physics

Hořava–Lifshitz gravity

Hořava–Lifshitz gravity is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hořava–Lifshitz gravity rather than just read about it. In short: Hořava–Lifshitz gravity (or Hořava gravity) is a theory of quantum gravity proposed by Petr Hořava in 2009. It solves the problem of different concepts of time in quantum field theory and general relativity by treating the quantum concept as the more fundamental so that space and time are not equivalent (anisotropic) at high energy level.

Key takeaways

Hořava–Lifshitz gravity belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Hořava–Lifshitz gravity to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Hořava–Lifshitz gravity from memory before moving on to harder problems.

Reference excerpt

Hořava–Lifshitz gravity (or Hořava gravity) is a theory of quantum gravity proposed by Petr Hořava in 2009. It solves the problem of different concepts of time in quantum field theory and general relativity by treating the quantum concept as the more fundamental so that space and time are not equivalent (anisotropic) at high energy level. The relativistic concept of time with its Lorentz invariance emerges at large distances. The theory relies on the theory of foliations to produce its causal structure. It is related to topologically massive gravity and the Cotton tensor. It is a possible UV completion of general relativity. Also, the speed of light goes to infinity at high energies. The novelty of this approach, compared to previous approaches to quantum gravity such as loop quantum gravity, is that it uses concepts from condensed matter physics such as quantum critical phenomena. Hořava's initial formulation was found to have side-effects such as predicting very different results for a spherical Sun compared to a slightly non-spherical Sun, so others have modified the theory. Inconsistencies remain, though progress was made on the theory. Nevertheless, observations of gravitational waves emitted by the neutron-star merger GW170817 contravene predictions made by this model of gravity. Some have revised the theory to account for this.

Detailed balance condition Hořava originally imposed the theory to satisfy the detailed balance condition which considerably reduces the number of coupling constants in the action.

See also Superfluid vacuum theory Causal dynamical triangulation Problem of time

References

External links Zeeya Merali, "Splitting Time from Space—New Quantum Theory Topples Einstein's Spacetime". Scientific American, December, 2009

Worked examples

Example 1 — a first encounter with Hořava–Lifshitz gravity

Start with the simplest possible case. Write down what Hořava–Lifshitz gravity claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hořava–Lifshitz gravity before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hořava–Lifshitz gravity ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hořava–Lifshitz gravity

In research: Hořava–Lifshitz gravity appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hořava–Lifshitz gravity in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hořava–Lifshitz gravity is common in secondary-school and first-year university syllabi. It links to neighbouring topics Quantum gravity, Relativity stubs, Theories of gravity, so understanding it makes those chapters shorter.
In everyday life: Look for Hořava–Lifshitz gravity outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Hořava–Lifshitz gravity in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Hořava–Lifshitz gravity means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Hořava–Lifshitz gravity out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Hořava–Lifshitz gravity in simple terms?

Why does Hořava–Lifshitz gravity matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hořava–Lifshitz gravity?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hořava–Lifshitz gravity.