B What speed and direction are we actually traveling?

[JMz]

Sometimes it seems difficult to explain that we can tell observers that are accelerating form observers that are not

I'm curious how this manifests, and what seems successful to get past it. I realize that most students don't have experience traveling at large fractions of the speed of light (as our OP-er wants), but is this any different from telling someone, "Even if we stood in a closet and turned off the lights, we would be able to tell which way is down"? A hypothetical unfamiliar individual might have to accept it on faith, but, really, that's just the way the world works.

 

[PeterDonis]

I'm curious how this manifests

You can measure proper acceleration directly with an accelerometer.

 

[JMz]

To clarify: I'm curious how this difficulty of explaining manifests: What do the students(?) say, and what works for them?

 

[julcab12]

The paradoxical aspect of the twin paradox is this: If all frames are equivalent, and all motion is relative, then what basis is there for saying that one twin will be older than the other when they get back together. To me, it ceases to be paradoxical (although it might still be puzzling) from the point of view of 4-dimensional spacetime.

Frames are never equivalent when measured. If two atomic clocks are started simultaneously(sort of) with one remaining on the ground and the other on a flight 10km above the Earth and its gravity, then the readings show two different results: the one at a distance would have “lived” less long by some nanoseconds than the one on the ground. Same goes with 3 clocks. Both of them move on opposite direction with equal path and distance and the other stationary would still show nano differences between clocks. The effect is very slight and not noticeable.

 

[Ibix]

Frames are never equivalent when measured.

I'm not sure what you mean by this. Frames are not something you can measure; they are a tool to analyse a situation.

If two atomic clocks are started simultaneously(sort of) with one remaining on the ground and the other on a flight 10km above the Earth and its gravity, then the readings show two different results: the one at a distance would have “lived” less long by some nanoseconds than the one on the ground. Same goes with 3 clocks. Both of them move on opposite direction with equal path and distance and the other stationary would still show nano differences between clocks. The effect is very slight and not noticeable.

I think you may have missed Steven's point. He was pointing out that the "paradox" lies in trying to treat this experiment as if all clocks were at rest in an inertial frame at all times. And one way round this is to drop the whole "frame" thing and just use intervals as you would distances in space.

 

[julcab12]

I'm not sure what you mean by this. Frames are not something you can measure; they are a tool to analyse a situation.
I think you may have missed Steven's point. He was pointing out that the "paradox" lies in trying to treat this experiment as if all clocks were at rest in an inertial frame at all times. And one way round this is to drop the whole "frame" thing and just use intervals as you would distances in space.

Yes. I'm just referring to small differences between two perceived object separated by a distance. Whether two spatially separated events occur at the same time – is not absolute depending on the observer's reference frame which is only a good approximation. Each observer may have her own way of defining a certain direction, incompatible with the choice of others. Same is true with clocks or things.

"In the Hafele and Keating experiment was the first proven evidence that atomic clocks register a dilation according to the Schwarzschild solution of the GRT. Clocks at rest with respect to each other, and at the same gravitational potential. They remain in sync, which just means that time can be defined unambiguously, and that talking of clocks is meaningful. If all sorts of reasonable mechanisms for keeping time were affected differently by gravity, or velocity, it would be hard to even define time clearly. The experiments show that his does not happen."

 

[stevendaryl]

Frames are never equivalent when measured. If two atomic clocks are started simultaneously(sort of) with one remaining on the ground and the other on a flight 10km above the Earth and its gravity, then the readings show two different results: the one at a distance would have “lived” less long by some nanoseconds than the one on the ground. Same goes with 3 clocks. Both of them move on opposite direction with equal path and distance and the other stationary would still show nano differences between clocks. The effect is very slight and not noticeable.

There is an ambiguity about what "frame" means. In General Relativity, some people would say that there are only local frames, not global frames. Locally, there is no difference between being close to the Earth and being high above the Earth. No experiment confined to a small enough region will tell you the difference.

 

[stevendaryl]

Yes. I'm just referring to small differences between two perceived object separated by a distance. Whether two spatially separated events occur at the same time – is not absolute depending on the observer's reference frame which is only a good approximation. Each observer may have her own way of defining a certain direction, incompatible with the choice of others. Same is true with clocks or things.

The analogy I was making was between paths in 3-D space versus paths in 4-D spacetime. Different paths through space connecting the same starting point and ending point can take different amounts of elapsed time (the time measured by your watch). In a similar way, different paths through spacetime connecting the same starting point (meaning a particular point in spacetime) and the same ending point (another point in spacetime) can take different amounts of elapsed time. There is no need to identify any particular "frames". What you need is a notion of the traveler's trajectory through space and spacetime---how he is progressing as a function of elapsed time.

 

[PeterDonis]

"In the Hafele and Keating experiment...

Where is this quoted from? Please give a reference.

 

[julcab12]

Where is this quoted from? Please give a reference.

https://www.researchgate.net/deref/http%3A%2F%2Fjournals.aps.org%2Fprl%2Fabstract%2F10.1103%2FPhysRevLett.116.063001
https://www.researchgate.net/deref/https%3A%2F%2Fwww.uam.es%2Fpersonal_pdi%2Fciencias%2Fjcuevas%2FTeaching%2FChou-Hume-Rosenband-Wineland-Science2010.pdf

Time Retardation

 

[PeterDonis]

https://www.researchgate.net/deref/http%3A%2F%2Fjournals.aps.org%2Fprl%2Fabstract%2F10.1103%2FPhysRevLett.116.063001
https://www.researchgate.net/deref/https%3A%2F%2Fwww.uam.es%2Fpersonal_pdi%2Fciencias%2Fjcuevas%2FTeaching%2FChou-Hume-Rosenband-Wineland-Science2010.pdf

Which of these references has the specific text you quoted? And where exactly in the paper is it?

 

[Chris Miller]

All motion is relative, so there is no absolute speed at which the Earth is moving through the universe. It doesn't matter, therefore, which direction you travel. It's only your motion relative to the Earth that matters.

Note also that at half the speed of light (relative to the Earth), the relative time dilation would only be about $1.15$. So, you're not going to get a lot of differential ageing with a 10-year flight - about 1.5 years.

I understand that velocity is relative, but it's also my understanding that there is an absolute/universal speed=0 of sorts: i.e., the co-moving frame of reference in which the CMB's wavelength is the same in all directions and in which maximum time has elapsed since the Big Bang.

Also, what the poster seems to be missing is that it's not his relative velocity that will allow him to have aged less upon his return to earth, but his path through spacetime.

 

[ZMacZ]

I'd say the sun goes in an outward spiral away from the center of the galaxy, much like a very very very very high orbit..and the Earth circles the sun..
But that's not all, because each galaxy is also spiraling outward from the center of the universe..

Now for what speed we have, here's a practical experiment..
Since we assume the speed of light to be an absolute, traveling from a point to another,
it will mean that every bit of speed we already have is deducted from the light speed..
So, when we have a vector A, and we are traveling along that vector,
any emission of light along that vector will be LS - speed along that vector..
While that is true the opposite will also be true, namely the relative speed from anything
along vector -A will be LS+ speed along vector A..
So, when we measure the time it takes for a beam of light along any 3 vectors being at
exactly 90 degrees from each other,we can calculate our actual speed and vector..

But do take into account that we also have computers that work along that same principle..

The best way to equalize their internal cronometers is equal distant to all three points of measurement, and as far away as possible.
(to minimize the differences in each vector towards the points of measurement..)
Since I do not have a lab, someone else will have to do this..

I'm certain that an alpha-like processing unit can measure time at 10Ghz, if not 100Ghz,
when specially designed for it..
At 10 Ghz the distance light travels will be 0.03 meters per Hertz,
but I'd say 3 mm per hertz is better to use..
Then when the light is registered, don't go for exact timings by a central processing unit,
but the number of ticks from each measurement processing unit..
I'd love to see the results..

note:Also, this experiment would also tell us if the LS is actually as constant as we assume it to be..
Since there would be an actual difference in ticks, if the test is one exact enough..
However, if there's no difference, than LS only is LS when measured from the point of emission..
(and that would mean that LS isn't exactly the barrier we think it is till now..)

I meant vector EB, EC and ED roughly equal to each other..not AB, AC and AD..sorry..
Note that If AB= (1,0,0) and AC=(0,1,0) and AE=(0,0,1), then AE would be x(1,1,1)..
(with x being as large as humanly possible..)

 

[Ibix]

I'd say the sun goes in an outward spiral away from the center of the galaxy,

I doubt that the path of a star in a system like the galaxy is anywhere near that simple. Do you have a reference for this claim?

from the center of the universe..

There is no such place.

when we have a vector A, and we are traveling along that vector, any emission of light along that vector will be LS - speed along that vector..

The speed of light is independent of its source. So all that follows from this is wrong.

I'd love to see the results..

There are tests of the isotropy of the one way speed of light. See the experimental basis of SR FAQ at the top of this forum.

note:Also, this experiment would also tell us if the LS is actually as constant as we assume it to be

We have actually tested this, you know. You might want to look up Bertozzi's video of accelerating electrons. It's on YouTube. He demonstrates that the electron speed gets closer and closer to c as he pumps nore energy in - but never reaches it.

 

[stevendaryl]

Since we assume the speed of light to be an absolute, traveling from a point to another,
it will mean that every bit of speed we already have is deducted from the light speed..
So, when we have a vector A, and we are traveling along that vector,
any emission of light along that vector will be LS - speed along that vector.

I'm not 100% sure what you're saying here, but it sounds like you're saying that velocities add linearly: If I am traveling at speed v relative to the center of the galaxy (say) and I send out a signal traveling a speed w relative to me, then it will be traveling at speed v+w relative to the center of the galaxy. Relativity specifically says that velocities don't work that way. Instead, the velocity relative to the center of the galaxy will be $\frac{v+w}{1+\frac{vw}{c^2}}$ (where $c$ is the speed of light and $c^2$ means $c \times c$). Note that if $v=c$ or $w=c$ then the result is $c$:

$\frac{v+c}{1+\frac{vc}{c^2}} = c$

 

[ZMacZ]

@Ibix, no matter what the test will stand..whether the LS is a true absolute or not..

@Ibix, you may deny the Big Bang, I however do not..

@Ibix, the speed of light may very well be an absolute, I'm not so sure about this, in any case the test will prove it's merit either way..
(this does not mean I say that it is not..only after a thorough test of this by me myself would I know, all other things are hearsay to me..)
(and yes, if LS is a true absolute it would mean that any speed we initially have would be added upon the time traveled to any measured point,
if and when vectors align, and be deducted when they are opposite..)

@stevendaryl, If the LS is a true absolute the test will confirm our current speed and direction..

@Ibix, If traveling at 0.5 LS we emit some light beam ahead of us, it would only travel forward along the vector we travel with
a relative speed of 0.5 LS, right ? So from a receiver's perspective along taht same heading would the LS not only be 0.5 LS ?
In other words, if the receiver would be 1 LY ahead from the emitter , but also traveling at 0.5 LS, would
the perceiver only see it after 2 years ? (1 LY/0.5 LS) I think so..you may differ if you want..
(but also, if it would NOT be 2 years, it would change the whole way light actually works, wouldn't it ?)

I mean not that the speed of light actually changes, but the perceived speed..

 

[Ibix]

@Ibix, you may deny the Big Bang, I however do not..

Where do you think I did that?

the speed of light may very well be an absolute, I'm not so sure about this

Check out the experimental basis of SR FAQ for extensive evidence.

LS is a true absolute it would mean that any speed we initially have would be added upon the time traveled to any measured point,
if and when vectors align, and be deducted when they are opposite..)

No it doesn't. See the velocity addition formula in Steven's reply.

If traveling at 0.5 LS we emit some light beam ahead of us, it would only travel forward along the vector we travel with
a relative speed of 0.5 LS, right ? So from a receiver's perspective along taht same heading would the LS not only be 0.5 LS ?
In other words, if the receiver would be 1 LY ahead from the emitter , but also traveling at 0.5 LS, would
the perceiver only see it after 2 years ? (1 LY/0.5 LS) I think so..you may differ if you want..

This is horribly confusing because it's not at all clear who is moving with respect to what, but I think you need to read the velocity addition formula again.

 

[stevendaryl]

@stevendaryl, If the LS is a true absolute the test will confirm our current speed and direction..

Okay. What you're saying is contrary to relativity. Physics Forums is for discussions of boring established theories, such as relativity, rather than being a place for you to post your own theory. Submit to a journal, if you want to publish your own theory.

 

[ZMacZ]

You guys really don't get it right ?

If each point of measuement from our lightpulse would be exactly one LY away in each axis, then they would be
receiving the lightpulse at exactly 1 year after emission right ?
But, if we're not, and the points of measurement are a relative distance constant to ours,
the time measured for each reception of the lightpulse woudl tell us about our own speed..
Since they would differ..
The experiment is BASED on the assumed fact that LS is constant, not to go AGAINST it..
In fact, it would confirm it's constant-ness..as well as tell us something about our velocity and vector..
(and also it would allow us to find out our near exact speed in the universe..)

 

[stevendaryl]

You guys really don't get it right?

What I get is that you don't understand relativity. What you're saying is contrary to relativity. There is more to relativity than the constancy of the speed of light.

 

[Ibix]

The experiment is BASED on the assumed fact that LS is constant

It's based on your failure to understand the implications of light speed being constant. Please get hold of a textbook - Taylor and Wheeler's "Spacetime Physics" is a good choice.

and also it would allow us to find out our near exact speed in the universe

Michelson and Morley's experiment was intended to do exactly this. Its null result is incomprehensible in the terms you are using; it's part of how relativity got started.

It's clear to me that you don't understand relativity. If you want to learn, Taylor and Wheeler is a good place to start and many people here will be willing to help you. If, instead, you prefer to dismiss over a century of detailed, carefully carried out, repeatable tests of relativity as "hearsay" and continue making up your own un-evidenced speculation based on misunderstanding of what relativity claims then that is your prerogative. It'll get you banned from here in short order, however.

 

[ZMacZ]

Ok then..here we go..

Now in one simple question, is this true, according to you...
(it'll help me with your responses as well..)

 

[Ibix]

Now in one simple question, is this true, according to you...

As noted, Michelson and Morley did this more thoroughly over a century ago, and avoided all the clock synchronisation problems you haven't noticed with your design.

The two setups will give identical results assuming the distances are measured using rulers at rest with respect to the source and receivers and the same clock synchronisation procedure is used. There is no universal rest frame, so the experiments are identical.

 

[ZMacZ]

@Ibix, does this means there will be no difference in ticks with my initial testing formation ?
(I know the answer already..)
(and the timing differences between the clocks are less than 0.000000001 % over a 24 hour period..)
(and that would be less than 0.3 mm..)

Also, I have already found a newer way to do it..

Scratch that, the 'newer' solution I came up with defeated itself again..

 

[stevendaryl]

Ok then..here we go..

I think that you really don't understand what relativity says about this thought experiment. I'm not exactly sure what your PE, PR1 and PR2 are supposed to be, but let's suppose that we have two planets that are at a constant distance of 2 light-years apart, according to their own reference frame. Call them A and B. Call their rest frame the AB frame. You have a rocket ship C that is traveling at 90% of the speed of light in the direction from A to B. Let's call its rest frame the C frame. When C gets half-way between A and B, it sends out two light signals: one to A and one to B. Then:

According to the AB frame:

1. The distance between A and B is 2 light years.
2. A is traveling at speed 0.
3. B is traveling at speed 0.
4. C is traveling at 90% of the speed of light.
5. The light arrives at A one year later.
6. It also arrives at B at the same time, one year later.

In this frame, the signals arrive simultaneously.

According to the C frame:

1. The distance between A and B is only 0.87 light years.
2. A is traveling at 90% the speed of light away from C.
3. B is traveling at 90% the speed of light toward C.
4. C is traveling at speed 0.
5. The light arrives at A 4.36 years later.
6. The light arrives at B 0.229 years later.

In this frame, the light arrives at B 4 years before it arrives at A.

 

[Ibix]

does this means there will be no difference in ticks with my initial testing formation ?

Assuming that you have three receivers equal distances from one source, all mutually at rest, and you've synchronised all three clocks using the same procedure then you will get the same results from all three arms under all circumstances. Have you looked up the Michelson and Morley experiment yet?

 

[ZMacZ]

@stevendaryl..yes..you got it..
But did you know it will also work at our non-relativistic speed, the same way ?..and that's how measuring time by the LS
will tell us about our actual speed in the universe..

@Ibix, nope...all the clocks are synchronized beforehand, and nope, the number of tciks since start will differ, except when we're not moving in the universe at all..
And yes, I looked those up too..but the thing with reflecting is this, whatever speed 'advantage you have on the way there, you'll get against on the way back..
But..when it's only one way and then measured, it'll be different, UNLESS we are all standing still and Earth doesn't move in the universe..

To me it's a mixed blessing...one one hand it will be expecting that LS is true constant, and thus
travelling faster by any means bound by LS will always yields a speed lower than LS..
On the other hand it will enable us to know exactly which way we are going and at what speed..
(So..!YAY! for measuring our speed, or !YAY! for FTL..but they are almost mutually exclusive..)

 

[stevendaryl]

@stevendaryl..yes..you got it..
But did you know it will also work at our non-relativistic speed, the same way ?..and that's how measuring time by the LS
will tell us about our actual speed in the universe.

No, that's not correct. You can figure out your speed relative to some other slower-than-light object, but there is no such thing as "actual speed" in relativity.

Look, what you're saying is contrary to relativity. It's your own theory. We don't discuss people's private theories in Physics Forums.

 

[Ibix]

number of tciks since start will differ, except when we're not moving in the universe at all..

No it won't - Michelson and Morley demonstrated that. You are contradicting experiment, which means you're off in fantasy land.

And yes, I looked those up too..but the thing with reflecting is this, whatever speed 'advantage you have on the way there, you'll get against on the way back..

True.

when it's only one way and then measured, it'll be different,

False. It's impossible to make a one-way measurement of the speed off light without assuming your answer. So the speed can be anything you like. It depends on how you synchronise your clocks - and if you use the same synchronisation procedure all three arms will give the same result.

 

[berkeman]

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