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If you were on a train that was travelling at 60 mph and you threw a ball (inside the train) and the ball was travelling at 10 mph (inside the train), then the ball would objectively be travelling at 70 mph. Any observer (outside the train) would be able to understand why it looks like 10 mph inside the train and 70 mph outside the train.
Are you with me?
Okay, so the same thing does not happen with light! If you turn on a flashlight (inside the train), the light would be travelling at 670,616,629 mph regardless of whether the train was stationary or moving. So an observer outside the train would see the light travelling at the same speed as an observer inside the train. Even if the train was some supersonic invention from the future, the light inside the train would still be travelling at 670,616,629 mph - not 670,616,629 mph plus the speed of the train. And both inside and outside the train, observers would see the light as travelling at that speed. That’s the big thing to get hold of!
How can this possibly be the case?
The answer is that time itself actually slows down when we are in motion. At low speeds, the effect is negligible, but the closer we get to the speed of light, the more the effect becomes observeable, until, when we are travelling at the speed of light, time stands still. If we were able to go faster than the speed of light, we would be travelling backwards through time.
So it’s woven with space?
You have two observers, moving directly opposite each other.
Each has a flashlight pointing back at the other.
The speed of the light from those torched is the same for both observers.
(Instead the light would be red-shifted.)
Add a third observer, stationary to one and moving towards the other. As the third observer passes that observer, the speed of light from their flashlight never changes, and it’s the same speed as from the other two. (Instead it would go from being blue shifted to red shifted.)
This adds substantially more questions than it answers.
-Scientists after discovering General Relativity
I think it’s neat that Newton is taught first. As in: gravity is a function of mass. Because that works in so many scenarios.
But then you learn that gravity bends light and that photons have no mass.
So… Gravity isn’t a force, it’s more like going downhill… in the dimension of time.
Okay, but what if I go the speed of light minus 1m/s and shine a flashlight, can I catch the photons?
Light ALWAYS travels away from you at the speed of light no matter how fast you and your flashlight are going. Something has to give and that is time. It may look to outside observers, not traveling that fast along with you, that light is going 1 m/s faster than you… but you would also appear to be moving in super slow motion trying to reach out for the beam. The faster you go the slower time moves for you.
You’re already traveling at the speed of light minus 1m/s relative to a reference frame that’s traveling away from you at that speed.
Wow, and I’m in my pyjamas while doing it
Whilst we’re at it can someone explain;
A) a photon travels at the speed of light because it is massless, right? But doesn’t E=mc² teach us that mass and energy are somewhat interchangeable? How can a photon have energy but no mass?
B) I’m willing to accept it as fact when smarter people tell me that FTL is impossible because, amongst other things, it will break causality. It makes perfect sense to me that the universe needs to have some unbreakable rules for things to remain consistent. But I’ve never been able to grasp exactly why it would break causality.
But doesn’t E=mc² teach us that mass and energy are somewhat interchangeable?
There are two ways to read that equation. Either you use the instant mass, that changes with the object’s speed and has an undefined result for anything moving at the speed of light (this is the one Einstein wrote in awe about), or you use the rest mass and that’s the half of the equation that doesn’t consider the object’s speed (this is the one everybody actually uses).
On the second case, the complete equation is E² = (mc²)² + (pc)² where p is the momentum.
But I’ve never been able to grasp exactly why it would break causality.
It’s what I might call inferred logic. It’s not that traveling faster than light “breaks causality” it’s that light is observed to be constant in all reference frames. From that observation you build a system of motion which requires that nothing can travel faster than light. In that mathematical system, if you break that rule then the math says that you must go backwards in time to go faster than light.
So the “causality” is a side effect of the math system you created based around the rule that light is constant in all reference systems.
It’s like if you created a math system where you observed A=1 and let B=1 and state that A+B=2. Then someone says but what if A+B = -1. Your reply would be that means B = -2 because we observed A = 1 and B can be anything so if we add A + B and get -1 B must be -2. If B represented time you’d say, “Well that means you have negative time!” But it’s just the math and if you give physically impossible inputs you get physically impossible outputs.
A) from a photon’s perspective extrapolating relativity, zero time passes from when it is created to when it is absorbed. Essentially the two points are connected and the interaction is pure causality. This is part of the quantum nature of matter and can be understood as the manifestation of the probability of the two points interacting. An outside observer in three dimensional space doesn’t see the folds in higher dimensions that allow these interactions. We observe a time difference from the source to the target but our observation itself requires similar prabalistic folds in higher dimensions to make such observations, so the effect is never in isolation but as a combination that cancels the paradox. Of course I’m just making this all up, but it sounds good, eh? I’m a little bit high. Sorry.
For a photon itself, time don’t exist, for a photon it is everywhere in the universe at the same time. Inly for an observer it need x time from A to B with a finite speed.
Real shit.
