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I've always had trouble with the speed of light and frame of reference examples. The light moving inside the train is moving from the person on the train to th

I've always had trouble with the speed of light and frame of reference examples. The light moving inside the train is moving from the person on the train to the mirror and back again. That is a fixed distance inside the train. Why does it matter what the observer outside the train sees? If a person on the train threw a ball in the air and caught it, to an observer outside the train it would also appear to travel a further distance in the same amount of time. But we would say that it's because the train's speed affects the ball's speed. But we don't say that about light. Explain like I'm five ?

superposeur 4y ago
Short answer: only things like light waves moving at the universal speed c have the same speed in all frames, not objects like balls moving at slower speeds. Longer answer. The principle of relativity doesn’t claim that particular objects have the same speed in different frames, it claims that the fundamental laws of nature take the same form in different frames. In the 1860s Maxwell wrote down a set of four equations governing ripples in the electromagnetic field (=light waves). They naively seem to only be exactly true in one frame of reference since they predict the light waves move at one particular speed c. Einstein made the leap to suppose that the Maxwell equations continue to take the same form in all frames of reference, so that each observer sees the ripples moving at the same speed c. This doesn’t apply to balls because the fundamental laws make no such prediction for the speed of a particular ball. (Obviously! You can hold the ball, or throw it.) Hence the ball being different speeds in different frames is just something about a particular ball — it doesn’t contain a grander implication about the laws of nature.
jodrellblank 4y ago
We don't say that about light because the train's speed doesn't affect light. The train can push the ball faster. It travels faster, catches up to the ball, and pushes on it. But if light is 'the fastest speed there is' then the train cannot go faster, cannot catch up to it, cannot push push on it. Say for a moment that the train can push light faster, there is a new 'fastest speed'. What changed so that light could go faster? Maybe overcoming air resistance? or friction? Or mass, removing mass makes sports cars faster, right? So let's remove all air resistance and all friction and all mass, and now we're thinking about as fast as anything can possibly go in this universe. When there's nothing left to remove, nothing to overcome, that's "c" the speed of light in a vacuum. So why can't the train push it faster than that? The 'speed of light' is a poor name for it, it's the speed anything can be communicated in any way - the train engine pushes on the wheels and the track pushes back and there is friction and the axels push on the train body and the train body pulls on the carriages and the carriage floors pull on the chairs which move the passengers and all those connections carrying the 'information' of the pulls and pushes through the metals are at fastest carried by electromagnetic interactions at 'the speed of light' that make up bumps and knocks. The train can't push light faster because the train can't go faster because none of the train's parts can move faster than light to gain any extra speed to push with. At best the train and light are side by side, neither gaining nor losing on each other. Except, the train has mass and light doesn't so it's always slower. And that's not mentioning that spacetime isn't 3D space with a timeline ticking beside it like a video time, as a 4th dimension, it's more connected than that in ways I don't understand at all. Take the simplest clock concept, a photon of light bouncing between two mirrors, once per second. You say the speed is how fast the light is going. The time is how long it takes the light to bounce once. And the distance is how far light goes in a second. You can't speed light up (see above). If you move the mirrors closer together have you changed the duration of a second? Or sped up light? Or shrank distance? Easy, you moved the mirrors closer together, right? Now consider what the observer outside the train sees - the information of what's happening is communicated to them by light coming to their eyes. And what they see from the way light moves is that the mirrors have been moved closer together. And you on the train see that the mirrors haven't been moved closer together. And both are correct. Neither is imaginary, a con, a magic trick, an optical illusion. So what happened to the clock on the train? Did the speed of light change? Did the distance between the mirrors change? Did the duration of a second change? Or did the universe go mad? Answer: relativity; we agree that taking away all friction, all mass, all air resistance, leaves some maximum speed that things can fly through the universe, we agree that all massless things including light travel at that speed and call it 'the speed of light', and from there different observers see distance and time differently, and that's OK. We must start to talk in terms of 'frames of reference', whose observation are we working from?

superposeur 4y ago

Short answer: only things like light waves moving at the universal speed c have the same speed in all frames, not objects like balls moving at slower speeds. Longer answer. The principle of relativity doesn’t claim that particular objects have the same speed in different frames, it claims that the fundamental laws of nature take the same form in different frames. In the 1860s Maxwell wrote down a set of four equations governing ripples in the electromagnetic field (=light waves). They naively seem to only be exactly true in one frame of reference since they predict the light waves move at one particular speed c. Einstein made the leap to suppose that the Maxwell equations continue to take the same form in all frames of reference, so that each observer sees the ripples moving at the same speed c. This doesn’t apply to balls because the fundamental laws make no such prediction for the speed of a particular ball. (Obviously! You can hold the ball, or throw it.) Hence the ball being different speeds in different frames is just something about a particular ball — it doesn’t contain a grander implication about the laws of nature.

jodrellblank 4y ago

We don't say that about light because the train's speed doesn't affect light. The train can push the ball faster. It travels faster, catches up to the ball, and pushes on it. But if light is 'the fastest speed there is' then the train cannot go faster, cannot catch up to it, cannot push push on it. Say for a moment that the train can push light faster, there is a new 'fastest speed'. What changed so that light could go faster? Maybe overcoming air resistance? or friction? Or mass, removing mass makes sports cars faster, right? So let's remove all air resistance and all friction and all mass, and now we're thinking about as fast as anything can possibly go in this universe. When there's nothing left to remove, nothing to overcome, that's "c" the speed of light in a vacuum. So why can't the train push it faster than that? The 'speed of light' is a poor name for it, it's the speed anything can be communicated in any way - the train engine pushes on the wheels and the track pushes back and there is friction and the axels push on the train body and the train body pulls on the carriages and the carriage floors pull on the chairs which move the passengers and all those connections carrying the 'information' of the pulls and pushes through the metals are at fastest carried by electromagnetic interactions at 'the speed of light' that make up bumps and knocks. The train can't push light faster because the train can't go faster because none of the train's parts can move faster than light to gain any extra speed to push with. At best the train and light are side by side, neither gaining nor losing on each other. Except, the train has mass and light doesn't so it's always slower. And that's not mentioning that spacetime isn't 3D space with a timeline ticking beside it like a video time, as a 4th dimension, it's more connected than that in ways I don't understand at all. Take the simplest clock concept, a photon of light bouncing between two mirrors, once per second. You say the speed is how fast the light is going. The time is how long it takes the light to bounce once. And the distance is how far light goes in a second. You can't speed light up (see above). If you move the mirrors closer together have you changed the duration of a second? Or sped up light? Or shrank distance? Easy, you moved the mirrors closer together, right? Now consider what the observer outside the train sees - the information of what's happening is communicated to them by light coming to their eyes. And what they see from the way light moves is that the mirrors have been moved closer together. And you on the train see that the mirrors haven't been moved closer together. And both are correct. Neither is imaginary, a con, a magic trick, an optical illusion. So what happened to the clock on the train? Did the speed of light change? Did the distance between the mirrors change? Did the duration of a second change? Or did the universe go mad? Answer: relativity; we agree that taking away all friction, all mass, all air resistance, leaves some maximum speed that things can fly through the universe, we agree that all massless things including light travel at that speed and call it 'the speed of light', and from there different observers see distance and time differently, and that's OK. We must start to talk in terms of 'frames of reference', whose observation are we working from?