Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

PO is now at the Physics Department of Bielefeld University!

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

205 submissions , 163 unreviewed
5,082 questions , 2,232 unanswered
5,354 answers , 22,791 comments
1,470 users with positive rep
820 active unimported users
More ...

  Does spacetime really exist in quantum gravity?

+ 2 like - 0 dislike
2443 views

If there are no localized observables in quantum gravity, does spacetime really exist, or might spacetime really be an illusion?

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Jim
asked Jun 20, 2011 in Phenomenology by Jim (50 points) [ no revision ]
If there are no observables in quantum gravity, then it means that the theory of quantum gravity is useless. It does not imply that space time does not exist or is an illusion. I vote to close this question because the question doesn't make sense.

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user QEntanglement
@QEntanglement: partially true but I feel like the question could make sense. More or less everyone agrees that the classical notion of space-time should break down at Planck scale and get replaced by some non-commutative version. There is lot to be said on this topic.

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Marek
I think the question should be restated to be more specific because "there is a lot to be said on this topic."

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user QEntanglement
I think questions on "really exist" are border metaphysics, or at least philosophy. I could answer on those lines: "do you really exist or are you a construct of my mind". After all for each of us the only "existence" of the "other" comes through convolutions upon convolutions of electromagnetic pulses that end up in our brain with the illusion "there is something out there" :) . I would also vote to close, unless it changes to "what is the experimental evidence that ...." , because we start with a framework where existence and reality are not in question.

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user anna v
He did not say "no observables", he said "no localised observables"

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Philip Gibbs
This question should be reformulated as: how do you reconstruct a space-time from the observables admitted in quantum gravity, namely the S-matrix in flat space, or the AdS boundary theory in asymptotically AdS spaces? This is a topic of very active research.

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Ron Maimon
@Jim: I agree the question could be reformatted for clarity and will probably recieve more attention if it were ask in this way Postulate: If there are no localized observables in quantum gravity, does spacetime really exist, or might spacetime really be an illusion. Scientifically Relevant Question: How do you reconstruct a space-time from the observables admitted in quantum gravity, namely the S-matrix in flat space, or the AdS boundary theory in asymptotically AdS spaces

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Argus

2 Answers

+ 1 like - 0 dislike

Andrew Strominger thinks spacetime is an illusion. It's all a Cosmic Hologram at the future boundary of spacetime at the end of time. It projects out the illusion we see around us.

This post imported from StackExchange Physics at 2014-03-07 14:34 (UCT), posted by SE-user Maya Lila Brahman
answered Feb 2, 2013 by Maya Lila Brahman (10 points) [ no revision ]
+ 1 like - 0 dislike

I think this question is a bit too broad, as it depends on the theory you're talking about.

Lubos Motl has just posted a new article about (a similar to this) idea on TRF.

Well, you do have spacetime in string theory, for example. But as Lubos Motl mentions in the article linked, the metric tensor isn't exactly that well-defined at the stringy scales. To quote the article:

However, quantum gravity doesn't allow you things like that. The metric tensor is only good and well-defined in an effective description of quantum gravity. At shorter distances, it just ceases to be a good observable. Well-defined observables in quantum gravity are different; the gauge fields in the $\mathcal N=4$ Yang-Mills theory involved in the most famous example of the AdS/CFT correspondence are an example. The matrices $X,P,Θ$ in Matrix Theory are another example.

Same with things like Twistor theory, where the real space is the space of twistors.

In LQG, though, there is indeed a vierbin; though.

answered Aug 31, 2013 by dimension10 (1,985 points) [ revision history ]
edited Jan 31, 2015 by dimension10

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar$ysicsOver$\varnothing$low
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
Please complete the anti-spam verification




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...