Treatment of Relativity in "Las Leyes de la Termodinámica" Movie

[Nantes]

TL;DR Summary

A film that blends romance and physics in a very interesting way poses a rather weird relativistic consideration.

Background: There is a very interesting Catalunyan film on Netflix called "Las Leyes de la Termodinámica" (The Laws of Thermodynamics) which is perhaps the world's first hybrid of a physics documentary and a romance film. The main character is a physics professor who falls in love and attempts to explain everything that happens in his love life through the optics of the laws of physics. It is smattered with commentary from actual physicists as well. Very interesting movie that everyone here should watch!

Discussion: There is a sequence beginning at 54:30 where the characters are in a Gay Pride parade and one of them is dancing on top of a bus. The bus stops, but suddenly has to leave again, which makes the guy lose balance and fall. The film discusses the "point of view paradox" of how two observers see the same event differently: people on top of the bus see him falling in a straight line, whilst people on the ground see him falling in a parabola, since the bus is moving. Because the parabola is necessarily longer than the straight line, the people on the ground see him "falling faster" than the people on the bus.

Then the film states: "The two observers have to see exactly the same event, because the speed of light is constant. And this is exactly what Einstein realized. Einstein proved that not only space contracts and stretches depending on the position of the observer, but time itself is also relative".

The film's statement that the guy is "falling faster" for the people on the ground is a bit confusing to me. He's only observed as having a horizontal velocity component, which is not perceived by people on the bus, but the vertical component of the velocity in both situations is the same, no? Shouldn't the vertical component be the only one that counts in this scenario?

Or maybe the film is alluding to the fact that light would take different amounts of time to reach each observer's eyes because, for a given person on the ground, he's getting farther or closer away with each second. But the rays of light that reach both observers are not the same, so there is no reason in my mind why they would have to match up. Their emissions are separate occurences to me, so it is fine that light takes a little longer to reach one than the other.

Background on me: as you can tell, I'm not a physicist, I just have considerable interest in it and in science in general, and probably more basic knowledge on relativity on than your average joe.

 

[Halc]

Because the parabola is necessarily longer than the straight line, the people on the ground see him "falling faster" than the people on the bus.

He faster relative to the people on the ground because he has higher speed in that frame, so he moves further in a second than in the bus frame. The parabolic path has nothing to do with that since I can come up with a scenario where the shorter path is the parabolic one.

Then the film states: "The two observers have to see exactly the same event, because the speed of light is constant.

That's nonsense. They'd have seen the same event even if speed of light wasn't constant. The situation illustrates Galilean principle of relativity, but not Einstein's special relativity, which follows from constant light speed. There's no illustration of length contraction, dilation, clock issues, or whatever in the guy on the bus thingy.

He's only observed as having a horizontal velocity component, which is not perceived by people on the bus, but the vertical component of the velocity in both situations is the same, no?

Under Galilean relativity, yes, the vertical component is the same in both frames. If the bus was moving at relativistic speeds, that would not be the case.

Or maybe the film is alluding to the fact that light would take different amounts of time to reach each observer's eyes because, for a given person on the ground, he's getting farther or closer away with each second. But the rays of light that reach both observers are not the same, so there is no reason in my mind why they would have to match up. Their emissions are separate occurences to me, so it is fine that light takes a little longer to reach one than the other.

They don't seem to be illustrating that with this scenario since they're not taking into account nanosecond differences in delays due to differing distances.

 

[PeterDonis]

Shouldn't the vertical component be the only one that counts in this scenario?

It depends on what you're interested in. If you're interested in the downward acceleration of the person relative to the ground, then the vertical component is the only relevant one. But if you're interested in the overall speed of the person relative to the ground or the bus, both components are relevant.

Their emissions are separate occurences

The emission of light rays from the falling person at a particular instant of his fall is the same for everyone. But light rays emitted at that particular instant that go to different observers will arrive in different places (since the observers are in different places) and can take different times to arrive.

 

[PeterDonis]

Then the film states: "The two observers have to see exactly the same event, because the speed of light is constant. And this is exactly what Einstein realized. Einstein proved that not only space contracts and stretches depending on the position of the observer, but time itself is also relative".

This seems fairly garbled to me, and does not bode well for any claim on the film's part to be "part physics documentary". Overall this looks to me like a good illustration of why it's better to learn physics from actual textbooks.

 

[Ibix]

The film's statement that the guy is "falling faster" for the people on the ground is a bit confusing to me. He's only observed as having a horizontal velocity component, which is not perceived by people on the bus, but the vertical component of the velocity in both situations is the same, no? Shouldn't the vertical component be the only one that counts in this scenario?

His vertical speed is the same for everybody in Newtonian physics. In relativistic physics it is lower for observers on the ground than on the bus. In both Newtonian and relativistic physics his total speed is higher as seen from the ground since he has a horizontal velocity component. So he's moving faster, but whether you regard that as falling faster or not is a matter of semantics.

Then the film states: "The two observers have to see exactly the same event, because the speed of light is constant. And this is exactly what Einstein realized. Einstein proved that not only space contracts and stretches depending on the position of the observer, but time itself is also relative".

This is just bizarre. Two observers have to see the same event because only one thing happened. Their descriptions will differ though, and relativity allows for some more subtle and complex differences than Newtonian physics, but the fundamental assumption that "only one thing happened, we just describe it differently" underpins all of science.

Also "space contracts and stretches" is a terrible description of anything in relativity. It's somewhere between "badly confused" and "wrong".

I have to agree with Peter - you really need a textbook to learn physics.

 

[A.T.]

The situation illustrates Galilean principle of relativity, but not Einstein's special relativity, which follows from constant light speed. There's no illustration of length contraction, dilation, clock issues, or whatever in the guy on the bus thingy.

Mainly this.

But also: What is that "m/sg" unit?

 

[Ibix]

But also: What is that "m/sg" unit?

Second is segon in Catalan and segundo in Castilian. "Sg" is an informal abbreviation in Castilian (and I guess in Catalan) analogous to "sec" in English. So "m/sg" would translate as "m/sec", which isn't good (they should use SI standard symbols if they're going to use SI units) but isn't as terrible as the rest of it.

 

[Nantes]

This seems fairly garbled to me, and does not bode well for any claim on the film's part to be "part physics documentary". Overall this looks to me like a good illustration of why it's better to learn physics from actual textbooks.

I have to agree with Peter - you really need a textbook to learn physics.

Haha, of course you do, but it's still amusing to see how the basic physics explanations line up with and are used to explain what is going on romantically in the movie. For example, there is a segment that connects the character's gradually failing relationship and how they gradually fall out of love for each other with the concept of entropy and how useful energy in a closed system inevitably gets lower over time, and how it's hard to put the pieces back together once something breaks.

It's a movie, for goshsakes. I don't think it's meant to teach physics for someone new to it, but to amuse people who already know it (and possibly to exasperate people who really know it :P) .

 

[David Lewis]

Point taken but the explanations still have to be rigorous and scientifically correct even when you are learning for amusement, entertainment, or just the joy of knowing.

 

[PeterDonis]

It's a movie, for goshsakes.

Discussions of movies as movies belong in the General Discussion forum, not the relativity forum.

Since we appear to have answered the only actual physics question that was raised, this thread is closed.