Thought experiment: how would physics develop without Einstein?

[Duda Jarek]

A valuable thought experiment from neighboring thread - how would physics develop if there would be no Einstein, especially regarding gravity?

While modern physicists might say that without him there would be "weeping and the gnashing of teeth", his introduction of general relativity a century ago seems to be event of extremely low probability (?) - so what if it wouldn't happen?

Or imagine some hypothetical other civilization e.g. SETI is looking for - would it automatically get to GR? If not, how their physics would develop?

 

Beside Newton's gravity, there was also known GEM before ( https://en.wikipedia.org/wiki/Gravitoelectromagnetism ) - just take Maxwell's equations to gravity, making it Lorentz-invariant theory. "The best confirmation of GR": Gravity Probe B in fact has confirmed GEM as approximation of GR.

Without Einstein we would probably develop further Heavisides' GEM - adding corrections to Lagrangian (like in "Standard Model approach"), starting with EM-GEM coupling to bend photon trajectories by Sun ... the big question is if gravitational time dilation could be explained by such slowing down?

GEM has no renormalization problem, trivially unifies with the rest of physics as it is just another F_munu F^munu in Lagrangian - many approaches to solve this problem like string theory might never appear (?)

 

Where the problems would start - which experiments leave no doubts that we have to use GR instead of GEM?

Which cannot be repaired with added terms to GEM Lagrangian?

[Strange]

9 minutes ago, Duda Jarek said:

his introduction of general relativity a century ago seems to be event of extremely low probability (?) - so what if it wouldn't happen?

Why do you think this? It was based on contemporary physics (known deficiencies in Newtonian gravity, special relativity*, etc). He worked with others to develop the theory. It might have been another 5 years or another 50, but someone would have worked it out sooner or later. (There were already geometrical interpretations of Maxwell's equations, for example.)

* Special relativity didn't require Einstein, either. All the component parts were there, the most significant thing he did was to pull them together with an underlying mechanism. Again, someone else would have done that sooner or later.

12 minutes ago, Duda Jarek said:

Without Einstein we would probably develop further Heavisides' GEM

Until it was found to be inadequate. Then it would have been replaced. (See also the Rutherford and Bohr models of the atom. Or science in general.)

[Duda Jarek]

Indeed special relativity did not require Einstein, as it was already e.g. in EM/GEM Maxwell's equations.

General relativity introduced intrinsic curvature of spacetime, while earlier it was thought to be flat.

So which experiments bring certainty that spacetime is curved?

Sure we need it e.g. to bend photon trajectories, but looking e.g. at glass of water reminds that there are also other ways to bend photon trajectories - like some slowing down of EM propagation in presence of strong gravitational field here (some EM-GEM coupling), what would also lead to some gravitational time dilation effects.

Edited August 11, 2020 by Duda Jarek

[joigus]

My opinion is yes, GR would have been found without Einstein. It would have taken a different order of consecutive realisations about different aspects of the theory.

The field equations would have been named the Hilbert equations for gravity, probably. Formulated in a more mathematical language, and taken some decades to infer everything about photons red-shifting and bending, etc.

And had Einstein been a woman, it would have taken a decade longer for everybody else to realise that she was right.

Had it been a guy in Papua New Guinea, we still wouldn't know.

And had it been a woman in Papua New Guinea, we would never know.

[Duda Jarek]

But which experiment would bring certainty that we need to give up flat spacetime?

From https://en.wikipedia.org/wiki/Tests_of_general_relativity :

- Perihelion precession of Mercury is GEM: "Gravitoelectric effects" + Lense–Thirring precession

- Deflection of light by the Sun - would need some EM-GEM coupling: slowing down of EM propagation in presence of gravitational field,

- gravitational time dilation - clocks are usually based on EM propagation e.g. in atoms, so slowing down of EM would slow down clocks ... the question is other interactions: what do we know about gravitational time dilation e.g. for muon decay?

- Gravity Probe B tested GEM,

- gravitational waves - also in GEM ...

... black holes - sure we have objects of extremely high density, but how to distinguish what is happening inside?

Edited August 11, 2020 by Duda Jarek

[Strange]

34 minutes ago, Duda Jarek said:

But which experiment would bring certainty that we need to give up flat spacetime?

All of them?

You are suggesting an odd view of science. A more meaningful question would be, what experiment would have falsified the model of spacetime in GR (answer: any of them producing a different result from GR).

Maybe you should be asking what experiment would falsify GEM. I can't answer that because I am not familiar with it. But I note that your source says:

Quote

The most common version of GEM is valid only far from isolated sources, and for slowly moving test particles.

https://en.wikipedia.org/wiki/Gravitoelectromagnetism

So, if we had started off down that route, sooner or later an experiment or observation would have shown it to be wrong. (We are still hoping for such a result for GR but, unfortunately, it hasn't happened yet.)

35 minutes ago, Duda Jarek said:

From https://en.wikipedia.org/wiki/Tests_of_general_relativity :

- Perihelion precession of Mercury is GEM: "Gravitoelectric effects" + Lense–Thirring precession

- Deflection of light by the Sun - would need some EM-GEM coupling: slowing down of EM propagation in presence of gravitational field,

- gravitational time dilation - clocks are usually based on EM propagation e.g. in atoms, so slowing down of EM would slow down clocks ... the question is other interactions: what do we know about gravitational time dilation e.g. for muon decay?

- Gravity Probe B tested GEM,

- gravitational waves - also in GEM ...

... black holes - sure we have objects of extremely high density, but how to distinguish what is happening inside?

These are all vague, hand wavy, qualitative claims of similarity. If you want to defend GEM as an alternative to GR, then you need to show, in mathematical detail, that it produces the same match to observations as GR (or better).

If you want to attempt that, then you would also need to do it in the Speculations forum.

[Danijel Gorupec]

Thinking out loud... At the time, Einstein did not have to feel embarrassed because he didn't know how to reconcile his theory with QM. If one would be developing GR some 40 years later, he would be under some pressure to provide a theory that includes QM. Some guys/girls might abandon their work out of frustration... So maybe, we won't ever have GR as it is now, but we would directly have (eventually) a version given with QM in mind.

[Strange]

57 minutes ago, Danijel Gorupec said:

Thinking out loud... At the time, Einstein did not have to feel embarrassed because he didn't know how to reconcile his theory with QM. If one would be developing GR some 40 years later, he would be under some pressure to provide a theory that includes QM. Some guys/girls might abandon their work out of frustration... So maybe, we won't ever have GR as it is now, but we would directly have (eventually) a version given with QM in mind.

Nice point.

On the other hand, maybe the difficulty of reconciling it with QM wouldn't be realised until the theory was largely done.

[Markus Hanke]

11 hours ago, MigL said:

Still, all these experimental falsifications of GEM would not have taken place until the middle of the last century, Markus.
That would have made GEM the ruling gravitational theory for about 40 years before people noticed enough of a problem to search for different formulations.
Whereas SR was just waiting to be pulled together and published ( probably by H Poincare ) when A Einstein beat him to it in 1905, GR might have taken up to a century without A Einstein.
We might just be at the stage now, where A S Eddington was in 1920; one of three people in the world who understood GR.

Yes, I agree.

[Maestro99]

without Einstein the Physics would have taken a different route.

[joigus]

11 minutes ago, Maestro99 said:

without Einstein the Physics would have taken a different route.

Any arguments, or are you after founding a new religion and we're supposed to nod and follow you?

[Duda Jarek]

General relativity has this huge problem of being non-renormalizable ... while it wasn't an issue when it was introduced, a few decades later it would make GR extremely difficult to accept - there would be needed very strong experimental evidence to give up renormalizability - which experiment could it be?

GR can be seen as a sequence of corrections in Taylor expansion - without Einstein we might now slowly introduce such succeeding terms ... 

[Markus Hanke]

17 hours ago, Duda Jarek said:

which experiment could it be?

Have we not answered this already earlier on this thread? Any scenario where the non-linearity of GR cannot be neglected - so essentially the strong field regime - will display effects that differ from the weak field approximation. Even the article on GEM which you shared mentioned some of these effects.

17 hours ago, Duda Jarek said:

GR can be seen as a sequence of corrections in Taylor expansion

I just see it as the simplest possible metric theory that fulfils all relevant self-consistency criteria.

17 hours ago, Duda Jarek said:

General relativity has this huge problem of being non-renormalizable

This is an issue only so long as one tacitly assumes that GR must be the classical limit of some quantum field theory - which implies the assumption that gravity works in the same way as the weak, strong, and EM interactions. But actually, there is nothing in physics that indicates that this must necessarily be the case. Also, it is actually trivially easy to write down a QFT for a spin-2 boson interaction, but it is just as easy to see that such an attempt yields something that is physically meaningless. So clearly, gravity doesn't work in the same way as the other forces, hence it is not a surprise that it isn't renormalizable.
Personally I think gravity isn't a fundamental interaction at all, so attempting to apply the usual quantisation schemes to it is simply an error on our part; a category mistake, if you so will.

[Duda Jarek]

Marcus, the experiments in https://en.wikipedia.org/wiki/Tests_of_general_relativity use low order corrections if seeing GR as Taylor expansion - a civilization without Einstein could be just adding such succeeding terms.

Extrapolating to GR from such corrections seems quite nontrivial and might be tough to accept due to non-renormalizability for a civilization which had developed QFT first.

For really convincing arguments we should rather think of qualitative consequences for intrinsic curvature, like black hole horizon or wormholes ...

[Markus Hanke]

2 hours ago, Duda Jarek said:

Extrapolating to GR from such corrections seems quite nontrivial and might be tough to accept due to non-renormalizability for a civilization which had developed QFT first.

It would certainly be non-trivial. However, what I was trying to point out is that, if someone develops QFT first, then they would very quickly realise that a standard QFT for spin-2 bosons that couple to energy-momentum (a reasonable ansatz if one wants to find a model for gravity) yields something that is physically meaningless. So they would begin to wonder if perhaps gravity can't be described via a QFT at all. This would eventually bring them to consider metric theories instead - and GR is the simplest example of that. 

2 hours ago, Duda Jarek said:

For really convincing arguments we should rather think of qualitative consequences for intrinsic curvature, like black hole horizon or wormholes ...

Yes, this is the strong field regime I mentioned in the last post.

[Brahms]

On 8/11/2020 at 5:57 AM, Duda Jarek said:

A valuable thought experiment from neighboring thread - how would physics develop if there would be no Einstein, especially regarding gravity?

While modern physicists might say that without him there would be "weeping and the gnashing of teeth", his introduction of general relativity a century ago seems to be event of extremely low probability (?) - so what if it wouldn't happen?

Or imagine some hypothetical other civilization e.g. SETI is looking for - would it automatically get to GR? If not, how their physics would develop?

 

Beside Newton's gravity, there was also known GEM before ( https://en.wikipedia.org/wiki/Gravitoelectromagnetism ) - just take Maxwell's equations to gravity, making it Lorentz-invariant theory. "The best confirmation of GR": Gravity Probe B in fact has confirmed GEM as approximation of GR.

Without Einstein we would probably develop further Heavisides' GEM - adding corrections to Lagrangian (like in "Standard Model approach"), starting with EM-GEM coupling to bend photon trajectories by Sun ... the big question is if gravitational time dilation could be explained by such slowing down?

GEM has no renormalization problem, trivially unifies with the rest of physics as it is just another F_munu F^munu in Lagrangian - many approaches to solve this problem like string theory might never appear (?)

 

Where the problems would start - which experiments leave no doubts that we have to use GR instead of GEM?

Which cannot be repaired with added terms to GEM Lagrangian?

Physics itself does not really develop, as all the math, mass, space and time were always there.  What develops is human understanding of the math, since Einstein had many collaborators or opponents all working on the same issues the differences would be less than they would be more

[Strange]

1 hour ago, Brahms said:

Physics itself does not really develop, as all the math, mass, space and time were always there.  What develops is human understanding of the math

Physics is the human understanding of the mathematical models and how they relate to observations and measurements.

[Duda Jarek]

5 hours ago, Strange said:

Physics is the human understanding of the mathematical models and how they relate to observations and measurements.

Indeed, hence it is extremely important to reflect what of it really comes from nature - is universal, should be also found by hypothetical other civilizations ... and what is just an artifact of physics being a social construct, for example evaluating concepts based on number of articles people can write about them - this way promoting exciting solutions and forgetting the simple ones.

[swansont]

!

Moderator Note

Tangential discussion on the details of GEM has been split

https://www.scienceforums.net/topic/122841-gem-split-from-thought-experiment-how-would-physics-develop-without-einstein/

 

 

[Endy0816]

I wonder if we couldn't have figured it out sooner actually. Maybe we're the thought experiment of a world where one Joel Schmidt came up with it instead.

[Duda Jarek]

Extraordinary claims require extraordinary evidence - there is a lot of general talk here, but I am asking about some details: convincing experimental evidence and argumentation.

Imagine civilization without Einstein: weak field effects can be introduced "the Standard Model way": as sequence of corrections - first terms of Taylor expansion of GR.

What experimental evidence could be used to convince them that spacetime is not flat but needs intrinsic curvature: that only non-renormalizable GR can explain it?

There are needed strong field effects e.g. using black holes - but how such argumentation could look like?

Edited August 17, 2020 by Duda Jarek

[Strange]

3 hours ago, Duda Jarek said:

Extraordinary claims require extraordinary evidence - there is a lot of general talk here, but I am asking about some details: convincing experimental evidence and argumentation.

You have been given details that you have chosen to ignore.

[Duda Jarek]

Where???

There was some criticism of GEM, which accordingly to a few Wikipedia articles like https://en.wikipedia.org/wiki/Gravitoelectromagnetism was confirmed by Gravity Probe B, its examples are frame dragging, Lense-Thirring.

But if you have some allergy to this well established GR approximation, then just take a few first terms of Taylor expansion of GR - low field approximations, which are used for calculations for most of these experiments

Without Einstein they could be seen as found succeeding terms added in Lagrangian - like they did for Standard Model (maybe also first Taylor terms of some TOE?)

Please elaborate how without Einstein one could extrapolate from these terms that we need to introduce intrinsic spacetime curvature in non-renormalizable theory?

[dimreepr]

2 hours ago, Duda Jarek said:

Where???

I think the point is, when???

[swansont]

I'm not sure what the premise is of the OP: what happens if Einstein doesn't come up with GR (he tragically dies in 1906, or something), or if GR doesn't exist at all.

If the former, then the matter of someone else coming up with it at a later date has to be a possibility.

If the latter - basically you are discussing alternatives to GR that are ultimately incorrect. According to the link, GEM was introduced a few decades before GR was finalized. What were the reasons it was not pursued further? I see nothing in the link about it explaining the advance of the perihelion of Mercury. This was a problem known in 1915, and solved by GR. Would an alternative theory have taken hold in a GR vacuum, if it could not explain this effect? Anything that could not would start out with the handicap of knowing it was not complete.

In addition, someone would have likely investigated light bending around the sun during an eclipse at some point, and showing that Newton was wrong. And model that could not explain this would likewise be considered incomplete, or wrong.