# B Great Collider & Naturalness

1. Aug 27, 2016

### cube137

I read the following interesting article about the Great Collider in China. Could the $10 Billion Great Collider become reality as the main adviser Arkani-Hamed main purpose of it is to find Naturalness? It's like him telling China "LHC didn't find naturalness, could you construct the$10 Billion Great Collider so maybe we can find it?" (see the bottom for my comments and some questions)

https://www.quantamagazine.org/20150922-nima-arkani-hamed-collider-physics/

"As things stand, the known elementary particles, codified in a 40-year-old set of equations called the “Standard Model,” lack a sensible pattern and seem astonishingly fine-tuned for life. Arkani-Hamed and other particle physicists, guided by their belief in naturalness, have spent decades devising clever ways to fit the Standard Model into a larger, natural pattern. But time and again, ever-more-powerful particle colliders have failed to turn up proof of their proposals in the form of new particles and phenomena, increasingly pointing toward the bleak and radical prospect that naturalness is dead.

Still, many physicists, Arkani-Hamed chief among them, seek a more definitive answer. And right now, his quest to answer the naturalness question leads through China. Two years ago, he agreed to become the inaugural director of the new Center for Future High Energy Physics in Beijing. He has since visited China 18 times, campaigning for the construction of a machine of unprecedented scale: a circular particle collider up to 60 miles in circumference, or nearly four times as big around as Europe’s Large Hadron Collider (LHC). Nicknamed the “Great Collider,” and estimated to cost roughly \$10 billion over 30 years, it would succeed the LHC as the new center of the physics universe. According to Arkani-Hamed and those who agree with him, this 100-trillion-electron-volt (TeV) collider would slam subatomic particles together hard enough to either find the particles that the LHC could not muster or rule them out, rescuing or killing the naturalness principle and propelling physicists toward one of two radically different pictures: that of a knowable universe, or an unknowable multiverse."

My physicsforums friends. If the following were simply not true:

1. Supersymmetry
2. Extra Dimensions
3. Multiverse
4. Dimensional Transmutation

Couldn't we say that in the Big Bang. The very unlikely constants of nature and fine tuning were simply designed that way by who knows what. If this were true. Would it be the end of physics?

2. Aug 28, 2016

### Staff: Mentor

There has to be something beyond the SM - it breaks down at high energies, it does not include gravity, and it does not have a dark matter particle. Those things are not a matter of fine-tuning, they are fundamental problems with the SM.

If it is none of the options you listed, it has to be something else.

3. Aug 28, 2016

### cube137

There is a higgs field that contribute mass-energy to the vacuum.. but yet general relativity is flat cosmologically wise. Isn't this obvious something is wrong with either qft or general relativity? It's like in a family there is a weighting scale and a box weight only a few grams.. yet the parents are arguing whether the box has bar-bells insides or heavy iron. Even a child can say the box is just near empty and doesn't have bar-bells and the parents may be suffering from some mental aberrations (like brain damaged). Isn't this a similar in our physics today?

4. Aug 28, 2016

### Staff: Mentor

We don't have a proper QFT of gravity, so we don't know. We might misunderstand how to get an energy density in QFT that we can plug into the equations of GR. Or the different contributions happen to cancel extremely well, and then we are back to fine-tuning.

5. Aug 28, 2016

### cube137

Either or both of QFT and GR may just be effective field theory. But they say they are valid in their applications up to the planck scale.. then below it.. quantum gravity takes over... but this Higgs field not affecting General Relativity is above the planck scale.. so can we say even our effective field theory of QFT or GR is wrong way above the planck scale??

6. Aug 28, 2016

### Staff: Mentor

We know that they are just effective theories. So it is not the end of physics - there is certainly something new to discover.

Neither the SM nor GR can tell us what happens beyond the Planck scale. In the worst case, they stay good approximations until that, but they might break down earlier.

7. Aug 28, 2016

### cube137

Mass-energy stress tensor should curve spacetime. Higgs field or other quantum contributions are also considered mass-stress energy.. but they don't curve spacetime. Therefore I was asking if this means they already break down earlier than planck scale.. or are you saying this higgs field not able to dent spacetime is a planck scale thing.. how come it's not above it?

8. Aug 28, 2016

### Staff: Mentor

We don't know that, especially because we don't know what the cosmological constant in a universe without Higgs would be. Same for all the other particles which would naturally be expected to contribute to an overall energy density. In QFT there is no way to calculate that in a unique way because QFT does not care about absolute energy scales.

9. Aug 28, 2016

### cube137

Yes, I read the Baez paper.

Is there any experiment where they tested different orders of mass-stress energy to see which one doesn't register in the Einstein Field Equation? they could make solid thing out of this mass-stress energy and see if it falls down to earth.

Whatever. Can we say that either QFT or GR already break down before the planck scale because the higgs field is not registering on the EFE or putting any dent on spacetime? Is your answer "Not sure".. or "No.. QFT and GR only break down at planck scale" (if this is the answer, do you consider higgs field or quantum contributions as planck scale phenomena?) Or are physicists kinda hoping and secretly wishing that Higgs field still register on the EFE but canceled by other quantum fields and insisting henceforth qft/gr doesn't break down above the planck scale? Please be specific with the reply so I'd not get confused. Thanks a lot!

10. Aug 28, 2016

### Staff: Mentor

That question and suggested experiment don't make sense at all.
We know the effect of matter on spacetime because we can see the effect with and without matter. We cannot remove the QFT fields from spacetime.

No we cannot.

The SM (QFT is more general, and not a specific physics model...) or GR might break down below the Planck scale, but we do not know.
We know the SM works well up to the LHC energies, which is way above the scale of the cosmological constant already. If this constant can be calculated at all, we don't understand how. This statement is independent of the question whether the SM breaks down at 10 TeV or at the Planck scale.

11. Aug 29, 2016

### cube137

https://arxiv.org/pdf/astro-ph/0005265v1.pdf

Have you read the famous Steven Weinberg Cosmological Problem paper? Something that escapes my understanding is the so called Anthropic Principle. Even if there were multiple Multiverses, all of them should instantly warp because of the huge quantum contributions that is 120 magnitude way off. It's as if there was atomic bomb explosions in all multiverses.. but in one of them, the bomb runs in slow motion that lasts 10 years instead of milliseconds.. this doesn't make sense.. or in the Anthropic Multiverse scenario.. did they just ignore the quantum 120 magnitude off contributions? I can't understand this part in the paper or the general Anthropic multiverse argument.

12. Aug 30, 2016

### Staff: Mentor

Not in exactly one, just in a very small fraction.
It is possible, but it would not be a good explanation.

13. Aug 30, 2016

### cube137

What? Let's use the pure cosmological constant scenario. In each multiverse.. the universe would warp suddenly when it feels the 120 magnitude quantum contributions.. are you saying in our universe, it warp slowly.. but why when it's feeling the full 120 magnitude quantum contributions?

14. Aug 30, 2016

### Staff: Mentor

What?

If there is a huge amount of different universes (potentially an infinite number), all with random values for the cosmological constant from 0 to the Planck scale, then some of them will have very small cosmological constants.

That is not a good model because it does not provide a deeper reason why the constant is so small, but it is a possible model - and maybe there is no deeper reason.

15. Aug 30, 2016

### cube137

But the cosmological constant can't be small because of the 120 magnitude quantum contribution.. therefore even with 10^10^10, etc. universes.. they will always start with minimum of 120 magnitude so all should immediately warp.. well?

16. Aug 30, 2016

### Staff: Mentor

Of course it can. GR has a free parameter in the model, this free parameter can cancel the contributions from other fields. In addition, those contributions can have some cancellation as well. The natural scale where we would expect the contributions to be is the Planck scale, but it does not have to be there.

17. Aug 30, 2016

### cube137

Oh.. what is the specific name for the free parameter in GR that can cause it to become rigid such that 120 magnitude quantum contributions can indeed be cancelled (or suppressed)? So I can inquire this specific term in the GR forum.

18. Aug 30, 2016

### Staff: Mentor

It is called cosmological constant and it is a free parameter in GR.

19. Aug 30, 2016

### cube137

But the cosmological constant only expands.. it can't make spacetime become so rigid that it can suppress the 120 magnitude quantum contribution.. can it? Unless you meant the cosmological constant can become negative and contract spacetime enough to cancel the huge quantum contribution?

20. Aug 31, 2016

### haushofer

Just curious: Did people consider QFT's in deSitter spacetime and try to build a standard model with it? How would the cosmological constant be renormalized in such a theory?