Why You Can't Travel Backwards in Space

[@johnmorkunas6707]

So Heraclitus was right. "You can't step in the same river twice."

[@petros_adamopoulos]

"For it’s not the same river and he’s not the same man.”

[@hectobreak8097]

If you vibrate furiously you'll statistically, eventually, be at the exact same position you were a small delta time ago. You won't know when that's gonna happen, but you know it will eventually, probably, happen.

[@xtaltia]

sounds like a fun experiment for april 1st, for any science teachers reading

[@xtaltia]

@@renedekker9806 no, that's interesting! thanks!

[@MrCmon113]

I find the idea that I'm not eventually gonna be everywhere an infinite amount of times ridiculous. Sure the universe can be infinite, but then there's also infinite me's. It's inevitable.

[@sciverzero8197]

Correction: systems do not _often_ rearrange themselves into a previously ordered state. It is neither impossible for them to end up by chance in their original configuration, nor is it true that it has never happened. It is merely true that it should not be _expected_ to. Likewise, in this case, I feel the argument here about how there is no location that is non-relative is merely an assumption based on the fact that we need to define relationships between where things are to determine location. I do not agree. If we had perfect knowledge of not only where everything is, but where it has ever been and will ever be, and when it will be there, you can use those dimensions of time and space to form a coordinate system, and while allowing for non-matching coordinates along the time axis, you can thus return to a spacial location. The location will be the same because it will have been defined as being the same according to the coordinate system, regardless of whether that location in any way currently resembles what it did previously in time. Even if we take into account the possibility that the coordinate system is itself changing over time, we can simply convert the change of the system itself as another axis like we did with time, and use the new system as our definition of location. We can continue to do this across infinite dimensions and axes to eliminate infinite variables that prevent may get in our way, and in the end what this means is that location and its subjectivity are things we have defined and have control over. We defined location as how some place is related to some other place, but that is not the only way to define location and it is not the only way we have ever defined location. The problem is to do entirely with the perception of location, which is an unnecessary variable. One does not need to know their location for it to be their location, and similarly one needs not be able to understand how to measure it for it to be true. There are many things we have measured that were previously impossible for us to measure, that we have done only because we changed how we define the process of measuring or understanding those measurements. Thus things that were "impossible" to us once, were all the same, still always true, in spite of our definition of it being that it is false, and so applies to location. Relativity is not the truth... it is a possible truth and is not the only possible truth, nor a purely mutually exclusive truth, as it inherently depends on us to define it as true.

[@khuti007]

No man ever steps in the same river twice, for it's not the same river and he's not the same man. Heraclitus

[@dananorth895]

You cannot think the same thought or same emotion twice. You can only aproximate them.

[@lilindie2467]

I travel back in time yesterday and I found I existed in different possible timelines .. Now I am dead .... writing this msg on 30th November 2020

[@askerali5794]

Where are you now? Please reply

[@thomasslone1964]

You're also gay

[@notgabby604]

There is a thing called momentum that you have to reverse if you wanted to travel back in time, like the momentum of everything. The thing that was moving forward you would have to apply a force to make it go backward and vice versa. That's a lot of work you would have to apply on a universal scale.

[@MrFram]

But momentum is relative like velocity, so momentum can only be reversed relative to some reference frame - a stationary object reversed relative to a moving one will be sent flying at twice the speed to catch up to it

[@doomtho42]

“so what does this all mean?” for the most part, absolutely nothing

[@cadrantedeestrelas8320]

Incredibly similar to the phrase said by Heraclitus of Ephesus: "You cannot step twice in the same river".

[@geordan6740]

@@gamevalor He sakd that in referencde to how the river constantly flows and thus is never the "same". Think more plz

[@crawkn]

The CMB Dipole suggests that there are absolute positions in space. We are moving about 600 kps relative to the CMB, which is only about 4 times the maximum speed reached by the Parker Solar Probe, meaning there's no theoretical reason we can't position something stationary relative to the universe, move away from that point, and return to it.

[@idesel]

Damn, this dude is taking no prisoners when comes to this science stuff.

[@UniverseSinking2011]

Well done, and great points. There's a saying that you can never step in the same river twice. The water flows, fish swim, gravel tumbles, branches float by, and so on. It is a close enough approximation of the same river, but it is not exactly the same. That's true of your house, too. You come home at the end of the day, and many things that you don't notice (or maybe do) have changed. Some paint chipped off there. A newspaper landed on a flower and smashed it. A window now has bird poop on it. The roof is hotter and some composite has come off and rolled into the rain gutter. Wasps started a nest in the eaves. And thousands of other details, mainly small -- but maybe big! The newspaper maybe broke a window, or a large tree branch landed on your roof. You can never return to the same exact house as it existed in the point in spacetime from where/when you left it.

[@alphaomega1089]

Glad someone understands spacetime. Your logic hasn't fallen on deaf ears. One thing about AE's equation states forward motion isn't possible without breaking that light barrier. Tweaking it will allow forward motion. However, that breaks his law on time travel. Only conclusion: one speed one direction.

[@neopalm2050]

No, actually, for any spacelike vector, an object's 4-velocity may be oriented with any inner product sign with respect to that vector (i.e. toward or against it), but for a timelike or lightlike vector, an object's 4-velocity may only be oriented one way with respect to that vector, depending on which way that timelike/lightlike vector points. You may travel against any space-directed 1-form, but not against forward-pointing time-directed 1-forms (which come from the exterior derivative of your "time" coordinate).

[@intrance96]

@CEO of Unpopular Opinions Also my first thought, lol

[@artielandfish2104]

This is a common sense video. Because if you could genuinely travel back in time, it would exactly be like a video or song played backwards. All your thoughts, your breathing patterns your body chemistry would have to be acting in reverse. All of that is going on when you play a video backwards. But you can not see it because it is all internal and not visual. But that is exactly what is going on when you play content in reverse. There was an iconic rap group in the 90s called the Pharcyde. They had a video that they came out with called "Drop". That video is actually a timeless classic. If you watch that video and find out how it was composed, you will see how it correlates to this video about space and time

[@EpizodesHorizons]

Thanks for this video @dialectphilosophy. For a long time now, I thought that motion in space (both backward and forward) was silly. Of coarse all matter only travels forward! I've been thinking about doing a video myself, but yours is very good. Thanks.

[@robertbeaman5761]

I tried to explain this when talking about traveling backwards in time and how you would end up dead in space and not on Earth. People we're very confused.

[@danielsnyder4426]

Yes we're.

[@robertbeaman5761]

@@danielsnyder4426 were

[@theblankchannel1752]

@@robertbeaman5761 Traveling back in space-time, because they are linked, you would end up on earth at its previous state (coordinates in space-time). Traveling backwards in space, while the arrow of time isn't reversed, well, then you would end up dead in space. which is pretty much the point of this video - you can't, because space-time coordinates exist only in relation to initial inertial frame of reference, which must be at absolute rest where absolute rest is impossible for something that exist and also it's also relative...

[@555RavenCrow]

Depends, if you use consensus science and pick any old constant in some made up equation and call it a "dimension" you can travel, maybe it won't happen like that.

[@siqueirabarros]

The important thing is that you can do, in space, things you cannot do in time. That is all that matters.

[@new-knowledge8040]

If you and your spaceship were travelling across space in the left direction, at say 260,000 km/s, but you then fired your rocket engines while pointing toward the right direction, and for some time you had obtained a spatial velocity of say 130,000 km/s, where in space you were located a minute ago, is now ahead of you. Thus, you have travelled backwards in space.

[@lilindie2467]

At your house dumb ass

[@Nuclearburrit0]

Backwards relative to what?

[@tyrjilvincef9507]

Relative to your mom, obviously.

[@Nuclearburrit0]

@@tyrjilvincef9507 oh well in that case going backwards is easy. *hugs mom*

[@aaronmorris1513]

I just choose my coordinate system centered on my home.

[@alexjohnward]

Which is accelerating...

[@aaronmorris1513]

@@alexjohnward Not with respect to the origin of my coordinate system.

[@alexjohnward]

@@aaronmorris1513 your co-ordinate system is accelerating though, not ideal! Lots of fictitious forces to live with!

[@aaronmorris1513]

@@alexjohnward Accelerating with respect to what?

[@alexjohnward]

@@aaronmorris1513 acceleration isn't comparative, it's absolute, and can be detected with a set of scales. If you stand on the scales in your bathroom and it shows a "weight" (actually a force), your home is accelerating with respect to something.

[@frankyjayhay]

Some of these ideas are down to language and definitions. To 'travel in space' suggests space is a 'thing' like a fabric that you move 'through', in other words with respect to. Space has to be a total featureless void (as you are in fact saying) for relativity to hold, there's nothing there to travel through in the first place. You can only move between objects so you're only altering the configuration of you and the objects. Then of course you can reconfigure things back to the original position. As for the mug even if you could put it back together exactly in absolutely every respect you're only making a perfect copy of the past in normal time, you're not going back in time. To go back in time you'd have to obliterate any evidence or memory that the mug broke from the universe or you'd have two pasts: one where it broke and another where it didn't, again we need to sort the semantics before delving into the physics.

[@soyjakchud]

I thought so too.

[@mindstorms8888]

Or you are always in the same spot because we're the center of the universe and everything is moving relative to you :P

[@duytuannguyen98]

Mark W That means Galileo could be wrong =)) . I think the solution for all of these is we have to figure out what truely fixed is in the whole universe.

[@Pauluzzs]

@@duytuannguyen98 speed of causality is fixed. Time and space are relative.

[@aaronmorris1513]

@@duytuannguyen98 Nothing is fixed, or everything is fixed. All frames of reference are equivalent.

[@gaemr_o5147]

You’re quickly becoming my favorite KZbinr, really informative yet there’s still a lot of style to your videos

[@kavalkid1]

This is great! Any movement in space is relative to every other position in space.

[@MacSmithVideo]

This video makes less sense the more I think about it. I wonder if he's changed his mind on this given his newer videos. This one makes some of the same mistakes that he criticizes other videos making.

[@jimmypk1353]

LOVED your choice of aircraft, The T-38 TALON!

[@deepthishenoy2160]

Thats a new perspective!!

[@PulseCodeMusic]

Surely absolute spacial location is an ill defined concept. 'Where are you in space?' is not a sensible question, that is the whole idea of relativity no? All we need to do is define what ever coordinate system/reference frame is most useful and then there is nothing stopping us from returning to a prior location in those coordinates. Also if you want the most global reference frame possible choosing the cosmic microwave background seems like a pretty good shout. We'd need a pretty bad ass rocket to get us where we were yesterday but theres nothing in physics that would stop us. (maybe in biology haha, might involve some serious G force).

[@PulseCodeMusic]

@LL Yeah that is my point. There is no such thing as 'the same place' so to say I cant go there is meaningless.

[@gaemr_o5147]

@@PulseCodeMusic I get that but it’s still interesting to think about. Also I think the main point of the video wasn’t really about going back to your old position but about that neat relationship between space and time

[@KaiseruSoze]

If you move from a point to the "next" point in any direction you change positions. Period. Local or not. Or if some object changes position with respect to you, it changes position. Who moved? Who should move to recover the previous "position". Also if you ignore other frames to turn around is irrelevant you still have to increase your distance from where you are to where you're going. You can't draw a line with a negative length.

[@harrywilson1660]

I am always at the centre of my own inertial frame, so not only can I travel back in space, I never left in the first place.

[@siarez]

But can you travel backward in time "locally"?

[@mementomori7160]

I think yes, it'd require you to put everything back in the state it was in the past, which would require energy you could take from the rest of the universe, kind of like putting in energy to cool down your fridge, but you'd have to know everything about the system

[@MrNicePotato]

@@mementomori7160 You still can't because second law of thermodynamics still holds in any isolated system. Unless you are talking about quantum systems with very few particles. Then you may return to the exact state in the past.

[@mementomori7160]

@@MrNicePotato We're talking about reversing time locally, and I said you'd have to take the energy from the outside, therefore we don't consider it as an isolated system that reverses time on it's own, that's why I think it is possible, just like cooling inside of your fridge by heating up the outside of it

[@saniancreations]

@@MrNicePotato You can, because as he says it "would require energy you could take from the rest of the universe". So long as the entropy created in the outer universe is equivalent or greater than the entropy that was reduced by resetting the objects locally, the total entropy still increased. Imagine having a hot meal on a cold plate. The meal slowly cools down until the heat of the warm meal is equally spread out across the meal and the plate (more entropy). Then you rewind time to when the meal was still hot and the plate was cold (less entropy). If this takes no energy at all, then you just reduced the total entropy in the universe and broke the second law of thermodynamics. But, if the action of rewinding time for the meal created an equal amount of entropy elsewhere (inside the device you used to perform the rewind?), then it's fine because the total amount of entropy in the universe did not go down. It decreased in one place, but increased by an equal (or greater) amount elsewhere. It's just that your _local-time-rewind-device_ needs to - _will_ - release enough heat (or other form of entropy) to compensate for the entropy reduction caused by the local time travel. I like to think that rather than heat, a local-time-travel-device would have to increase the passage of time on some other object. Just as an AC produces more heat than it removes from the air, a device that manipulates time could rewind one thing, but would have to fast-forward another, and by more than was rewound. Could be a neat science fiction plot. Like a little rewind gun with an "entropy battery": You aim and shoot to rewind an object which fills the entropy battery as the object is rewound more and more, but you can't go too far or the entropy battery will explode. Then if you want to rewind more, you must first release all entropy from the battery by fast-forwarding other objects.

[@bmpixy]

Yeah. Physically grab the hands of the clock, and turn then back.

[@Aesthetycs]

If you are capable of instant travelling beyond the speed of light, then you would be able to travel back, in a local segment in time.

[@ThraxxGaming420]

Thanks for the existential crisis! 👍

[@sb-sm8ib]

Does it not refer to absolute position in space?

[@TheErraticTheory]

This assumes that global space is not related to your temporal space and thus time dilation couldn’t exist, but it does. Otherwise we would experience different time dilation when the Earth’s spin move with the motion around the sun or against the sun. But the GPS satellites don’t have to take this into consideration. Because in reality local spacial frames of reference govern out total motion

[@laz7354]

Great Scott !

[@Aesthetycs]

Good point. Fundamentally because while most assume the physical space as a vector space, it is in fact only an affine space.

[@anatolesokol]

This argument won't work for you in a court, as for the judge and the evidence of you coming back to the crime scene later is in fact the same place.

[@saniancreations]

Any position can only be defined in terms of where it is relative to another object. The court prefers to use the crime scene as reference point.

[@krzyszwojciech]

We don't know what space _is_ and how it works exactly (how it grows, or whether its parts rearrange for instance), we could even add the issues of ontology of time (eternalism, presentism, ...) and identity of objects. All in all, you should remain agnostic on whether you can or cannot travel to some point in space that you've occupied earlier (with a spaceship for instance).

[@curiousmind9287]

Earth, sun, galaxy etc are moving in locked orbits. But what if you are traveling in a spaceship and do not experience any gravitational force so you can chose freely where to move. Then, unless you believe that space itself is moving, you can get back exactly where you were.

[@-_Nuke_-]

I want this question answered too!

[@will19125]

@@-_Nuke_- It is impossible to not experience gravitational forces

[@waynesaban2607]

So, is it possible when adding up all relative motions, all matter is moving at the speed of light ?

[@PacômeHégésippeAdélardLadislas]

And moreover the notion of objects of human size is already an approximation for a mess of molecules gigling around and exchanging themselves constantly, such that the person going back home isn't really the same as the one going away from it

[@BPJJohn]

Him: "Much like trying to fit the pieces of a broken Teacup back together." Me: "Hold my Teapot."

[@walterchavez3081]

Watching your various videos has helped me a little to understand the differences between absolute and relative space and time. Would you do a video that in simple terms explains the debate between Newton and Leibniz about absolute and relative space and time? Maybe that was what you are talking about at 3:07 to 3:24. My mom wonders why Leibniz would say absolute rest is not possible, is that partially explained in the Newton's spinning bucket with water video you made?

[@SIM2014]

Considering this makes it easy to understand what it means to always then be moving in space-time.

[@worstedwoolens]

Those places where information theory bumps into physical reality always make my stomach drop

[@Epoch11]

I am so glad someone made a video about this because I've been thinking about this for a long long time. If you want to travel back in time you have to return all the matter back to where it used to be and also account for the space created or let's call it the expansion of space and contract that as well. I guess I'm not the only idiot who thinks about pointless stuff like this.

[@clementvining2487]

No it isn't pointless unfortunately the majority thinks the other way. And they are wrong you are correct.

[@bartelsmore6285]

I feel the same. I’ve been thinking about this ever since I started to understand Relativity theories. I’m so glad I found this video. I’ve always wondered why high level physicists still talk about this incorrectly. This simple idea builds on and makes Einsteins theory of Spacetime. more easily digestible

[@claudiaarjangi4914]

Nah, completely normal ( unless I'm not normal either😁)

[@sailingmohican2767]

looking in the hubble space telescope is technically watching a movie of that planet or sun in real time from the past as it happened 100 years ago if that planet is 100 light years away..so you are actually seeing the matter from 100 years ago interacting we can't go back but we can observe the past 😉 Einstein was wrong we can't go forward or back no matter what we do we can observe the past that's all and there is zero physical proof that time would stop for a person on a space ship traveling light speed our observation of that ship would appear to stop because the light can't travel to us. But there is no proof or logical reason why the person in the ship would age slower or there clock would magically tick slower .as spock would say that's illogical..lol

[@claudiaarjangi4914]

@@sailingmohican2767 You completely correct with looking in the past.. Though it is 'proven' by General Relativity that if you were traveling through spacetime at the speed of light ( which really nothing with mass can) your time / clock would stop..⌚ It's the same situation with GPS.. They have to always take General Relativity time dialation into account or GPS would never be distance accurate .. 🛰️GPS satellites are traveling around the earth, through 'space-time' faster than we on the ground are, so their time/ clocks tick a little slower ( tiniest difference, but enough to throw out distance accuracy )slower than ground clocks do, so the GR calculations adjust for the difference.. ( With this next bit it helps having a space-time axis diagram to picture what I mean.. ( An L shape, with time⬆️ & space➡️ ) & remembering that our movement / travel always adds up to the speed of light through 'space- time'.. Not thru space alone nor through time alone , but both added up ) So, now if we are traveling through spacetime at the Speed of light.. Because we ( with mass) don't go fast compared to the S.of L., nearly all of our movement through space-time is through time (We experience 1 sec/sec along the time axis) but not much movement through space ( stuck going slow on a planet, or on the space axis).. But.. If you travel fast, 🚀zooming through space-time so fast, at the speed of light, suddenly all of your movement is through space,🌠 or on the space axis, leaving no movement left over to experience time.. That's cos the speed of light is not about light, it's about the speed of causality.. The fastest that anything can happen.. I don't reckon we'll ever go back in time, but forward in time , or changing our time compared to someone else happens all the time.. 🤦‍♀️ Despite the crappy explanation it's true.. 🤯It so cool & freaking amazing how spacetime works.. Basically it's saying that the faster you go through spacetime, the slower your clocks tick/ run.. As in satellites clocks all run slower than any clocks on the ground.. The bigger the difference in speed, the bigger the clocks are out.. It is actually taken into account for so many things.. 🌏☮️♾️

[@jonathanhuerta5460]

I really want to time travel so bad because everything used to be good. My childhood days the 2000s

[@gablit-gt8kk]

But it's impossible

[@-_Nuke_-]

omg that hit so hard!

[@MrCmon113]

There's so much wrong with this. Firstly you're following some kind of cringe presentism...

[@dorax1876]

It always wonder me that The place where I was born is too far away from where am I now !

[@sedorriku8021]

There is actually a mathematical path around 2 rotating black holes that woud let you end the path before you started it

[@philhos4052]

Neat

[@-_Nuke_-]

what?

[@punkypinko2965]

That was incredible. So simple and thought provoking.

[@AntiCitizenX]

Your entire argument is predicated in the existence of absolute positions and motion through space. That’s obviously not what we mean. Since space and distance are intrinsically relative, we can absolutely “return” to a location in space. All we have to do is return to the same relative distance between objects of reference-a point you even acknowledge at the end of the video.

[@MultiVigarista]

You don't get it. One can return to an object or place locally but not to the same position in space that object was when you left. That's what he acknowledges at the end... Everything is moving at once, you can only return "home" because both you and your house are moving at the same rate is relation to the planet, so it seems like everything is perfectly still, but the planet is moving through space, so you'll never get back to same position in space. get it now?

[@AntiCitizenX]

@@MultiVigarista No, YOU don't get it. Read my comment again. The reason you can't "return" to a point in space is because there was never a "point in space" to begin with. It is literally meaningless to think about space in this way. And since that's not what we mean when we say "point in space," the argument itself is a gigantic fallacy of equivocation built on a false premise.

[@MultiVigarista]

@@AntiCitizenX we can call it "general region in space" then, if that makes you feel better I guess. For all it matters we can be talking about a region 5 m^3 in 3d space the argument still holds, you can't return to it... 😅

[@TactileTherapy]

@@AntiCitizenXthat doesn’t make sense by virtue of Pauli’s exclusion principle

[@donovangomez8114]

Our sun is a positron in another system lol

[@adaeptzulander2928]

I agree. Notice how the spatial structure of the universe seems to mirror as it scales up or down. An atomic nucleus or particle is exceedingly small compared to the spatial size of the overall atom. Same with stellar systems. Same with large structures such as galactic clusters.

[@teletubby-g1v]

Well, I guess a discussion about CTCs (van Stockum/Goedel 1937/1949) and the self-consistency principle by Novikov (mid-1980s) could be way more interesting.

[@pratoarancione7646]

The equivalence is here bad presented: the question is not if you can return to the exactly point of departure, but if you can go back, or go laterally or go forward, in short: if you can choose a direction and a toward, yes, you can, even if you are transported by space flow. But with the time you haven't these freedom degrees, you can only eventually slow the time or accelerate it by moving you near to ou farer from a mass (scalar modification of the time speed), without vectorial degrees of freedom: direction and toward. So, not: the time and the space are not equivalent and are not "dimensions".

[@incoporealmage294]

now, "progressing backwards" finally makes sense.

[@gabriellitewski7988]

If we don't time travel we'll be stuck in the Era we don't like forever

[@Raptor302]

Doesn't this establish that time is an emergent property of matter moving through space, just as a shadow is an emergent property of light being blocked?

[@marcoantoniazzi1890]

Or a shadow is an emergent property of light being "broken".

[@narasimhasithurea2271]

If you think position as a relative concept I think we can travel back in space .( Something similar : you cannot step back into a same river)

[@jeremiahmullikin]

This channel is a rare gem.

[@bryandraughn9830]

The "co moving" reference frame is one way to look at it.

[@gryphonschnitzel7140]

but not being able to travel backwards in space is not as absolute as not being able to travel backwards in time. Just because lets say the processes of entropy are reversed in a system it doesnt mean it has traveled to the past, it just reaarranged. The same is not the case with space, except space itself is moving inside another dimension or wtf idk

[@petevenuti7355]

This reminds me of that pickup line, where that 900+ year old man was talking to that 17 year old...what show was that?

[@claudiaarjangi4914]

Yes, this is Exactly what I thought..The only thing that makes sense 💯

[@PhysicsNextBook]

Position is defined in refence to a frame. So this is just being obtuse...we might as wll say we can't move in space at all since we are always at the origine of our own rest frame. Also you are refering to an inconvenience as an impossibility.

[@gman8563]

But a frame is defined via your position relative to other objects… so you still wind up with the same problem, the lack of a “global” frame

[@Pauluzzs]

@@gman8563 exactly. The point of this video is that you cannot go back in space, cos space is ever changing.

[@gabriellitewski7988]

Why did I go overboard with my addictions!?

[@sunilsheoran5730]

Time travelling is going back in time not in going back to home suppose you can understand

[@shanmugavel1381]

Super explain vro keep it up ❤

[@jialixx]

This is really inspiring! Thank you.

[@Fixundfertig1]

Just two videos, but already subbed

[@khiyronamerson9455]

Take 1 shot when u hear SPACE 2 for TIME

[@haddow777]

Under those philosophical conditions, time wouldn't allow being able to be in the same space even if you could physically go back there anyways. Entropy would ensure you wouldn't be the same you when you made it back. For instance, the last heard, which is admittedly a while back now, that the human body changes every cell in its body every seven years. So, if you did actually manage to get back to an exact point in space you existed in before, likely none of the atoms that made you up would make it back there with you, just your consciousness.

[@jeancorriveau8686]

I had thought of your idea over ten years ago. Nice that someone else agrees too.

[@eitasaulo]

Great !

[@daubert4892]

Isn't it possible to define an absolute frame of reference of motion relative to the cosmic microwave background?

[@AndrewBrownK]

the CMB is different depending on where you observe it. Obviously as the biggest and farthest observable thing in the universe, it is impractical to get a significantly different perspective on it. But hypothetically, just as the constellations would look different in a different solar system, the CMB would look different from a different galaxy (especially very distant galaxies) hm.... actually the more I think about it this doesn't really dispute your point... I mean sailors use the stars to navigate. So to repeat your question, why can't a space traveler use the CMB to gauge their motion? I mean the entire CMB will be blue/red shifted hemispheres depending on your direction of travel. Maybe it is a dumb distinction but "relative to CMB is still relative". Relativity doesn't stop you from comparing reference frames, it just says that no reference frame is privileged or more authoritative than others.

[@donpeters7080]

If an event is marked by the emission or reflection of a photon, then an observer at a distance will view that Photon. If the light is in fact a wave, it expands in a spherical surface at c. If the observer wants to vies the event again, he would have to travel faster that the speed of the wave, c, but this is impossible. It doesn't matter how fast the observer travels, he can never go back to the emission or reflection of that photon or wave...

[@shugyosha7924]

Rather than it being impossible, it sounds like we have no definition for places in an absolute sense. You can travel back to your home if you define your home as being the place where your house is. The definition of the place is anchored to your house, so wherever the house is that's where we agree your home is. But what does it mean to go to where your house was 1 second ago? We don't have any definition of where that place is. We could say that since your house 1 second ago was in the past, that the place should also be 1 second in the past, in which case we can't go there and it would be impossible. Alternatively we could say that it's the position of your house relative to the earth that your house had 1 second ago (e.g. in latitude and longitude), in which case you could go to "the same place", despite the earth moving relative to the sun and so on. Whether it's possible or impossible depends on what we mean by going "back" or a place being "the same".

[@gabriellitewski7988]

I can't go back to the past to redo what wrong in whatever year I want

[@UnoAluminio]

"You can't never go home again".

[@takomamadashvili360]

I was like: "mom, come pick me up, I'm scared"

[@knightrider585]

And this is why you should never think too deeply about time travel in fiction.

[@MikeTooleK9S]

I thought about this as a kid, in terms of the movies back to the future

[@firstjedi2]

Wow great channel, I like it.

[@bidyutbikashhazarika481]

Well, one should understand that physics is local! There is no point in imposing global constraints in every conceivable scenario. You can move to and fro in space, but not in time. Period.

[@bartelsmore6285]

The point is that it’s the same as time. You can move through time at different rates relative to other matter, as you can move through 3D space at different speeds, but you can never move backwards in either.

[@cykkm]

Cf. Einstein's Hole argument, that bothered him for many years. Some locations may not be “locations” in the full sense of the word, i.e. be possible to assign coordinates to. That, or not fully covariant. As for traveling backward in space... in which theory? Such a theory requires, at the least, to define what is "traveling," "a location,” and a distinction between forward and backward (which you've noted so keenly). Incidentally, the theory does not require a notion of time; the “traveling” subsumes it, and simply time isn't enough. These prereqs are easy to satisfy, but it's helpful to keep in mind that they ain't givens. Very good point, thanks for bringing ti up!

[@dialectphilosophy]

We stole the idea for this video from a passage written in a textbook we came across once. Later, we discovered Einstein had written about the very same thing in the prefaces to a discourse he had written on relativity. It seems like he had borrowed it from elsewhere as well, so it is certainly not a new argument. It does raise the issue of the relationship between space and time, which, even from a classical standpoint, can be viewed as blended to a certain extent. We feel that was what was most compelling about the topic... also of course, that it hints at why both absolute space and absolute time are impossible, or at least unknowable.

[@garychap8384]

But hold on... there is a perfectly valid frame of reference in which I can go back to the exact same place I was earlier. What you're actually saying, is that there is no absolute frame to contradict me. Okay... fine! So, here I am ... inarguably back where I was : ) Of course, whether the "me" that goes back can be said to be the same "me" that left, is a far knottier problem : )

[@rreece90]

Minor nitpick, but it would be better to use metric units in a scientific video. Or at least have both.

[@iganpparamarta8813]

Can we just observe the past? I want to watch pyramid being built and Hannibal destroying Romans.

[@rafelg94]

Well if you somehow teleported to 2300 lightyears -or whatever- from here and looked towards here with a telescope, then yes; you could watch those events. When we see the Sun, it is how it was 8:20 min before we saw it, since it's what it takes for the light to travel the distance to the Earth. I have a question for it, though. Is it really the past? From what I understand, after learning so much about relativity, the speed of time equals the speed of light, so we aren't seeing the "past" of the Sun, only the present that the light tells us is present.

[@iganpparamarta8813]

@@rafelg94 thanks for replying but now I have a headache haha not your fault it’s just my brain (or lack there of)

[@MrCmon113]

The apparent observability of the past is what makes it the past.

[@robertjamesdamien]

If we are watching or moving relative to time. what kind of reference frame that time would use to see us if we travel approximately at the speed of light ? SOMEONE PLEASE ANSWER THIS FOR ME 🤔🤔🤔🤔

[@philoso377]

Nice video and presentation. OK we got the message. So what is the point to return to the previous location in space?

[@davidcarter8012]

This is only physically coherent assuming the existence of the Ether

[@haddow777]

With respects to no fixed points in space existing, I've always questioned that. Current science stipulates that light travels at a constant speed. That means, nothing adds speed to it when it is emitted. If you are driving at 50kph, the light coming out of your headlights isn't traveling at light speed + 50kph. This by definition means that fixed points in space exist. When a photon is emitted from an atom, that photon decouples from any speed or direction that atom was traveling and instead travels from the exact fixed point in space that atom existed in at the moment the photon was created. Until it is absorbed or affected by something else in the universe, it will always be traveling from that fixed point at the constant speed of light. This implies that any light emitted on Earth should automatically have a Doppler effect built into it from the speed and direction the Earth is moving compared to the fixed positions those photons are traveling from. This should also mean that if we could build light sources with accurate enough emitters and sensors accurate enough to detect these extremely minute Doppler effects, we should be able to calculate Earth's exact speed and direction in relation to these fixed points in space, irrespective of having to be relative to any other matter in the universe. Fixed points in space may not exactly exist for matter, what with space time stretching and compressing. It would have to exist for light though, because light's speed has to be defined by some zero nonmoving state to base its speed off of.

[@silentobserver3433]

All this is saying is the obvious (for the modern humans) fact that there is no such thing as "absolute space" in a very convoluted way. Like, the entire concept of having an "absolute position" is meaningless, what are you going to measure it against? And if you have a fixed local coordinate system (like measuring positions relative to CMB), then travelling backwards is trivial. This is not true with time, however: no matter what local reference frame you use, you cannot travel backwards in time even locally. So this video just creates a false similarity where there isn't one.

[@haddow777]

That's not entirely true about no fixed points in space. Light has a constant speed. When a photon is emitted, it is decoupled from any sorts of speeds its emitting atom was traveling at. The atom cannot add its speed to the photon. This means, light is emitted from a point of zero speed, an absolute point in space. As such, light should have at least some Doppler effect created in it by its moving light source that should be able to give the exact speed and direction that source is moving compared to some universal fixed position.

[@silentobserver3433]

@@haddow777 Sorry, that's not how relativity works. Sure, an atom cannot add its speed to the photon, because photons always travel at the speed of light, but that doesn't automatically create a "point of zero speed", that's just relativistic speed addition, c + anything = c. And the speed of the source is still recorded in the photon by the means of Doppler shift, except it's not absolute, Doppler shift only exists for an observer traveling relative to the source

[@haddow777]

@@silentobserver3433 worded weirdly, but I think I get your point. You can't detect the Doppler effect the emitter builds into the light beam because the detector is moving as the same speed and in the same direction as the emitter, so it undoes the Doppler effect. Kind of like sound with train horn. If a train is moving quickly, the sound of the train horn should sound the same if you are ahead of it or behind it while on the train. Still, maybe detecting the Doppler effect isn't feasible in that situation, but I don't think it changes things for the implication of light speed. Not being able to hear the Doppler effect while on the train doesn't mean the speed of sound isn't constant. When a sound is emitted, by the very fact that the train does nothing to affect that speed, that means each sound wave's origin I space is fixed. Whether that train blasts its horn when standing by a sign or when passing the sign at a hundred miles an hour, that horn sound would reach the same distance after 5, seconds, it would just be at a different pitch. If light really is a universal constant that travels at the same speed, in the same medium, everywhere in the universe, then there has to be a value it is set against. A fixed point. Okay, detecting a Doppler effect is a flop. What about two trains moving at the same speed down a straight track. If both trains had horns facing each other, with conductors watching each other. Then, when one signals, they both blast their horns. Each will signal when they head the other's horn. The rear train will signal first, because the sound from the front train has a shorter distance to travel to the rear train that the sound from the other horn. Okay, ya, it wouldn't be that much difference, so a more provide instrument than a human ear should be used. Still, the math supports it is. So, instead, we could come up with a bit more complicated of a solution involving satallites. There could be 3 pairs of satellites. Each will have both laser emitters and detectors that have extremely precise time measuring capabilities. Each pair could start off attached together, where they could sync their clocks. Each pair would be set for a different axis, but perpendicular to a high precision pulsar. Each pair could decouple and put distance between each other while keeping their clocks in check with the pulsars. Then, each could fire at predetermined time intervals at the other. It could be coordinated to the pulsar timing, which should, if both satallites are using the same algorithms, should detect the signal from it at the same time. This could help them fire their lasers at the same time towards the other. Like the trains, if both satellites are traveling along their respective axis at any sort of speed, the lasers should be detected by both satallites after differing intervals of time. Adding distance between each satallites should increase the discrepancy. With 3 sets of satallites covering 3 separate axis, an overall speed and direction should be calculatable. Probably over complicated, but I figured bigger distances would make it easier to detect.

[@silentobserver3433]

@@haddow777 Yep, if light really was traveling in some stationary media, like sound does, that's exactly what you would observe. In fact, that's what everyone thought how light worked in like the 1900s, the ether theory. So they did experiments trying to measure exactly that - differences in arrival times of light signals depending on the direction. Specifically Michelson-Morley experiment. And guess what, it detected literally no difference. "Front train" and "rear train" both "blasted their horns" at exactly the same time, even when measured to the required accuracy. That's what relativity is derived from: speed of light in vacuum is a constant, and doesn't travel in any kind of universal ether, it just somehow travels at the speed of light for any kind of measurement you could think of, in a way that is completely against any fixed points in the universe

[@haddow777]

@@silentobserver3433 okay, I somehow doubt that their clocks were accurate enough back then to make a calculation properly. Also, you have to have two clocks, which I don't think they had the capability to sync properly back then. The test only works if you have two independent lights going in opposite directions to two independent detectors, but with the ability to accurately compare the travel times of both. If you bounce the laser back using a mirror, it nullifies any gains or losses as both directions would average out. I used sound because it is a constant speed. It doesn't matter that it's in a medium and light isn't, although, that's as long as you don't believe quantum field theory it is correct, because that sounds a lot like the old either theory. The entire crux of the matter is that they both have constant speeds. Yes, I know the speeds change in different mediums. The speed of sound is different on Venus compared to Earth. It's part of the reason why I placed the experiment in space, to escape that point being pushed. The light would be traveling in the near vacuum of space. The satellites could be positioned quite a lot farther away from each other than they could on Earth, which would make the need for extreme precision less necessary. Also, it allowed the satellites to coordinate using an independent timing source to solve any sort of relativity time clock issues. Okay, all that out of the way. No matter how light travels, if its speed is genuinely a constant speed, what you are describing cannot be true. If two satellites are position in a fixed position compared to the Earth and there is zero different in the travel times of any of the 6 lasers, that would be absolute proof that light does not travel at a constant speed. It would be moving at a constant speed relative to the emitters and the detectors, which means they added their speed to the beams. That is not a constant speed. If that was true, then light from other galaxies would be coming at us in all sorts of different speeds, based on whatever speed they were moving relative to us.

[@Elrog3]

Hold up. This is an argument from ignorance (a common fallacy). Not knowing what direction in space the spot you were at some finite amount of time in the past was does not equate with it being impossible to move to that location. It only means that if you do it, you won't know it.

[@dialectphilosophy]

Good point, and bravo for bringing that up! Is it merely a sort of epistemological confusion to equate an inability to define a past place to ontological unreality of that place? If something cannot be known to truly exist, then empirically speaking does it really exist? These sorts of questions are the true fun of the philosophy of physics!

[@tkimaginestudio]

@@dialectphilosophy If there is a reality that exists independently of being observed or known about -- I hope we all agree on that -- then events, like an object returning to a prior location (whatever that means in expanding spacetime), occur with or without someone being able to determine whether they have happened. It is of course still entirely reasonable to take the view that one should not concern oneself with the unknownable. I agree with the latter but I don't see how the admissions of one's epistemological limitations would impact on the ontological state of affairs.

[@Returntonature145]

Agree.Without a direction,even with time,we will never know if we reached the place we were in

[@tkimaginestudio]

@@Returntonature145 Facts don't care about our knowledge. It is good to know about one's limitation to know things, but it is hybris to think things don't happen because we are unaware of them. The same nonsensical school of thought led some to believe that a wave collapse needs a conscious observer. To me, this is no better than people in the middle ages thinking the universe revolved around the earth.

[@miku4j]

so this is why some dad can't travel back to home after buying milk

[@angeldude101]

From what I can tell, the earth's orbit around the sun, and the sun's orbit around the Milty Way, etc. are all inertial reference frames in GR. Therefore it is perfectly legal to define our coordinates such that the galaxy is moving around the sun, and the sun around the earth, and the earth is standing still. Under this coordinate system, it is possible and very easy to return to the same point in space. Every point in space is the center of the universe, so why not pretend that your house is the center of the universe?

[@paradiselost9946]

i think its easier, if maybe making for a short video if done so, is to say that ANY direction you travel... its forward. its also back. its an oxymoron. a sphere with no identifying features has a top, a bottom , a front, a rear, a left, a right, but they are just words, and if it moves in any way whatsover a new area is now "top", "bottom", etc... its so obvious that its sort of not worth contemplating. a slight deviation to the left? your just traveling forward in THAT direction, thats all. and its always forward. "backwards" is just a term we use for dealing with our physical body, being stuck in space and defining it as unit lengths, giving it coordinates and directions. i walk backwards, and now my back is moving forward in space. im moving forward in space. "front and back" arent really objects as such. just descriptions to orient oneself in relation to an object. what IS up? what IS front?

[@axle.student]

Nailed it :) The only direction from a single point at the center of an abstract future sphere is "away" from the center or forward (In space-time) :) > I guess another way of saying it would be that we are either stopped or have a positive velocity. We can't really have a negative velocity in space or space-time :)

[@7sbunny505]

That's true only for the person who is seeing us far away from the earth.But for us it will be same

[@smokey04200420]

This is the 4th video I watched on your channel. I already had a headache from not being able to stop my mind from thinking about physics, but now it’s worse because I can’t tell what you’re trying to prove with your videos or if you’re just messing around for the sake of being funny or if there’s a point you’re trying to make by being funny.

[@annamalayadevi]

Just a thought...u actually can...here we are considering an object which moves a head...along with earth and galaxies...but an observer is required to see this...what is the object doesnt move forward or moves at a slower speed ....now this cannot be achieved physically....can only happen at the level of thought,...dats why right meditation is only the answer ... because the answer lies beyond physical imagination.....

[@jespermikkelsen7553]

I love this. We all have a path, a lifeline, which is predetermined, from start to end. Free will is an illusion. We feel time as a river that cannot be tampered with. But we feel that we ourselves determine our movements in space. But this is an illusion. A very convincing illusion. However, why there is a difference between our perception of the temporal and the 3 spatial dimensions is still a mystery.

[@coopergates9680]

It's all abstract anyway. Dimensions and quantities are useful for explaining observations and evidence but don't definitively manifest themselves. We don't really know what 'existence' means or where to draw the universal line between conscious and unconscious.

[@MrCmon113]

You can't put a mug right next to air in time (disregarding quantum physics).

[@tyedee7552]

@@coopergates9680 What absolutely exists is what all observers agree upon. An observer is a coordinate system on the 4D Lorentzian manifold of spacetime.

[@coopergates9680]

@@tyedee7552 It's our best guess and still philosophical, you can't point to an x, y, or z axis (let alone an origin) sitting there among celestial bodies, just imagine it. "All models are wrong, but some are useful."

[@lyrimetacurl0]

Came thinking this would be the same argument as you can't have a negative speed (but you can have negative velocity). Which might be related to negative energy. Also interesting that in 2D random movement will certainly result in returning to the same location - maybe the 2nd law of thermodynamics isn't necessarily true in 2D (or especially 1D) due to the dimension itself.