Eli5. Why is gravity classified as a weak force when it can bend and trap light?
In: Physics
There’s a theory, within the many worlds theory, that gravity is shared across the multi verse which is why it’s so weak.
Well, stating that gravity is a force on it’s own is a contentious thing. Especially if you are talking about bending and trapping light. It’s like saying that a circle is a weak force. From the standpoint of general relativity, gravity isn’t even a force, but rather a consequence of a curved spacetime.
From that standpoint, light doesn’t bend. Light propagates in accordance to the shape of spacetime, or I guess you can say it travels in a straight line.
It’s like saying that our spherical planet is somehow a force because the shortest distance between Alaska and New York by plane is a curved line on a map. It’s only curved if you look at one projection. Gravity is the same, but it gets harder to visualize because you need to think of a higher dimension projection.
But to answer the question, gravity is considered weak because the apparant force that gravity exerts is much much lower than strong forces do. It’s 10^38 times weaker than the strong interaction for example. This difference is obvious when you can use tiny magnets to hold papers on the fridge. You’ve got the mass of the entire earth pulling on the paper, and this little tiny magnet is able to hold the paper from falling down.
Think of forces like a wave that dissipates with each wavelength. Strong forces are like a very high intensity wave with a short wavelength, they’re really strong close up, but fall off quickly. Gravity is like a very low wavelength, it’s not strong at all, but because of that it retains more of its influence much further.
It’s important that gravity is weak, it is what stops us from collapsing into black holes ourselves. The other forces like electromagnetic force and strong interaction keep masses separate. If gravity were the strongest, all matter would just immediately fall together into a black hole because it would overpower the forces that kept it apart. But gravity being stronger doesn’t really make sense. Rather, if you consider gravity as a force, you know it’s weak because all matter doesn’t collapse together, and the other forces act more strongly to keep things separate in spacetime.
When you lift something off the ground, you are overcoming the earth’s entire gravitational pull on that object, because gravity is a *biiiiiitch*.
Right but can a magnet bend space? Also a black hole is said to be very small, just extremely dense. So couldn’t a small amount of very dense matter out pull a very large magnet?
Which is stronger, a planet’s worth of gravity? Or a one gram refrigerator magnet?
That’s why. The magnet is a strong force. Gravity just applies over larger scales. It bends light because it bends space. The light technically doesn’t bend- it keeps going in a straight line! If you draw a line on paper and then bend the paper, did the line change? Or did the paper?
Take a small magnet. Put that magnet close to a nail. It picks it up. Congratulations, your magnet has just exerted a larger force on that nail than the entire Earth beneath it. That’s why Gravity is considered weak, because the other forces are much stronger.
It’s weak in comparison to the other forces. You can pick up a paper clip with a magnet. So the magnetic forces of a magnet that you can hold in your hand is stronger than the gravitational force of the entire Earth.
Gravity is considered weak because it is much weaker than the other fundamental forces. Sure, gravity can distort spacetime, but that requires a huge amount of mass. Compare it to the electromagnetic force. A magnet the size of your fingernail can easily hold up a paperclip, keeping it from falling to the ground. Gravity is so weak that the gravitational force of the entire Earth is weaker than the electromagnetic force of the magnet.
You can not just alter but reverse the trajectory of light with a hand mirror using the electromagnetic force, while the gravitational force of the entire sun only slightly bends its trajectory
You can pick up a coffee cup really easily despite an *entire planet* trying to pull it down against you. Gravity generally doesn’t bend nor trap light in any noticeable amount until you’re approaching star-levels of mass.
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