Okay, but what if I go the speed of light minus 1m/s and shine a flashlight, can I catch the photons?

A bit late to the party, but I'll try anyway!

So, first, speed is distance over time. Miles per second, kilometers per hour, whatever.

Consider a person rocketing by a planet in a little spaceship at a good fraction of the speed of light. To amuse themselves, they're bouncing a ball between two paddles on opposite walls of their craft. The ball describes a path like:

O--------O

--O----O

-----O

Of course, to a person on a planet they're blasting past, the path looks different - the ship moves a long way between each bounce, so they see:

O----------------------------------O

-------O------------------O

----------------O

The thing is, both of these are correct from each point of view - from each reference frame. For the shipboard person, the ball moves the width of the ship, and for the planetside person, it covers the distance the ship traveled in the bounce (plus some for the width).

Now, swap the ball for a photon, which always moves at the same speed. The distance the photon travels from the two points of view - the two reference frames - is different, so the time component of the photon's measured speed must change as well because the photon's speed remains the same! Each side sees the photon moving at the same speed, despite the difference in distance traversed each pov sees - which means each must also have a different measurement of the time involved!

So, time is compressed on the spaceship relative to the planet - from the ship, the planetside observer is moving very fast, while to the planetside observer, the space pilot is moving in slow motion. The speed of the photon is universal - it's the distance it travels between bounces, and therefore how long it takes to bounce, that differs between their perspectives.

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.

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.

The speed of light isn't even really a speed, it's a property of space itself.

Ok, firstly I'm not very smart. Secondly I don't understand the meme AND don't understand the explanations in the comments.
Can anyone actually dumb it down so a stupid person like me can understand 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.

And remember, a photon of light does not experience time. Time only applies to mass. When a photon is emitted traveling at the speed of light, it is eventually absorbed by something, eglight from the sun hitting your eye. The photon of ligjt, from the photon POV is emitted and absorbed in the same instance.