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Dear Dr B: If photons have a mass, would this mean special relativity is no longer valid?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

Dear Dr. B: What are the requirements for a successful theory of quantum gravity?

�I've often heard you say that we don't have a theory of quantum gravity yet. What would be the requirements, the conditions, for quantum gravity to earn the label of 'a theory' ?
I am particularly interested in the nuances on the difference between satisfying current theories (GR&QM) and satisfying existing experimental data. Because a theory often entails an interpretation whereas a piece of

Dear Dr. B: What is the difference between entanglement and superposition?

The only photo in existencethat shows me in high heels.

This is an excellent question which you didn�t ask. I�ll answer it anyway because confusing entangled states with superpositions is a very common mistake. And an unfortunate one: without knowing the difference between entanglement and superposition the most interesting phenomena of quantum mechanics remain impossible to understand � so

Dear Dr Bee: Can LIGO�s gravitational wave detection tell us something about quantum gravity?

�I was hoping you could comment on the connection between
gravitational waves and gravitational quanta. From what I gather, the
observation of gravitational waves at LIGO do not really tell us anything about
the existence or properties of gravitons. Why should this be the case?�

~ Bobak Hashemi



�Can LIGO provide any experimental signature of quantum
gravity?�

~ Ervin Goldfain



�Is

Dear Dr B: Is string theory science?

This question was asked by Scientific American, hovering over an article by Davide Castelvecchi.They should have asked Ethan Siegel. Because a few days ago he strayed from the path of awesome news about the universe to inform his readership that �String Theory is not Science.� Unlike Davide however, Ethan has not yet learned the fine art of not expressing opinions that marks the true science

Ask Dr B: Is the multiverse science? Is the multiverse real?

Kay zum Felde asked:

�Is the multiverse science? How can we test it?�
I added �Is the multiverse real� after Google offered it as autocomplete:




Dear Kay,

This is a timely question, one that has been much on my mind in the last years. Some influential theoretical physicists � like Brian Greene, Lenny Susskind, Sean Carroll, and Max Tegmark � argue that the appearance of multiverses in

Dear Dr B: Can you think of a single advancement in theoretical physics, other than speculation, since the early 1980's?

This question was asked by Steve Coyler, who was a frequent commenter on this blog before facebook ate him up. His full question is:


�Can you think of a single advancement in theoretical physics, other than speculation like Strings and Loops and Safe Gravity and Twistors, and confirming things like the Higgs Boson and pentaquarks at the LHC, since Politizer and Wilczek and Gross (and Coleman)

Dear Dr. B: What do physicists mean when they say time doesn�t exist?

That was a question Brian Clegg didn�t ask but should have asked. What he asked instead in a recent blogpost was: �When physicists say many processes are independent of time, are they cheating?� He then answered his own question with yes. Let me explain first what�s wrong with Brian�s question, then I�ll tell you something about the existence of time.
What is time-reversal invariance?

The

Dear Dr. Bee: Why do some people assume that the Planck length/time are the minimum possible length and time?

�Given that the Planck mass is nothing like a minimum possible mass, why do some people assume that the Planck length/time are the minimum possible length and time? If we have no problem with there being masses that are (much) smaller than the Planck mass, why do we assume that the Planck length and time are the fundamental units of quantisation? Why could there not be smaller distances and

Dear Dr. B: Can you make up anything in theoretical physics?

�I am a phd-student in neuroscience and I often get the impression that in physics "everything is better". E.g. they replicate their stuff, they care about precision, etc. I've always wondered to what extend that is actually true, as I obviously don't know much about physics (as a science). I've also heard (but to a far lesser extent than physics being praised) that in theoretical physics you