The Biggest Ideas in the Universe | Q&A 6 - Spacetime

[@steliosp1770]

Dr Caroll, I just want to say a huge thank you for being who you are and doing what you do. Doing all these videos, podcasts, public lectures on topics that are definitely deep and profound but only scratching the surface in a sense so that us lay-people can follow along and you simply dont get tired or bored of it; you just love it for the sake of promoting science and enabling people to be curious about the world. It's people like you and Brian Greene, for example, that give me hope for the future in this mad world.

[@NyxSilver8]

Yeah, atheists are the intelligentsia, long live atheism in this mad, mad world.

[@shingai_munyuki]

Man, I have never heard this type of insight. I keep replaying it to comprehend. Absolutely amazing.

[@dozog]

Those Q&A are so great together with the lectures. Thanks Dr. Carroll.

[@garyinbellerose]

Dr. Carroll, I'm reading your book "The Biggest Ideas in the Universe: Space, Time, and Motion", and it is absolutely fascinating but hard to follow at times. Pairing the book with these videos makes it that much easier. Thank you so much for all that you are doing along these lines! Looking forward to the next book in the series.

[@Cemselvi1988]

Sean Carroll is the James Bond of Physics

[@onepieceatatime]

Thank you for making these videos! It's an honor to be in your light cone.

[@philochristos]

So, like, if space can bend and stretch, what exactly is space? General relativity makes it sound like space is a substance. If space is expanding is there such a thing as space density, and does it change?

[@Zoolooman]

I've been really enjoying this series, Dr. Carroll. Thank you.

[@stevenschelling8452]

Genuinely astounded this is free, your physical insight is exactly the level beyond pop sci that my brain has been craving lately and I feel like some questions I’ve had about special relativity for years have been cleared up. Thank you so much for the mind expansion, I’ve bought both BIitU books and can’t wait to learn more.

[@rajkumardhakad8773]

I don't know what to say, but I'm having a good teacher after a long time. Thank you Sir.

[@stridedeck]

If there is no changing, Sean says there is still time due to the space between objects and bodies. But this space in between objects are still a part of change as light is needed to change position from one object to another. No change means no change in position of photons. No time as then there is no space to decipher between an object and space around it.

[@captainpints]

This is a good format eh? Hope it continues.

[@pb4520]

Wonderful and please keep these going! Thankyou !

[@pwatsky]

Thanks for taking the time to do these videos and answer questions. Those watching are being given a treat..Thx again.

[@woody7652]

Thank you, Sean! Long live the tachyons.

[@zaphodbeeblebrox2817]

bartender says "what'll it be?" a tachyon walks into a bar.

[@ToriKo_]

Around 30:00 the whole setting the speed of light to 1 thing clicked for me. So it seems like our universe has a speed limit. A limit of how the universe updates, not just the velocity of light. Speed is just a distanced travelled / the time it took to travel that distance. Light travels at 1 Lightyear / year. As our universe has a speed limit, we can think of there being a top speed, and every other possible speed in the universe is just a fraction of that speed. So we set C to 1, and every other velocity is usually a small fraction of 1.

[@joandesanto9316]

I'm really enjoying these videos. I am not a scientific person, in any way, but I am really liking the learning process. I am more of an artistic type, but in our universe, all things are related. Thank you, for what you do.

[@CalvinHikes]

"You shouldn't priveledge your being able to notice something happening, in order to say something exists." 2:25 Thanks for making a fair argument for God in your discussion. I knew you had it in you.

[@MattOGormanSmith]

It's an equally fair argument for the fairies at the bottom of your garden, or the spiders in your brain.

[@coreymorgan6000]

Thank you for answering my question re Oscillating Photons with no elapsed time. That was clear and very helpful

[@justdata3650]

I had a related question to your 2nd answer about photons which is, if there is no time at the speed of light/causality does that mean that light can never spontaneously change/decay theoretically since events must happen over time?

[@coreymorgan6000]

Not a physicist, but I'm of the understanding that you are correct: photons cannot change/decay because they are mass-less and move at the speed of light. The fact that neutrinos were found to decay informed physicists that neutrinos have mass, albeit very small, and move close to but not quite at the speed of light.

[@JohnDlugosz]

Multi-photon interactions do occur. At sufficiently high energy, 2 or 3 photons can turn into an electron/positron pair. That's just the time-reversed reading of annihilation.

[@justdata3650]

@@JohnDlugosz Yeah but now you are talking about things interacting, even if it is photon on photon action (can particle physics sound dirty?) When a photon hits a particle, an electron for example, things happen. The question is really about a photon meandering all on its lonesome.

[@JohnDlugosz]

@@justdata3650 Nothing ever happens to a particle by itself. When you look at a decay, for example, you are really simplifying a diagram that involves a virtual particle pulled from the vacuum. E.g. a W boson will interact with a virtual neutrino, producing an electron (or positron) and giving energy to the virtual neutrino's partner to become real. See en.wikipedia.org/wiki/Two-photon_physics The interesting thing about a Feynman diagram is that it can be turned in any direction and describe legal interactions. So hold it diagonally, and you'll see it describes 1 photon spontaneously turning into 3.

[@royalbloodedledgend]

Just finished From Eternity to Here. Very insightful but some parts are quite hard to understand. I still don’t get them

[@giuseppe1926]

at 12:20 talking about building a clock made of photons, I don't understand why if two (or more) photons start interacting that system is no longer massless and hence can't move at the speed of light.

[@peterb.559]

Sean seemed to have missed mentioning that the speed of light does change depending on what's it's going through. The constant c is just the speed of light through a vacuum; light slows down when it goes through glass, water, and even air.

[@ThomasGutierrez]

Wonderful lectures. The whole series is great. I have to quibble a bit about the characterization of an electron not changing around 10:38. It will have a chiral L-R oscillation with a frequency set by its mass as it interacts with the Higgs field. In this sense, a stable "static" particle like an electron does have a natural clock even in its own rest frame.

[@corypride]

the fixed and exact speed of light finally explained. I've been waiting a lifetime--and I'm an old person. thank you!

[@kredwol2103]

Dr Carroll, the speed of light is equal to the smallest possible distance divided by the smallest possible duration of time. Plank length/ Plank time. Just so happens to be around 3*10^8 m/s.

[@TanioDiazSantos]

Thanks for the answers. The one about the "internal life of a photon" was fun :-) Looking forward to the next big idea.

[@bingcom5250]

Been waiting for this !

[@davidseed2939]

at 24:59 we can choose v1 =c and then vtotal=c regardless of v2

[@eziowayne]

Thank you so much for this series!

[@michaelschmidt1223]

[25:30-25:58] Where can I find the show notes or errata on KZbin?

[@TheFojar]

He just completely reworked my mental image of a travelling electromagnetic wave. I'd always thought of it as an oscillating particle or wave tracing a path up and down as it moved through space, rather than an unchanging wave crest that moves past a point being observed, followed by a trough, etc. I feel like I've seen depictions of the alternating electric and magnetic fields that get this very wrong.

[@donbasparklane]

Thank you! 🙏🙏

[@gregmw]

Perhaps it is a technical point, but an electron is constantly exchanging weak hypercharge with the Higgs condensate via Yukawa coupling - this gives it its measurable rest mass - doesn't that mean it does, in fact, change and evolve with time?

[@roros2512]

01:23 I think this analogy is not appropriate. I think the masses in space are the analogy to the events in time, so, if there are masses, there is space, if there are events, there is time. As you can measure the distance between two masses in space, you can measure a lapse between two events in time

[@KrisPucci]

His response to the first question about whether time is possible without change is interesting to me. In his book From Eternity to Here, he discusses that time emerges from entropy. Either that or I completely missed the point of that book.

[@AlphaFoxDelta]

Thank you very much for these

[@Petrov3434]

You are mentioning your "notes" about each episode. I simply can't find them anywhere... Where exactly are they? Many thanks

[@justdata3650]

I had this other question about fundamental units which is, since the discovery of quarks that have down to 1/3rd eVolts why do scientists still use electron volts when it seems the more fundamental unit would be quark volts where 1eV = 3qV? Ditto about spin, why still use 1/2 spin when you could make 1/2 the fundamental unit of 1? Your talk is awesome as always by the way - thanks.

[@cazymike87]

Just ask the USA why they are keep using feet , miles , pound , fahrenheit ! Just ask why does some people keep driving on the left side of the road , and some on the right ! Just ask why does the flow of current is assumed to be from positive to negative direction. The flow of electrons is from negative to positive. Etc

[@justdata3650]

@@cazymike87 I get what you are suggesting but I don't think these are equivalent. Physicists routinely change light to equal 1 instead of 299e3 km/s, i.e. change it to what they call natural units so these people are used to changing units however they want for the sake of ease of calculations. This is a very different mindset to your average Joe who's just used to using gallons for fuel purely out of habit and convention and doesn't give it 2nd thought. There's probably some convention in their thinking but I suspect the primary reasons are more complicated than that and are probably centered around convenience of common calculations.

[@statichackx]

Isn't the reason the speed of light/c is exactly the speed it is, those 2 fundamental constants in maxwells laws? We don't know why those are what they are, but they constrain c to what it is right?

[@Psnym]

Thank you for answering @9:55 a question I got to ask (and get answered) on an AMA some months ago. Yay mindscape!

[@ZAMsChannel]

Maybe you should have mentioned, that the speed of light is "just" the square root of 1/the product of the contants of permeability (µ0)and dielectricity (epsilon0). Sorry for not having the symbols correctly ...

[@trucid2]

Speaking of building a clock purely out of light, isn't there gravitational interaction between photons? Setting aside the question of whether you can make a clock out of gravity and photons, will such a gravity-bound mass of photons move at the speed of light? Why or why not?

[@johnsongibbs6567]

Dr. Carroll, I have some thoughts I would like to share in the hope to hear your opinion. I have formed this belief through years of investigation as an amateur, however, with a sincere interest and ambition to be shot down from the very top.. so here it goes.. Time is a measure of progress. Everything progresses through time independently and the progression of all things through time is directly affected by temperature. Temperature affects the rate of progress through time because time has wave properties. The wave properties of time account for the prevalence of wave mechanics in nature. The electromagnetic field exists because time has wave properties and electromagnetic radiation is a ripple in time through space. Mass is a stable resonance of space and time. The stable resonance of mass impedes the flow of time and creates gravity.

[@ToddDesiato]

An electromagnetic field has wave properties. Time is simply what you measure with a clock. Wave-like properties are not required to define time.

[@davidchung1697]

I have a question. It seems to me that the theory of Special Relativity says the speed of light is the limiting speed of all objects without really explaining the reason. Is the reason why an object cannot travel faster than the speed of light because the object itself is made of waves at the quantum level? If so, is the velocity of the object just the group velocity of the waves that comprise the object?

[@incoathwetrust4612]

Special Relativity postulates that the speed of light is the same in all inertial reference frames. The speed of light being a limiting speed for all massive objects is just a consequence of that .. It has nothing to do with quantum mechanics.

[@AlyssaMichelleSoap]

You're spot on David, there are no particles only discrete wave packets.

[@davidchung1697]

@@incoathwetrust4612 I understand that the SR says the speed of light is the same and I understand the Lorentz transformation, etc. To me, that feels more like a mathematical description, rather than an explanation.

[@robertshirley2645]

Hi professor, what is the speed of light without the speed of universe? If we subtract the speed of universe from the speed of light, can we say that the speed of light is decelerating because the speed of universe(expansion of space-time) is accelerating?

[@stanrogers5613]

That's trickier than you're imagining. The universe isn't expanding in the way that you think; it's just that things are getting further apart. Point A is not moving away from point B, but there is more distance being injected between them. This absolutely does not happen anywhere in human experience. When things get further apart in "real life", one or both of the things has to be moving. You can't get B increasing in distance from A at a rate of one metre per second just by putting spare metres between them carefully at an appropriate rate. But that's what's happening (as near as anyone can tell) with universal expansion.

[@danielcox8819]

@@stanrogers5613 Not my subject and yet I've found a supporting experience in the supernatural, life after death experiences. A not so bright man, a Dull, after he died was taken out of this universe and shown the multiverse by God. He testified to the multiverse as an infinite to him number of cigar shaped universes. It's like that 50 year old man with a 3rd grade level education speaking fluent latin or the little boy as the subject of the movie The Exorcist who fluently spoke ancient Aramaic, how did the Dull know our universe is cigar shaped?

[@robertshirley2645]

@@stanrogers5613 I believe the way you described it , is perfectly normal. Because every galaxy is in free fall towards the center of galaxy cluster and clusters are in free fall towards the center of superclusters and so on!(it is my guess) therefore, we see them falling apart from us and we observe the increase in the distance between them. But my question was not based on this idea of free fall in structures of the universe. I used to think that by expansion of the universe, they mean that the fabric of universe is expanding . Is this assumption false?

[@robertshirley2645]

@@stanrogers5613 also, if only distance between the galaxies increases without moving any of two galaxies, that would mean that the light between those two galaxies must be decelerating in order to cancel out the acceleration of increase in that distance!

[@justinwinter4908]

Sorry I'm sure its obvious, but where are the show notes Sean was referring to in the video?(25:55)

[@dclark92064]

It's not obvious to me. I have searched around and Googled it. Can't find anything. Does anyone know where the show notes are?

[@justinwinter4908]

@@dclark92064 when you watch the video, tap the screen and an icon appears in the top right hand side with the information

[@ecsciguy79]

When you presented the question "Why is the speed of light the value it is?" I was imagining an answer about how it's derived from epsilon nought and mu nought. Should we instead see those two constants as derived and c as fundamental?

[@JohnDlugosz]

The speed of light is just a scaling value, and properly has a value of 1. The fundamental constant is called "alpha" and is dimensionless.

[@GGoAwayy]

Max Tegmark wrote in "Our Mathematical Universe" about the idea that maybe all values for fundamental constants in the wavefunction of the universe have "real" representations, and its just anthropic principle that these particular values allowed for enough fun stuff to happen to lead to us. Those values could be part of our address within a larger multiverse.

[@WEPayne]

Why do we say all inertial frames are equal? One is obviously special, namely the frame where the CMB has no redshift. But the CMB came from the singularity, which we believe defines the origin of spacetime. So how does the singularity define an inertial frame??

[@ryankelly936]

Hello, I think there might have been a minor mistake in your response to one of the questions. If you were to "draw" light cones in 3 dimensions of space (and one dimension of time) you would draw concentric spheres around a point, not a cone. IE, before an "event" at a point, the light sphere would be shrinking, until its a point, then the light sphere would expand. Not sure what the phrasing of the question was, so maybe I am mistaken.

[@ryankelly936]

@@michaelsommers2356 Hey, sorry my explanation was not clear. You are absolutely correct, the light cone in 4d is the "4d cone". My explanation was an attempt to illustrate what this 4-d cone is. Let me try to explain it again. In math we have a notion of an "n-sphere" and I will use wikipedia's definition of an n-sphere (en.wikipedia.org/wiki/N-sphere) in my explanation, Here goes: In 1-d space and 1-d time, a light cone is a collection of concentric 0-spheres (2 pts a distance r from the "center") which are drawn in the time axis; so you get a 2-d cone which has a point at an "event" and rays coming out at 45 degrees in either direction from the point. If we think of this as though we are at a position and experience this over time, then we would say that there is a large 0-sphere of light which is shrinking to a point (when it is a point is when the light actually hits where we are in space and is when we actually see it) then the sphere expands away from us. What I am trying to illustrate can be explained another way. Given the 2-d drawing of a light cone, as in the videos, imagine sweeping a horizontal line upwards(in time); the intersection of this horizontal line with the light cone is 2 points, ie a 0-sphere. This can kind of be thought of as concentric 0-spheres. In 2-d space and 1-d time, a light cone is a collection of concentric 1-spheres (ordinary circles with radius r) which are drawn in the time axis; so you get a 3-d cone which has a point at an "event" and rays coming out at 45 degrees in any direction off the x-y plane from the point. If we think of this as though we are at a position and experience this over time, then we would say that there is a large 1-sphere of light which is shrinking to a point (when it is a point is when the light actually hits where we are in space and is when we actually see it) then the sphere expands away from us. What I am trying to illustrate can be explained another way. Given a 3-d drawing of a light cone imagine sweeping a plane upwards(in time); the intersection of this plane line with the light cone is a circle, ie a 1-sphere. This can kind of be thought of as concentric 1-spheres. In 3-d space and 1-d time, a light cone is a collection of concentric 2-spheres (ordinary spheres with radius r) which are drawn in the time axis; so you get a 4-d cone which has a point at an "event" and rays coming out at 45 degrees in any direction off the x-y-z cube from the point. If we think of this as though we are at a position and experience this over time, then we would say that there is a large 2-sphere of light (ordinary sphere) which is shrinking to a point (when it is a point is when the light actually hits where we are in space and is when we actually see it) then the sphere expands away from us. What I am trying to illustrate can be explained another way. Given a 4-d drawing of a light cone imagine sweeping a cube upwards(in time); the intersection of this cube line with the light cone is an ordinary sphere, ie a 2-sphere. This can kind of be thought of as concentric 2-spheres. Also when I say Cube, I mean a 3-d space, like an infinitely large filled in cube, couldn't think of a better word... hope this clarifies what I was trying to say! Let me know if you disagree or now understand what I was originally saying.

[@tanaymehta7603]

Can you elaborate on tachyons

[@dazphd]

From the perspective of one photon in the group, relative to itself, are all other photos moving at the speed of light?

[@dazphd]

I see this is not taking Proff. Carroll’s advice!

[@dougg1075]

Sean!!! You have a mullet:) Tennessee Top Hat Kentucky Waterfall! Business in the front , party in the rear!!! I idolize you man! Super smart

[@viewer3091]

Is there more than One dimension of Time ? Or is every Planck Length of Time a Dimension ?

[@ezsparky]

Is time granular or pass by in discrete chunks? Is the movement of a particle granular or happen in discrete chunks?

[@GGoAwayy]

I think it's supposed to be smooth but nothing important happens at distances shorter than planck length or durations shorter than planck time.

[@ajjcooper]

Love this series, but you did both say: nothing is happening, no time is passing (re a photon) ... and that time exists even if nothing is happening. Is this a difference between time passing and time existing? I guess I struggle to differentiate between no time passing and nothing happening but time existing.

[@JohnDlugosz]

Greg Egan wrote a "hard sf" novel set in a universe with two timelike dimensions. It's called dicronouts or something like that. The math supplement is free to read on his website.

[@mokopa]

These videos are to me like cold water is to a thirsty man on a hot day.

[@NyxSilver8]

Yes, atheism is like that, it's wanting more cowbell. I've got a fever and the only prescription is more atheism.

[@Destrolll]

Can an universe still have time if is static? That is the only way to be static. What does it even mean to be static if there is no time?

[@morsedruet1832]

Can we segregate time from space and vice versa?

[@seifharidi]

I think the question is not if there is no change there is no time. Could the question be: does change define the arrow of time either Newtonian or relativistic.

[@danielcox6143]

In 1999 for not much more than a second I left my body and met God in His Shekinah Glory, that one second was eternity. I experienced in that single moment (now - the present _then)_ everything I ever experienced in my life all at once. I was about 38 years old. So, all of a sudden I'm whooshed out of my body and experiencing everything i ever did, all of my past was accelerated into a consolidated eternal moment. That's what it is to be glorified, to die and go to heaven, that's the reward. I could see how God knew the future the same way as LaPlace's Demon. I knew as I was known.

[@GGoAwayy]

In a branching multiverse, identical spacetime coordinates exist in different places of that multiverse structure, whatever its topology looks like. Thinking about this moment in time but in a different branch feels sort of like moving "sideways" in time to me, even if its not actually traversible. (Writing sideways just made me think about the Mindscape episode with Lera Boroditsky.) What would an accurate spacetime coordinate look like that actually specified your multiverse branch? Wouldnt you need more than one time dimension to specify "Which May 3rd, 2020?"

[@yodajimmy2574]

From the very first minute: Then time can be an emergent property arising in any one such spatial dimension.

[@yodajimmy2574]

At such condition, time simply converges to being another spatial dimension of locations. The equation t^2-x^2 and pythagorean theorem become equivalent.

[@amitkhambekar3994]

Thank you Sean!

[@danielcox6143]

All thanks go to atheism, the biggest idea in the universe!

[@idesel]

Is there an animal that is significantly faster than a human, I mean in terms of it's day to day experiences, for instance if it were to have a small chat with a friend it will take like a second as compared to maybe 5 minutes a human will have.

[@divyabhmishra]

Although I get what you say about there not being two time dimensions, but the logic that this will give us enough freedom of movement to open the possibility of meeting ourselves may not be right. Even in one dimension of space, we can go back and forward at any speed or acceleration, however in time we get to only move in one direction and at one speed in a specific frame of reference. So just like entropy gives us the arrow of time in one dimension, there could be restrictions that disallow some of the paradoxes even with multiple dimensions of time? It's a question not a statement :)

[@jmoak311]

Why is the speed of light squared in many equations? a la: e = mc^2

[@matkosmat8890]

Hello, Sean, thank you for this series. I am far from understanding everything you talk about, but it's very interesting for me to try, having no background in Physics. My question is basic: why would a spacetime be curved if it has nothing in it? Isn't matter what causes curvature, to begin with?

[@henrymortensen7478]

E=mc2. Mass has an equivalent amount of energy and vice versa. When they measure the energy of empty space or vacuum, there is some energy there which therefore has mass. This is called vacuum energy or the cosmological constant, also known as dark energy.

[@gcewing]

Not just matter, but any form of energy -- and curved spacetime itself has energy. So spacetime can do interesting things all by itself.

[@robertw1871]

I have a suspicion that time itself is where modern mathematical physics fails, just having a basic geometry seems a highly unlikely description of it’s true nature. Length contraction dictates oscillating speed relative to the 3D feild, but is that all time is.

[@ssshurley]

Amazing 2 dims of time answer!

[@TheDummbob]

to the question about time: What i we're considering quantum mechanics? In order to get a quantum machanical system where really nothing ever changed we would need the system to have 0 total energy, because that would be the only way to stop the time evolution operator to change the system, since even 'stationary' states with finite enrgy will have a phase oscillating in time. Then we might argue that a system with zero net energy is a system thats not real, it has no physical existence. To stop the time evolution we need to get rid of energy, since energy generates time evolution

[@ulenrich]

The Planck length is part of the calculus of so many phenomenons. But if alien bros are on an earth moving fast, so fast that lengths are smaller: Is their Planck length smaller also?

[@viralpatel7969]

How do I ask questions? Where do I go ?

[@Shalkka]

In the two time explanation neither of the time dimensions worked as a one-way street. Closed space-like loops are not a problem. It is also unclear if two times leads to closed pathlike curves but the two time theory doesn't have closed timelike curves how is a two time theory still (wouldn't they by defnition not be time dimensions?). But I did end up connecting what physicist get uncomfortable about. In Achron if you have a paradox it will cycle throught its states without anything "classically physical" happening, it's all dynamics in the time-domains. To someone that really wants to think about only one time having two things attributed to the same time makes it seem like an inconsistent theory (Is the result 2 or 4? pick which is your prediciton). So why does quantum mechanics being reserved about predicting what happens trigger the same revulsions or does it? Was that part of the point of einsteins that it can't be the final thoery?

[@BeltramMilos]

To what is speed of light linked at, to be as it is 300,000,000 m/s. ???

[@BeltramMilos]

@@michaelsommers2356 Dear Mr. Michael Sommers. For example. The speed of the fish in water is linked to its power, hydrodinamics and to the density of the fluid, water. To what is speed of light linked at ? Kind regards. Miloš

[@BeltramMilos]

@@michaelsommers2356 Thank You very much Mr. Michael Sommers. I've read the Wikipedia link you directed me to. The Maxwell's equations. Hmmm. To much 3D Calculus and integrals and derivatives. He is one of the five most greatest. ... And there is also an ''Beltrami'' Eugenio. Ha. Ha...OK: Free space, vacuum is ideal, and rapresent an no obstacle (medium) for the speed of light. OK. But. In theory, what could we do, what should we change, in equations, to increase the speed of light ? Thank You for your kind answer in advance and receive best regards. And yes, I've looked to some KZbin videos in which scientist almost stopped the traveling of light....Miloš

[@BeltramMilos]

@@michaelsommers2356 Thank You again. And for Your patience. But in THEORY..what could we change.??? I hope forward. Miloš

[@BeltramMilos]

@@michaelsommers2356 Thank you very much Mr. Michael Sommers. And there are about up to 36 physical constants with fixed values. And thanks to them, all exists, we are alive and conscious. Nature is Genius. Cosmos is Genius. Filled with energy and quantum fields. And accelerating... I feel sorrow only for Universe. It will come a death also for it. In 10 to power 500 yrs. What an Cosmic injustice. Sencerely. Miloš

[@MrHeroicDemon]

Why did youtube take so long to suggest, I manually researched your channel after watching your lectures for a long time, watched all Veritasium, perodictable videos, deep sky, sixty symbols, numberphile, vsauce, and tons more smaller channels that isn't Brady. Steve Mould, Mark Rober, and I was like, "I wanted a more understanding of physics in all scales". Searched Sean up after remembering he was at the end of Veritasium video and explained difficult things nicely, as I picture it, fields were always in my mind, like how we are in air, or if we were in water/liquid, I imagined the universe as we know it to be again in a field/air/liquid type thing, which would either move too fast or too small, or too large and too slow.... I enjoy these thoughts, and using math to try and make sense of it. P.S. I've been watching perodictables since it was new, ive known these scientists for a long time, just took a long time to get through all the things I enjoy. Now time to binge watch Sean Carroll.

[@TheDummbob]

To the statement that electrons can't experience time, because they would lack an essential quality that is needed to "experience" time, namely the change of an internal structure of some sorts: I would disagree that they lack this essential quality, because the wavefunction describing the state of electrons is all the time oscillating, even for the electrons at rest, since the rest-mass-term in the Hamiltonian generating the time evolution of the electron wavefunction will generate an oscillating phase no matter the momentum or state of the electron. There will always be the possibility to experience the passage of time through this phase oscillation

[@TheDummbob]

@@michaelsommers2356 I got the Impression that he based this statement (that elctrons don't experience anything at all) on the argument that you would need *at least* some kind of internal change in order to be able to experience anything (I know that to him this would be necessary but not sufficient for "experience" to emerge) Thus he ruled out the possibility for internal experience of electrons. I disagreed that you can rule it out on that basis, because as I said, there is an internal oscillation of the wavefuncion going on, generated by the energy of the system.

[@TheDummbob]

@@michaelsommers2356 Well sorry then maybe I misunderstood your first reply, but it seemed to me that you didn't really react to the point that I made. Thus I assumed that you misunderstood my point. but maybe you weren't interested in it in the first place. Anyways, do electrons experience anything? Well, the word "experience" has no clear meaning here, so it doesn't really make sense to answer it. But what I have in mind when I use the word "experience" is something intrinsic, something that can't be "observed" by interacting with something on the "outside" thus it is an unscientific question to ask about the experience of electrons, and thus it borders on ideology to plainly answer the question with a "no". And I think it is important to distinguish ideology from science, science should not be abused

[@TheDummbob]

@@michaelsommers2356 I would say it's wrong to think that experience would imply a person. To me admitting the possibility that "dead" parts of the universe could have some kind of experience is not a personification, and it's not unscientific. (it's a-scientific if you will) For example, nowadays people are more prone to believe that animals have inner experiences, but one could argue that this is mainly because people can percieve similarities between themselves and animals. They can see similarities in their behaviour and it makes it plausible that animals can have experiences just as we do But there is no direct evidence of an "internal experience" happening in animals. Still it seems reasonable to believe so. The fact that we don't believe this with inanimate matter might just be our failure to "see ourselves" in dead matter. They don't act and react like ourselves, thus they must be dead inside. This is basically the same line of reasoning that made descartes declare that animals are empty machines (they don't act like we do, they are not "reasonable") I think this is just as much a logical fallacy as the "unscientific personification" that you put forth in your argument, just the other way around. Actually, I think it's like this: You are saying I'm personifying dead matter when I say that it could have an experience, thus alluding to some kind of spiritual world (which probably goes against your world view) But I'm saying: you are personifying people/animals by saying only those could have experiences, making people (or "higher animals" ) special in a way, saying there is a qualitative difference between living things and dead things. I think it boils down to the question of whether one thinks that such a qualitative difference is actually just an emergent difference, just a difference in form. I don't think that a new quality like this can arise by mere reordering of matter. and there is no scientific evidence for or against that statement. Thus it's still unscientific to believe one way or the other

[@TheDummbob]

@@michaelsommers2356 schade

[@Robinson8491]

Doesn't time have a helicity? It only moves forward. So why would it change when you add the second dimension? They would both still only move forward right, why would it suddenly be allowed to move backwards like the original helicity suddenly is not applicable anymore? You're changing the rules!

[@GGoAwayy]

Time exists without change because if you could travel back in time, you'd have to keep traveling back further and further to get back to when something did happen. If you can say "1 second since the last change in the universe" and also say "1 billion years since the last change in the universe" then obviously time is still passing.

[@Caleb-zu1pk]

Thank you, sir

[@dizy3513]

I still cannot get over a feeling that mathematics forces us to quantize physics... a circle has a finite structure but we calculate infinite... nope I can see it, it has ended clearly ... coastline paradox ... explain how we quantify without using infinite please ...

[@AlyssaMichelleSoap]

The lemniscate is the symbol for infinity, right? It's not my field, but maybe fully self explained like God (?). Your question reminds me of Dawkins proof for atheism, *God is Too Complex to Exist.* I think he's begging the question of God being material. It's easier to say, IF everything is material then God does not exist.

[@MyWissam]

Thank you.

[@lesliedellow1533]

Imagine the heat death of the universe. Nothing is moving, and nothing is ever going to happen again. Space will still exist, but in what sense could time and spacetime be said to still exist?

[@Nice-Y]

I'm surprised no one asked about reference frames from the twin paradox.

[@keithmccann6601]

How time causes gravity: So I think this is right but it’s only 'what I think’ and I’m not qualified so trust me accordingly :) - ‘space’ and ‘time’ are at right angles ….. and everything moves at the speed of light - light moves through ‘space’ at the speed of light and matter moves through ‘time’ at the speed of light - when you move through ‘space’ you deflect your speed through ‘time’…….. You probably hate the idea that ‘matter’ moves at the SOL - but it definitely does - it's just that most of that speed is through the 4th dimension - the more massive a thing is the more it moves through 'time' and ages faster - the less massive a thing is the more it moves through ‘space’ and ages less - massless things have their vector entirely in the ‘space’ direction and so don’t age at all So everything, ‘massive’ and ‘massless’ has a speed component in 'time' and a speed component in 'space' - and they add up to ‘C” - Very massive things move quickly through time and warp space a lot - less massive things move more slowly through time and warp space less - massless things don’t move through time at-all and don’t warp space at-all - so, in-fact, photons can be thought of as standing still everywhere and ‘everywhen’ simultaneously, while matter zips past at the SOL on a space / time vector directly proportional to its mass……. So far so good? - Ok now consider this - In a hypothetical universe without ‘matter’ you have only ‘massless’ things - so just photons - and since photons don’t experience either ‘time’ or ‘space’ - then, QED, - in that universe there is neither ‘time’ nor ‘space’ - that universe is dimensionless!!! - think about that - a dimensionless universe is neither big nor small nor anything at all - it has no measurable size at all…..!!! You must introduce some slower stuff - ‘matter’ - to get a ‘relative speed differential’ and thereby give birth to the concepts of ‘time’ and 'space' (spacetime) - and it’s that ‘relative speed differential’ that pulls and warps the ‘fabric of spacetime’ and gives rise to 'gravity' - so: Energy - Mass - Time - Space - Gravity - in that order - it’s that tension between ‘space’ and ‘time’ and ‘mass’ and ‘acceleration’ that causes the curvature of spacetime that we perceive as gravity!!!

[@joyecolbeck4490]

Thankyou

[@Mike-zf4xg]

Hmm. Not that I don't agree with his position of the abuse of privilege dealing wrt to time's existence, but I feel by using the angle that spacetime is a fundamental part of relativity is also adding bias and privilege. Absolute time is fundamental to Newton's framework, but this in fact turns out to be incorrect. Walking this line is tricky. Spacetime could be emergent from some other fundamental structure ie there are conditions where it doesn't fully manifest.

[@DeclanMBrennan]

Greg Egan wrote a novel called Dichronauts set on a world with *2 time dimensions* . However one of them really behaves more like a hyperbolic space dimension. There are various pages exploring the maths and physics here: www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html Some of the consequences are quite bizarre.

[@idesel]

Does the Christian apologist concept of their god being outside space and time make any sense in reality?

[@omarino99]

Hey Sean, thank you so much for these videos. I was wondering, have you already had your eyes on this recent paper? iopscience.iop.org/article/10.1088/1367-2630/ab76f7, it seems to want to predict the weird features of quantum mechanics by allowing the existence of particles moving faster than the speed of light. Would be amazing to know what your thoughts are on it.

[@AlyssaMichelleSoap]

Dfpolis (KZbin) has two videos on demythologizing quantum theory. He says there are no particles only discrete wave packets.

[@florh]

yeah, I disagree with the actual question "is time possible without change?" erm, if time is merged with gravity as einstein suggested, and everywhere in the universe you've got virtual particles popping up, everywhere entropy is getting higher, well, none of those things can happen without time... in a black hole where time supposedly stands still, that black hole in its entire, is still moving, so even when time halts, it's still there working as it should, so in my mind, there goes the assumption of a black hole having a singularity as a core, where time doesn't exist, what I do believe, is time runs a heck of a lot slower. I don't even agree with the number of spatial dimensions, i mean, take m-theory for example, when I ask chat gpt, "why exactly do forces need their own dimensions?" and then you get an answer that they give all forces spatial dimensions for them to work, 1 for time, 1 for em, 2 for the weak force and even 4 for the strong force. But my answer is intriguing when I say, why can't all those forces just like time just share the dimensions of gravity to work? Instead of giving all those forces their own dimensions, why not just change the levels of degrees of freedom for them to work? You can spin a tetrahedron on all 4 of it's axes at the same time, if you consider each line through the corner and center of the tetrahedron a axis, then they are at 120° from one another, and if you give each axis it's own stepper motor and battery, you can even spin a tetrahedron on even more axes at the same time. So why the need for extra spatial dimensions in the first place, when it comes to string theory, since m-theory is a part of string theory? You physicists make everything too complicated, try the simple things first (believe me, because all of you didn't for hundreds of years, i'm the one without a degree, without any titles, without any experience in that field, that invented the gravity-assisted-self-sustaining-mechanical-regulator/generator, or in other words, a solution for global warming and energy. (still have to build it though)

[@BenKrisfield]

'm pretty keen on the idea of 2D time - time, anti time... You would need two things, something pushing spacetime forward, and something pulling stuff-time back. Back to what... Of course, that's not part of Einsteins theories. But 2D time solves that pesky "back to future" paradox.

[@graybryan9521]

Your challenge for next week, should you decide to accept it, is to get a set of clippers and guards online, look at KZbin and admit that anyone can cut their own hair and do so. :)

[@markthebldr6834]

Yep. My wife is my new barber. Its waaay cheaper.

[@Robinson8491]

When you say that time is only a meaningful dimension if you can see things changing, aren't you then principally guilty of adhering to the supposedly no longer adhered philosophical stance in physics called operationalism (I learned a new word yesterday)? You are then for instance excluding the possibility of a random quantum fluctuation, or some tunneling effect from a different universe. Just saying' ;-) I love philosophy

[@wattooz]

Awesome

[@protoword10]

But profesor, if any “change” doesn’t exist, than time can’t be measured! If so, than, to me it doesn’t exist ether...Act of measuring would be enough for time to flow...

[@Jesus.the.Christ]

Does time exist apart from gravity? Is time a part of the manifold of Space (spacetime) or is it emergent, like gravity? I don't know how you could test this.

[@sanjuansteve]

The standard illustration of a sphere of mass pushing 'down' on a 2D grid of 'space-time' (as in the thumbnail of this video) is simply incorrect and deceptive. The grid should be getting pulled inward toward the center of mass of the sphere in all directions to be correct, and is much better illustrated with a 3D grid. Animating this and moving a semi-transparent Earth-like sphere back and forth through a 3D grid would clearly illustrate this, and even more clearly if gravity (in the animation) was turned off and on while moving the sphere through the grid as well.

[@llaauuddrruupp]

Terrific, but maybe turn up the sound more next time?

[@oursecretlord9008]

You can do that at home on your machine.

[@josephlombardo1246]

The answer to the first question about time existing without change feels intuitively lacking. The analogy of two planets separated by a vacuum is not the correct analogy because things change along that dimension. Fields, gravitational potential/spacetime curvature, the planets themselves, etc. The correct analogy would be a spatial dimension in which literally nothing was different. If you imagine everyone existed in the x and y dimension, and then the z dimension was just uniformly the same wave function everywhere. All fields would be the same along the z-dimension, the curvature would be the same, it would evolve the same way in time. If you moved in the z direction, nothing would change for you. In fact, you wouldn't be able to move in the z-dimension, because You couldn't experience that z dimension. It would have no effect on the world. It would effectively not exist. It is impossible to have a ruler that measures in the z-dimension. This is why time does not exist without change. At the heat death of the universe, in a static universe, there is no clock to measure time, and there is no time.

[@GGoAwayy]

But you could imagine being a minute into the heat death of the universe as well as being a year into it. (Ignoring that if you still existed and were doing stuff then its not the heat death yet.) If you had a time machine and could go backwards, you'd have to travel back further to get to before the heat death, so time is still elapsing.