When sailing boats pushed for speed they ended up hitting an unexpected speed barrier. As you increase velocity the break wave created by the bow of the ship elongates until the length of the ship is at 1 wavelength, then the hull drag prevents further acceleration. For a 50 meter ship it’s about 17 knots. You can get much faster lifting the boat from the water as you gain speed with an underwater wing, the current max speed was set 47 years ago at ~276 knots. But that’s only because they can remove the hull from the high drag environment and is extremely dangerous to attempt to break. The speed of light is nothing like that because spacetime itself can stretch and squish, I just wanted to talk about boats for a bit.

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.)

Nope. In your frame of reference, they will still be moving at the speed of light.

No

That’s not a physics statement btw. I just think that you, personally, are too slow to be able to do that. Offense intended.

You can catch the photons just fine without needing to go absurdly fast. Just put your hand in front of the flashlight beam, and you’ll catch lots of them.

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.

I think the answer is that you are only traveling at almost light speed from reference frame of your start position.

The light of the lamp travels at light speed from your own reference speed which to you in a vacuum is 0.

Anyone correct if wrong please?

No. Ever heard of the doppler effect?

Light behaves like a wave. But instead of hearing sound in a higher/slower pitch relative to the source, you’d be seeing a different color.

If you’re going at the speed of light minus 1 m/s and you turn on a flashlight, the beam emitted by the flashlight will be travelling at the speed of light, according to your measurements. Time passes slower the faster you travel.

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.

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.

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.

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) https://www.youtube.com/watch?v=wiBsfvW5AWY

B) https://www.youtube.com/watch?v=i88ipC8GebA

And just for good measure on a more intuitive understanding of special relativity, https://www.youtube.com/watch?v=Zkv8sW6y3sY

I love this guy’s videos; the best I have ever seen difficult physics concepts explained, based on intuition and not equations.

Regarding B someone linked this a week or so ago. It’s kind of technical but it does explain it https://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel

They would both agree on the same speed of light in a vacuum by independently measuring the coefficient of refraction for the medium.