What If Our Understanding of Gravity Is Wrong?

PBS Space Time
10 Nov 202118:27
EducationalLearning
32 Likes 10 Comments

TLDRThe video script explores the concept of dark matter and the possibility that our understanding of gravity may be flawed. It delves into the history of dark matter, the evidence supporting its existence, and alternative theories such as Modified Newtonian Dynamics (MOND) proposed by Mordehai Milgrom. The script discusses the limitations of MOND, its inconsistency with other physical laws, and the development of more sophisticated models like AQuaL and TeVeS. It also highlights a new relativistic version of MOND that attempts to address previous shortcomings. The video concludes by acknowledging the ongoing debate between dark matter as a particle and modified gravity theories, emphasizing the excitement of scientific exploration regardless of the outcome.

Takeaways
  • 🌌 The concept of dark matter was proposed to explain the observed rotation rates of galaxies, which should disintegrate if only visible mass's gravity were at play.
  • 🚀 Vera Rubin's work in the 1960s showed that galaxies rotate faster than expected, suggesting the presence of unseen mass, termed dark matter.
  • 🔍 Despite extensive searches, dark matter has not been directly observed, leading to alternative theories questioning our understanding of gravity.
  • 🤔 The possibility that gravity might not follow Newton's laws under all conditions has led to Modified Newtonian Dynamics (MOND) and other modified gravity theories.
  • ✨ Albert Einstein's General Theory of Relativity describes gravity as the curvature of spacetime, which differs from Newtonian gravity in strong fields.
  • 🔧 MOND introduces a minimum possible acceleration that gravitational force can produce, modifying the drop-off rate of gravitational force with distance.
  • 🔗 MOND successfully reproduces observed galactic rotation curves without invoking dark matter, but struggles to explain phenomena like galaxy clusters.
  • ⚖️ MOND and its variants have faced criticism for not aligning with established physics, particularly conservation laws and general relativity.
  • 🌟 The Tully-Fisher Law, which relates a galaxy's rotation speed to its luminosity, is well-explained by MOND, even though it was not designed to predict it.
  • 📐 Jacob Bekenstein and Mordehai Milgrom's AQuaL and TeVeS theories attempt to reconcile MOND with relativity, but still encounter issues, such as instability and cosmological inconsistencies.
  • ⏳ A new relativistic version of MOND, RelMOND, proposed by Constantinos Skordis and Tom Złosnik, allows the scalar field to change behavior over time, potentially addressing some of the previous issues.
  • 💥 The Bullet Cluster, where light matter is separated from dark matter during a collision, poses a significant challenge to MOND and modified gravity theories.
  • 🧐 While the majority of experts favor the particle dark matter model, MOND theories have influential supporters, and the debate continues to be unresolved.
Q & A

  • What was Vera Rubin's discovery regarding the rotation of spiral galaxies?

    -Vera Rubin proved that spiral galaxies are rotating so fast that they should fling themselves apart, assuming they are held together by the gravity of their visible mass alone. They would need at least five times as much matter to provide the gravity needed to hold these galaxies together.

  • Why is dark matter hypothesized to exist?

    -Dark matter is hypothesized because the gravity of visible matter is too weak to hold galaxy clusters together or to bend the path of light to the degree seen in gravitational lenses. It is believed that about 80% of the mass in the universe is completely invisible to us, which is termed as dark matter.

  • What is Modified Newtonian Dynamics (MOND)?

    -Modified Newtonian Dynamics (MOND) is a theory proposed by Mordehai Milgrom in 1982 that suggests there exists a minimum possible acceleration that can be produced by the gravitational force. In MOND, the force or acceleration drops off with distance squared until, at very low values, they start to plateau out.

  • How does MOND attempt to explain the observed phenomena without dark matter?

    -MOND modifies the laws of gravity to account for the observed phenomena without the need for dark matter. It suggests that at very low accelerations, the gravitational force does not decrease with the square of the distance but instead levels off, which can explain the flat rotation curves of galaxies without invoking dark matter.

  • What are the three criteria a new hypothesis like MOND needs to meet to be taken seriously?

    -A new hypothesis like MOND needs to: 1) give the right answer in more than one special case, eliminating the need for physical dark matter in other observed phenomena; 2) be consistent with other known laws and theories of physics that are experimentally verified; and 3) make testable predictions beyond the phenomena it was designed to explain.

  • How does the AQuaL theory attempt to reconcile MOND with the rest of physics?

    -AQuaL, which stands for 'a quadratic Lagrangian,' is a reformulation of MOND using Lagrangian mechanics. It introduces a second scalar field to gravity, which helps obey conservation laws and allows the theory to work with relativity, thus addressing some of the inconsistencies of MOND with the rest of physics.

  • What is the Tully-Fisher Law and how does it relate to MOND?

    -The Tully-Fisher Law describes a tight relationship between the speed of rotation and the luminosity of spiral galaxies, with brighter galaxies spinning faster. MOND, when tuned to account for flat rotation curves, automatically predicts this relationship, which was an unexpected outcome and is considered a success for the theory.

  • What is TeVeS and how does it improve upon AQuaL?

    -TeVeS, which stands for Tensor, Vector, Scalar gravity, is a theory introduced by Bekenstein that describes gravity with three fields. It improves upon AQuaL by fixing the problem with gravitational lensing and addressing the causality-breaking nature of faster-than-light waves in the scalar field.

  • What is the main cosmological problem with MOND and TeVeS?

    -The main cosmological problem with MOND and TeVeS is that they struggle to explain the observed lumpiness in the cosmic microwave background radiation, which suggests that dark matter was able to collapse and form structures in the early universe, something that regular matter could not do due to radiation pressure.

  • What is the current stance of the scientific community on the debate between dark matter and MOND?

    -The majority of experts are firmly in the dark-matter-as-particle camp. Although dark matter has not been directly detected yet, there are many possibilities for what it might be beyond the standard model of particle physics. However, some prominent physicists support MOND theories, and the debate continues.

  • What is the Bullet Cluster and why is it a challenge for MOND theories?

    -The Bullet Cluster is a system where galaxy clusters have collided and the dark matter appears to have been separated from the light matter. This observation supports the idea of dark matter as a distinct physical entity and poses a challenge for MOND theories, which require a different explanation for the observed phenomena.

Outlines
00:00
🔍 Dark Matter and the Questioning of Gravity

This paragraph introduces the concept of dark matter and the possibility that our understanding of gravity might be flawed. It discusses the work of Vera Rubin and the evidence suggesting the existence of dark matter, which is inferred from the behavior of galaxies and gravitational lenses. The paragraph also explores the idea that instead of dark matter, there might be an alternative theory of gravity that can explain these phenomena without invoking unseen mass. It mentions Modified Newtonian Dynamics (MOND) proposed by Mordehai Milgrom as a theory that modifies our understanding of gravity at very low accelerations, potentially eliminating the need for dark matter.

05:05
🌌 MOND's Performance and Bekenstein's Contribution

The second paragraph examines the limitations of MOND, particularly its inability to fully account for all the evidence pointing towards dark matter, especially in galaxy clusters. It also highlights that MOND does not align with other established physical laws, such as conservation laws and general relativity. However, the paragraph then describes how Jacob Bekenstein, known for his work on black holes, collaborated with Milgrom to reformulate MOND using Lagrangian mechanics, which led to a version of MOND that obeys conservation laws and is consistent with relativity. This new formulation, known as AQuaL, faced its own challenges, including issues with gravitational lensing and causality, leading to further refinements and the development of TeVeS (Tensor Vector Scalar gravity).

10:08
🌟 TeVeS and the Cosmological Challenges

This paragraph delves into the problems associated with TeVeS, including the potential for instabilities in the presence of matter and issues with cosmology. It discusses how the cosmic microwave background radiation suggests the existence of dark matter and how MOND theories, including TeVeS, struggle to explain the early formation of structures in the universe. The paragraph then introduces a new relativistic version of MOND proposed by Constantinos Skordis and Tom Złosnik, which allows for a scalar field that changes behavior over time, potentially addressing some of the cosmological issues.

15:09
🌌 The Debate Over Dark Matter and Modified Gravity

The final paragraph summarizes the ongoing debate between the dark matter particle theory and modified gravity theories like MOND. It mentions the Bullet Cluster as a significant challenge for MOND and acknowledges that while the majority of experts favor the dark matter particle theory, the MOND theories have not been dismissed due to the expertise of their proponents. The paragraph concludes by emphasizing the excitement of the scientific pursuit, regardless of which theory prevails, and the potential for new discoveries beyond the standard model of particle physics or general relativity. It also includes a thank you note to CuriosityStream for supporting PBS and a brief introduction to PBS's new medical show, Vitals.

Mindmap
Keywords
💡Dark Matter
Dark matter is a hypothetical form of matter that is thought to account for approximately 80% of the mass in the universe. It is invisible to electromagnetic radiation, which means it does not interact with light, making it undetectable by current technology. In the video, dark matter is central to the discussion as it is proposed that our understanding of gravity might be flawed rather than the existence of this invisible mass.
💡Gravity
Gravity is the force that attracts two bodies towards each other, and it is the dominant force at a cosmic scale. The video questions the current understanding of gravity, suggesting that if our laws of gravity are incorrect, it might explain the phenomena currently attributed to dark matter without the need for additional mass.
💡Vera Rubin
Vera Rubin was an astronomer who provided significant evidence for the existence of dark matter. In the early 1960s, she demonstrated that spiral galaxies rotate so fast that they should disintegrate if held together only by the gravity of their visible mass, implying the presence of additional, unseen mass.
💡Modified Newtonian Dynamics (MOND)
MOND is an alternative to the dark matter hypothesis, proposed by physicist Mordehai Milgrom. It suggests that at very low accelerations, the force of gravity no longer follows an inverse-square law but instead plateaus. The video discusses MOND as a theory that could explain the observed phenomena without the need for dark matter.
💡General Theory of Relativity
Albert Einstein's general theory of relativity is a theory of gravitation that describes gravity not as a force, but as a consequence of the curvature of spacetime caused by mass and energy. The video mentions that while Newtonian gravity breaks down under strong gravitational fields, leading to the need for general relativity, it might also break down under very weak fields, which is a concept explored in MOND.
💡Tully-Fisher Law
The Tully-Fisher Law is a relationship in astronomy between the rotation rate of a spiral galaxy and its luminosity. The video highlights this law as a surprising tight correlation that, when MOND is used to explain the flat rotation curves of spiral galaxies, automatically gives the correct relationship between rotation speed and luminosity, which was an unanticipated outcome of the MOND theory.
💡AQuaL
AQuaL, which stands for 'a quadratic Lagrangian,' is a reformulation of MOND using Lagrangian mechanics. It was developed by Bekenstein and Milgrom to make MOND consistent with conservation laws and to work with relativity. The video discusses AQuaL as an improvement over MOND, although it had its own issues, such as not correctly predicting gravitational lensing effects.
💡TeVeS
TeVeS, or Tensor, Vector, Scalar gravity, is a theory proposed by Bekenstein that adds a scalar field to gravity, aiming to resolve the issues with AQuaL and make MOND consistent with general relativity. The video describes TeVeS as a model that behaves like Newtonian mechanics on solar system scales, like MOND on galactic scales, and like general relativity for gravitational lensing.
💡Cosmic Microwave Background Radiation
The cosmic microwave background (CMB) radiation is the thermal radiation left over from the time of recombination in Big Bang cosmology. The video discusses how the CMB radiation provides evidence for the lumpiness that indicates how matter, including dark matter, pulled itself together under gravity in the early universe, which is a challenge for MOND theories.
💡Relativistic MOND (RelMOND)
RelMOND is a new relativistic version of MOND proposed by Constantinos Skordis and Tom Złosnik. It allows the scalar field to change its behavior over time, addressing some of the cosmological issues faced by previous MOND theories. The video mentions that RelMOND is a promising development, but more work is needed to see if it can fully explain phenomena such as galaxy clusters.
💡Bullet Cluster
The Bullet Cluster is a pair of colliding galaxy clusters, which provides strong evidence for the existence of dark matter. The video briefly mentions the Bullet Cluster as a challenge for MOND theories because when the clusters collide, the dark matter appears to separate from the visible matter, suggesting that dark matter cannot be explained by modified gravity alone.
Highlights

The search for dark matter has been ongoing for over half a century, challenging our understanding of gravity.

Vera Rubin's work in the 1960s showed that spiral galaxies rotate too fast to be held together by visible mass alone.

It is estimated that at least five times more matter is needed to explain the observed gravitational effects in galaxies.

80% of the universe's mass appears to be invisible, leading to the hypothesis of dark matter.

The possibility that our laws of gravity are incorrect is explored as an alternative to the existence of dark matter.

Modified Newtonian Dynamics (MOND) proposes a minimum possible acceleration that can be produced by gravity.

MOND can explain the observed rotation curves of spiral galaxies without the need for additional mass.

MOND has faced challenges in explaining phenomena like galaxy clusters and has had issues with conservation laws.

The Tully-Fisher Law, which relates the speed of rotation to luminosity, is an unexpected success for MOND.

Jacob Bekenstein and Mordehai Milgrom reformulated MOND using Lagrangian mechanics to address its inconsistencies.

TeVeS (Tensor, Vector, Scalar gravity) was introduced to resolve issues with MOND and general relativity.

Cosmological issues, such as the cosmic microwave background radiation, pose a challenge to MOND theories.

A new relativistic version of MOND has been proposed to address issues with the early universe and structure formation.

The Bullet Cluster presents a significant challenge to modified gravity theories, as it shows dark matter separated from visible matter.

The debate between dark matter as a particle and modified gravity theories is unresolved, with experts divided.

The principle of least action is a fundamental concept that underlies much of physics, potentially leading to a theory of everything.

CuriosityStream, a subscription streaming service, offers documentaries and non-fiction titles on various topics, including science.

Vitals, a new PBS medical show, aims to provide accurate health information and bust medical myths.

Transcripts

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Dark MatterGravityEinsteinCosmologyMONDModified GravityAstronomyScientific TheorySpacetimePhysics