Existential physics: answering life's biggest questions - with Sabine Hossenfelder

Join theoretical physicist Sabine Hossenfelder as she draws on the latest research in quantum mechanics, black holes, string theory and particle physics to explore what modern physics can tell us about the big questions. Watch the Q&A here: • Q&A: Existential physi...
Sabine's book "Existential Physics: A Scientists Guide to Life's Biggest Questions", is available to purchase now: geni.us/oBMXZv
Subscribe for regular science videos: bit.ly/RiSubscRibe
Do we have free will? Is the universe compatible with God? Do we live in a computer simulation? Does the universe think?
In this talk, Sabine explores counterintuitive insights, and interviews with other leading scientists, to reshape your understanding of science and the limits of what we can know.
This talk was filmed at the Ri on 19 August 2022
0:00 Introduction - what is existential physics?
3:54 Does the past still exist?
9:55 The block universe
11:40 Can information be destroyed?
16:42 The measurement problem
17:27 Black hole evaporation
21:20 How did the universe begin?
29:17 Are we part of a multiverse?
34:53 Can particles think?
Sabine Hossenfelder is a physicist and science writer.
Sabine is currently a research fellow at the Frankfurt Institute for Advanced Studies. Sabine's research focuses on the foundations of physics, but her interests extend to the philosophy and sociology of science.
Sabine partly works as science writer. Her writing has been published, among others, in 'Scientific American', 'New Scientist', 'Nautilus', 'Aeon', and the 'New York Times'. Sabine has an active KZbin Channel dedicated to communicating current science news and scientific methods in general.
--
A very special thank you to our Patreon supporters who help make these videos happen, especially:
Andy Carpenter, William Hudson, Richard Hawkins, Thomas Gønge, Don McLaughlin, Jonathan Sturm, Microslav Jarábek, Michael Rops, Supalak Foong, efkinel lo, Martin Paull, Ben Wynne-Simmons, Ivo Danihelka, Paulina Barren, Kevin Winoto, Jonathan Killin, Taylor Hornby, Rasiel Suarez, Stephan Giersche, William Billy Robillard, Scott Edwardsen, Jeffrey Schweitzer, Frances Dunne, jonas.app, Tim Karr, Adam Leos, Alan Latteri, Matt Townsend, John C. Vesey, Andrew McGhee, Robert Reinecke, Paul Brown, Lasse T Stendan, David Schick, Joe Godenzi, Dave Ostler, Osian Gwyn Williams, David Lindo, Roger Baker, Greg Nagel, Rebecca Pan.
--
The Ri is on Patreon: / theroyalinstitution
and Twitter: / ri_science
and Facebook: / royalinstitution
and TikTok: / ri_science
Listen to the Ri podcast: anchor.fm/ri-science-podcast
Our editorial policy: www.rigb.org/editing-ri-talks...
Subscribe for the latest science videos: bit.ly/RiNewsletter
Product links on this page may be affiliate links which means it won't cost you any extra but we may earn a small commission if you decide to purchase through the link.

[@TimBitts649]

Sabine is a gem. The kind of teacher we all wish we had.

[@BonsaiBurner]

For an intellectual, Sabine always comes off as intellectually honest and having a "real" conversation. This is refreshing and appreciated in an ever polarized world.

[@georgesheffield1580]

Because she is

[@EllyTaliesinBingle]

because she's a woman

[@BonsaiBurner]

@@EllyTaliesinBingle authentic people come in all forms and are to be cherished, no need to exclude half the humans from the pool of people with the qualities because of sexism.

[@lrvogt1257]

I really appreciate how Ms Hossenfelder insists on what we don't know or can't know... yet. Her KZbin videos are excellent

[@schmetterling4477]

Except that many of the things that she insists on to be unknown are quite trivial. She simply relies on you not having read the literature on those topics.

[@davidpantoja676]

Just love that I can watch content like this at my leisure.

[@edwemail8508]

Thank you RI for bringing us a great lecture from Dr. Hossenfelder.

[@arasharfa]

Sabine is so extremely clear in describing complexities, i love her lecturing style.

[@hugegamer5988]

Probably because she attempts to disbelieve her own two hands at least once a day to make sure her understanding isn’t built on unfounded principles. Because of it she sniffs out the caveats and exceptions and frames them with a consistent and descriptive world view. I’m just afraid as an excellent German scientist steeped in dry dead pan humor she isn’t breaking enough stereotypes.

[@mrblackmamba117]

@@hugegamer5988 I had to read this more than once but it made me laugh when i understood what you said hahaha PS: That's the definition of a true scientist studying concepts humanity isn't able to grasp, yet.

[@iv0447]

As a former physics student I find Sabine's videos refreshing. I can recall when I first learned about a concept and why I still find it interesting, basic curiosity about the world. What I find most amazing is that the physical laws haven't changed as much as we have. I am an entirely different person today than the kid who liked reading about new particles and worm-holes at the library after school. Then I dreamed of discovering something that would change the world, now I am not sure technology is really the right means to do it. Science can tell us what we can do and how to do it, but not if or why. It is important to understand where we live and what we are made from, what is time...etc. But a more important question for me now is, what is consciousness, its function and how does it fit with what is being observed? I would like to see more on this....many "big" scientists have contemplated these questions, even without a theory or proof. Yes, That guy again : ) Sabine, you have a crush on Einstain!

[@rajendranegi7330]

Isn’t the object and the observation within the observer (consciousness) itself?

[@paultraynorbsc627]

👩‍❤️‍👨

[@captainoates7236]

Sabine. I am in awe of you trying to cover most of what I have learned over several years of watching your videos in one go to an audience. Your communication skills no no bound.

[@bobbabai]

I think the no-no bound is the edge of current understanding

[@lawman3966]

I'm pretty sure that a no-no bound is what holds a person in place while the person's spouse (or duly selected professional) dispenses physical punishment.

[@jamesyoungerdds7901]

The New York Post reported that scientists had found a boundary to Dr. Hossenfelder's science communication skills and her ability to convey complex topics in an approachable way. But when she studied the literature and consulted with colleagues in the field, it became apparent they had not in fact done that. It was merely a manipulation of the data. And upon further examination, the results were not repeatable.

[@dionysianapollomarx]

That's a double negative, Sir. Not exactly a positive statement without the correct spelling of "know." But, I agree with your intended praise.

[@hugegamer5988]

@@dionysianapollomarx you’re failing to not apply logic there buddy.

[@plasmaburndeath]

OMG I am so excited that Sabine got to be in person lecture 😀 I discovered her work and her KZbin Channel a few weeks ago and now she is popping up everywhere heh.

[@FLScrabbler]

Pleased I discovered her and her channel about two years ago... 😇

[@michaelhunter2136]

I like simple stand-up presentations like this rather than videos with sound effects and flashy video editing. This was a special treat for me.

[@Hithere-ek4qt]

Yes, I agree Wholeheartedly.

[@deborahferguson1163]

I agree too. More relatable and enjoyable! (Though I have liked the other presentations)

[@spirosmatthaios2297]

Sabine is always impressive and inspiring! Allways to the point and "down to earth" despite being an astrophysicist!

[@vauchomarx6733]

Technically her main field is not astrophysics anymore, but phenomenology and foundations of quantum mechanics. But I agree with the sentiment, Sabine is brilliant!

[@cosminvisan520]

@@vauchomarx6733 No. She is just a mumbo-jumbo materialist.

[@klammer75]

@@cosminvisan520 I do not think those words mean what you think they mean🤔😉🤓

[@cosminvisan520]

@@klammer75 They clearly do. Consciousness is all there is.

[@rubiks6]

@@cosminvisan520 - Ummm ... okeydoke. (rolling eyes) That's a bit contrary to the empirical evidence, wouldn't you say?

[@needheartranken]

Everytime I get crazy contemplating about what reality is really, I look at Sabine's videos and gets calmed. Thank you, Sabine. My existential crisis saviour! We DON'T KNOW, and I think that's enough. Living as a human in the blip of time is an amazing ride!

[@northernhemisphere4906]

Good point.

[@JohnPretty1]

Reality is a word. Existence is sensory.

[@_lumos4029]

@@JohnPretty1 existence is also a word

[@koenth2359]

4:20 Off by a factor 3600. But never mind, fastfast.

[@zetacrucis681]

just a slip of the tongue

[@gefginn3699]

Sabine has a great gift for delivery of tough topics. A great teacher.

[@Mando4Dan]

Much thanks to Sabine. Big concepts translated to an easy to understand, honest presentation about what our collective observations teach us. (And clarity about what they don't).

[@ramkumarr1725]

Good reading.

[@crowlsyong]

You and Matt O'Dowd are keepin it real for us all. Thank you.

[@CrowMagnum]

The "for all we currently know" bit is something that needs to be said more. So many scientists state current theories as truth instead of the best theory we have. Please keep saying it and don't bury it in a footnote.

[@DarkSkay]

Absolutely agree. Of course, it would be impractical to relativize every single detail that may or may not be seen differently in the future, in coming centuries of research and philosophy. A special case are arguments built on non-falsifiable assumptions that should ideally always be declared, unless evident from context or part of common sense. Finding a good balance here is even more important when venturing into what could be called "mataphysics" (*), where philosophy, metamathematics, belief systems and other perspectives can build on experimental and theoretical results gained in physics.

[@liralenx3513]

I agree, too. I've been a fan of Dr. Hossenfelder for several years now, for precisely that reason. That was because physics had reached the point where empirical science was in the process of being discarded as unnecessary. She was the first physicist I read who objected to that, probably at great cost to her professionally.

[@didack1419]

@@DarkSkay Can you give examples of "special cases"? "Metaphysics" is too much of a fuzzy word for any of this, I know that you know that by your own clarification, but regardless.

[@DarkSkay]

@@didack1419 Perhaps following are decent examples of non-falsifiable assumptions: 1) "information cannot be created or destroyed" 2) "in our universe 'true randomness' resp. 'divine dice' do not exist" 3) "the cause of inertia will always remain hidden" 4) "the flow of time in the universe is synchronized by clocks, which are driven by another clock, which in turn is driven by yet another clock...etc." 5) "observing or not observing has no influence on events"

[@steveflorida8699]

@@DarkSkay "finding a good balance" might very well venture into "metaphysics". Case in point --- the origin of Life (living organisms). Scientists know WHEN Life appeared in the evolutionary timeline, but do Not Know HOW Life was introduced into lifeless molecules. Having said that, Life is Not Inherent in mechanistic atoms and lifeless molecules. Also, physics cannot quantify the Values of... love, beauty, goodness and truth. The human condition, experiences and nature is more complex, than mere scientific analysis.

[@wacomtexas]

Thank you Sabine for a great talk, and for reminding us that what we don't understand is as important as what we think we understand..

[@AlexanderWerner]

Sabine talking at the RI. Like Christmas and Birthday at the same day! Superb!

[@eljcd]

At last! I wanted to see Sabine in the hall for years, thank you!

[@rayoflight62]

Congratulations Dr. Hossenfelder for the great intervention!

[@MrHelkeys]

Sabine in RI. Definitely on my must-watch list.

[@bernardputersznit64]

Wonderful! - So many diverse ideas al brought together in one talk - Thank you Sabine, you are the best 🙂

[@davidhowell2269]

Thank you so much for such an interesting and thought stimulating talk. Thank you also for making the trip to the RI, I hope they are looking after you!

[@spiralsun1]

Yes, Rhode Island is beautiful this time of year 😂

[@carlosalbertoteixeira375]

How great to see you here, dear Sabine! Grüß dich from Niterói, Rio de Janeiro, Brasil!

[@WyomingGuy876]

Sabine, I love your lectures! Don't stop 😁

[@stevefrayne]

I love your strict adherence to the scientific method. It is best shown in the 16 seconds from 20:51 to 21:07 where you focus on what we KNOW (presume true) and go out of your way to minimize your own speculation. So many physicists irresponsibly speculate on these matters well beyond the frontier of science but not you. That is why you are gaining popularity among serious supporters of the scientific method. Thank you for keeping us from fanciful daydreaming and calling that science. Gratefully yours, SF

[@schmetterling4477]

Sabine's papers seem to be almost entirely speculative. I am not aware that she has worked much on some serious data driven physics.

[@Syntax753]

This is outstanding! What a wonderful presentation

[@ThePepzo]

Great talk Sabine! Thanks Ri!👍👍👏👏🙏🙏

[@nagendrabala9756]

Sabine is a great explainer of physics 🥰😎😎

[@goduniverse1024]

She’s the best in the world 🌎

[@gastonlagaffe9156]

And, I think, a reliable one.

[@cosminvisan520]

@@goduniverse1024 Fanboy.

[@carlospenalver8721]

There’s several signs that serves as a telltale of “if” or “if not” someone may just be a genius , and that’s when they say “ I” or “we” don’t know. She’s definitely extraordinary. Someone who thinks they’re genius never if rarely admits to something they’re not sure about trivial or complex so, yeah she’s not full of bullcrap.

[@ChaineYTXF]

she's probably a good teacher

[@shuchko]

Thank you for inviting Sabine, she is my favorite science communicator

[@MichaelHarrisIreland]

A breath of fresh air blowing through the houses of science. ....from Ireland.

[@schmetterling4477]

Sabine? Sabine is the most boring physicist in the world. ;-)

[@Thomas-ws6lk]

​@@schmetterling4477 and you are the most boring Sucker. Not need to work? Hope I don't feed you with tax pay ❤

[@jamesyoungerdds7901]

Thank you for such an amazing presentation. You have such a huge fan base of people who care about your unbiased take on physics and reality. If you've ever watched John Carmack talk about video games and technology - he just cares about pursuing what's true and real, and I get the same vibe. Really grateful for your dedication! 🙏

[@HystrixLignum]

Sabine seems to be a little nervous and slowed down the Speed of light a little bit... but she's briliant as ever.

[@SubroutineLtd]

A slip of the tongue but yes, as you also say, quite brilliant. In the debates I've seen, she is able to hold her ground against anyone and stays remarkably unfazed, nomatter who they are.

[@bytefu]

That's just the nature of a live talk, in contrast to a recorded video, where you can record the lines multiple times and pick the best takes.

[@eswn1816]

3,600 times slower!?!? (I noticed too... 🥺)

[@barnicle77]

Yes, and she sped it up too (or she stands very close to people). At 5:24 she said it would take a fraction of a nano second for light to reach her from someone else. People! Light travels at 1 foot per nano second ... sorry to those of you that only use metric.

[@jamesyoungerdds7901]

Ha, I noticed that too, but it kinda adds to her charm. I was watching the video thinking, I can relate - I'm very nervous speaking in front of audiences, so I'm sure I'd make at least that same slip. Love it.

[@francescos7361]

Thanks for this contribution .

[@JOED338]

I love these series. Can learn so much.

[@davidkent2804]

Why is intellectual honesty so rare? Thank you, Sabine

[@michaelfried3123]

because some science disciplines have been hijacked by philosophy and pseudo-science.

[@davidkent2804]

@Edward Armstrong No proof - personal observation about the media and the way it seems to affect people

[@manalzd764]

Sabine is amazing as always. So excited to listen to this!

[@cosminvisan520]

Preaching her materialistic dogma ?

[@smlanka4u]

Yes, she is amazing. Science is simple than we think. The universe is locally simple, and it depends on relationships between emptinesses. Moments of time at each location could make the relative time and distance. And the conversation of energy law could make Big Bounces and other cyclic processes. Scientific explanations must predict both the past and the future. Therefore, it requires reliable scientific calculations to confirm scientific explanations. If some observations infer that the universe was a tiny singularity with infinite density, then that prediction should be able to predict the previous state of the tiny singularity in order to call it a good scientific prediction.

[@cosminvisan520]

@@smlanka4u Nobel prize for you!

[@williamhatfield8935]

A fascinating talk and exposition of the sorry state of modern Physics with its plethora of false assumptions. Instead of asking how did the Universe begin the question should be is there any reason to believe that the Universe had a beginning. We should not be asking if information is lost as it enters a Black Hole but why the myth of the Black Hole has continued for sixty years. Einstein inevitably pops up with his set of insoluble equations which can only be solved by making a series of assumptions depending on what result you would like to have. His equations contain as a main element G which is assumed to measure gravitational attraction when it can be easily shown to be a repulsive force by carrying out Cavendish’s classical big G at the surface and deep underground. The deeper you go the lesser the value of G so obtained. CMB measures the radiation of black objects in the Universe, of which there are many and easily seen and nothing to do with some creationist cult. And so on.

[@cosminvisan520]

@@williamhatfield8935 Universe doesnt exist. "Universe" is just an idea in consciousness.

[@CosmosNut]

Thank you for another remarkable presentation about the current state and enduring and current issues.

[@Turbohh]

Honest, frank, humble and fascinating. Thank you.

[@doncourtreporter]

Light travels 300,000 km per second and she knows that. She misspoke at 4:23. That never happens with her. Sabine is a treasure.

[@zbigniewprzedpelski7221]

Nobel prize 2022 in physics on enanglement which travels faster tjen light

[@doncourtreporter]

@@marope Hahaha. Sadly, you are likely correct. Peace.

[@doncourtreporter]

@@zbigniewprzedpelski7221 Non-sequitur. I am correct.

[@boring7823]

Hopefully this now means she will remember that a nice measure of lightspeed is 30cm/ns.

[@beattoedtli1040]

Naaa, c=1

[@StaringCompetition]

Subbed to both RI channel and Sabine, did a bit of double take there. Wonderful.

[@hybridepigenes]

I love your approach and educational perspective. You are infinitely inspiring.♥️♥️♥️♥️♥️

[@gilliebrand]

A great lecture, thanks for sharing. 👍

[@TheSonicfrog]

A brilliant talk at a historic venue, well done! FINALLY the answer to life's biggest questions ... we ... just ... don't ... know ....

[@mercurius8725]

Bring back content like this, this is great

[@erikfinnegan]

Inspiring talk. And I am also very much pleased with the visual material !

[@tresajessygeorge210]

THANK YOU...!!!

[@laioren]

Dr. Hoss is a BOSS! Love her, love her work.

[@George_slough]

I love Sabine's videos and dry sense of humour!

[@JohnChampagne]

Re Sabine, what's not to love?

[@Dbean48]

I do enjoy your intro to physics..Thank you.,

[@Operakid]

Thoroughly enjoyed. Great stuff.

[@markcampbell7577]

Now depends on the velocity and the relationship between the objects. The object or subject that is moving at any speed has a different time interval. The coast to coast airlines experience an increase in interval of about 30 minutes. The elapsed time difference between the stationary time and moving time is 30 minutes. As real as a normal travel experience.

[@robertromero8692]

I like the way Sabine is willing to answer "we don't know" to some of these questions. It would seem that there are limits to our knowledge.

[@kensurrency2564]

we are pattern-recognizing creatures. if one thing is constant throughout human history, “i don’t know” is a consistent theme. so, does that imply that there will always be some mystery for us to seek an answer? are knowledge and understanding an infinite process? i think that’s quite probable.

[@kx4532]

Thanks for it!

[@TurboJon]

Sabine is an excellent presenter and clear, logical thinker. Very much enjoy her talks and books. Looking forward to reading this new one. Thanks.

[@leematthews6812]

Yes, it was a Friday night, and there was a tube strike, but even so, it was disappointing to see so many empty seats. I hope Sabine makes a return trip sometime, I would certainly be interested in attending another of her lectures.

[@freddyjosereginomontalvo4667]

Sabien good and excellent video. I love your channel.

[@SeeSawMacaw]

Thanks Sabine! Thanks RI!

[@paxesmab]

You are my hero Sabine! Thanks!

[@Mikey-mike]

We can thank our lucky stars for Sabine. Good one, Sabine.

[@mindyourself7063]

I'm in agreement that the "popular" trending topics of modern physics simply push the mathematics beyond their assumptions and constraints. Strikingly reminiscent of the creation stories conjured as our ancestors projected to model existence. I'm encouraged that Sabine is humbly speaking to this issue with perhaps more decorum than I can muster given the disservice that mis-information many in this field capitalize on.

[@user-yv8bw3zf6n]

May I ask what kind of scientific background you have?

[@daarom3472]

I think we have to distinguish the hard work of the thousands of unknowns grinding out measurable results and carefully stacking mathematical explanations from the 10-20 popular physicists talking about multiverses and the like (Kaku, Green, Dyson etc.). These people do not represent the entire physicist community yet are always assumed to be by Sabine/Erik Weinstein. Not saying Physics is perfect, but we have to accept that its harder and harder to provide empirical evidence for our theories and most of the low-hanging fruit has been plucked. It's extremely unlikely we'll ever see guys like Newton, Maxwell or Einstein again as any progress will require an insane amount of specialization and collaboration.

[@mindyourself7063]

@@daarom3472 Agreed. The distinction is important and I'm heartened to see I'm not alone in tiring of the attention seeking subset seeking celebrity with such conviction. I share in your short list here as a good start for case in point.

[@datchentai3047]

I really enjoyed this talk. A very nice overview of these existential physics questions made easier to understand by the lay person

[@schmetterling4477]

Which of these questions was existential to you? By that I mean more important than where next month's rent will come from or how to get a girlfriend? ;-)

[@andrewcliffe4753]

Brilliant lecture

[@woodchuck9]

Sabine in the house! Amazing 😊

[@nealkonneker6084]

I really appreciate your willingness to say "we don't know". Thank you. Real scientists like you may someday restore the credibility of "science" squandered by those who misuse it.

[@sfinxwojerz]

science and academia are to different things.

[@malcolmdavis-zl4xy]

Superb, as always!

[@mrtransmogrify]

You know an academic and an explorer and a knowledge seeker when they talk semi-confidently and they know LOTS of things, yet they don't know... 👍🏽 we need more ppl like Dr Hossenfelder... There probably are bt they get pushed to the background by those confident-talkers.

[@danieljohnmorris]

Very happy Sabine has joined your roster!

[@archstanton1628]

The speed of light seems to slow down hugely for Sabine 😁

[@claudiozanella256]

3600 times.

[@DeathBender]

great lecture !

[@RockAristote]

Sabine is the simply the best!

[@hartwigkoppi901]

Hello! I have a question to the multiverse theory: If all universes are expanding (maybe infinitely) like ours, is there no risk that they are colliding? And what gravitational impact must they have to each others?

[@MentalFabritecht]

It would depend on what space they lie in. They could be embedded in a much higher dimensional space where they would not necessarily intersect each other... it gets pretty wild 😅

[@elisabethloxley6124]

I have seen Sabine in a discussion with Michio Kahu and Roger Penrose. She and Roger concluded that the ‘multiverse’ is not science, but conjecture.

[@johneonas6628]

Congrats Sabine.

[@rudolfquetting2070]

The Multiverse and the Flying-Spaghetti-Monster have one thing in common: Both principally can’t be refuted by observation. Hence, there must be a consistent theory, which can do without. And Sabine just follows Ockham’s Razor, whe she says, good science can easily do without multiverses.

[@bh2020_]

Sabine's perspective is very interesting. I listen to her often.

[@simonrigac8201]

This is what I've been waiting for! Finally a refresher for the already a bit boring RI.

[@irfanbanning5393]

Every discussion of simultaneity I've seen makes it dependent upon the observer, but I think it ought to depend on causality. Using the train thought experiment, if the train is an inertial frame, and the lights are controlled by equal-length circuits to a common trigger, then the flashing is simultaneous, no matter how the light is seen by observers.

[@davidsalvia6294]

You are almost right. There is a mistake in the video. The flashing lights needed to be outside the train, then the flashes would arrive at the same time for the outside observer and at different times for the woman on the train.

[@daylesuess552]

Your missing the point. Simultaneously to who? Time behaves differently for every observer. If there are no observers what and who determines or decides time. If there are no particles and no waves there can be no measurement of time. What would you be measuring and what would be an interval of time? E.g. No turning of the earth for a day, no vibrations of an atom to mark a second. Time would have no meaning.

[@davidsalvia6294]

@@daylesuess552 I'm not missing the point. Sabine is misquoting an Einstein's gedanken experiment.

[@realfreedomofspeechoriginal]

@Irfan I don't think the past still exists at all. We are only seeing the light, not the actual physical event. People seem to be equating seeing an event with actually experiencing an event.

[@APaleDot]

You don't need an equal length circuit. You can set each light on a timer such that they flash at the same time from the point of view of the person on the ground. The person on the train will see them flashing out of sync with each other.

[@anonymous.youtuber]

❤ this talk !

[@katg-gk5ox]

Very nice job! Brava!

[@Nocontextturkishboi]

she’s the queen/icon of popular science! I admire her holistic thinking and storytelling skills!! Even though many scientific problems and perpexling paradoxes that always blow people’s minds were mentioned, i didn’t lose my focus or have existential crisis moment 😂 she was really careful and sensitive while choosing the words!!! Also her mental & emotional stability is so consistent even in the subject that everyone can go crazy and feel excitement ( such as multiverse)

[@protocol6]

The problem I have with block time is that if you ask "does the past exist," implicit in that question, thanks to the tense, is "still" or "now." Asking if 1 minute ago exists now is the same as asking if 18 million kilometers along your path behind you is where you are. Unless your path through space-time is cyclic, the question has already answered itself in the negative. The past exists at its time, not at a different time. You can argue over whose numbers you assign and in what coordinate system all you like and it won't change that.

[@jeffkaylin892]

If "now" exists then I'll agree that the "past" exists. However, to make a VERY subtle distinction, I believe "now" is existing, not exists. Something about present perfect or present progressive verb meaning. I don't believe the present leaves a wake behind it. I'm called a Human Being, not a Human Is.

[@maximumentropy1863]

Is "now" equal, exact, a precise finite moment in "time" everywhere, in all that exist (including void or empty vacuum", in the all encompassing term Universe of infinitum, not observations from the speed of reflection (by some detections: eyes, made devices or other "natural" way) or the speed of the expanding Universe from infinite different points in the Universe?

[@DrDeuteron]

Does now exist at the andromeda galaxy? 2.5M light years away. Approaching at 0.1% c. Lorentz transform says right now in andromeda their right now says it’s 4522 AD on earth. In a similar galaxy receding it’s 478BC. Who is right?

[@protocol6]

@@DrDeuteron The light you see arriving from Andromeda is old, yes. That's got nothing to do with what's going on in Andromeda now. Your inability to see it is a separate issue.

[@protocol6]

@@maximumentropy1863 The manifold that I would call "now" isn't entirely free from local deviations in curvature so you'll always have to argue over whose numbers to use. Those defects are mostly due to gravity's effects on the rate at which clocks tick. Distance from the origin also affects the rate at which clocks tick so gravity's effect on time is just as much to do with that region of the manifold being closer to the origin. In that sense, you could say more than one time exists simultaneously but it's a small amount and spatially separated so it's trivial compared to the idea of block time. The moon, for instance, is a bit older than the likely collision event that created it on Earth. When we go there, we travel into the future more than normal. When we come back we travel into the future less than normal so Earth catches up with us since the Earth is, in one sense, in the moon's past and it's not just the mutual issue with light delay. That's a very different notion than block time, however, that's just a lumpy surface. I see no evidence that the surface isn't simply connected and infinitely thin, though thinking of it in terms of the minimum proper interval between two events at any given point is useful, particularly as that 4 volume is invariant so you can use that to work out the rate clocks tick at any given distance from the origin.

[@tcarr349]

Sabines presence at this great hall is Spectacular.

[@Inpreesme]

Thank you

[@bartvenken7138]

What I always wonder about re the speed of light: imagine two fotons going opposite directions, is their relative speed (in reference to each other) still the speed of light? If so, how does this happen?

[@russellsharpe288]

Yes. According to Galilean (and Newtonian) physics if two particles are travelling at velocities u and v then their velocity of the second relative to the first is just v - u. But in Einsteinian relativity this relative velocity is instead (v - u) / (1 - uv/c²). (c is the speed of light of course) So eg if two particles were travelling each at half the speed of light but in opposite directions, their velocity relative to each other would be (0.5c - (-0.5c)) / (1 - (0.5c)(-0.5c)/c²) = (0.5c + 0.5c) / (1 + 0.25c²/c²) = c / 1.25 = 0.8c. In your case with two photons, each travelling at the speed of light, we get (c - (-c)) / (1 - (c)(-c)/c²) = 2c / (1+1) = c.

[@BulentBasaran]

@@russellsharpe288 Applying relative velocity concept to photons would very quickly lead to absurdities. E.g., imagine two photons going in the same direction (u = v = c = 1). Their relative speed would then be: (1 - 1) / (1 - 1/1) = 0/0. To stay clear from such inconsistencies, we better apply relative speed calculation only to inertial reference frames with v < c. Also keep in mind that to a photon, time never passes. So, speed is not even a meaningful concept from the perspective of a photon. Light is special :-)

[@schmetterling4477]

@@BulentBasaran The distance between two wave fronts of the same light wave stays constant in every coordinate system (it's just not the same because of Doppler shift), so the scenario is neither absurd nor particularly complicated.

[@RubensteinIan]

if two trains are moving 60 miles an hour in opposite directions, how fast are the trains going?

[@schmetterling4477]

@@RubensteinIan That depends on the observer, kid. You need to pay attention on school. ;-)

[@D-Fens]

This channel always has the real intelligent and quality content I desire 😅👍

[@cosminvisan520]

Neah. It just has materialistic mumbo-jumbo.

[@scott1285]

Watching Sabine talk about chucking turkeys into a black hole in thanksgiving

[@a.randomjack6661]

@@cosminvisan520 You reLLY should stick i you lala realm 🦄👻👼 This universe makes you too whiny

[@cosminvisan520]

@@a.randomjack6661 Universe doesnt exist. "Universe" is just an idea in consciousness.

[@solexxx8588]

Great talk.

[@mpperfidy]

Sabine is always interesting and educational. This is the first lecture I've seen from her, being a longtime follower of her YT channel. She's possibly even more impressive as a lecturer.

[@schmetterling4477]

Not really. She is much better at tearing down than at teaching. How can I tell? I have read some of her papers. They are not very good. She lacks a clear grasp on the topics she likes to write about as far as I can tell.

[@tom-kz9pb]

Sabine seems like a fellow traveller with the same condition that many of her viewers probably share, something that you could call "ontological wonder sickness". As I would describe it, "ontological wonder sickess" is when you have a burning passion and curiosity, and really, really want to know the answers to unanswerable questions, like "What is time?" "How did the universe get here?" "What is the ultimate, deep reality that causes the laws of physics to be what they are?". We cannot answer the questions, but cannot stop the itch to keep asking them, so we helplessly chase our own tails, ridiculously like a kitten, in a whirlwind fury, Maybe there are highly advanced aliens watching us, and finding it hilarious, yet it still seems somehow like the most noble endeavor of life.

[@atarahashirazi1941]

You should put this on a t-shirt. I would buy that. It's quite accurate.

[@ThePurza]

Good point. Accepting that we just don't know many many things seems too hard for some, hence all the crazy theories out there

[@douggale5962]

I have seen first hand that anything that can happen, will happen. In software, I have seen bugs that are incredibly unlikely occur, occurring. You have to completely seal off the possibility and make its probability zero, or it will eventually happen, no matter how unlikely it is.

[@herwighuener3256]

Quite true. Serious working to defend agains such things requires time, and managers do not like time being spent. But see the bright side (of life?!) of it: There is a branch of the multiverse, where a software-error in *every* ICBM in the world renders the thing non-functional, so there is a happy mankind downstream of history in at least some universes!

[@botboy0]

@@herwighuener3256 assuming the multiverse does exist ^^

[@herwighuener3256]

@@botboy0 As soon as you get used to it, you cannot help "believing" it. So, SH ist quite right in comparing multiverses with some sort of religions. However, physicists do not burn non-believers on the stake. That must mean something. I have to forward a more precise statement: whenever I personally speak of multiverses, I think about the Everetian kind and the Max Tegmark kind. All others: maybe, or maybe not. I am not fanatic about it. Not in this universe.

[@ianseaweed]

Now this is gonna be a treat

[@pasadenaphil8804]

Awesome! Great book too!

[@SimonBrisbane]

Sabine is almost the exact same age as me. Talking about relativity , this leaves me feeling like I’ve wasted my life!

[@99guspuppet8]

You have wasted your life…… Sabine has wasted her life…… Let’s all go to Sugar rock Candy Mountain

[@JmWro]

5. I think that particles, well atoms to be precise, do think and they are conscious. I'm not saying that there is proto-consciousness or hidden properties. I just think that we need proper definition of consciousness. You can not say that we don't know what consciousness is. We invented that term. So the definition should be: consciousness is the ability of the system to respond to the environment. So single electron is not consciousness. But hydrogen atom is. It can absorb light and the re-emit it in it's own frequency (hydrogen line). And all other systems are just more complex so they are more consciousness, more inteligent. There is no magic. It's just growing complexity. If you don't agree with that you should then tell when consciousness started? Were your parents conscious? Grandparents before them? Neanderthals before them? Dinosaurs before? Bacteria in oceans? RNA? Carbon dioxide molecule? Where you draw the line? Only logical approach is to draw the line before the first system that started this whole evolution. And that was a single hydrogen atom.

[@theunifiedtheorycom]

No, its actually more ‘hidden’ than this! Mind is always conscious. There is no such thing as unconsciousness. And the hidden aspect everyone is looking for is the ‘Fulcrum.’ Mainstream physics will have to understand the electric wave to this level of detail if they want to understand the unified theory. Lauren Dove, author of ‘‘The Design Equation - The Unified Theory and the Mathematics of Hidden Dimensions.’

[@stn5334]

First hominids What do we gain from this redefinition?

[@APaleDot]

But single electrons _can_ respond to their environment, after all a hydrogen atom can only absorb light _because_ the electron gets pushed to a higher energy state by a photon.

[@JmWro]

​@@APaleDot I would argue that in such case you just increase potential energy of the whole electron, so the electron itself is not internally modified. But I might be wrong. Then we can say that single electron is also conscious, sure, why not :).

[@rubiks6]

Around 13:30, you begin talking in detail about "information" and you talk about how a state can in principle be projected forwards and backwards in time. I was wondering how uncertainty impacts these projections and how uncertainty impacts the existence of the past. (Time is the strangest of all things.)

[@j.477]

for me it's not depressing at all when she keeps on destroying my scifi-dream-bubbles, even though very often I have to first go through a seconds long phase of disappointment, when Sabine is saying FTL travel isn't very realistic without negative energy or sth like that... but whenever I listened to Sabine, she takes out all the nonfacts and what's left are facts that I can trust. This gives me a real good sensation of clarity and a sense that things have been put in the right order again...she relentlessly separates the wheat from the chaff ... I really appreciate that. thanks!

[@PaulSmith-pi4om]

Fab, It is the first time i have heard a Sabine lecture without her mentioning cheese. Balls to carrots and cats. MORE CHEESE!

[@MattHare]

She finally performed where she should be a long time ago. Mock me but to me, she's the next Feynman. Witty, funny, no selling or taking BS. She's also musical, just like Feynman was (well, in a way, at least)

[@uku4171]

Feynman did groundbreaking research

[@stephenhicks826]

Sabine is the best Physics / Science explainer on the internet. This was a wonderful talk.

[@schmetterling4477]

Also full of technical errors. ;-)

[@aleksandrpeshkov6172]

@@schmetterling4477 Enters Optional Chronology : " I challenge You to provide the notion of... Particle..."

[@schmetterling4477]

@@aleksandrpeshkov6172 Enters that you didn't pay attention in school. ;-)

[@aleksandrpeshkov6172]

@@schmetterling4477 Enters MeterLingam : " I am relative to... Myself..."

[@schmetterling4477]

@@aleksandrpeshkov6172 Enters total idiocy. ;-)

[@marinogrivonlaverde6963]

You are one in a kind! I feel illuminated .