaps that is why God can see the end from the beginning?

[@jajssblue]

Wow! Really great and approachable explanation!

[@AK-ny5bz]

As someone posted somewhere: DARWIN : Everyone is Relative Einstein: Everything is Relative.

[@mrspidey80]

Alabama: Everyone's a relative.

[@st.hansen]

Do such a "plane wave explanation" depend on transverse EM plane waves having some definite length in space? If so, must not the length be the same for all light, since light is bent the same way regardless of its frequency? I know the plane wave was only introduced as a metafor, but think it could be interesting with some further explanation.

[@paulhinrichsen8628]

Thank you for another GREAT episode. I must admit to being absolutely STUNNED by you suggesting that a variable speed of light (due to gravity) is a possibility. Although not a career physicist, I have studied GR for a number of years and became intrigued by the possibility that we can either describe the universe, using a constant speed of light, and a curved spacetime, OR a variable speed of light, and a flat spacetime. It is just encouraging to hear that you Matt, also recognize this as a possibility. OK so we are not re-writing GR just yet but its an interesting possibility.

[@pixartist8190]

This one and the last video are absolutely mind-blowing

[@Ceelvain]

I've always wondered what would be the mass of a photon based on its bending path according to classical newtonian mechanics.

[@SkyDarmos]

The mass cancels out in all equation for free fall and for deflection.

[@SkyDarmos]

In Newtonian gravity and Einsteinian gravity all objects fall at the same rate. That includes photons, no matter if classical or quantum.

[@SkyDarmos]

Both in Newtonian and Einsteinian the pathway of the photon depends only on the speed of light and on the gravity of the sun.

[@43-manoranjan22]

Can put a video about how electromagnetic field affected by the gravity

[@OuroborosVengeance]

Same as regular matter, the electromagnetic field is embbeded into spacetime. What happens to matter, happens to electrons too. Weird how its different for photons tho. I guess the most technical explanation is in QFT lenguage, possibly showing us the field curving due to energy-induced spacetime curving (it would even curve itself due to self-interactions, regardless of massive objects in the universe, i guess). How the fields interact with themselves and with other fields, is something i think we could learn if we would to study QFT, i would love to. I wonder if there is an actual gravitational "field" in the same sense as there are quantum fields. Maybe this "curving field" dont interact the same way as in QFT

[@43-manoranjan22]

@@hyperduality2838 does electromagnetic field affected by the gravity as same as mass . For example the mass of the electron or proton curves the space-time but the difference in the charges does causes any change in curve of the space-time

[@OuroborosVengeance]

@@hyperduality2838 interesting. I have think about this fact a lot by myself. Interesting to read you ans find my own thoughts in your words

[@OuroborosVengeance]

@@43-manoranjan22 i think that QFT is actually having a problem with this and they are trying to solve it using renormalization, bcs fields, without renormalization, would curve *themselves* a lot, but thats not what we see when we measure them, they manifest as vibrations in euclidean space, except when close to massive object, but again, without a massive object in a given patch of space, they still should curve, but they dont. This is an open problem. However im not sure, im not an expert and probably i lack a lot of info, its just my thoughts and interpretations of what i know

[@alistairferguson6590]

Wow, nice upgrade in the visuals

[@BB-cf9gx]

Brilliantly explained without a single equation.

[@CalebTerryRED]

Question: the effects of Gravity pulling down are indistinguishable from the effects of an accelerating reference frame accelerating you up, but can the same be said of other forces like electromagnetism? Similarly, would a positively charged body feel time dilation near a powerful negative force the way massive objects do near black holes?

[@Jatt2613]

I left this comment on the last video, and since you mentioned it again in this video but I had the same question, I'm asking again in hopes that you might answer my comment in the next video. I'm still stuck on the sort of chicken/egg-iness of time causing gravity. You say gravity slows time, which converts some of the temporal velocity into spacial velocity toward the mass, like a planet, but then that causes gravity. But gravity already had to be there for time to slow. So how does it all get started? How can time be slowed by a gravity that it creates?

[@sasshole8121]

This is how I understand it: Gravity itself just warps spacetime, but gravity does not create any attracting force in itself. The deeper you are in the gravitational well, the slower the time dilation. The attracting force comes from the differences in time dilation that gravity creates, as shown with the kayaks in the last video. So mass causes gravity which warps spacetime which causes differences in time dilation which causes the attraction. This is new to me too, so take what I said with a grain of salt.

[@Jatt2613]

@@sasshole8121 That may be what it is, and, if so, makes sense, but if that's it, he's using "gravity" to mean the spacetime warping force and "gravity" to mean the attractive force, so it's a bit confusing when he's using the same term for both. Hopefully he clears it up for sure next video!

[@sasshole8121]

@@Jatt2613 Yeah, it is confusing. When most people hear gravity, they think of the attraction of gravity not the warping of spacetime. So, it is difficult to distinguish the two.

[@DeeFeeCee]

I'm still confused as to how matter even affects spacetime in a way that it could bend time.

[@richardbraakman7469]

@@DeeFeeCee Even more confusing: it doesn't even have to be matter. A sufficient amount of concentrated energy will bend spacetime the same way. Even if it's potential energy! I can't figure that one out.

[@screamingbuddha2865]

Great episode. So clear. Thank you.

[@WanDeLay4]

That was sick. Thanks Matt and rest of SpaceTime team!!! Love you guys so much

[@7Alberto7]

"But this contradict everythings i just told you" Me: "exatly what a was thinking"......

[@zlm001]

If the energy of light is decreased due to gravity induced redshifting, what happens to the light's lost every? Is momentum transferred to the source of the gravity? Does a transfer of energy also occur when light is bent by gravity as there is a change in direction?

[@DeeFeeCee]

I would think so! But I think acceleration (change in direction) also requires energy, so maybe it all cancels out? Or maybe that doesn't get taken into account with geodesics…

[@justindelima5607]

Reflected light can cause acceleration (see: laser propulsion concepts), so I would assume that the energy is, in fact, transferred.

[@Melki]

Thank you Matt, PBS, and patrons

[@WhitefirePL]

Somehow I find the ending really beautiful and profound in this episode. Way to go!

[@AuthenticDarren]

That was actually clearer than last week's episode. My head is aching far less :/ .

[@captainpuffinpuffinson4769]

Be a physicist Make huge wrong assumptions on the universe Get the correct answer anyway ... Profit? Discovery!

[@rmsgrey]

It's not as coincidental as the episode made it sound. The pre-relativity black hole calculations were based on theories derived from close observation of the real world, that match relativity to very high precision under normal circumstances. There's also the point that we know that at least one (probably both) of general relativity and quantum mechanics is wrong since they're fundamentally incompatible, despite both being fantastically accurate in their predictions in their own domains, so we have good reason to suspect that the black hole predictions of general relativity are themselves based on an incorrect theory, which would make it a case of two (slightly) wrong theories both giving the same answers.

[@captainpuffinpuffinson4769]

@@rmsgrey it was a joke...

[@FelipeKana1]

These animations are getting better and better

[@Xurreal]

I needed this. More than I realize. Thank you!

[@let4be]

Who said light bends? It always follows a straight line from the wave's perspective, it's space itself that becomes curved in the presence of energy ;)

[@grah55]

"the presence of matter" fixed it for you.

[@let4be]

Energy or matter, makes no difference. With enough energy(say in the form of photons) focused into tight space you could theoretically create matter with mass ;) Even the light itself(it's carrying energy) should curve space it's moving through but this added curvature is just negligable small to measure

[@grah55]

@@let4be I don't believe anyone has ever stated that electromagnetic radiation has its own gravity. The field interaction of a photon is apparently entirely em field. Yes it should be theoretically possible to convert electromagnetic radiation into an object with mass but... I've never seen it and it's probably some weird reverse interaction where when you'd usually see a photon emerging from a quantum event, the photon goes in and adds mass to something. But that doesn't mean it carries mass. e=mc^2 but the c^2 part cannot be overlooked.

[@grah55]

@@let4be Infact to elaborate on the e=mc^2 part, remember that c is the speed light and massful objects can't reach that speed but can get very close. Infact, as we add more speed to an object it's mass actually increases. That is where the perceived increase in gravity of a highly kinetic energetic object comes from. You're right to assume that adding more energy to object increases it's gravity but remember, this only works on object that have mass, not light itsself.

[@EvilSnips]

Last time I was this early the electromagnetic force and the weak force were the same thing!

[@spacetardigrade8277]

Last time I was this early time didn't exist......so there's no last time.

[@EvilSnips]

@@hyperduality2838 Super awesome list!!! thanks for posting!

[@rupakrokade]

This video answers some important questions about light.. incredible!!!

[@saritp101]

Today I learnt that relativity is relative! Thank you PBS Space Time 😀