# Gravitational waves, tides and the end of universe

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

• Gravitational waves are not required to produce tides. A wave phenomenon has to be separated by multiple wavelengths from its source and it has to show a well defined dispersion relation before we start talking about "waves" to begin with. In case of the roughly 24h tidal motion the wavelength of the responsible gravitational waves would have to be larger than the solar system, which is clearly not the case. – CuriousOne Jan 21 '16 at 22:55
• Gravitational waves are not the same as static gravitational gradients. – Daniel Griscom Jan 21 '16 at 23:13

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.

• I am not an mathematician but after allot of googling I think I understand what a gradient are. en.wikipedia.org/wiki/Gradient explains it but I think this is easier to understand: en.wikipedia.org/wiki/Contour_line#Isopleths. But the maps of gravitational waves will be dynamical. What I mean is that detecting gravitational waves from the Moon and the Sun can be done white your eyes. Low frequency waves from other planets maybe can be detected by GPS satellites when computers compensates for sea leaves cause by winds and currents. Maybe the nine planet can be detected this way. – Ubmeje Jan 23 '16 at 0:32
• Solar System radiates only about 5000 watts in gravitational waves. The dissipation of energy by tidal friction on earth averages about 3.75 terawatts. Tidal is force not radiation. Answer accepted. – Ubmeje Jan 23 '16 at 23:47

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.

• I am not so good in maths but for photons higher frequency is higher energy. Amplitude is higher closer to the mass? thanx for the link need to readit more. – Ubmeje Jan 22 '16 at 20:33