Halc

Both had the comma, and yes, I missed it. Thanks for the clarity. I do read screens without glasses since they don't help at all.

On 8/12/2024 at 10:39 PM, MigL said:

The acceleration, other than at the launch, is always -g, downwards.

On 4/21/2025 at 1:11 AM, amaila3 said:

So the acceleration is not zero — it's -g, directed downward .

I know know the topic is old enough to have given birth by now, but the acceleration is +g downward or -g upward.

The way you've both said it says the accelerates upward acceleration is negative, which would be true of something repelled by Earth.

There are no superluminal jets.  The effect in question was simple motion more or less towards Earth, and it only requires a velocity in our direction of > 0.5c.   So let's say a space ship begins a journey to Earth from a point 3 LY away, at a constant speed of 0.6c.  At time 0 (years), it departs, to arrive at time 5, but we don't see that departure for 3 years since it takes 3 years for light to get to us from that far away.  So at time 3 we observe the departure from 3 LY away and at time 5 it gets here.

So to us it looks like the ship took only two years to go 3 LY, superluminal, right?  No, since time for light travel was not taken into account in that calculation. Ditto for the jets, which only appear fast due to this sort of Doppler compression.

On 1/24/2018 at 5:54 AM, OroborosEmber said:

When i said move forward i meant in time. Times the only thing you CAN move forward in.

Motion is a change in coordinate spatial location over time. 'Motion through time' isn't really a defined thing. Under a 3D model, a thing evolves in place as time progresses. In a 4D spacetime model, a thing traces a worldline through spacetime and is everywhere present on that line. There is no motion, no progression in such an abstraction.

42 minutes ago, curious_mind said:

so I am also curious about this question, and here is my theory, which is supported by Einstein's special theory of relativity about how the closer an objects velocity is to the speed of light (c=299,792,458ms^-1), the perception of time for that object (relative) to other objects, slows down.

42 minutes ago, curious_mind said:

so here's my theory: if the closer you get to the speed of light(speeding up),your perception of time slows down,

Both of these are worded like speed is some kind of absolute property. Remember to word it as "relative to a given frame, the closer an objects velocity is to the speed of light, the coordinate time for that object relative to said frame, slows down". So say relative to the frame of a muon, Earth is moving at near light speed and clocks on Earth run much slower than the clock on the muon. Yes, it has a clock. Note that I also removed the word 'perception' from your sentence since the time dilation is a coordinate effect, an abstract computed thing, not actually perceived by anything anywhere (per the first postulate of SR). Your 'theory' violates this explicitly and contradicts empirical evidence. Perception is always of proper time, not of coordinate time.

42 minutes ago, curious_mind said:

assuming this is true; which evident supports it, muons which using conventional science shows that it should be impossible for them to travel as far as they do with their half life and all that, do it regardless, why not the opposite.

Yea, why not the opposite?  Or rather, why do you think not the opposite?  I referenced the muon just above.

42 minutes ago, curious_mind said:

I'm not suggesting going back in time, simply constricting it a bit, same as dilating time by speeding up, why not constrict time by slowing down?

Not sure what is meant by this. Proper time (that which clocks measure) is a measure of a time-like interval. It isn't something that slows, not for anybody.
 

42 minutes ago, curious_mind said:

also a fun fact for anyone who enjoys this and doesn't already know, but if an object could travel at the speed of light, in theory, in its own time reference no time would have passed from the moment it started moving at that speed, until it slowed down. so it would be instantaneous for the object, but for outside observers it still takes time. 

This is a popular concept, but is wrong. It essentially says "If [something impossible], I can conclude anything I like" since it's not even wrong, sort of like asking how long Earth's orbit would take to change if the sun suddenly ceased to exist.

On 1/24/2018 at 3:17 AM, Strange said:

There is no known method for travelling backwards in time. (If you could, the pilot would get younger rather than age faster.)

Actually, I did it just now, and you're right, you get younger.  I have photographic evidence of it. Anyway, I'm back so I could post this.

I do balk at the term 'pilot', which makes it sound like I needed an aircraft or something.

33 minutes ago, KJW said:

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

 

That's a classical radius, and I said that size was a classical concept.  Swonsont beat me to it.

To quote the site:

"The classical electron radius is a combination of fundamental physical quantities that define a length scale for problems involving an electron interacting with electromagnetic radiation. It links the classical electrostatic self-interaction energy of a homogeneous charge distribution to the electron's relativistic mass-energy."

I don't entirely get that, and I certainly don't know how that sort of thing can be measured to 11 significant digits. Nevertheless, defining this size is 'useful', so there it is.

To also quote the same site and show what I was talking about:

"According to modern understanding, the electron is a point particle with a point charge and no spatial extent."

10 hours ago, Khanzhoren said:

I don't entirely agree with you because one also talks about the size of an atom, a molecule, or a solid

A solid is definitely classical. But what I should have said is that size doesn't apply to fundamental things like an electron.  All such things are quantum, but technically a horse is a quantum thing as well, so not all quantum things are without meaningful size.

9 hours ago, Khanzhoren said:

 in my opinion , "singularity" or "object with size 0" cannot realy exist because of the quantum uncertainty principle.

'Singularity' doesn't mean 'object of size zero'. It means conditions under which the equations no longer produce meaningful results. It means a different theory (or different coordinates, or something else) is needed to describe what goes on under said condition. You note this below.

As for zero size object, size is a classical concept and doesn't really apply to quantum things.  The uncertainty principle loosely says you cannot know both momentum and position at the same time. Neither references a size.

9 hours ago, Khanzhoren said:

And as  it is known GR is incomplete because, in particular, it is not a quantum theory. So the real solution to the problem related to singularity should be from quantum description.

A quantum description probably doesn't work either since it cannot describe the spacetime curvature. A unified theory would really help.

On 12/21/2024 at 5:36 AM, abhinav7s said:

The same way, violet should have been the primary colour in RGB and hence it should've been RGV. Thomas Young initially proposed the primary colours to be Red-Green-Violet but James Clerk Maxwell favored changing violet to blue.

First of all, I support blueshift exactly due to it being the highest frequency primary color.

Secondly, assignment of the primaries is not a matter of arbitrary choice. It is a human thing, the three colors to which the human eye cones are optimally tuned. A different creature would assign totally different colors.  Are our eyes truly tuned more to violet than blue?  If so, then Young has a point.

We have three primary colors (RGB) and three pigments (CMY), the colors of the ink in the cartridges, each of which absorbs one of the three primaries and reflects the other two.  Squirrels can actually see yellow and would immediately be able to distinguish a banana from an object painted with yellow pigment, which is no more a color on the spectrum than is magenta which fits nowhere on the rainbow. Magenta is simply the absence of human green.

3 hours ago, KJW said:

Yes, it is worth keeping in mind that even as we sit in front of our screens, there are frames of reference relative to which we are moving at arbitrarily high speeds even if those frames of reference do not correspond to any observer or object.

There's always an object stationary in almost any arbitrary frame choice, even if it's just a muon or neutrino somewhere.

Observer, no, but observers in relativity don't actually observe anything except instruments, which can be done by anybody regardless of motion.

For instance, a fast approaching clock is observed to run fast despite actually running slow in the observer's frame. His role is to run the the computations and provide a name for the frame, neither task requiring any actual observation.

As for my assertion that say length contraction is not a physical effect, there are examples that can demonstrate it so. Rotation is absolute so via rotation, coordinate effects have physical consequences.  A spinning ring will fit through an identical (*) ring not spinning. That's real contraction and not just coordinate like the barn pole thing is.

Another example is a circular train track packed with cars.  As they pick up speed, more cars will fit in the same track who's circumference is unchanged, despite the fact that relative to any one train car, the track just below it is shorter and one would think that relative to the cars, fewer would fit in the same contracted circle.  Not so.

* Unless really thin (2D), a spinning 3D object cannot be identical to a non-spinning one.

5 minutes ago, KJW said:

No, but I had to dispel the view that if Falcon 2 is more massive from the perspective of Earth then Earth is less massive from the perspective of Falcon 2.

 

But the perspective of Earth was not mentioned.  Just "co-ordinate mass of the Falcon 2 is approaching infinte at 99.99%c", which is true of me now relative to some frame. No fancy ship needed.

But sure, if that speed is relative to some other object, then relative to the ship, it is the moving object (Earth??) that gains coordinate mass.

Gian also implies that acceleration and/or energy is required for something to have a large coordinate speed. This isn't true at all since several examples have been given of Earth moving at nearly c. It's a coordinate effect. Nothing is physically different in such a frame.

2 hours ago, KJW said:

No. If an object is at rest relative to the Falcon 2, then its mass in the frame of reference of the Falcon 2 will be the rest mass of the object.

Note also that in the frame of reference of the Falcon 2, the mass of the Falcon 2 will be the rest mass of the Falcon 2. Also, in the frame of reference of the Falcon 2, the mass of the earth, which is moving at 99.99%c relative to the Falcon 2, will be approaching infinite.

Pretty much that answer, yes, except I don't remember Earth being involved in the question.

I had just chosen a frame where the ship (and Earth too) were moving at .9999c  But yes, in the frame where the Falcon is at rest, its coordinate mass and mass are identical, by definition.

Tidal disruption (the breakup of moons that fall below the Roche limit) is essentially Newtonian physics and has been fairly well understood since the 19th century. It has little if anything to do with gravitational waves and more with physical stress on orbiting objects.

Earth for instance puts out a total energy of about 200 watts in the form of gravitational waves. It has plenty of tidal gradient to say destroy the moon if it ever gets close enough (it will be destroyed before this happens), but that 200 watts will not register on any detector we make.

Yes, an orbiting GW detector will presumably be more sensitive than LIGO, but would lack the redundancy of the multiple GW detectors on Earth unless they orbit several of them. Not sure how much redundancy is needed in space where trucks driving nearby are not going to trigger false positives.

3 hours ago, geordief said:

how can  one measure the difference between them

OK, it does seem to be a measurement topic, and not one of expressability or predictability

Measurement of planets is hard due to the long delay between where it is and where you see it.

1 hour ago, studiot said:

There is a difference between measuring after the event and prediction before the event.

The OP didn't seem to reference prediction

1 hour ago, geordief said:

Is my question ,then connected to entropy?

Your OP seemed to have little to do with entropy. Your card shuffle example was one of a chaotic function, but the deck seems no more entropic before or after the shuffle, as opposed to if you play 52 pickup.

1 hour ago, geordief said:

I wasn't sure where to post really but I often post in relativity  and thought it might be quite  close

This seem to have nothing to do with relativity theory. Choice of coordinate system was necessary even in Newton's physics.

1 hour ago, geordief said:

but I think you may be saying that the Sun's reference  frame "includes" that information

The sun's frame is an accelerating one, more complicated.  I chose an inertial frame, but I didn't compute many numbers in it. Just >180 and 'faces the other way'.

38 minutes ago, studiot said:

You also need a direction, at minimum.

This is true, and you need two of them, not just one.  Given distant stars not in the system, we have that reference. The OP said to ignore the gravitational influence of the rest of the galaxy, but that doesn't mean we don't have external references.  Without it, picking a stable reference is possible (since rotation is absolute), but not as easy.

19 minutes ago, geordief said:

the concept seems to run foul of the  idea that there is no absolute time or space.

But I chose the CoM as the reference. That's not an absolute reference, sure, but the relationships between the planets can be derived from each planet's coordinate relative to that CoM.  Measuring it all is another problem since there's nowhere to be that sees where everything is at a given time since the distances are so large.

Well, you posted this in relativity, so it needs to be stated that you seem to be referencing states at times relative to some inertial frame, say the frame of the center of mass of the collection of objects, which is stable in isolation.

The question seems to be how to express the states 1,2,3

34 minutes ago, geordief said:

I think it is unarguble that changes have happened  but can we measure that change for the system as a whole or  just by comparing individual  components of one system against  individual components  of the other system.

Sure, in state 2 the Earth object has rotated just over 180 degrees and is facing the other way. It has also moved around the solar system just like all the other objects.  I referenced the solar system center of mass, so given that reference, each of the objects has a position relative to that at each of the times 1,2,3.  It's a stable point of reference, so nobody is worrying about saying where Earth is relative to Jupiter since both have moved.

Measuring it is another thing, but measuring doesn't seem to be your question.

 This two-year old topic started with the below comment.  I've not read almost any of it, but I see a lot of word salad that seems unrelated to this original question.

On 12/9/2022 at 2:01 PM, DimaMazin said:

If we want to accelerate very long train,

instantly accelerating each railway carryage into a moving frame at v, then...

1) No rigid object can instantly change its velocity without breaking, per Bell's spaceship scenario.  You can accelerate it over time (finite proper acceleration), and how much time that takes depends on what clock is used to do it.

There are limits. I worked out the minimum time it takes to move a 100 LY rigid train a distance of one light day, being stopped at beginning and end of trip. It takes almost 2 months and cannot be done faster without violating rigidity. It cannot be done at all without applying force to all parts of the object instead of say pulling it with an engine up front.

Now regarding this latest post, forgive me if I am unaware of any context that might help make sense of any of it.

1 hour ago, DimaMazin said:

Let's use velocity u as distance

How can a velocity be treated as a distance? This seems meaningless.

1 hour ago, DimaMazin said:

addition of any velocity, even when it is as nonsimultaneity bigger than c

How does a simultaneity or a nonsimultaneity have a size?  Two events not simultaneous in some frame would have a time difference in time, but that difference would be a time, not a speed. You are seemingly comparing a time to a speed, which is total nonsense.

How is any of the posts (in any of 2024) relevant to the topic?  Is this just one of those blogs left open to keep the forum from filling up with dozens of crazy topics from one user?

1 hour ago, Gian said:

looks like co-ordinate mass is a location

No, not at all. As swansont points out, the term is used to refer to mass issues involving frames of reference.

Mass of any kind is a resistance to acceleration. Coordinate mass is simply a coordinate dependent mass that you've been speaking of:

1 hour ago, Gian said:

in getting to c, mass  of my spaceship becomes infinite

Say your proper (physical) mass is 80 kg and so is your coordinate mass relative to the frame of your shoe.  Relative to the frame of some muon, your mass is still 80 kg but your coordinate mass (some sites call it relativistic mass) would be say 500 kg. Notice that all we did was an abstract change of reference frames (coordinate systems) and there was no requirement for energy or acceleration. Mass is physical and frame independent: it is 80 kg in any frame.  But coordinate mass is a frame dependent abstraction,  ranging from 80 on up to any arbitrary value depending on the coordinate system chosen.

Notice also that no mention of location was made in any of that.

4 hours ago, Gian said:

engines given to me by the Time Lords.

Time lords live in a universe with significantly different laws of physics. Same for Star Wars/Trek.

The problem is that there are two kinds of mass: proper mass (frame invariant) and coordinate mass (frame dependent). Use of the term 'mass' was sort of ambiguous until around 1950 when the term was formally assigned to mean proper mass. But using it to mean coordinate mass (as Einstein did) persists in pop sources to this day, and chatbots will likely still use it since those pop sources are the larger percentage of its training data.

The article is nice, but makes a lot of errors, some of which are just in clarity, but some actually wrong. The wonderland example for instance treats a bicycle as a brick instead of a system with moving parts.

The cycle wheels contract if they rotate, so the spokes would become slack and the bicycle would fail (collapse) if a plodding walking pace was approached.

The article treats speed as absolute, which violates special relativity. You're moving at 0.99c right now relative to some muon.  No fancy acceleration or Time Lord engine required. Do you mass more because of that?  No. Hence their choice of what 'mass' means.

It starts with: "nothing can move faster than [the speed of] light", which should read that it's only true relative to an inertial frame.  For instance, if I shine a light to a reflector on Mars and measure the round trip, it will go faster than c. Not much, but it will, and it is due to spacetime not being flat between here and there, so inertial frames are not applicable.

Reading more, I see SR, 1905 "begins with the astonishing experimental fact that c never changes". This is wrong. It is a premise, not a fact, and to date it has never been experimentally verified.

It says that an object's mass increases with speed, but that's a coordinate effect, not a physical one, so only the coordinate mass thus increases, and they don't correctly say that.

It later says 'ship time is flowing at less than 7/8 the normal rate' which is confusing. Ship time is at 100% per the first postulate of SR. The wording makes it sound like time dilation is a physical effect instead of a coordinate one. Again, does your clock run slow because it is moving at .99c relative to some muon?  No, it doesn't.

Discussion of a more general version of E=mc2 is given by other posters.

The bottom left of the 2nd page says something crazy about a journey to 283 light years away taking just over a year from Earth's point of view. That's totally wrong. It might take that much proper time on the ship, but that's not an Earth observer.

1 hour ago, Gian said:

Am I right that I have to divide the stationary mass by this number to get the mass of the Falcon at 99%c? which gives the Falcon's mass as about 1,430,000kg

The Lorentz factor is almost exactly 7. Some call the 1.4 million kg the relativistic mass but technically the mass of the ship is the frame invariant proper mass of 200,000 kg.

That's just a terminology thing. The ship relativistic momentum goes up to γvm, so you can compute its kinetic energy by multiplying that by v.

Also note that once you've given your ship a specific mass, one has to wonder how it accelerates without losing any of it, which is why most hypothetical scenarios avoid any mention of masses since it introduces so many problems that are not illustrative of the point of the exercise. Not sure how your fusion engine produces thrust. Most fusion reactors only produce heat, and maybe electricity from that, neither of which immediately translates to thrust.

So anyway, unless you are firing this object out of an insanely long rail gun, any calculation of energy usage needs to factor in where the energy is produced and where it is going, and what is left at each point.

1 hour ago, Gian said:

energy needed to shift her at this point,

Don't know what that phrase means. You talk about energy, but then follow it up with a specification of power (so many joules per sec).  If the ship is accelerating at 1g, then it's power consumption is presumably proportional to its current mass, so integration is needed. One also has to factor in energy imparted to the reaction mass, and all that scaled by some kind of efficiency rating. The most efficient engine might be something like an ion drive, but those don't put out enough force for 1g of acceleration.

200 tons is seemingly not enough to provide life support for Joanne for all those years, let alone any left over for trivialities like propulsion. Perhaps it is really good at recycling.

7 hours ago, Gian said:

if I just flew the Millennium Falcon 2 in a big circle by firing the stabiliser jets to keep us on the circular heading

Acceleration in physics is a vector change in velocity over time (and is not a scalar increase in speed, the dictionary definition). That change can only be in one direction, so if you're using lateral jets and still are maintaining 1G of proper acceleration then you're just finding an inefficient way of accelerating at some angle.

Anyway, if you're getting up to .99c in the solar system frame, your circle at 1G would be at minimum far larger in diameter than the distance to AC, and you'd be using your main engines pointed sideways to maintain the turn radius, with no increase in speed.  This would take at least 7 years to get up to this speed linearly and far more if you maintained a circle centered on something like Earth. It would take decades to do one 'orbit', one lap of this huge circle.  Once you 'cruise' and go straight, you'd be heading away from your destination since at no point in that circle are you heading toward it.

Fastest way to get there at fixed acceleration is straight out and back, but getting up to a speed higher than your ability to stop in time is just wasted effort.

7 hours ago, Gian said:

Either that or my gf and me would have to bear switching the acceleration to 2G

It would take at least 3g to get up to .99c and still stop 4 light years away

34 minutes ago, Gian said:

But would there be anything to be gained in travel time ship's time if I first took the Millenium Falcon into orbit around our Sun, accelerating by 1G upto 99%c

You can't orbit anything much faster than its escape velocity at 1G, so no, dropping to the sun requires one to lose energy rather than gain it, which is what you want to get to AC.  OK, technically a ship always loses energy as its fuel drains, but I'm talking about the mechanical energy (potential and kinetic) of the payload.  So orbit of anything is likely just a waste of time since one cannot exceed some low speed while going in circles, at least not if acceleration is confined to 1G.

Suppose the sun could be condensed into a neutron star or black hole. Presuming Joanne doesn't mind a little (a lot) of tidal stress, one could orbit such a thing at super high speed near c but it would take a lot of time to drop into this orbit. And then it gains you nothing getting to AC since all that kinetic energy must be wasted over a long time just to get back to where Earth is orbiting, all the energy being expended not accelerating to AC, but rather just climbing back out of that gravitational well you were in.  When back at Earth, no net speed remains. The trip is no shorter.

6 years 1 way Earth time, 3.58 years on the ship. The figures reflect more the actual distance and not just 4 light years exactly.

If it was 4 light years exactly, it would be 3.46 years on the ship.

56 minutes ago, joigus said:

It seems to be an either cookies, else subscribe wall.

I won't accept such cookies normally, but I pasted the link in an incognito window and it let me in without challenge.

It seems to be a Newtonian calculator and yields 1.18c if I put in 10k hours at 1G

Really, there are very good calculators for relativistic space travel.  One of the best:

https://gregsspacecalculations.blogspot.com/p/blog-page.html

That one presumes fixed proper acceleration, not coordinate acceleration like a Newtonian calculator would use.

1 hour ago, swansont said:

1g acceleration gets you to 0.99c in a little less than a year (earth frame)

That's coordinate acceleration, and 1G of that for almost a year would kill poor Joanne, and it could not continue at all for a full year.

I do realize the OP did not specify explicitly, but 'comfortable' was used, so my figure is based on a comfortable 1G proper acceleration, and that takes almost 2.7 years (ship time) to get to that speed and around 6.8 years Earth time.  Fixed proper acceleration can in principle be kept up indefinitely.

Oh, and your link is behind a paywall, or at least a subscribe wall. I could not view it.

I'm afraid that you cannot get to that speed (relative to Earth) in only half the distance to your destination (at least not at 1G). Perhaps a target further away like Tau Ceti, which is almost exactly how far you'd get if you got to .99c and immediately started slowing.

You'd be back home in 27 years 2.3 months.  To Alpha Centauri and back you'd be gone only 12 Earth years, but would only reach 0.95c

Tell Joanne to skip most of the luggage and have laundry facilities onboard.

Thank you for ignoring everything that matters and finding something to nitpick about.

5 hours ago, studiot said:

I see no reference to the flashes beeing separated by "negligable distance"

So do you have a reference for this claim ?

Says the guy who doesn't back his claims.

I did not claim it was a quote. Throughout the small paper, each lightning strike is treated as a single event in two-dimensional spacetime. Were that not the case, the entire conclusion would be invalid.

You also only showed the first page, just before he describes the relevant bits.

So I had a long car ride today and used 20 minutes of mental free time to consider the case of a 100m (proper len) ship passing really close to the observer.  How big would it appear when going by at 0.8c?  Turns out it appears larger. The observer would measure 165 meters in the photo he takes when the center of the ship passes him, and if he condescends to compute anything more, he can deduce 100m proper length from that.

The middle of the ship (moving left to right) is directly in front of the camera as specified. The front half of it appears to extend 15m to the right and the rear half 150m to the left. Mind you, this is what the photo shows, not where the ship actually is when the photo is taken. The photo cannot show where the ship actually is since the camera cannot be everywhere present along its length simultaneously. The right kind of camera can, but we're using a wide angle camera which constitutes a single point of view (as does any telescope). We presume it takes a 360 degree image of everything with sufficient resolution to see what we need to.

The further away the camera is from the middle, the longer the front appears and the shorter the rear appears, approaching 30m each if the camera is infinitely distant. That's why I put my camera a light year away in my earlier example, but during the car ride I wanted to work out the limit when the ship passed arbitrarily close.

So your camera was quite close, and you claim it shows a 60m ship. You need to justify that claim with a description of how that is done without positing a magic device that just somehow knows. Use mathematics. I can do that with mine, although I have not yet done so since I did it all in my head.

I used an ship with length 8 (natural units) and then scaled that up to our example of 100m.

From a light year away, the photo shows the name of the ship printed on the rear of the rectangular prism shaped thing.  From a photo taken nearby, this name is not visible.  I think the camera needs to be at least 23 meters away for the name to be in the photo, if I computed that correctly. None of this paragraph is relevant to determining the ship's proper length except to illustrate that the photo will show a distorted image, making the task not entirely trivial.