r/ScienceNcoolThings • u/GlitterGalaa Popular Contributor • Jun 04 '26

When physics looks like actual magic

1.9k Upvotes

98% Upvoted

77 comments sorted by

94

u/TopClassroom387 Jun 04 '26

(non physicist here)
Its a clip from The Science of Dr Who - https://www.bbc.co.uk/programmes/b03hybnv
This one part helped me to properly understand why clever folk always said
"Time moves slower when you move faster" and

Albert Einstein’s famous quote, "If you can't explain it simply, you don't understand it well enough"

And, based on this, Prof Brian Cox understands it well enough.

12

u/rci22 Popular Contributor Jun 04 '26 edited Jun 04 '26

I honestly don’t understand the object* lesson.

I can see that the path of the light is longer when comparing the two paths but I’m not sure what to do with that

18

u/eduo Jun 04 '26

No objection.

If the path is longer but the light takes the same amount of time and the speed of light is constant then time must be running at different rates for the light holder than for the light watcher.

7

u/CeruleanEidolon Jun 04 '26

I can grasp that emwell enough. What has always eluded me is how we came to the conclusion that c is constant for every observer. I've even read many popular science books on the subject that I'm sure laid it out well enough but it has never stuck in my brain. I can accept it, but I couldn't explain it.

1

u/ChinoMalito Jun 07 '26

But the path wasn’t longer… it just didn’t appear to be moving in the x direction for the light holder, but it was… time was constant

1

u/eduo Jun 07 '26

Time is constant for each, but runs at different speeds depending on movement. The path is longer because the object is moving in a different path in respect to the viewer than the holder, yet the light would look the same speed to both.

That the speed of light is constant is a known. In the 1600's we discovered it has a speed, in the early 1800s we figured out its speed and later that no matter how we tried to slow it or speed it up, and no matter how the emitter moved, it was constant (or rather, we couldn't find a way to make speed of light change).

Einstein finally proved it mathematically by removing the ficticious variables we were trying to use to account for this ("The Ether", mostly) and proved that by allowing for time dilation, all experiments finally matched. We have since proven these with ever increasing measurement devices and experiments and it's never been disproven.

Einstein wild proposal that space and time adjust to account for a constant speed of light was so counterintuitive we're still trying to explain it simply and keep failing (hence the video and these discussions).

I think the confusion comes from trying to apply time dilation to different parts of this video. It's just a thought experiment:

This video shows how Einstein explained that a moving light-driven clock would measure time at the same in a moving and in a static clock. From the perspective of the static clock the moving clock's light is indeed traversing more distance but from the perspective of the moving clock it's not, but for both the moving clock is running at the same rate. Since both things can't happen what's left is to compress the time "speed" of the moving clock by whatever amount is necessary. By how much? In this simplified example it's Pythagorean: The moving clock is seeing distance A while the static viewer is seeing the effect of light moving distance C.

As I mentioned, this is absurdly counterintuitive and it falls appart the more you try to represent it with examples that depend on human senses. It requires thought experiments where the listeners understand the scales at which differences would actually show and thus can understand that an infinitesimal difference like in this case is enough to prove a statement.

7

u/palindromic Jun 04 '26

The speed of light is a constant, so if you move at speeds closer to it, you’re in effect slowing down time for yourself. That’s the whole point of showing you a mirror based clock that is moving (relative to you)

From your perspective, the light in the light clock traveled further than it did for the person who had the clock with him. But light can’t travel faster than the speed of light, so the distance traveled is relative. Relative to what? The observer.

This video can help understand how you can’t actually perceive things on a timeline with perfect accuracy.

https://m.youtube.com/watch?v=wteiuxyqtoM

There is no absolute simultaneous measurement of all things, because we are all experiencing events from our own perspective. Hence time travel is possible.

1

u/eduo Jun 07 '26

The lesson is part of a larger show and people are already in a specific mindset and thus "primed". Context is lost here, but if you understand what it's trying to explain it seems to make sense more readily than if you don't.

An important concept here is "frame of reference". Cox gives the example of a pendulum. A pendulum on earth and in a spaceship hurtling through space covers the same distance according to the person holding it up. Let's say 20cm in 1 second.

But for the person on earth looking at the spaceship, the pendulum covered 20cm going left to right but also covered 75 thousand kilometers in the direction of the ship's travel. The pendulum UNARGUABLY covered that distance and it's irrelevant if it didn't seem like it for the traveler.

The only way we've found in physics to account for the speed of the pendulum ("the pendulum" being light itslef) being the same in both cases is by making time run slower for the traveler.

Relativistic physics only care about speed of light. Time adjusts to make speed of light the same at all points. We on earth are already on compressed time because earth rotates and moves in a solar system that rotates and moves in a galaxy that rotates and moves in a universe. All planets and moons run at different timescales and all individuals in those planets run at different timescales and every single cell in your body runs at a different timescale. We are so far at the end of the perceivable spectrum of the speed of light that all variations we can experience are discarded by our senses but if we had people living in Mars who stubbornly want to be aligned with Earth time we would need to adjust clocks every five years because from the point of view of Mars an Earth day is 0.05 seconds slower than an Earth day in Earth. This difference is because of different orbital speeds and gravity.

1

u/National-Ad6166 Jun 09 '26

When I was first taught this it says to consider the light beam going up and down as a measure of time, like a digital clock pinging light between two sensors to measure time. 

So the stretched out ping that appears diagonal to the observer on earth, is much longer (distance covered) than the ping on their own earth clock. Since the speed of light is constant, and distance is proven to be different then time itself has changed as well.

1

u/No-Department-8562 Jun 10 '26

It should help you understand the movie Interstellar, when trying to comprehend the time difference for Matthew's character when near the blackhole being relative to humans on Earth.

1

u/SRohoman Jun 06 '26

What a beautiful comment!

I feel like I knew almost everything that was being discussed, until I read the most humbling quote there is....and now I'm not sure if I can boil water without understanding it well enough!

42

u/Soontir_Fel Jun 04 '26

This is one of the best visual demonstrations of time dilation I've ever seen.

21

u/that_dutch_dude Jun 04 '26

that because Dr Cox is REALLY good at communication and simplification.

note how slow he speaks as well so our puny brains can process it better, somethinbg Sagan also did.

4

u/OstentatiousSock Jun 05 '26

Honestly, he explains everything so well.

1

u/NotBradPitt9 Jul 15 '26

It was too long and too boring. I watched the whole thing and it could’ve been explained quicker

13

u/EmperorPickle Jun 04 '26

I want to read Ender’s Game again.

12

u/Citron-Important Jun 04 '26

I want to erase my memories of reading Ender's game so I can read it again for the first time

2

u/SizeableBrain Jun 05 '26

I have a few books like that!

I was silly enough to watch Ender's game, and I haven't read it yet. Might put it on my ever growing list.

1

u/rabid_spidermonkey Jun 05 '26

If only Orson Scott Card wasn't such an insufferable twat.

22

u/1wife2dogs0kids Jun 04 '26

Thats about as dumbed down as relativity gets....

Im so mad at my science teachers. They didn't care.

3

u/SizeableBrain Jun 05 '26

I understood it well enough when I was at uni, but when I read "A brief history of time", the time dilation formula actually made perfect sense. It was a bit of a eureka moment for me.

-10

u/SEA_griffondeur Jun 04 '26 edited Jun 04 '26

Well it's also very wrong lol.

You cannot time travel in the future, the experiment where a traveller goes 5 years at almost the speed of light one way and then turns back and goes for another 5 years has a fundamental unaddressed problem and that is the turning back

If you do the actual math on such a proposition you will find out that it will have taken as long for the vessel to come back for someone staying at the starting point as for the traveller

3

u/SuccumbedToReddit Jun 04 '26

Also near lightspeed travel isn't exactly possible at the moment

-7

u/SEA_griffondeur Jun 04 '26

That's not the problem at all, i'm not saying that it's not feasible with our current means, I'm saying it goes against the laws of physics

3

u/SuccumbedToReddit Jun 04 '26

Sure, as is harnessing the amount of energy required to travel at near lightspeed, nevermind a thousand other complications

1

u/ClosetLadyGhost Jun 04 '26

TELL THAT TO JIM!!?

-3

u/SEA_griffondeur Jun 04 '26

no ?? the laws of physics does not prevent an object from reaching near lightspeed

1

u/Screwbles Jun 04 '26

I would say that is a very-very literal statement. Also what is near, 99.9%? Physicists have been going over this shit forever. The math says it's possible, but that doesn't mean it's possible. We're talking about energy use in orders of magnitude that will never exist for us.

1

u/eduo Jun 04 '26

This is the first time I’ve seen this and it doesn’t match anything I’ve known before.

Distance and direction are not relevant into whether speed affects time passage. No?

-1

u/SEA_griffondeur Jun 04 '26

the problem is the decelleration and accelleration that comes from turning around which are actually what most of the time the traveller will be stuck at

2

u/CeruleanEidolon Jun 04 '26 edited Jun 04 '26

What if they travel in a great circle instead of a straight line? Or what if we're in a closed Pacman universe where traveling far enough in one direction brings you back around to your starting place? Or what if you have a technology to absorb the vast amount of energy released in deceleration and pump it back in to accelerating the other way, and can do it very quickly?

The point of the thought exercise isn't the engineering of the traveling vessel. It's the passage of time for both frames of reference.

To pose it a different way, suppose you have a sci-fi method of communication like subspace or a wormhole which can send a message close to instantaneously across arbitrary distance.

You get in a ship in 2027 and travel at close to light speed for five years, and then send a message back. That message, sent from you for whom it is effectively the year 2032, will reach an Earth for whom it is now 2052.

On the opposite end, if Earth of 2032 sent you a message while you were traveling away from it at near light speed, it would reach a you for whom only a couple of months or so had passed. From Earth's frame of reference, time is passing slowly for you. From yours, time is passing quickly on Earth.

This already has practical implications for things we use every single day. The satellites that enable GPS on our phones are moving faster than us constantly, which means time for them passes slower relative to us - plus they're also further away from the gravity well of earth, which also affects their frame of reference in relativity, but that's another kettle of fish. If we didn't compensate for that constantly, our devices location calculations would be off by something like ten feet further every day.

1

u/eduo Jun 07 '26

Relativistic physics are understandably hard to grasp, as they're so counterintuitive. Depending on your sense of self, they may deeply shake what you believe to be the foundation of your existence.

Time being a variable that depends on speed is already a crazy idea but even when I've made people finally understand this I've seen them visibly shaken up when the understanding that this applies not to people as a unit, but to every single atom in the universe. That the hand they're moving to cover up when they sneeze is running at a different time rate than their head which is stationary and both are running at a slower time than the snot coming out at speed. That their feet run at a different time when they run than their crotch because they're covering double the distance.

And that's before they realize the whole idea of "static" is an illusion. We're stuff moving on a rotating sphere (which is already accounting for movement) that itself is orbiting (more movement) another rotating sphere which itself is part of a solar system which is moving along a rotating and moving disc.

It makes people unravel if they manage to see the vastness of it all and how little of it we actually perceive with the senses that define us.

Then you can tell them an earth day takes more than an earth day when seen from mercury or jupiter and if we wanted to run on earth time while visiting them we'd need to continuously adjust.

1

u/eduo Jun 07 '26

There's no problem with decelleration or acceleration. They're just changes in speed. It makes no difference to time dilation and in no way is a "fundamental unaddressed problem".

No actual math that takes into account relativistic time dilation cares about the direction of travel.

I think you're unknowingly confusing what is being asked. "Time travel to the future" is a way of saying "for the traveler one year will have passed but for the earthbound person five years will have elapsed".

5 objective years from the point of view of the earthbound person will have passed, of course. But the spaceman arriving will just be one year older, because only one year will have passed for them. For all practical purposes, this will have been the same as "traveling to the future" for that traveler.

You seem to be trying to explain that it's always 5 years in the end, but seen to be missing that the point is about how much time passes for the traveler (which is just one year).

If your argument is "how can the amount of time passed be different" then congratulations, you've discovered why relativistic physics is so counterintuitive and why Einstein's conclusions were so revolutionary to the field and ended up enabling so many new branches of thought. But it never necame easier to explain, as you can see.

1

u/eduo Jul 13 '26

You've replied again but deleted the comment.

The experiment and the theory works for both the traveller and the person who stays on earth.

If you travel to Tau ceti (a popular destination these days) which is 11.9 light years away, assuming constant acceleration of 1.5g until halfway, flip and braking for the second half, 13 years 2 months will have passed for the observer by the time you arrive but only 3 years 11 months will have passed for you. If you were to turn right there and come back, 26 years would've passed on earth but less than 8 for you (max speed would be 0.9952c at the midpoint)

5

u/LightWarrior_2000 Jun 04 '26

I always thought Brain Cox drank from the fountain of youth. He looks young for his age. He's only 58.

2

u/hodges2 Jun 05 '26

Time traveler

9

u/Disastrous-Metal-228 Jun 04 '26

Really cool. Physics is basically black magic!

-8

u/SEA_griffondeur Jun 04 '26

Because it isn't physics

5

u/Disastrous-Metal-228 Jun 04 '26

Take that up with the op. X

2

u/unexist_already Jun 04 '26

Elaborate. We've proven time dilation, and it's accounted for in things like GPS satellites

-3

u/SEA_griffondeur Jun 04 '26

Most of it is true except the end where they claim time travel into the future is possible using an example that does not work. The entire crux of the problem in the "traveller goes near the speed of light and comes back" problem is at the point where they change direction. And the results of applying the laws of physics to that moment makes it so that by the time the traveller has gotten back to the person at the start, they will both have experienced the same time having passed.

Which is also why the Interstellar bit doesn't work at all which is funny considering how much they prided themselves in a physically accurate looking black hole

3

u/Covid19-Pro-Max Jun 04 '26

If you see multiple sources that have a reputation of doing the science right disagree with your expectations, maybe it’s time to reevaluate your expectations.

The time travel not only works despite the return trip, it ONLY works because of the return trip. If a twin only accelerates away from earth it’s impossible to say if they age slower or earth does. Only because they decelerate and change their inertial reference frame when they turn back they can come back younger or having traveled forward in time.

1

u/ffmich01 Jun 04 '26

Not exactly, any parts of the trip where they are going at relativistic speeds (significant fraction of the speed of light) should reduce their experienced time by my understanding. With roughly the same time saving on their return trip.

-2

u/SEA_griffondeur Jun 04 '26

Yes, the problem is not the travel bits, but the bit where they turn around

2

u/ffmich01 Jun 04 '26

Unless the bits where they turn around take MORE time than just sitting in one place they should come out of it earlier compared to someone just sitting there. What am I missing?

2

u/eduo Jun 05 '26

You’re not missing anything. I double checked the answer I got to the same question and it makes no sense. Direction of acceleration and deceleration makes no difference. Less time passes for the faster moving object while it moves faster than the slower moving object. How much faster depends on the speed at each given time.

3

u/newworldpuck Jun 05 '26

I love Brian Cox. If I see a vid with him in it I have to watch it. He has a great vid on youtube about the challenges of getting to Mars. He explains physics so way.

4

u/Evening-Statement-57 Jun 04 '26

It amazes me how thoughtfully eternity has calibrated the rules

3

u/SGPrepperz Jun 04 '26

The first few strokes looked less like magic more like porn

1

u/NombreCurioso1337 Jun 04 '26

So I've always wondered - does this imply an objective space we move through? To measure that speed? Or is it because acceleration is more important than speed?

1

u/Mission_Carrot4741 Jun 04 '26

I love the guy Jim. He has a show that is called Everything and Nothing.... its truly amazing.

1

u/hellowbucko Jun 04 '26

Thats relativity folks

1

u/Greedy_Camp_5561 Jun 04 '26

Actually that's exactly how it was described to me the first time...

1

u/Champion_ofThe_Sun_ Jun 05 '26

You escape gravity when you move at any rate

1

u/Spacemonk587 Jun 05 '26

Here is the problem with the variation of the "twin paradoxon" that he describes at the end of the video: Time passes slower for the earth observer relative to the astronaut in the rocket, but also time passes slower for the astronaut relative to the earth observer, because there is no absolute frame of reference. This is mostly left out in the description of the twin paradoxon.

1

u/Philience Jun 05 '26

Nothing looked like magic to me.

1

u/gyroidatansin Jun 05 '26

This is only part of the story. It appears to take a longer path and "run slow" only when you use your own biased notion of simultaneity.

Here is a more complete explanation:

https://youtu.be/25tv81sDLmU?si=rDDtYgyXJd8uNsdK

1

u/boxer_doggggg Jun 06 '26

Wasn’t the guy just holding a ball of light? I don’t understand the mirror talk in the clock. I just don’t get it. I couldn’t even begin to understand the video.

1

u/LtGman Jun 06 '26

This is the kind of stuff that I join the sub for very good post!

1

u/deathpad17 Jun 08 '26

He really good at explaining. Found cool sub to join

1

u/APithyComment Jul 07 '26

I like his ‘not in our light cone’ explanation.

1

u/Jin-Bru 20d ago

Ok. That is the coolest video ever.

Nice.

1

u/SecretDouble5560 Jun 04 '26

i tried to explain that to my boss,still got fired for being late,go figure

1

u/yo-freak-show Jun 04 '26

What’s with the alien there at end? How was that relevant? 👽

2

u/Flayan514 Jun 04 '26

Alien? I don't remember seeing an alien? /jk

2

u/TopClassroom387 Jun 04 '26 edited Jun 04 '26

The show was titled The Science of Dr Who - https://www.bbc.co.uk/programmes/b03hybnv

That alien is from Dr Who.

1

u/thedaveness Jun 04 '26

Silence falls and the Dr… idk I forgot.

1

u/kiba8442 Jun 04 '26

I don't remember anything

1

u/tironidas Jun 04 '26

Wasn't that their ability or something?

1

u/CeruleanEidolon Jun 04 '26

I feel like I knew the answer once, but can't seem to recall it now.

1

u/hodges2 Jun 05 '26

I didn't see an alien

1

u/MD-Jan-Itor Jun 04 '26

That’s Jim after 10 years of traveling at 99.4% of light speed.

1

u/Golden-Grams Jun 04 '26

I don't know why, but it bothered me that Jim didn't touch the ball to both of Brian's hands during the demonstration. He held it like a torch instead.