Einstein's description of gravity just got much harder to beat • Hacker News Board

Re: Einstein's description of gravity just got much harder to beat

#61

Post by hoveringhen » Sat, Oct 03, 2020, 11:50 AM UTC

bsaul wrote: post #18

niho wrote: post #17

Earlier quoted context omitted.

> I often think about why/how medieval artists couldn't quite grasp how to depict depth in their paintings (as a lateral thinking technique). The explanation for that is generally agreed on to have more to do with the aesthetics of early Christianity influencing Medieval practices, more than changes to human perception (source: I have a masters degree in Art History).

could you give more on that ? i can imagine how religious dogmas can influence a lot of things, but representation of depth ??

Not exactly what they mentioned, but check this out [1]. It talks about the perception of time in the middle ages, and how merchant time slowly substituted the impractical notions of church time. A professor of mine who was talking about medieval paintings and their ideas of space and time suggested this paper.

[1] https://journals.sagepub.com/doi/10.1177/053901847000900411 - Church Time and Merchant Time in the Middle Ages - Jacques Le Goff

Re: Einstein's description of gravity just got much harder to beat

#62

Post by bryanrasmussen » Sat, Oct 03, 2020, 12:48 PM UTC

Taek wrote: post #2

Actually, it just got easier to beat. What happened was scientists found a new way to measure the effects of gravity that can substantially distinguish between different proposals to unify general relativity with quantum mechanics. The article says it got "harder" because the theory of general relativity wins every measurement, and that means the search space for potential solutions is much smaller - you have more co…

Perhaps it should have said:

"Einstein's description of gravity just got much less likely to be beaten"

Re: Einstein's description of gravity just got much harder to beat

#63

Post by Kednicma » Sat, Oct 03, 2020, 12:58 PM UTC

henriquemaia wrote: post #56

nestorD wrote: post #45

Earlier quoted context omitted.

As others have said, I believe the lack of depth in medieval art is much more a matter of conventions than a matter of lack of theory or physical abilities (see [0]). The one thing that convinced me of that is the sudden jump in realism in Egyptian art during the reign of Akhenaten (Amarna period, see the famous Nefertiti bust for a great example). The new king asked for more realism and he received it, it just took…

Don't take this personally. Your comment just gave me an itch I had to scratch. There are some words in people's explanations that always bother me. These are 'just', 'mere', merely' and the like. Why? Because they're not explaining, but explaining away. It's like saying: x is trivial, so it's not as important as you think. But reality is much more complicated than that. For even simple things are hard to fathom for…

To expand on this, Pirsig tears apart "just" as a completely useless word which reaffirms the speaker's belief but also trivializes the listener's inability to understand. "just" comes from ancient Latin "ius" and it means "I swear" or "I believe". When somebody says "I was just doing something", they are swearing a light oath that they were doing something.

Back in context, what the parent is saying is that the grandparent provided a "just-so" story: The parent finds it easy to believe that history happened in a way which neatly lines up with the story that they are telling. Did earlier humans know about perspective? Grandparent believes that it's obvious.

Re: Einstein's description of gravity just got much harder to beat

#64

Post by Kednicma » Sat, Oct 03, 2020, 1:13 PM UTC

irrational wrote: post #21

RcouF1uZ4gsC wrote: post #4

> Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics. Maybe the universe just has a giant if statement.

Why must the ultimate theory of the universe encompass both? Do we know that there is an ultimate theory of the universe? Could GR and QM be wholly separate with no unifying connection?

The color of gold is better explained by an ad-hoc combination of SR, GR, and QM, than by any one of them alone. SR and GR explain the planets' motion better than Newton and Kepler alone. QM explains pi bonds in molecules like benzene better than classical chemistry. GR compensation is required by GPS satellites and receivers.

So it doesn't have to be a combination of GR and QM, but it does have to be something which is at least as good as GR and QM respectively in their existing domains of explanation.

Re: Einstein's description of gravity just got much harder to beat

#65

Post by nestorD » Sat, Oct 03, 2020, 1:38 PM UTC

henriquemaia wrote: post #56

nestorD wrote: post #45

Earlier quoted context omitted.

As others have said, I believe the lack of depth in medieval art is much more a matter of conventions than a matter of lack of theory or physical abilities (see [0]). The one thing that convinced me of that is the sudden jump in realism in Egyptian art during the reign of Akhenaten (Amarna period, see the famous Nefertiti bust for a great example). The new king asked for more realism and he received it, it just took…

Don't take this personally. Your comment just gave me an itch I had to scratch. There are some words in people's explanations that always bother me. These are 'just', 'mere', merely' and the like. Why? Because they're not explaining, but explaining away. It's like saying: x is trivial, so it's not as important as you think. But reality is much more complicated than that. For even simple things are hard to fathom for…

The word "just" in "it just took a reset in conventions" was intended as ironical (but it might not come across clearly in the text). A change of conventions is a large change that is no trivial matter to undertake.

Re: Einstein's description of gravity just got much harder to beat

#66

Post by brabel » Sat, Oct 03, 2020, 2:11 PM UTC

pja wrote: post #36

brabel wrote: post #29

Earlier quoted context omitted.

Photons have zero mass so we know them to produce zero gravity, no?

Energy gravitates. Photons have energy, therefore photons gravitate. Photons only have zero /rest mass/, not zero total mass.

Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience time, so rest does not even make sense for a photon.

Re: Einstein's description of gravity just got much harder to beat

#67

Post by T-A » Sat, Oct 03, 2020, 3:33 PM UTC

cinquemb wrote: post #54

T-A wrote: post #49

Earlier quoted context omitted.

In your quote of the abstract, you conveniently left out the last sentence: "The treatment is purely classical, but motivated by links between acausal structures and quantum theory." This is not unification of QM and GR, it's a classical construction which the author speculates might lead to such a thing. Such things have been around for a long time; the obvious (and far more progressed) example is string theory.

What you quote doesn't deny "no compatability" of the author… (I didnt say it was THE unification theory of QM and GR, you somehow inferred that, why? i dont know) and irrelevant when the author points out where the overlaps lie (what I quoted last). I could add that quote and it would change nothing as far as I'm concerned what the authors stance is on "no compatability". Hell, even in the second quote even mentions…

> What you quote doesn't deny "no compatability" of the author

What I quote shows that the author knows and acknowledges that he's describing a classical construct. The question of "compatability" (sic) is not even addressed.

> I didnt say it was THE unification theory of QM and GR, you somehow inferred that

I did not. Here's what I wrote: "This is not unification of QM and GR, it's a classical construction". Not the absence of any "THE", whether capitalized or not.

> I could add that quote and it would change nothing as far as I'm concerned what the authors stance is

Because you do not understand what the author is saying. He's presenting a purely classical model which displays some features (in particular charge quantization) also seen in quantum mechanical ones. That does not make it a quantum theory, any more than the discrete eigenfrequencies of a vibrating classical string make a classical model of vibrating strings a quantum theory.

Re: Einstein's description of gravity just got much harder to beat

#68

Post by jlokier » Sat, Oct 03, 2020, 3:46 PM UTC

brabel wrote: post #66

pja wrote: post #36

Earlier quoted context omitted.

Energy gravitates. Photons have energy, therefore photons gravitate. Photons only have zero /rest mass/, not zero total mass.

Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience time, so rest does not even make sense for a photon.

That makes sense in English, however "rest mess" is also a technical and mathematical term, and in that terminology it has the value zero for photons.

Re: Einstein's description of gravity just got much harder to beat

#69

Post by jlokier » Sat, Oct 03, 2020, 3:49 PM UTC

brabel wrote: post #29

computerfriend wrote: post #9

Earlier quoted context omitted.

Rather it is the belief that they fundamentally must be compatible that is driving this research. Currently we don't know how to answer questions like "what is the gravitational field around a photon?".

Photons have zero mass so we know them to produce zero gravity, no?

We can actually measure the mass of photons.

If you shine light on an object, the light pushes on the object and the object is accelerated. If the object absorbs the light, there's a certain amount of pressure, and if the object reflects the light, there's even more pressure.

This has practical applications. Optical tweezers are made of light's force acting on small objects. "Optical tweezers are used in biology and medicine (for example to grab and hold a single bacterium or cell like a sperm cell, blood cell or DNA), nanoengineering and nanochemistry (to study and build materials from single molecules), quantum optics and quantum optomechanics (to study the interaction of single particles with light)" (Wikipedia).

Conservation of momentum means the light has mass by definition, and this photon mass can be measured by multiplying the mass and acceleration of the object.

Not only does this mean photons have gravity in the theory of GR, because all mass/energy does, it means they attract each other in a vacuum. In theory, you can make photons orbit each other due to their gravity.

Re: Einstein's description of gravity just got much harder to beat

#70

Post by andrewflnr » Sat, Oct 03, 2020, 6:44 PM UTC

jlokier wrote: post #69

brabel wrote: post #29

Earlier quoted context omitted.

Photons have zero mass so we know them to produce zero gravity, no?

We can actually measure the mass of photons. If you shine light on an object, the light pushes on the object and the object is accelerated. If the object absorbs the light, there's a certain amount of pressure, and if the object reflects the light, there's even more pressure. This has practical applications. Optical tweezers are made of light's force acting on small objects. "Optical tweezers are used in biology and…

Radiation pressure is due to momentum, not mass. I have no idea where you got that idea, since the same sort of sources that teach you about these concepts are very emphatic about photons being massless.

Here is one of the 3-5 interesting facts I remember from my modern physics class: Two photons going the same direction have no mass, but two photons in different directions do have mass. Unfortunately I don't remember the math well enough to explain it. Maybe this means your right about being able to put photons in orbit around each other.