# How exactly does light's wavelength expanding over time mean space expands?

I already understand the notion, but what I don't get is how that leap is actually logical. Wouldn't you have to assume the photons and atoms are somehow "attached" to the fabric of space? It seems like the same thing as saying Earth's atmosphere is attached to it's crust.

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Changing wavelength of light does not necessitate expanding space.

Particles are 'attached' to space (in a way), by inertia.

Drakkith
Staff Emeritus
Does it have anything to do with light being an EM wave and matter not?

Changing wavelength of light does not necessitate expanding space.

Particles are 'attached' to space (in a way), by inertia.
Is there like...any direct evidence of that?

Drakkith
Staff Emeritus
Is there like...any direct evidence of that?
Which part? Light's wavelength can easily be changed by gravity and motion of the receiver relative to the emitter.

Which part? Light's wavelength can easily be changed by gravity and motion of the receiver relative to the emitter.
I get how light's wavelength is relative to position, but is it proven that the bending of space itself streatches/shortens wavelengths? Does string theory even attempt to explain that?
Or I guess, "how" does the fabric of space bending mean light itself bending and not just some relativity property of measuring it?

Drakkith
Staff Emeritus
I get how light's wavelength is relative to position, but is it proven that the bending of space itself streatches/shortens wavelengths? Does string theory even attempt to explain that?
Or I guess, "how" does the fabric of space bending mean light itself bending and not just some relativity property of measuring it?
Are you referring to the redshift of light due to expansion of space, the red and blueshifting of light due to gravity, or the way gravity alters the path of light?

Are you referring to the redshift of light due to expansion of space, the red and blueshifting of light due to gravity, or the way gravity alters the path of light?
I guess gravity can alter the "path" of light, but isn't that still just us measuring relative frequency based on position? How does gravity exactly "stretch" or "attach" to photons?

I guess gravity can alter the "path" of light, but isn't that still just us measuring relative frequency based on position? How does gravity exactly "stretch" or "attach" to photons?
Who exactly are you "quoting"?

The deflection of light has nothing to do with 'measuring relative frequency'. See http://www.einstein-online.info/spotlights/light_deflection

Relativistic effects which occur in a region of space-time (be it cosmological redshift or gravitational redshift) effect the matter/energy occupying that region---including light. When space is dilated, so is light.

Who exactly are you "quoting"?

The deflection of light has nothing to do with 'measuring relative frequency'. See http://www.einstein-online.info/spotlights/light_deflection

Relativistic effects which occur in a region of space-time (be it cosmological redshift or gravitational redshift) effect the matter/energy occupying that region---including light. When space is dilated, so is light.
But how is it that whenever space warps that light also warps? How could matter be attached to the supposedly not even physical fabric of space as to make it so that "when the fabric of space stretches x distance, so does the wavelength of a photon" (I know it's not actually that proportional)?

Drakkith
Staff Emeritus
But how is it that whenever space warps that light also warps? How could matter be attached to the supposedly not even physical fabric of space as to make it so that "when the fabric of space stretches x distance, so does the wavelength of a photon" (I know it's not actually that proportional)?
The warping of spacetime that you normally hear of is not responsible for the redshift or blueshift of light. The expansion of space is what causes redshift over extreme distances. Near to massive objects such as stars the path of light is altered because the shortest distance between two points, the geodesic, has changed. This does not alter the frequency of light as it passes other than normal blueshift as it heads toward the object, and redshift as it heads away. At any given distance before and after passing the object the wavelength is the same.

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The answer is complicated. Really complicated. As in four month long fourth year university level course complicated. The whole issue of light paths and wavelength is endlessly subtle because wavelength is relative to the velocity an observer present at a point in spacetime the light goes through. Light has a wavelength with respect to the chosen coordinate system, but it can be misleading to rely on that. For instance, given ANY light wave, even one moving through curved space, it is possible to construct a coordinate system around its path such that its wavelength with respect to the coordinate system does not change. It is all very interesting stuff, but an explanation that was satisfactory to me anyway required a lot of less sexy math and physics—about a years' worth at full tilt, whether in university or by self-study. Issues involving curved spacetime are best dealt with after addressing flat spacetime, which is complicated enough. It is best dealt with after learning calculus, linear algebra, and classical mechanics.

russ_watters
Mentor
I get how light's wavelength is relative to position, but is it proven that the bending of space itself streatches/shortens wavelengths? Does string theory even attempt to explain that?
Or I guess, "how" does the fabric of space bending mean light itself bending and not just some relativity property of measuring it?
It is pretty much the same phenomena as a doppler shift.

sophiecentaur