In the Back to the Future universe, we know what is needed for time travel: a flux capacitor, 1.21 gigawatts and a travel speed of 88 miles per hour (I know there are more requirements than that but I do not want to get bogged down by tiny details).  If any one of these three things is absent, per the story's logic, time travel will not occur.  Here is my problem.  Doc was hovering in the DeLorean when it was struck by lightning, sending him back to 1885.  How?  He was not traveling 88 mph, he was traveling 0mph. Did the filmmakers just overlook this glaringly obvious problem, or is there an in-universe explanation?
121946
At the end of Back to the Future II, a lightning bolt hits the DeLorean which sends it back to 1885.  At the time though, the DeLorean wasn't moving; it was relatively stationary hovering in the storm.  Yet, it's emphasised repeatedly that the DeLorean must reach 88 miles per hour (which becomes a key plot point in Back to the Future III).  So, considering the DeLorean wasn't traveling at 88mph when the lightning struck the DeLorean sending it to 1885, what is the in-universe explanation of how it travelled there?  Some other factors to bear in mind:   If a lightning bolt was sufficient to send the DeLorean time traveling, then the 88mph business would be irrelevant Note that there are the firey tracks left after the DeLorean time travels in the '66' shape.  The thing to note about this though is that we never see these before the DeLorean disappears.  These tracks are always appearing after the DeLorean has traveled away.  This would seem to indicate that the DeLorean did move, but following the activation of the time circuits.
100151
Hypothetically, if you were to travel say, 6 months into the past with no spatial displacement, you would end up floating in space because the Earth would be on the opposite side of the sun.  I know when it comes to sci-fi, they can't account for everything and I don't expect them to. So I'm not nit-picking or anything, I'm just wondering if this is something that has been addressed to any detail, as it seems like it would have to be a major component of any time travel.
82411
I'm talking about this nonsense:   Now, I'm aware that there are problems with the practicalities (or possibilities) of methods involving exotic matter with negative mass, and that kind of thing has been addressed here:   But in this case my question is different and more specific. Sweeping all the other problems of constructing such a beast under the rug, wouldn't this violate causality?  SR says that the simultaneity and relative ordering of events is dependent on the relative motion of the observer, but that no matter how it works out, if event A causes event B, no observer will see B first. One of the many problems with FTL travel (or signals) is that if any information is sent faster than light, there will exist an inertial frame in which the signal arrives before it is sent, a blatant violation of causality. (I think we can all agree, can we not, that the threshold should be extremely high before breaking assumptions that we're in a causal universe?)  So how is any scheme to travel faster than light, even if it may locally satisfy the equations of GR, not a blatant violation causality according to SR?  Is my understanding of GR incomplete and there's something in it that obviates the causality implications of SR? Or am I correct that FTL claims should be dismissed with extreme prejudice on this basis alone? (Pending a serious replicable experiment showing causality to be breakable.)
108355
I think it's well established that for the basic premise in the Back to the Future Trilogy to work, we have a system of alternate timelines.  If this is the case though, then why would Marty in Back to the Future (the original) begin to fade?  Logically, there would simply be no Marty in the alternate timeline created when Marty travels back to 1955 for the first time; why should this affect the original Marty from the original timeline?
95455
"Now we turn our attention to what happens in $S'$, in which the particle is at rest and the wire is running past (toward the left in the figure) with the speed $v$. The positive charges moving with the wire will make some magnetic field $\ B'$ at the particle. But the particle is now at $rest$, so there is no magnetic force on it! If there is no magnetic force on the particle, it must come from an electric field. It must be that the moving wire has produced an electric field. But it can do that only if it appears $charged$-it must be that a neutral wire with a current appears to be charged when set in motion."  So i got this from Feynman's lectures in physics volume 2 pg 13-7 and 13-8. I find it hard to believe that the particle experiences no magnetic force due to the moving wire although it's stationary. Since this is classical and not quantum electrodynamics if we assume the charged particle to have inertia, then by Newton's second law, the particle will remain at rest until acted upon by an external force, which i assume is the $magnetic$ $wind$ created by the moving wire past the stationary charged particle. Could someone please clarify??
79363
I'm in trouble with time dilation: Suppose that there's two people on the Earth (A,B), they are twins and each other has a clock. (So they are at the same reference frame). B travels in a spaceship and is orbiting around the Earth, as B's speed has increased there's some time dilation. So it's supposed that if A "looks" to B, B's clock will run slower than A's, and viceversa. And if one day B decides come back to the Earth, he will be younger than his twin and the hour of his clock would be different (earlier than A's clock) because of time dilation.  My first question is, why each person see the other's clock as the slow one? (In fact, I found some information that may solved it but I'm not sure if it's the right answer, and if it is, I don't understand it. -> It's because everyone thinks about himself as the reference frame. In other words, that for B, he is the stationary and who moves is A, so he sees A's clock run slower as B is "stopped" and the same for A. B is who is moving and A the stationary.)  The other question I have is that if it's true that each one sees the other as the slow one there's something that i missed out. We all agree that B (after going to the space and came back to the Earth) he is younger than his brother. So, why B don't see A moving and aging very fast if A see his brother and his things going very slow? Let's imagine they were looking to each other all the time, if both of them see the other going slowly once they meet and "discover" B is the young one... how is that possible?  [I think that in one episode of Cosmos of Carl Sagan (though may be I'm mistaken) he said that a neutrino "borned" in the Big Bang could have seen the creation of the Universe until today in few seconds due to its high speed, here starts my doubts: or i misunderstood something or there's contradictory information]  (I don't know anything about physics, only what is taught at high school as I'm a teenager so it'd be better to answer with no kind of calculation as I'd not understand it.)
62532
In Back to the Future Part II, Doc picks up Marty in 1985 and takes him to 2015. Then old Biff realizes the Delorean is a time machine. Old Biff steals the Delorean and travels from 2015 to 1955 to give the 1955 version of himself the 2015 sports almanac.   When old Biff then returned to 2015 from 1955, shouldn't he have arrived in a way-different 2015? Instead, he arrives in the same 2015 that he departed, where Doc and Marty are still running around as if nothing changed.  Here's pic of Doc Brown in BTTF2 explaining how Old Biff created new timeline:    I think this is a problem created by perspective. When Old Biff takes the Delorean back in time, we viewers do not go along for the ride. We're left hanging in a 2015 that becomes obsolete when Old Biff changes 1955. Seems our 2015 has two options: continue to exist as an alternate to the 2015 Old Biff created, or cease to exist. Either way, still seems our 2015 would be unreachable by Old Biff and the Delorean on their return from 1955.
5709
With the use of a time turner, that opens a whole "Pandora's Box" of possibilities.  While the use of one can be problematical, when the Ministry of Magic found out how dangerous Voldemort was, why didn't they simply use a time turner to go back and stop him before he became dangerous?  (I know this does open a lot of plot holes, but witches and wizards don't seem to worry about laws of science, so it would seem strange that magic that seems to easily supersede laws of physics (like brooms that fly without propulsion) would worry about the problematical issues of time travel.)
99221
If the gravity well steepens to infinity at the point of a singularity, then so does the "time well".  Understandingly, the inward falling energy/matter is not aware of this change in time relative to the "outside" universe, locked as it is in its local reference.  How does the inward falling energy/matter ever "physically" reach the singularity?  It seems like the man counting to 100, going half way each time.  He gets closer and closer, but never actually arrives 100% there.
15048
In , Arthur, Ford, Zaphod, Trillian, and Marvin have just stolen Hotblack Desiato's stunt ship after leaving Milliways and traveled into the past 2 million years before their own time.      ”Trouble is,” said Ford, ”that the one instrument in this whole ship that   is giving any reading is worrying me. If it is what I think it is, and if it’s   saying what I think it’s saying, then we’ve already gone too far back into   the past. Maybe as much as two million years before our own time.”   As they realize that this ship is about to crash into the sun, they discover a teleportion device (their only means of escape) and leave Marvin behind to control it so they can escape despite the fact they have no idea where they might end up. Arthur and Ford end up on the Golgafrincham ship which shortly thereafter crash landed on pre-historic Earth (i.e. roughly 2 million B.C.), and Zaphod and Trillian end up back on The Heart of Gold.      ”I trust you had a pleasant meal?” said Zarniwoop to Zaphod and Trillian   as they rematerialized on the bridge of the starship Heart of Gold and lay   panting on the floor.   As I understand from the beginning of the book, at this point the Heart of Gold had been left with Zarniwoop alone on the ship in Arthur's "own time" (in the 1980's). There doesn't really seem to be any explanation as to why the teleportion device sent Zaphod and Trillian 2 millions years into the future as far as I can tell.   So, am I misinterpreting something to the story and the gang really left The Heart of Gold 2 million years before their own time, or did the teleportion device also act as a time machine when Zaphod and Trillian stepped into it? Or perhaps did Zarniwoop have The Heart of Gold travel back in time?
17875
I've always been a little confused by the fact that when a crew member dies they don't just use the transporter history to bring that person back. I'm aware that this wouldn't make for good TV, but I was wondering if there was a more technical reason.
41101
In the last scene of the movie,       Leo gets into the pod and returns back to the electromagnetic field.  He is pushed back in time - the display of the pod starts somewhere  around 2700 and reaches somewhere around 2100. (The original time when  Leo left earth was 2029) The past earth seems to be now inhabited by  Apes and somehow Thade is considered a hero in this world.   How the heck did this happen? What is the meaning of this ending?
55533
I just watched the movie  and started pondering time travel.  While pontificating and stretching my limited remembrance of AP Physics, science documentaries, and YouTube videos I remembered the laws of Conservation of Energy and Mass.  Would that not make time travel impossible?  You can't send something elsewhere in time as you would be destroying energy and/or mass, by removing it from the current time.  And it would be impossible to arrive elsewhere in time, as you would be creating energy and/or mass.  I know this is theoretical and a real answer can't be found till someone invents a time machine.  I'm just curious as to what people think of that.
99997
So, I just saw  by Veritasium that discusses how it's impossible to measure the one-way speed of light from a light source to a detector, since it's impossible to synchronize clocks in a fashion that would prevent a directional difference in the speed of light from altering clock speeds via relativistic time dilation in such a fashion that it would conceal directional differences in the speed of light. As a result, it's only possible to measure the two-way speed of light, where light passes from a source to a reflector and then back to a detector located at the light source, and it's impossible to rule out a directional difference in the speed of light, which could be as extreme as light traveling at c/2 in one direction, and instantaneously in the other. However, Veritasium goes on to briefly mention that if there is true, we would observe stars in the direction of the instantaneous speed of light in real time, rather than through a time delay of hundreds or thousands of years. During his papers on relativity, Einstein noted this possibility, and assumed that it traveled at equal speeds in all directions; as a result, this assumption has apparently been called the &quot;Einstein Synchronization Convention&quot;. This suggests to me that if the Einstein Synchronization Convention is untrue, and there is a directional difference in the speed of light, we should be able to notice this through astronomy; if the light takes less time to travel to us from some directions than others, we should see older stars and galaxies in the directions where light travels more quickly than in the directions where it travels more slowly, since we'll be observing them as being closer to &quot;real time&quot;. For instance, if you had galaxies 3 billion light years away from us, but it took light 2 billion years to reach us in one direction and 4 billion years to reach us in the other direction, you'd expect the galaxies in the 2 billion year direction to look 2 billion years older than the galaxies in the 4 billion year direction. Is this correct, or would a variation in the speed of light invalidate our methods of measuring distance and/or the age of celestial objects? Have there been any astronomical studies investigating this, and if so, what have they found? To be clear, I am specifically asking about detecting directional variations of the one-way speed of light using astronomy. I don’t want answers that discuss variations of the speed of light due to the velocity of their emitters (which was disproved by experiments prior to Einstein). I don’t want answers talking about clocks or simultaneity, since they’re not really directly relevant to this question.
37767
Temporal Agents were supposed to keep eyes on timeline. Whenever timeline was altered and Temporal Agents failed to prevent it, it was due to advanced tech of other factions of Temporal Cold War.  Narada was powerful in front of 24th century tech &amp; past, but not in front of 31st century tech. And, it had no Temporal tech (that's why Nero didn't try to return back to 24th century). So, there was no way for them to escape detection etc.  Why didn't Temporal Agents stop Nero?
87563
The  of March 01, 2013 put it's distance at 14 hrs 04 mins 23 secs of light-travel time from Earth. A  (earlier today) tweet says it is 14 hrs 04 mins 22 secs of light-travel time from Earth. An  tweet put it still further out at 14 hrs 04 mins 25 secs of light-travel time.  The Voyager 2 tweets on Voyager 1's proximity to Earth too has reduced similarly to  earlier today from  reported on Feb 28 2013  It could quite simply be a typo ... If not a typo then my curiosity is piqued; I was under the impression the Voyagers are outbound from the Solar System now without any major in-system activities now.   How is it the Voyagers are closer to Earth than before?
67358
I am not a physicist, however, when Barry is running/flashing, he gets to talk to the team back in HQ - How?  When he talks back to them, I have a feeling, his voice needs to be distorted, even if not possible, as everything is ultra fast with him.  Isn't there some law in physics, that the movie is breaking, by allowing him to talk normally (vocal cords, breathing) while moving at 1000's steps per seconds (or whatever?)