<p>LIGO has a mind boggling sensitivity. What would it take to directly measure the expansion of spacetime, along the laser beam? I suppose it could be done at interplanetary scale.</p>

CC BY-SA 3.0
2016-04-08T12:47:17.903Z
LIGO is a differential measurement in two perpendicular arms at relatively high frequencies. It wouldn't work if the arms expanded by the same amount at the same time.
ProfRob
CC BY-SA 3.0
2016-04-08T13:10:09.297Z
@Rob Jeffries Well, maybe they don't ;) Couldn't the increased redshift be detectable? That would be a very different instrument thoug. Is the LIGO precision tech transferable to other instruments?
LocalFluff
CC BY-SA 3.0
2016-04-08T13:56:03.767Z
I think I partly answered that recently  http://astronomy.stackexchange.com/questions/14411/can-we-tell-how-fast-bodies-are-moving-away-by-measuring-their-frequency/14422#14422 see also http://physics.stackexchange.com/questions/170525/how-does-a-particular-galaxys-apparent-recessional-velocity-change-as-the-unive/170610#170610
ProfRob
CC BY-SA 3.0
2016-04-08T20:37:11.210Z
But space doesn't expand on such small scales as the beam. Space only expand on super-galactic scales. Or am I misinterpreting your question?
pela
CC BY-SA 3.0
2016-04-09T15:22:04.783Z
@pela Doesn't all spacetime everywhere expand and accelerate? Is it not a matter of sensitivity to measure it at ever shorter distances? If different regions or directions of spacetime expand differently, then a LIGO-like concept might catch it? If space expands equally in all directions, then there is no reference, right? But what about time with its single direction and constant speed.
LocalFluff
CC BY-SA 3.0
2016-04-09T18:44:54.167Z
No, space only expands if the energy density is small enough. In the Solar system, inside galaxies, and even in galaxy groups, the mutual gravitational attraction of matter prevents expansion. You might think that the Hubble constant of 70 km/s/Mpc could be translated into 3e-5 cm/s/AU between the Sun and the Earth, but this is not the case. You could say that space is coupled to matter. On the even smaller scales of everyday objects, electromagnetic forces prevents the expansion. On the other hand, in the voids between galaxy clusters, space expands faster than the average expansion.
pela
<expansion><gravitational-waves><interferometry>
Can the expansion of spacetime be directly measured with laser interferometry (like GW can)
LocalFluff
# Can the expansion of spacetime be directly measured with laser interferometry (like GW can)

(Asked by: username_0 on 2016-04-08)

LIGO has a mind boggling sensitivity. What would it take to directly measure the expansion of spacetime, along the laser beam? I suppose it could be done at interplanetary scale.

**Tags:** expansion, gravitational-waves, interferometry

> **Comments:**
> **username_1**: LIGO is a differential measurement in two perpendicular arms at relatively high frequencies. It wouldn't work if the arms expanded by the same amount at the same time.  
> **username_0**: @Rob Jeffries Well, maybe they don't ;) Couldn't the increased redshift be detectable? That would be a very different instrument thoug. Is the LIGO precision tech transferable to other instruments?  
> **username_1**: I think I partly answered that recently  http://astronomy.stackexchange.com/questions/14411/can-we-tell-how-fast-bodies-are-moving-away-by-measuring-their-frequency/14422#14422 see also http://physics.stackexchange.com/questions/170525/how-does-a-particular-galaxys-apparent-recessional-velocity-change-as-the-unive/170610#170610  
> **username_2**: But space doesn't expand on such small scales as the beam. Space only expand on super-galactic scales. Or am I misinterpreting your question?  
> **username_0**: @username_2 Doesn't all spacetime everywhere expand and accelerate? Is it not a matter of sensitivity to measure it at ever shorter distances? If different regions or directions of spacetime expand differently, then a LIGO-like concept might catch it? If space expands equally in all directions, then there is no reference, right? But what about time with its single direction and constant speed.  
> **username_2**: No, space only expands if the energy density is small enough. In the Solar system, inside galaxies, and even in galaxy groups, the mutual gravitational attraction of matter prevents expansion. You might think that the Hubble constant of 70 km/s/Mpc could be translated into 3e-5 cm/s/AU between the Sun and the Earth, but this is not the case. You could say that space is coupled to matter. On the even smaller scales of everyday objects, electromagnetic forces prevents the expansion. On the other hand, in the voids between galaxy clusters, space expands faster than the average expansion.  

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