Wikipedia:Reference desk/Archives/Science/2020 June 21

= June 21 =

"Pretty" the flower
File:Pretty the flower.jpg. Since the team here are so good at this...This plant started growing in my garden, from the foliage I initially thought it would become a bluebell. It then produced this flower which has finally opened this morning. Does anyone know what it is please? My child has been watering it and has named it "Pretty". Thanks — Preceding unsigned comment added by Anton.Brad (talk • contribs) 10:13, 21 June 2020 (UTC)
 * I'm having trouble displaying the picture I uploaded to commons. Any help would be appreciated. Thanks  — Preceding unsigned comment added by Anton.Brad (talk • contribs) 10:40, 21 June 2020 (UTC)
 * You mean this? ←Baseball Bugs What's up, Doc? carrots→ 10:56, 21 June 2020 (UTC)



It's a lily (Lilium), probably an Asiatic hybrid, though there are very many varieties of those. This is the google images page for Asiatic hybrids. PaleCloudedWhite (talk) 11:02, 21 June 2020 (UTC) The variety 'Abbeville's Pride' looks similar (especially on the third image down on that link). It is also quite a short variety (your pic looks like a short variety, though the shot angle makes it a bit difficult to gauge). PaleCloudedWhite (talk) 11:17, 21 June 2020 (UTC)
 * Thank you very much all. This has been resolved. — Preceding unsigned comment added by Anton.Brad (talk • contribs) 11:39, 21 June 2020 (UTC)

Time travel
The Earth is always in motion around the Sun, which in turn goes around the galaxy, which in turn is moving. If I go forward or backward in time by one minute to my present location, I should be inside Earth or in outer space. I made further comments on the Time Travel talk page before being referred here. Are there any discussions of this locational displacement problem in time travel discussions? I came to Wikipedia hoping to find a sentence or two about this. Geographyinitiative (talk) 21:08, 21 June 2020 (UTC)


 * Moving relative to what? You appear to be assuming that [A] the frame of reference where the star field is, on average, fixed is the frame of reference that a time machine will use, and [B] that the time machine only changes the time, never the location or velocity. Because time machines only exist in Science Fiction, we have no way of knowing whether your assumptions are true or false. To further confuse things, time is not a constant when discussing two observers with different locations and velocities relative to each other. --Guy Macon (talk) 21:16, 21 June 2020 (UTC)
 * Thanks for your response. Moving relative to other galaxies and stars and planets. I am sitting 'motionless' in my room, but I am not where I was a minute ago. If I went back in time to 500 BC right now, I would be outside the solar system if I didn't move faster than light to the position of the earth at that time, right? The point is, the time travel sci fi doesn't seem to delve into the locational shifting that would be required to visit ancient or future Earth: ancient and future Earth are not 'here'. Tell me if I am still getting it wrong, but I think this would be mentioned in philosiphocal or scientific discussions about time travel, no? So that's what I want to see some info about on Wikipedia. Geographyinitiative (talk) 23:10, 21 June 2020 (UTC)
 * I know I've seen this as a plot point in some time travel stories, but most stories just assume that your motion through time includes corrections for such spatial movement. The T.A.R.D.I.S. from Dr. Who includes such spatial adjustments in its very name. --Khajidha (talk) 23:22, 21 June 2020 (UTC)
 * Yes, if this is a question about science fiction, then it's a matter of what the author decides for story purposes. But if that's what you want then it's off-topic for the science desk. --76.71.5.208 (talk) 23:29, 21 June 2020 (UTC)


 * Re: "If I went back in time to 500 BC right now, I would be outside the solar system if I didn't move faster than light to the position of the earth at that time, right?", wrong. Again you are ignoring relativity and assuming one particular frame of reference. Maybe the time machine uses the earth as a frame of reference. You don't know.


 * Re: "The point is, the time travel sci fi doesn't seem to delve into the locational shifting that would be required to visit ancient or future Earth: ancient and future Earth are not 'here'." Wrong again, for the same reason. You say "would be required" as if you knew what would be required. You don't.


 * Re: "Tell me if I am still getting it wrong", you are getting it wrong. You think you know aspects and limitations of an invention that does not exist and may never exist. You don't.


 * An author can use any assumptions he wishes. For example. Larry Niven assumes that his transfer booths change location but not time or velocity and further assumes an advanced technology that can compensate for small velocity changes but not large ones. Nothing in the laws of physics supports any of that, but it was convenient for the plot to assume that you can use the transfer booth to go from your job in San Fransisco to your home in the rocky mountains, but would still need a ship to go to mars or the asteroid belt. --Guy Macon (talk) 00:01, 22 June 2020 (UTC)


 * Since all backwards time travel stories are fictional, the universe in which they operate is also fictional. For example, that universe could be geocentric. ←Baseball Bugs What's up, Doc? carrots→ 02:48, 22 June 2020 (UTC)


 * "People need to be aware that there is a range of models that could explain the observations… For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations… You can only exclude it on philosophical grounds… What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that." - George F. R. Ellis, Scientific American, October 1995, Vol. 273, No.4, p. 55


 * "I have had your newspaper on my desk for weeks, hoping to find time to write you. You say in your headline (Fall), 'Six Physicists Say it is Pos­sible' that the earth stands still.  I don’t know who your physicists are, but the situation is much simpler than they seem to think.  It is not just possible, it is a fact.


 * It is also a fact that earth does not stand still. The fact that makes facts of those two apparently conflicting statements is that, as Einstein said, there are no milestones in the Universe, and thus no absolute stan­dard of rest or motion that makes such categorical statements mutually exclusive."


 * --David Park
 * Professor of Physics, Thompsonville Physical Laboratory
 * Williams College, Williamstown, Massachusetts


 * Source:--Guy Macon (talk) 06:33, 22 June 2020 (UTC)


 * Wow! I don't know if my original thoughts hold up, but my original question is not as important as what I have just seen in these quotations. I am sure that the quotations in the comment above this post need to be put somewhere in Wikipedia for the layman readers like me to see. What would you all say is the Wikipedia article that a reader would turn to to understand that physicists can and do take a point of view in which the Earth is both without motion and in motion at once? I know you can say "it's an innate fact so it goes on every page and on no page simultaneously", but I would like to really put these quotes in an appropriate spot on Wikipedia or at least create a sentence that uses them as a source. It seems like a significant and interesting viewpoint that readers ought to be made directly aware of. Geographyinitiative (talk) 07:18, 22 June 2020 (UTC)


 * "Look at that beautiful sunset!"
 * "Actually, the horizon is moving up."
 * --Guy Macon (talk) 08:53, 22 June 2020 (UTC)


 * Perhaps you are seeking Preferred frame and Absolute space and time. But you may be reading too much into this.  Ellis's "philosophical grounds" are not esoteric ones, but ones of parsimony.  Choose a non-inertial reference frame and your laws of motion become more complicated with the addition of fictitious forces. -- ToE 09:55, 22 June 2020 (UTC)


 * We also have Geocentric model. --Lambiam 10:08, 22 June 2020 (UTC)

Time and space are one and the same concept. By travelling in time you would also be travelling in space. You would move relative to your reference point...you would remain in your room, but be there 5 minutes before. — Preceding unsigned comment added by Anton.Brad (talk • contribs) 09:16, 22 June 2020 (UTC)

Johnny Alpha killed Hitler by using time bombs to shunt him a few minutes into the future, to when the Earth had moved, leaving him stranded in space. Iapetus (talk) 09:47, 22 June 2020 (UTC)

The thing is, "time travel" in fiction is generally a magical plot device. The creator is crafting a story, not taking you through a solution of the Einstein field equations. To look at some practical aspects, you and I and the whole Earth and everything orbiting it are in orbit around the Sun; from a Sun-centered frame of reference, we're in constant freefall around it. Changing our trajectory requires doing work, which takes energy. Any more-towards-the-hard-side science fiction will often at least take a stab at grounding things in realistic-sounding physics. "Time travel", described in terms of special relativity, means changing your spacetime four-vector relative to another observer. This involves, again, doing work. Relatively (*rimshot*) small changes don't take much; indeed, you and I do them all the time whenever we move around anywhere. But of course these changes are so incredibly tiny that we never notice. Bigger changes require more energy, lots more. "Time travel" of decades or more relative to a fixed Earth-bound observer requires so much energy that, well, here's the problem. If you can, at-will, "time travel" this much, you have access to so much energy that you are basically a god. You could turn Earth into vapor with a button press, using a relativistic kill vehicle. You could, if able to control and apply the energy more precisely, disassemble the Solar System except something (or someone) you choose, and reassemble it in a slightly different position. The obvious question then presents: if a character has such astounding power, why do they only use it to change their and/or others' worldlines? Of course the obvious "Doylist" answer is that it would totally wreck the plot. ("Why doesn't the Joker just shoot Batman?") And that's why it's called a "plot device"; its purpose in the story is to develop the plot. Real "time travel" forwards is perfectly possible and breaks no physical laws: just accelerate a lot, up to relativistic speeds. When you swing back around to Earth, you'll find that less time has elapsed on your clock compared to those who stayed on Earth, due to time dilation.

Fun related fact: which takes more energy? Shooting something from Earth into the Sun, or out of the Solar System? It's the first one. To get something to hit the Sun, as opposed to just kicking it into a different orbit around the Sun, requires cancelling out all the existing orbital motion that it has around the Sun. You have to bring it to a full stop, in a Sun-centered reference frame, so it falls into the Sun instead of continuing to fall around it. And remember that there's no friction in space; you can't just "hit the brakes". By contrast, kicking it out of the Solar System just requires adding an additional kick to speed it up more until it reaches the Sun's escape velocity, so that the Sun's gravity fails to pull it back. --47.146.63.87 (talk) 16:47, 22 June 2020 (UTC)
 * Could not a swing-by be used to use less energy in a trip destination Sun? --Lambiam 21:30, 22 June 2020 (UTC)
 * It doesn't change the total amount of energy "used up". There's no free lunch. What a gravity assist does is "recruit" a planet or other celestial body to do some of the work, which in turn means you don't have to carry the energy yourself as fuel. To decelerate, the craft flies "in front" of the planet's orbital path, which pulls the planet towards it, accelerating the planet's orbit and decelerating the craft's. And indeed, the Parker Solar Probe has been doing exactly this, making repeated flybys of Venus to decelerate so the probe can get into an orbit close to the Sun. Same principle, the probe just wants to get into a tight solar orbit rather than actually hit it. --47.146.63.87 (talk) 22:08, 22 June 2020 (UTC)
 * If you reason that way, then I say that by the Law of conservation of energy neither shot requires any energy. --Lambiam 06:40, 23 June 2020 (UTC)
 * True. For a particular course change either you put a lot of energy into changing the velocity of whatever comes out of the exhaust nozzle a lot, or you put the same amount of energy into changing the velocity of the planet you slingshot about a little. Either way everything balances out and both total energy and total momentum are conserved. You do save fuel and reaction mass though. --Guy Macon (talk) 07:11, 23 June 2020 (UTC)