Q.What are tides?

Think of the last time you stood in a crowded bus that suddenly braked. Your body lurched forward — not because something pushed you, but because your feet stopped while the rest of you kept going. Tidal forces work on the same principle, but on a cosmic scale. They are not a force that pulls you in one direction; they are a difference in pull across a stretch of space.

Here is the everyday intuition. Gravity weakens with distance. So the Moon pulls the side of the Earth facing it more strongly than it pulls the centre of the Earth, and it pulls the centre more strongly than the far side. That difference — the extra pull on the near side and the extra pull on the far side — is what we call a tidal force. It is not the Moon's gravity itself, but the imbalance of that gravity across the Earth's diameter.

Now, what does that imbalance actually do? It stretches things. The near side of the Earth is pulled toward the Moon, and the far side is, in effect, left behind — so the whole planet gets slightly squashed into an oval, with bulges on both the side facing the Moon and the side opposite it. Those bulges are the high tides. The water, being fluid, responds far more visibly than the solid rock beneath it, which is why we notice tides in the ocean and not in the ground under our feet.

Why does this matter beyond the beach? Because tidal forces are the reason the Moon always shows us the same face. The Earth's tidal pull stretches the Moon slightly, and over millions of years that stretching has slowed the Moon's rotation until it is locked — one rotation exactly matching one orbit. The same mechanism is at work on a grander scale: when a star wanders too close to a black hole, the tidal force across the star becomes so enormous that it tears the star apart into a long stream of gas. Astronomers call this spaghettification, and it is the most dramatic example of the same physics that lifts ocean water twice a day.

A few things worth holding onto:

• Tidal forces arise from the change in gravity over distance, not from gravity itself.
• They always stretch objects along the line toward the source of gravity, and compress them sideways.
• They explain ocean tides, the Moon's locked rotation, and the destruction of stars near black holes. …