@Zest

because intuitively if one thing is moving away from you and another thing is moving in the same direction, if you accelerate fast so that your relative speed is equal to the speed of light, is it possible to travel away from both of them at the same speed?

So this is your first mistake, the phrase “speed is equal to the speed of light” makes no sense. You can never have anything moving relative to you at the speed or light or faster (other than light of course or another massless particle).. You would have to rephrase this to say “near the speed of light” to make any sense.

If you and the object are moving at the same speed and direction and are moving at near the speed of light relative to the earth the earth would appear to be receding away from you at near the speed of light, lets say 98% of C.

Now if you left the object that is moving with you behind and now accelerated to 98% the speed of light relative to that object what you would see is that even though you increased your speed by 98% C relative to the object that was moving with you, relative to earth you only appear to have accelerated by 1.98% the speed of light. In other words after accelerating the object that was moving with you would now be receding away at 98% the speed of light and the earth would now be receding away at 99.98% the speed of light.

is this a time dilation thing?

Well, no it isnt a time dilation thing exactly, though time dilation is occurring it isnt really relevant to the people in the ship (who would be unaware of any time dilation presumably. but not completely unrelated either (if they had a telescope and looked at earth thigns would appear to be moving slower in terms of the people and clocks the faster they go). With special relativity you cant really ignore any one aspect wholly.

and would that only be from your perspective

Well not only from your perspective exactly, but only from certain perspectives. Obviously if a second ship were traveling along side you at the same speed and direction as you and accelerated along side you then they would see all the things you see from their perspective.

But yes that perspective is not shared by other observers necessarily. From your perspective (since you’re not accelerating) it feels and looks like you are stationary and it is the object and the earth that are moving instead. So to you on the ship the object is moving 98% the speed of light and the earth is moving 99.98% the speed of light. From the perspective of the object the object is stationary and you are moving away from it in one direction at 98% the speed of light and the earth is moving away from it in the other direction at 98% the speed of light. Likewise from earth the object appear to be moving at 98% and you appear to be moving at 99.98%

the relative speed by an observer who was traveling even faster than the other object you could be moving slower relative to all three things from their perspective because time would be going differently to them?

Direction matters here, but yes. If there was another object in the mix that was another ship and basically flew along side you and accelerated when you did, but lets say they stopped accelerating just a few moments before you stopped, so they are almost at the same speed and direction as you but not quite. From their perspective the object and earth would be moving marginally slower than from your perspective. However this new ship would appear to be moving very slowly away from you and in the direction you came from (towards the earth) while to the new ship you would appear to be moving very slowly away from them and in the direction away from earth.

@Science

@Zest

One added point, in the last part where we introduce a new ship. Time dilation has little to do with it, and its a perfect example of why.

The degree of time dilation is always proportional to the relative speed between two objects. Since you and the new ship are traveling at almost the same speed and direction and only differ slightly, therefore you appear to be traveling away from each other but at a very slow speed, time dilation between you and the other ship would be negligible.

Imagine the speed difference is so small you and the new ship appear to only be moving away from each other at about the speed of a car on the highway, 60 mph/kph. then the time dilation between you and that ship would be no different than the time dilation between your family you left at home and you in a car driving down the highway. In other words it would be imperceptibly small.

@Science

@freemo @Science okay that's pretty interesting but it does make me wonder, at near light speeds, would those objects that are moving apart from each other anyways not still be moving apart from each other at rather high speed, despite them both moving at close to the same speed relative to me? if all three objects stopped moving relative to each other then, how would that effect the distance between them? would it be different to different observers?

@Zest @Science

> would those objects that are moving apart from each other anyways not still be moving apart from each other at rather high speed

from all observers the distance between the earth and original ship will appear to be increasing, though not at the same rate.

> if all three objects stopped moving relative to each other then, how would that effect the distance between them?

You have actually just hit on the third component of special relativity, you already knew about time dilation as you mentioned it, and we already discussed velocity dilation (that velocity is relative to the observer just like time is), now you are basically realizing the third aspect of special relativity, that is, length dilation (formally called length constriction). the length of a thing is just as relative as the rate of time or the speed of the thing.

Depends on who out of the group is doing the decelerating in order to create a relative velocity of 0 between them.. If you are standing in between two spaceships each going 98% the speed of light in opposite directions away from you, and they both decelerated equally until velocity was 0, then the distance between you and teh ships and between the two ships from your perspective would not change (however the length of the ships themselves would). When the ships are moving at high speed they would be much short in the direction they were moving, specifically 19.9% of their normal length, as if they were "compressed" in that one direction. As they come to a stop their length would expand to their normal length again.

However the perspective from the moving ship is much more interesting, assuming the ship is the one that did the accelerating the entire universe, along the direction that it is moving, would be 19.9% shorter than it appears to you, so a star that would normally had been 1 light year away is now only 0.199 light years away and from the perspective of the person on the ship would take 1/5th the time to get there that it would have taken light to do the journey.

Similarly if we had two ships moving away from you at 98% the speed of light the distance from the perspective of the ship is 19.9% less from the ship to you or the ship to the other ship than it appears to you. However the moment the ship accelerates and becomes stationary to you the universe will expand to its normal length again and the distances seen from the people on the ship would agree entirely with your own measurement of distance.

@freemo @Science oh my god! it all makes sense now I'm actually literally crying thank you so much for explaining all this!

@Zest

This moment right here is what I live for...

Give it time though you will start to have questions againa nd feel like it stops making sense. Dont let that win, you got the idea, it does make sense, and when you realize the "gotchas" it doesnt mean you dont really understand, it just means there are some unusual scenarios that require one other piece of the puzzle to understand and incorperate...

I dont want to break up your tears of joy but take this curve ball (again don't think it means your thinking about it wrong, i promise you aren't), i will give you the answer if you'd like but its kinda the only time piece you haven't considered so worth exploring...

Remember that ship looks short to you and the universe looks short to the ship... so take this class paradox of that scenario and consider how you might resolve it logically... You have a barn that normally is more than large enough to hold the ship. The barn has two huge doors on opposite sides. Normally the ship is small enough to fit inside. Now as a ship flying at 98% the speed of light flys past you and flys through the barn (the doors are initial open) you very quickly in the instant it is inside the barn close both the doors, containing it inside, then open them again before the ship crashes intot he door, and it flys away.

Now because the ship is going 98% the speed of light it is 19.9% its normal length. This isnt a problem because the ship could fit inside at full size anyway. However from the ships perspective the universe is 19.9% shorter and the ship is normal size. So that means from the perspective of the people on the ship the barn is 19.9% its normal length and they should not be able to fit inside it and have both doors closed at the same time. Yet somehow from their perspective it all works out.. how is this possible?

You can also flip this around and presume that at normal lengtht he ship is too big to fit in the barn but when it is traveling at speeds and is 19.9% its normal length it can fit inside just fine, therefore to the stationary person they can still close the doors safely when it is at speed, but couldnt otherwise, and to the ship there is still an apparent paradox (it isnt really but there is something you need to realize to understand why)...

Let me know if you have any ideas as to how this is answered, or if you just want me to answer it for you I Can do that too.

@Science

@freemo @Science so, from the ships perspective the barn shrinks, which causes a problem, but from the barns perspective the ship shrinks, which doesn't cause a problem, or if it doesn't fit in the first place solves a problem but not from the ships perspective...
if it still works when the ship wouldn't fit normally but does at near light speed than it's not like they compensate for each other right?
maybe... the ship is shrinking for real but it can't tell from it's own perspective?
I don't know, at this point either a hint or you could tell me because I think that's the best I can do lol

@Zest

> so, from the ships perspective the barn shrinks, which causes a problem, but from the barns perspective the ship shrinks, which doesn't cause a problem, or if it doesn't fit in the first place solves a problem but not from the ships perspective...

Correct

> if it still works when the ship wouldn't fit normally but does at near light speed than it's not like they compensate for each other right?

There is something that compensates for the problem and makes it a non-issue. but your use of the term "for each other" suggests it is the length of one or the other compensating to make the situation work. That is not the case, the lengths as you and i have described them are correct as is and do not change in any way to resolve this paradox.

> maybe... the ship is shrinking for real but it can't tell from it's own perspective?

Well from the ships perspective it isnt shrinking, the barn is, from the barns perspective the ship is shrinking. Both of these are "real" from that perspective. To the barn the ship is really smaller, literally and truthfully. From the ships perspective the barn is literally shorter.

> I don't know, at this point either a hint or you could tell me because I think that's the best I can do lol

Does that count as a hint or shall I give the answer now?

@Science

@freemo @Science is the thing that compensates for it a function of one of the mechanics of relativity we discussed earlier?

@Zest

No it is something new we haven't discussed, though not of the same nature as the other things. Basically you are making an assumption that doesn't hold true anymore, but it isnt any sort of "dilation" of anything.

@Science

@freemo @Science well, it's very interesting and honestly I do think those are good hints, but I still don't know the answer so go ahead and tell me

@Zest

The idea is called simultaneity. What that means is that two events that appear simultaneously to one observer (in one frame of reference) is not necessarily simultaneous in another frame of reference.

In this case the two events are the closing and re-opening of each of the barn doors. While that occurs simultaneously from the perspective of the barn and the person doing it, from the perspective of the ship they are not. So while the ship's perspective looks like it cant fit into the barn, and in reality it cant fit in the barn from its perspective, it doesnt have to. From the ships perspective the doors are never closed at the same time. one door closes, then opens, then the other door closes and opens, all at the right time to ensure the ship passes through without crashing through a door.

So while from the ships perspective the ship was never enclosed (both doors shut with it inside) in the too-small-to-fit barn, from the barns perspective where it is more than big enough the ship was, in fact, enclosed inside for a moment.

@Science

@freemo @Science woah! but why though? that's actually really perplexing, I don't know of any reason why it would happen that way

@Zest

Its actually not that surprising you see this happen every day with other things, you just never see it happen at such short distances (which is only possible due to the high speeds)...

Imagine sound instead of light, which travels slow enough you can actually see the delays and therefore is easier to conceptualize. if you are right in the middle of two lightening strikes then you would hear them at the exact same moment, assuming you only used your hearing and not sight you would assume they were simultanious.

Likewise imagine you are much closer to one than the other, you would hear that one almost immediately and might not hear the other for a few seconds. Even if they happened at the same time it would seem to you, with your different frame of reference, as if they happened at significantly different points in time.

That's all that is happening here, just in a more confusing way because it isnt due to you being in a physically different location but because time and space itself is warped. But in principle its the same thing.

@Science

@freemo @Science trippy, so like, at those kinds of speeds physical events can reach your.. time? at a different time? it still kind of confusing...

@Zest

Yea, the thing to remember about relativity that is really the trippy part is that you arent just seeing things in the past because light takes longer to reach you (though that is how we talk about it)... but rather things far away from you are literally farther back in time.. with lightening we know the lightening hit much sooner than we hear it, and we think the same way about light. But in reality its that time is warped (or at least this is how I think about it, you wont usually hear scientists describe it this way)

Just think about it in simple terms, if im on a planet around a star that goes super nova and my clock was synced with the clock on a spaceship that is 1 light year away.. if i see the star go supernova on 2020 the space ship according to its internal clock (its stationary by the way) will record the event as happening at 2021 because it took a year for the light to get to him. We usually say its just time delay of light propagation as its easier.. but in a sense it literally didnt happen for that guy on the spaceship until 1 year later.

When you think of "what you see is literally the truth" and not just some optical illusion when it comes to special relativity then it all starts to make sense.

Its no different with the other stuff, one could say that time dilation isnt really happening but instead explain it by the fact that its like a doppler effect where things appear slowed only as an optical illusion because the thing is moving away from you as it produces the image so the "video" gets "stretched" out (like it would with sound).. but that isnt really the case, it is **actually** happening... that is true of everything, including simultaneity.

Once you understand all this weird things you see that you want to dismiss as optical illusions are literally real, then it all makes way more sense.

@Science

@freemo @Science hmm, it seems like both explanations make sense that way though. is there some example of real evidence that shows that this explanation is more accurate? because the barn riddle is a theoretical example, but I'm sure nobody has tested that directly, so even if it's a bit over my head, I'd like to hear about the thing they did do.
though honestly if it's just a large complex math problem maybe you do still need to dumb it down a bit lol. but either way what I'm curious about is why saying that the things literally happen at different times relative to different places makes more sense than just saying "well the light took this long to get here so we're just seeing it now".

@freemo @Science also does this have something to do with the speed of light being a constant even relative to your own speed? because I feel like if that's true about the time stuff, that would also explain a lot about how time and light and speed and all these things interact or like, why they would, but it would also really make me question the nature of light cus like, what the heck light? why do you get to have so much influence over physical stuff when you don't even exert (hardly any) force?

@Zest

The riddle/paradox can be found here:

https://en.wikipedia.org/wiki/Ladder_paradox

> but either way what I'm curious about is why saying that the things literally happen at different times relative to different places makes more sense than just saying "well the light took this long to get here so we're just seeing it now".

The reason is because its the only way the other things make sense without there being a paradox... If time **actually** dilates (and we have experimental evidence that proves that, it was proven on GPS systems for example) that means the actual time expiernced by different frames of reference must be actually different. If the time is actually and literally different then clearly the events must also occur at different times.

Because all 4 concepts, time dilation, velocity dilation, length constriction, and simultaneity are all interdependent on each other then either they are all an illusion , or they are all real, if you mix and match it you get logical paradoxes that make no sense. we can and have proven that time dilation is real as well as velocity dilation, as we can prove it in particle accelerators, so we know that the others must be real as well.

We have also seen length contraction inside colliders (in this experiment it was observed that gold ions were contracted to a thin disc like shape: https://www.physicsforums.com/threads/modern-physics-view-of-length-contraction.575495/page-3 )

However directly and experimentally confirming the ladder paradox itself, thats much harder and I know of no direct evidence of that. But as i said with the other elements being well proven it is the only logically consistent conclusion and matches the math.

@Science

@Zest @freemo @Science This is probably one of those situations where you really need to build things up axiomatically and not rely on intuition.

@swiley

I kinda disagree. Personally I have always found special relativity highly intuitive even from a young age (in 4th grade i had re-discovered the equation for time dilation after hearing my teacher mention how it worked as a side comment once and it sounded like an easy concept).

I think intuition goes very well with it but you must learn to have a new intuition of sorts and think about what it is like in space where you have no points of reference around you to consider what is or is not stationary in an absolute sense.

IMO it doesnt start getting to be intuitive until you tack on general relativity, then things are a lot harder to reason through. Length constriction fo special relativity can be a bit unintuitive too, but even that I think you can build up an intuition rather fast once you understand a few things without the need for math or axioms.

@Zest @Science

@swiley @freemo @Science tbh I completely don't remember writing the word intuitively and I don't really remember what I meant to type but I'm pretty sure that word wasn't supposed to be there. I think I meant to say initially actually, like, to establish the original speeds and then acceleration being a change in relative speeds cus that's the moment I was wondering about.

@Zest

When we talk abotu special relativity we usually dont talk about acceleration. We assume everyone has some speed and direction and that those speeds and direction are not changing. That is in fact what makes the difference between special relativity and general relativity. Once you get into acceleration then we have moved on to discussing the more complex ideas behind general relativity (though that is also where things get cool).

If you pretend acceleration doesnt have any effects and only think in terms of special relativity then under conditions where acceleration occurs the whole system breaks and you get logical paradoxes that dont make sense. General relativity resolves those paradoxes and describes the additional effects that take place when acceleration is introduced to the through experiment in some way.

@swiley @Science

@trinsec yea it can be for sure. Takes some getting use to.

@khird

Indeed, that is the intended meaning of course.. but often grossly misunderstood by people.

@Science

@freemo

I don’t know it’s a misunderstanding as much as just a carelessness or lack of rigour with the vector math (mixing observers without accounting for time dilation when adding velocities leads to errors at relativistic speeds). The sum of the velocity of B as observed from A and the velocity of C as observed from B is not necessarily the velocity of C as observed from A.

@Science

@khird

I’m not sure we can say its carelessness if they dont understand the math to begin with. Most people who misunderstand it likely arent even doing or considering it in vector math terms. I am more interested in the purely conceptual understanding that a lot of people are curious enough that they do consider it, but often get it wrong.

Understanding it really doesn’t require anything nearly as complex as vector math, and even if you do vector math you don’t even need to touch or incorporate time dilation directly (you can use energy equations and ignore time dilation and it works out correct). Sure if you want to calculate exact numbers in a complex scenario with multiple objects moving at different speeds and directions then vector math is going to be your go to (with or without time dilation).

Simply understanding intuitively as I have already done in some of the comments or even doing the math without consideration fo vectors by limiting our talks 2D is possible with just a basic intuition and very little rigour (the numbers I cited in some of my comments i just did simple math in my head, there wasnt a vector in site or anything to do with time dialation to cite those numbers).

@Science

@Zest

People without science backgrounds is **exactly** who my OP is trying to target. It is people like you who have a curiosity about it, dont want to necessarily do the complex math, but do want to understand it in basic terms, that I feel are neglected.

It is a surprisingly simple idea to understand, but it is very rare that anyone who knows the topic will enough will actually both to explain it to someone like you in simple terms. So there is a gap of ignorance despite the curiosity and I dont like that, I dont like ignorance particularly when people are more than willing to try to understand.

So by all means **please** keep asking questions and take this conversation as far as you would like to get some understanding of the topic.

> I have one other question since you said something about being absolutely stationary and I was under the impression that this wasn't really a thing that exists

When i say absolutely stationary I mean that the distance and direction between you and some object are fixed. Better wording would have been "perfectly stationary", as in relative to someone or something else.

> I was under the impression that this wasn't really a thing that exists. are there actually stationary objects?

Your impression was correct there is no concept of stationary outside of relative terms.

> s there any such thing as absolute motion that's meaningful in a way that's not dependent upon it's relativity to other objects?

No there is not, not in any sense what so ever. It always must be relative to some thing. There is also no point that could be considered "the center of the universe" or even "where the big bang happened" which one might think if there were could be used as an absolute reference point. But no such point exists. The whole of the universe is where the big bang occurred more or less. The center of the universe, in any meaningful sense, is always wherever you happen to be, quite literally.

@Science

@freemo @Science wow! okay, it's definitely amazing because this subject is something I always found super interesting but it never made sense until today. I think I learned everything I need to know to be satisfied for the time being, but I'm very grateful that you took the time to answer all my questions in a way that made it all make sense. I had actually heard of space expanding and contracting in this context but nobody ever put all the elements together for me in a way that showed their relationship to each other and that's what really made the difference to my understanding of it!

@Zest

I was in exactly your shoes for many years, almost exactly. I had all the basic ideas but they never fit together just right and no one who knew enough came along (until far too late) to help me put the pieces together. I'm glad I could do that for you.

@Science