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Posted 26 November 2007

Tidal Forces and the Universe

by Frikkie de Bruyn

We are all familiar with the tidal waves in the oceans caused by the Moon’s gravitational pull on the Earth. The side of the Earth facing the Moon is about 6 400 km closer to the Moon than the centre of the Earth. The Moon’s gravitational pull on the oceans nearer to the Moon is therefore stronger than its gravitational pull on the Earth’s centre. The difference is very small, about 3 percent, but it is enough to make the water on the side facing the Moon flow in a bulge. A bulge also forms on the other side of the Earth which is about 6 400 km further from the Moon than the Earth’s centre. The relatively smaller gravitational force exerted by the Moon causes the water to flow away from the Moon in a bulge. It is strictly speaking not correct to say “the tide comes in”. The tide does not come in but rather we are carried into the tidal bulge by the Earth’s movement. Even if the Earth had no oceans the effects of the tides would still be visible causing slight bulges in the rocky surface of the Earth. What are the tidal waves? Tidal forces (visible in the oceans as waves) occur when two massive objects (such as the Moon and the Earth) interact gravitationally. Whenever there is a difference in gravitational pull between the two objects, tidal forces can distort and even destruct the gravitationally weaker object. Tidal waves affect both rotational and orbital forces. The friction of the water in the oceans against the sea beds slows the Earth’s rotation and the days are getting longer by 0.001 percent per century. The earth’s gravity slows the Moon’s rotation so that it now rotates to keep one side permanently facing the Earth. We say the Moon is tidally locked to the Earth. Tidal forces cause the Moon’s orbit to grow larger and the Moon is receding from the Earth at about 3.8cm per year.

Other planets in our Solar System interact gravitationally in a similar way on the moons in orbit around them. However, tidal forces are not restricted to our Solar System, but are to be found across the Universe. Let us look at tidal forces between massive Jupiter like planets orbiting their stars even closer than Mercury’s orbit, between in falling objects and black holes, between galaxies, clusters of galaxies, super clusters and tidal forces exerted by dark matter. Astronomers have currently found about 300 planets orbiting other stars. Some of these planets are more massive than Jupiter with orbits smaller than Mercury’s orbit. These massive planets can create tides on the surfaces of their stars. They pull on the stars gravitationally disrupting the star’s velocity, causing it to move back and forth, toward and away from the Earth, so much that it can be detected from Earth. If we can detect it there must be some very powerful tidal forces at work. These very massive planets orbit their stars from 3 to 10 days. The stars often rotate on scales 20 to 30 days. The planets are moving faster than the surfaces of the stars are moving and this cause the stars surfaces to move out towards the planets. In extreme cases parts of the planets can be pulled away towards the stars ending up as part of the stars.

The planets are tidally locked to their stars and this can cause extreme convection flows on the planets with atmospheres. The sides of the planets facing the stars will be extremely hot and the opposite sides extremely cold. This will result in massive circulation of gasses at thousands of kilometres per hour. Planets getting close to their stars can have their atmospheres ripped off by the stars. This could also happen where two stars form a binary system. To understand this mechanism we must bear in mind that there is a point between the two stars where the gravitational pull of the stars balance, called the Lagrange-1 point. Gas from one star gets pulled towards the other star to the extent that it overflows the Roche limit (the minimum distance between the two stars where each hold itself together by its own gravity) and it ends up on the surface of the other star. It is this mechanism that causes a Type l A supernova when the gravitationally superior white dwarf star pull matter from a normal star until it exceeds the limit of 1.4 solar masses and explodes as a supernova.

The most extreme forms of tidal waves occur when matter falls into a black hole. Suppose an unfortunate astronought gets too close to a black hole and starts falling in. His feet will feel the gravitational pull of the black hole much stronger than his head. His whole body gets elongated because of the uneven pull of gravity. As he crosses the event horizon his feet first then his legs, his upper body and his head will be pulled apart until even molecules, atoms and particles will be ripped apart by the extreme tidal forces in a process known as spagettification. The astronought’s body gets elongated because the force of gravity on his feet is different from the pull of gravity on his head. This is a puzzle because it means that the force of gravity on one part of your body is different from that on another part. What happened to the principle that if you drop two objects at the same time they will fall at the same speed? It is, however, the centres of mass of the objects that fall at the same speed. The centre of mass in a human being is the belly button. If the astronought could hold tennis ball in his hand the part of the ball nearest to the black hole will get stretched out towards the centre of the black hole. The astronought’s feet fall faster than the rest of his body and his body gets stretched out because his feet are closer to the centre of the black hole than his legs. His body cannot keep itself together and gets pulled apart by the tidal forces.

Galaxies are subject to tidal forces if a gravitationally stronger galaxy is interacting with a smaller galaxy. Anytime you have two masses interacting you have tidal forces. The gravitationally weaker galaxy will invariably be disrupted completely. The dwarf galaxy Carina, currently in the halo of our galaxy is in the process of being shredded apart. Astronomers believe that Carina could have been an elliptical or spheroidal collection of stars but all that will remain of the galaxy is a stream of stars and gas. Two galaxies moving past each other with the edges closer to each other will become stretched out while the stars at the back of the galaxies feel less gravitational force and will lag behind. This could result in some spectacular results with arms of the two galaxies reaching out towards each other and the rest of the galaxies trailing behind as tidal forces elongate the galaxies.

Tidal forces are also at work when clusters of galaxies interact gravitationally. The clusters will be stretched out as they pull one another together. The merged clusters in the process of collision will settle down to spherical shapes. Instead of sprays of stars we get sprays of galaxies, stars, gas and dust and, the surprising thing, dark matter. This raises an interesting question: “How does dark matter impact on tidal forces?” Dark matter does not clump into objects in the way normal matter does. However, the gravitational pull of a cluster of dark matter pull objects towards it causing the objects to be stretched out or elongated. Dark matter itself can also get stretched out or elongated in the same way that normal matter gets affected by gravity. Tidal forces are at work whenever two bodies with a difference in gravitational force interact with each other. Think about this wonderful way of gravity at work next time you are on the beach.


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