TLDR;
This video explores the concept of redshift and its implications for our understanding of the expanding universe. It clarifies that redshift isn't a single phenomenon but rather a consequence of relative motion between observers and the source of light. The video argues that the common interpretation of "expanding space" stretching objects is a misconception. Ultimately, it concludes that while the universe is expanding, this expansion doesn't directly cause everyday objects, including ourselves, to expand.
- Redshift is not a single phenomenon but depends on the observer's frame of reference.
- The idea of "expanding space" stretching objects is a misconception.
- Everyday objects, including ourselves, do not expand with the universe.
Introduction to Redshift [0:07]
The video starts by introducing the evidence for the expanding universe: the redshift observed in the light from distant galaxies. This redshift, a shift towards longer wavelengths, is typically explained by the stretching of photons as they travel through expanding space. However, this explanation raises questions about whether expanding space also stretches atoms, molecules, stars, galaxies, and even ourselves. The video aims to address these questions by examining the true nature of redshift.
Three Types of Redshift [1:23]
Physicists recognise three types of redshift: Doppler redshift (due to relative motion), gravitational redshift (due to differences in gravitational potential), and cosmological redshift (observed over vast distances in an expanding universe). These redshifts appear distinct and are governed by different equations. The video questions how each type of redshift actually occurs and whether they are truly different phenomena.
Gravitational Redshift and the Equivalence Principle [1:57]
The video discusses gravitational redshift using the famous Pound-Rebka experiment, where photons sent up a tower were observed to be redshifted at the top. This redshift happens continuously as the photon loses energy climbing the tower. Einstein's equivalence principle suggests that this is equivalent to an experiment in an accelerating rocketship. The video then presents a thought experiment involving observers inside and outside the rocket to demonstrate that the observed wavelength depends on the observer's frame of reference.
The Observer's Frame of Reference [4:15]
The video explains that the observed redshift depends on the observer's motion relative to the source. Observers inside the accelerating rocket see a Doppler redshift because they are moving away from the source, while stationary observers outside see no redshift. This demonstrates that wavelength and energy are not intrinsic properties of photons but depend on the photon-observer system. The video then revisits the Harvard tower experiment, showing that a free-falling observer would see no redshift, further emphasising the importance of the observer's frame of reference.
Unifying Redshift [5:43]
The video argues that there aren't actually three different types of redshift, but only one. Gravitational redshift can be seen as a Doppler redshift in an accelerating rocket ship. The video then zooms out to cosmological scales to examine cosmological redshift.
Cosmological Redshift and Co-moving Observers [5:58]
At large scales, the universe can be treated as a homogeneous and isotropic cosmic fluid. The video introduces the concept of co-moving observers, who are at rest with respect to this fluid. While cosmological redshift is typically explained by the stretching of photons as the universe expands, the video proposes an alternative explanation: a series of Doppler shifts between neighbouring co-moving observers. Each observer sees the next one moving away, resulting in a small Doppler shift that accumulates over vast distances.
The Misconception of Expanding Space [8:49]
The video emphasises that redshifting is not something that happens to a photon itself, but depends on the observers at the point of emission and absorption. Therefore, there is only one type of redshift, described by a single mathematical framework. The idea of "expanding space" pulling on things and stretching them apart is a misconception.
Do You Expand with the Universe? [9:55]
The video answers the central question: do you expand with the universe? The answer is no, because on the scale of people, the universe is not homogeneous or isotropic. Local spacetime curvature is dominated by the Earth. Even in deep space, your body is held together by electromagnetic forces. Only if these forces were turned off would you expand, and even then, only due to the dominance of dark energy. The key takeaway is that redshifting photons does not mean space is expanding and stretching everything apart.
Conclusion [11:01]
The video concludes by reiterating that molecules, stars, galaxies, and ourselves are not expanding with the universe under normal circumstances. The video clarifies the misconception that expanding space is directly stretching objects.