Dear Dr. B: What is emergent gravity?

�Hello Sabine, I've seen a couple of articles lately on emergent gravity. I'm not a scientist so I would love to read one of your easy-to-understand blog entries on the subject.

Regards,

Michael Tucker
Wichita, KS�

Dear Michael,

Emergent gravity has been in the news lately because of a new paper by Erik Verlinde. I�ll tell you some more about that paper in an upcoming post, but answering

Dear Dr B: Where does dark energy come from and what�s it made of?

�As the universe expands and dark energy remains constant (negative pressure) then where does the ever increasing amount of dark energy come from? Is this genuinely creating something from nothing (bit of lay man�s hype here), do conservation laws not apply?

Puzzled over this for ages now.�

-- pete best



�When speaking of the Einstein equation, is it the case that the contribution of dark

Dear Dr B: What do physicists mean by �quantum gravity�?

[Image Source: giphy.com]

�please could you give me a simple definition of "quantum gravity"?

J.�

Dear J,
Physicists refer with �quantum gravity� not so much to a specific theory but to the sought-after solution to various problems in the established theories. The most pressing problem is that the standard model combined with general relativity is internally inconsistent. If we just use

Dear Dr. B: How come we never hear of a force that the Higgs boson carries?

�Dear Dr. Hossenfelder,
First, I love your blog. You provide a great insight into the world of physics for us laymen. I have read in popular science books that the bosons are the �force carriers.� For example the photon carries the electromagnetic force, the gluon, the strong force, etc. How come we never hear of a force that the Higgs boson carries?Ramiro Rodriguez�
Dear Ramiro,

The short

Dear Dr B: Why not string theory?

[I got this question in reply to my last week�s book review of Why String Theory? by Joseph Conlon.]

Dear Marco:

Because we might be wasting time and money and, ultimately, risk that progress stalls entirely.
In contrast to many of my colleagues I do not think that trying to find a quantum theory of gravity is an endeavor purely for the sake of knowledge. Instead, it seems likely to me that

Dear Dr B: If photons have a mass, would this mean special relativity is no longer valid?

Einstein and Lorentz.[Image: Wikipedia]

�[If photons have a restmass] would that mean the whole business of the special theory of relativity being derived from the idea that light has to go at a particular velocity in order for it to exist/Maxwell�s identification of e/m waves as light because they would have to go at the appropriate velocity is no longer valid?�


~ Brian Clegg

(This

Dear Dr B: Why is Lorentz-invariance in conflict with discreteness?

Can we build up space-time fromdiscrete entities?

�Could you elaborate (even) more on [�] the exact tension between Lorentz invariance and attempts for discretisation?

Best,

Noa�

Dear Noa:
Discretization is a common procedure to deal with infinities. Since quantum mechanics relates large energies to short (wave) lengths, introducing a shortest possible distance corresponds to cutting off