Gravitational waves, tides and the end of universe

Question

Is tides proof of gravitational waves with low frequency?

According to Wikipedia,

In physics, gravitational waves are ripples in the curvature of space-time which propagate as waves, travelling outward from the source. Predicted in 1916 by Albert Einstein on the basis of his theory of general relativity, gravitational waves theoretically transport energy as gravitational radiation.

Edit: Thank you all for your answers and great links. Maybe I have misunderstood gravitational waves. But would not neutron stars or black holes, orbiting each other and pointing the orbit plane to earth cause very very very very small tides?

Then I found this link from wikipedia: https://en.wikipedia.org/wiki/Tidal_locking So maybe until the hole universe is tidal locked it will expand and then starting to shrink until it just gravitational waves left?

Thanks

Answer 1

No.

Tides are caused by the gradient in the gravitational field. As you get further from the moon, the field drops as $\frac{1}{r^2}$ and the gradient changes as $-\frac{1}{r^3}$. If there is a gradient, then objects closer to the moon will accelerate towards it more rapidly than objects further away from it. The effect of this is nicely illustrated in an earlier answer to a question about tides.

There is no need to invoke (low frequency) gravitational waves to make this description. A static picture works just fine.

With that said - a gravitational wave would give rise to a gradient, which would therefore give rise to "tides". The lower the frequency, the smaller the gradient. In other words - good luck detecting them that way.

Answer 2

Gravitational waves can be emitted from a rotating object, but the object must not be axisymmetric. For example, a perfect sphere will not radiate gravitational waves, but a sphere with some sort of bulge may.

We can calculate the radiated energy from such a source (see for instance this paper). However, for gravitational waves to have effects on the same scale as tidal forces, the source must be spinning extremely rapidly - which is not the case for the Moon or Sun. Neutron stars that may emit gravitational waves of strengths we can detect spin many orders of magnitude faster than the Moon or Sun do.

Answer 3

In your edit you write: But would not neutron stars or black holes, orbiting each other and pointing the orbit plane to earth cause very very very very small tides?

My answer only refers to this: In fact, massive binaries are the strongest sources of GW anywhere. Much stronger than pulsars with mountains that are searched for with LIGO. From the period length of binary stars it follows that one has to search at frequencies in the range 0.1 $\mu$Hz to 50 $\mu$Hz.

On Earth, around 13,000 oscillations can be measured in the frequency range 1 $\mu$Hz to around 70 $\mu$Hz. A very strong peak is observed at 22.3643 µHz. At this frequency the moon deforms the earth and the atmosphere (tides). The moon, the sun and the planets generate further, much weaker resonance frequencies, which are precisely known and tabulated in The HW95 tidal potential catalogue, https://publikationen.bibliothek.kit.edu/160395.

In the spectrum of this frequency range (see https://vixra.org/abs/2311.0020) you can find these well-known frequencies and - especially in the lower range between 1 $\mu$Hz and 30 $\mu$Hz - other spectral lines, which are not included in HW 95. The two types differ when one determines and compares the frequency drift over a period of about 20 years. These could be gravitational waves.