Space-Time: The Biggest Problem in Physics

What is the deepest level of reality? In this Quanta explainer, Vijay Balasubramanian, a physicist at the University of Pennsylvania, takes us on a journey through space-time to investigate what it’s made of, why it’s failing us, and where physics can go next.

00:00 - The Planck length, an intro to space-time
1:23 - Descartes and Newton investigate space and time
2:04 - Einstein's special relativity
2:32 - The geometry of space-time and the manifold
3:16 - Einstein's general relativity: space-time in four dimensions
3:35 - The mathematical curvature of space-time
4:57 - Einstein's field equation
6:04 - Singularities: where general relativity fails
6:50 - Quantum mechanics (amplitudes, entanglement, Schrödinger equation)
8:32 - The problem of quantum gravity
9:38 - Applying quantum mechanics to our manifold
10:36 - Why particle accelerators can't test quantum gravity
11:28 - Is there something deeper than space-time?
11:45 - Hawking and Bekenstein discover black holes have entropy
13:54 - The holographic principle
14:49 - AdS/CFT duality
16:06 - Space-time may emerge from entanglement
17:44 - The path to quantum gravity

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[ahabkapitany]

0:17 seemingly minor, but important differentiation. The laws of nature do not break down. Our models of the laws of nature do.

[patrickhendron]

The only question we need

"Is that a mirrored fence?"

[kirigherkins]

my hobbies include watching quanta mag videos and pretending like I can even begin to understand what they're talking about
I am but a humble chimpanzee

[millamulisha]

Pretty wild that CFT/AdS is the easiest quantum gravity theory to explain to the public, even if it ultimately proves wrong. Goes to show how hard the problem of unification is.

[fuffoon]

Around the 5 minute mark I realized I'm really uneducated.

[raphaelkaume]

Thank you Professor Vijay and Quanta for this gem. The narration and animations are outstanding in the way you've explained the nature of the problem. You've given me a great way to think about various concepts that gave me a brain freeze, easily explaining manifolds and how the holographic principle might connect the quantum with the macro world...granted at very high level but just right for us curious general lay public. You're not overly complicated yet you're not dumbing down - this was just right and makes one want to dive even deeper.

[NothingverseOfficial]

The Casimir effect shows that empty space isn't truly empty but filled with fluctuating quantum fields. This phenomenon supports the idea that space-time itself might be a dynamic quantum entity rather than a passive stage for physical processes.

[scottbogfoot]

1:10 dude, that fence should be illegal!

[swollenrabbit]

very much enjoyed the video, but what's with the hilarious fence haha?

[eMbrys]

Nice! Hope Quanta Magazine continues to produce these mid-length explainer docus. It's great stuff.

I'd skip the high chirpy noises and such though, keep it a bit simpler on the audio engineering side.

[Jordan-xmwo]

Finding this while stoned at 11pm is exciting yet terrifying, but I have only a vague idea of what is going on

[vishalmishra]

Space-time curvature is all around us. Just throw a ball and watch gravity do its magic. Then, why do most videos on this topic sound so theoretical and abstract, when space-time curvature is so very visible all around us on the surface of Earth.

[googleyoutubechannel]

I feel like this is vastly overstating the importance of Ads CFT correspondence, and CFT in general, there's no reason to believe CFT is anything more than symbolic re-arrangement of properties and operators that some folks who are lost in math happen to think are good ways of describing the universe, even though we know those in Ads are 'wrong' in that Ads does not make correct predictions about observables... so while I think this approach is great, clearly 4d we've hit a wall with 4d spacetime and an every growing zoo of free params and 'fields' in QFT (and it still can't describe helium for god sake), CFT seems like a nothing-burger to me, it explains nothing, it simplifies nothing, it just looks like fun with algebra(s).

[speedybonsky]

sometimes i just question myself if this shi is real or just an abuse of zaza and lsd

[Wordsworth-og]

It turns out you don't need all that stuff you insisted you did.

[leontich]

As a physicist concentrated on experiments I would say the whole idea of "breakdown of laws of nature" is an empty sound. Useful to attract a broad audience though. The concepts of time, space, etc. depend on the situation you deal with. Those are useful approximations with no mysterious depth at all. There are no universal laws of nature that you can apply no matter what. An attempt to find them will fail because what science does it creates best possible models of its object of study. Specifically physics uses math, a lot of math, an enormous lot of math which is the core of physical theories. What circulates on YouTube is mostly interpretations of those theories. Those interpretations are useful only to those who actually knows math behind them. For other people they trigger wild fantasies. They give an impression that you understand how it works not knowing the theories themselves. But interpretations change, I myself create them when needed to better navigate the math. A good interpretation is really useful but never complete or universal.

Hello,

I'm a physics teacher and I really admire your work.

I'd like to know if I can use animated segments from your videos to re-record on my channel. I have no idea how to create those awesome animations!!

Without the animations, it's hard for the audience to grasp such a complicated subject.

My idea is to use some excerpts and re-record the content with my voice and my own script.

By the way, I'm only asking because I fell victim to a financial scam and lost about $20, 000. I'm desperate to recover that money somehow, in addition to my work with science textbooks. My channel is small and not even monetized... but I need to try something.

Warm regards from a fellow colleague,

Daniel

[uuu]

Einstein theories only descrube the observable effects of spacetime, while not actually telling us what spacetime is exactly.

[MuzaffarKrylov]

Bruh.... there is no such rhing as 'time!'

[charlesbrightman]

IN THE INTEREST OF FINDING THE THEORY OF EVERYTHING:
It seems to me that ANY theory of everything idea should be able to answer the below items in a logical, coherent, inter-related way. If that idea does not, then is it truly a theory of everything?

a. Numbers: Modern science does not even know how numbers and certain mathematical constants exist for math to do what math does. Surely the very nature of reality has to allow numbers and mathematical constants to actually exist for math to do what math does in this existence.

b. Space: Modern science does not even know what 'space' actually is nor how it could actually warp and expand.

c. Time: Modern science does not even know what 'time' actually is nor how it could actually warp and vary.

d. Gravity: Modern science does not even know what 'gravity' actually is nor how gravity actually does what it appears to do. And for those who claim that 'gravity' is matter warping the fabric of spacetime, see 'b' and 'c' above.

e. Speed of Light: 'Speed', distance divided by time, distance being two points in space with space between those two points. But yet, here again, modern science does not even know what space and time actually are that makes up 'speed' and they also claim that space can warp and expand and time can warp and vary, so how could they truly know even what the speed of light actually is that they utilize in many of the formulas? Speed of light should also warp, expand and vary depending upon what space and time it was in. And if the speed of light can warp, expand and vary in space and time, how then do far away astronomical observations actually work that are based upon light and the speed of light that could warp, expand and vary in actual reality?

f. Photons: A photon swirls with the 'e' and 'm' energy fields 90 degrees to each other. A photon is also considered massless. What keeps the 'e' and 'm' energy fields together across the vast universe for billions of light years? And why doesn't the momentum of the 'e' and 'm' energy fields as they swirl about not fling them away from the central area of the photon? And why aren't photons that go across the vast universe torn apart by other photons, including photons with the exact same energy frequency, and/or by matter, matter being made up of quarks, electrons and interacting energy, quarks and electrons being considered charged particles, each with their respective magnetic field with them?

Electricity is electricity and magnetism is magnetism varying possibly only in energy modality, energy density and energy frequency. So why doesn't the 'e' and 'm' of other photons and of matter basically tear apart a photon going across the vast universe?

Also, 'if' a photon actually red shifts, where does the red shifted energy go and why does the photon red shift? And for those who claim space expanding causes a photon to red shift, see 'b' above.

Why does radio 'em' (large 'em' waves) have low energy and gamma 'em' (small 'em' waves) have high energy? And for those who say E = hf; see also 'b' and 'c' above. (f = frequency, cycles per second. But modern science claims space can warp and expand and time can warp and vary. If 'space' warps and expands and/or 'time' warps and varies, what does that do to 'E'? And why doesn't 'E' keep space from expanding and time from varying?).

g. Energy: Modern science claims that energy cannot be created nor destroyed, it's one of the foundations of physics. Hence, energy is either truly a finite amount and eternally existent, or modern science is wrong. First Law Of Thermodynamics: "Energy can neither be created nor destroyed." How exactly is 'energy' eternally existent?

h. Existence and Non-Existence side by side throughout all of eternity. How?

* NOTE: Even General Relativity and the Standard Model of Particle Physics cannot answer these items in a logical, coherent, inter-related way. Shouldn't these above items also require accurate answers?