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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.
And that is what is meant by time dilation, and why Matthew Mcconaughey was younger than his grandkids. His balls took longer to bounce…
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.
Thanks for fucking up my mind so I can’t get anything done at work for the rest of the day lol
So it’s woven with space?
Yup moving through space faster means moving through time slower. Kinda like if you were traveling at a constant speed through a 2d grid, if you make your vertical speed faster, your horizontal speed would be slower, except replace the grid with spacetime where we’re pretty much always traveling through time at ~1c and space at ~0
They are one and the same.
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?Thing go slow - > speed act normal, speed add up between POV’s, speed make sense AF
Thing light fast like - > speed act fucky wucky, speed don’t add up at all between POV’s, speed so fast make time fucky wucky too
you and two friends, Al and Be, are outside at night time. Al has a flashlight and flashes it at you and Be. Be is 20 feet away from Al, and you’re 20 feet behind Be.
20ft 20ft Al..........Be...........you r1 r2The photons from the light Al has hits Be first, so he sees it first. it took .03 seconds(r1) to reach Be.
After that, the photons hit you and you can see it. it took .03 seconds(r2) to reach you from Be.
They were both .03 seconds because you are equally apart and the speed of light is finite.
so, how long did it take the light to reach you from AI?
In this example all 3 observers are in the same reference frame. That is, each of them are moving at the same speed. This is not what the meme is about. In this example the night from Al would reach you 0.06s after he lit it. Which is expected in Newtonian physics.
The problem of the speed of light being the same for all observers is when they move at different speeds.
The scenario is: Alice is inside a train moving at almost the speed of light, and Bob is outside it, looking at the train. Let’s set that light moves at 1m/s (for simplicity) and the wagon Alice is in is 1 meter long.
Alice is at one end of the wagon, and turns a flashlight on. Alice will see the other side of the wagon illuminated after 2 seconds (since the light has to reach the other end and bounce back). Since the wagon is 1 meter long and the speed of light is 1m/s.
However, when she turns the flashlight on, Bob is also looking. Let’s say that the train moves at 0.99m/s. In that case, after 1 second the light would have traveled 1 meter, but the wagon (and Alice) has traveled 0.99m, therefore the light is only 0.01m away from Alice.
To summarize: after 1 second, in Alice’s frame of reference, the light has just touched the other side of the wagon. Meanwhile, in Bob’s reference frame, the light has only moved 0.01m away from Alice.
I’m not following how this explanation is supposed to help with their question. Like, what are you trying to get at here?
- expecting klaxon sound * … is it 0.06 s? assuming it’s a straight line
correct!
this is because the speed of light is finite. meaning it cannot speed up or slow down on its own.
both you and Be are observing the light at the same speed, but at different times and different distances.
The fact that this gif has a brief pause between the “ex” and the “explain” really irks me
Ex…Term…Ah…Nate!!!
It’s there to convey the line’s intonation.
Yeah, so it should be “Ex” and then add to it “plain” The way it currently is looks like a stutter, “ex-explain”
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.
I’m not even mad because now I get why and how these competition multihull boats basically fly above water while keeping like 1% of the boat in water
Those ones are reaching a new speed limit as well, the cavitation around the hydrofoil starts at a certain speed/angle and stalls the foil, which then abruptly drops the hull back into the water.
I’ve been wondering how long before the equivalent of variable swept wings will be added to the cup boats.
So you’re saying, we need to jump the spaceship out from space/time. With a wing.
Still very interesting
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
It makes sense after accepting spacetime is mutable. Reference frames are merely referential localization.
Different phases (colours of light) move at different speed, and have different longevity, I think. So how they always thought those pretty galaxy nebula photos, were red? Actually turns out that phase of light just travels for a longer time. Lasts longer. OK, now someone correct me, because I have absolutely no idea what I’m talking about. Just bring up vague floating memories from an article i read once.
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 they have no mass, how do they push solar sails?
They have momentum, but not mass (ignore the other explanation, like yes, E=sqrt(m0^2 c^4 + (pc)^2), but so what? m0=0 for photons)
As you can see, momentum, p, is p=E/c, and we know that the energy for light is proportional to its frequency, f, E=hf (h is Plank’s constant). So, p=hf/c. When light is absorbed by a material momentum (and energy) conservation apply and it imparts p onto the object. If light is reflected it imparts 2*p, showing this is left as an exercize to the reader
They have mass. Everything that has energy has mass. They don’t have inertial mass but it’s just part of the equation
They have momentum, not mass, in relativistic physics you need something more complex than p=mv to describe momentum.
Okay, but what if I go the speed of light minus 1m/s and shine a flashlight, can I catch the photons?
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.
That may seem harsh to the casual reader, but it’s 100% true in this case.
Nope. In your frame of reference, they will still be moving at the speed of light.
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.
Physicists hate this one weird trick
without needing to go absurdly fast
… I would actually argue that for a human catching anything at c-1m/s is impossible. Except catching death perhaps.
Not going “absurdity” fast is more of a requirement.
You’re already going c-1m/s if compared to the right reference frame.
You are correct, can’t argue with that.
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?
Correct. Physics gets really, really weird at relativistic speeds. Something to keep in mind is that the speed of light isn’t actually the speed of light itself, but rather the speed of causality - the universe’s hard limit at which any interactions can occur. Even if you are traveling at .99c from a certain reference frame, space time itself warps in such a way that light traveling away from you is still measured at 1c
Any observations wouldn’t see anything but the whole light
I believe this is explained by Einstein in his example of the train in the Theory of Relativity
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.
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.
Regarding B… Yeah, I’ve tried and tried to find an intuitive explanation for this, and I’ve never yet found one. Basically the answer boils down to “Because the math says so” and every attempt to explain that in practical terms always comes down to drawing a light cone diagram and showing the causality paradox in geometric terms. I find this intensely dissatisfying because it’s really just visualizing the math without actually framing it in any way that’s truly comprehensible beyond “Trust me bro.”
This video, while a little over padded with fluff, does contain the best and most comprehensible version of this explanation that I’ve seen, but I’d love for someone to be able to put it in terms that actually make sense without just drawing lines on a graph and gesturing at them. Maybe that just isn’t possible. Perhaps the whole point is that FTL breaks the laws of the universe and our understanding of reality is necessarily constrained by those same laws.
Edit: OK, this video is a much better version of the explanation I described. I still find that explanation unsatisfying, but it is at least highly comprehensible.
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) 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.
Is this floatheadphysics? I was gonna link him, haha. I discovered him like a week ago and it’s been so fun.
It is! Seriously one of the best physics educators out there.
Agreed, floatheadphysics is doing an amazing job of explaining these concepts in a comprehensible way.
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.
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
I kind of despise the time cone answer because it doesn’t actually answer the question. It transforms the question into a picture and uses the picture as the answer which is circular logic.
“Why do thrown objects fall in a parabola?”
“See here’s a graph of a equation. If the object is outside the graph it’s not on the parabola and that’s why thrown objects fall in a parabola shape.”
I don’t think in this case the logic is circular, it just explains how the light cone shows that FTL breaks causality, and assumes that you’ll learn the math behind the light cone somewhere else. Maybe the author assumes the light cone can be better learned from other sources
But it doesn’t explain it. It is a graph of the equation. But that doesn’t explain the equation. That’s why I gave the parabola example.
Showing a graphical form of the equation is still only the question but in another format. That’s what makes it circular. If you ask why something is Y = X^2 and I only show you a graph of Y = X^2, that’s not answer. It’s the question in a graphical format.
it explains it using other formulas. It’s like if the teacher taught you trigonometry using algebra. They expect you to either already know algebra, or to learn it from somewhere else. You could say that the teacher is just restating trigonometric questions in algebraic format, and that might be a fair way to interpret it, but that also might be enough for people who already know algebra
For a photon itself, time don’t exist, for a photon it is everywhere in the universe at the same time. Only for an observer it need x time from A to B with a finite speed.
Everywhere along its path at once*
What if one observer is on one side of a medium that light passes through slower and another observer is on the other side? 🤔
They would both agree on the same speed of light in a vacuum by independently measuring the coefficient of refraction for the medium.
What if they’re stupid? And they hate each other?
What if they haven’t eaten and it’s like, almost 9pm?
What if the one observer was reading the other observer’s body language for clues??
What if God was one of us??
Real shit.
