Science & Technology

Don’t believe the critics, of course there is gravity in space

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The latest Star Wars epic hit cinemas last week and its reception has been somewhat mixed. One particular scene has been garnering more undeserved criticism and scorn than others. During the course of the film, the Rebels lead a daring bombing-raid is space. The criticism being, how can one ‘drop’ a bomb in space when there is ‘no gravity’ or when the ships are in so-called zero gravity? As one self confessed ‘Star Wars Fanatic’ wrote in the Sun newspaper “[s]ome Star Wars fanatics also got worked up about the opening scene, where we see a Resistance bomber dropping its payload to destroy a Dreadnaught…. how did the bombs fall in space, where there is no gravity, and how did the gunner, Paige Tico, survive opening the bomb doors to unleash the payload?”

Website, the Gamer, went as far as to name this the seventh worst error in the movie: “This whole space battle is happening in, well, space. And in space, there is no gravity. So how are bombs being released from bombers? Even if the door opens and the bombs are let loose, with no gravity to pull them down, they would just hover uselessly over the Dreadnought. It’s a cool moment, don’t get me wrong. But that still doesn’t mean it makes sense.”

Clearly, it is a common belief that there is no gravity in space, but what creates this misconception?

We all know that Hollywood films don’t have great track records when it comes to accurately representing science on the silver screen. Even movies that claim the title of science-fiction are most explicitly the latter and not the former. The key to tackling this lackadaisical attitude to actual science is the suspension of disbelief. There is no doubt that this is much easier to do in films with a completely fantastical setting than films which attempt to be based in the ‘real-world’. A good film establishes its tone early on, and as a result, we, as the audience, enter into an unspoken pact with the film-maker about to what level we must suspend our disbelief. This is the reason that we are able to forgive the ‘Asteroid-field chase’ scene in ‘Empire Strikes Back’, but we can’t forgive the statement about ‘mutating neutrinos’ in Roland Emmerich’s pot-boiler enviro-disaster movie ‘2012’. Sure, asteroids in collections such as fields are extremely well-spaced, thousands of miles apart, but we ignore this because it’s well established that Star Wars takes place in a magical universe where the laws of physics do not apply. Conversely, 2012 establishes itself in a world similar to ours, but with better teeth, thus we can’t forgive a particle physicist applying a biological phenomenon to neutrinos; a fundamental particle much smaller than a gene!

For the sake of completeness, in what follows I’ll refer to gravity as a ‘force’ when the theory of General Relativity showed that gravity should be considered as a distortion of space-time. The Newtonian description of gravity suffices for what follows.

So if there is gravity in space, why do astronauts float?

Probably the main reason why many people believe there is no gravity in space is that they have seen images, like the one above, of astronauts in orbit, floating, and heard the misleading term ‘zero-gravity’. Surely, the ability to hover mid-air implies not being under the influence of gravity, which most people view as making objects ‘fall’.

Imagine Ian and Astra are both in an elevator, one is on Earth and the cable has snapped and they are hurtling to their doom. The other is in orbit around the Earth in free-fall. Both have a ball and release it, finding that it does not accelerate to the floor. There is no way to conclude from inside the elevator which one is in which predicament.

Now imagine that the Earth-based elevator emergency brakes activate and at the same time the other elevator’s rocket boosters activate. The Earth-bound elevator is still under normal gravity, whilst the space elevator accelerates upwards. Can Ian and Astra determine which one escaped which fate?

Now the released ball doesn’t float, it falls, but is it falling because of gravitational force or because acceleration is pushing the floor up to meet it? There’s still no way to tell who is where. All this led Einstein to conclude that acceleration and gravity are indistinguishable.

Imagine another astronaut, Stella, orbiting Earth in her rather crude looking ship. This time Stella is climbing a ladder in their ship when they slip. Instead of falling she hangs in mid-air.

But the situation is different if the ship is accelerating. Hearing of Stella’s ladder mishap, Bruce intends to replicate it. Unfortunately, he doesn’t check if his ship’s rockets are firing and it is thus accelerating first.

The truth is, astronauts, float because they are falling. They and the vessels they are in are in a perfect free-fall. They are falling ‘around’ the Earth – that’s what an orbit is. They never reach the Earth because as they fall, as a result of the curvature of the earth, the surface of the planet is moving away from them at the same rate they are falling at. Gravity causes the spacecraft and all the objects in it to fall at the same rate, regardless of mass. This is why all the objects appear to be floating, they’re actually all falling at the same rate.

So what about ‘Zero Gravity’?

 

If there is gravity in space, why are objects in space often referred to as being in zero-gravity?Actually, the term is a complete misnomer. There is no physical body that isn’t feeling the force of gravity at any point, but because gravity obeys an inverse square law, the further from an astronomical body an object is the less force it feels. In fact, it falls off incredibly quickly. But that doesn’t mean that this force ever falls to zero.

 

The term ‘zero-gravity’ is one that is so misleading that NASA themselves don’t tend to use it anymore, preferring the more accurate term ‘microgravity’. Objects in space may indeed be, for all practical intents, weightless, but it is vital to remember that weight and mass and different concepts. Gravity and acceleration act on mass to create ‘weight’. It’s also important to note that Newton’s law of gravity applies to any bodies with mass, not just planets. moons or stars. The table in front of you exerts a gravitational force on you, it’s just that its mass is way too small to have a significant effect on you. One could imagine that the First Order’s huge star destroyers have quite a significant mass.

So, if material can ‘fall’ through space onto the surface of another object, does it ever actually happen?

The most obvious example of material passing through space from one body to another is the mass transference that occurs in a binary star system. A binary star system is a star-system of two stars orbiting around a mutual barycentre. It is approximated that 85% of the star-systems in the Milky Way are binary or triple-star systems, so our single-star system is less common than multiple star systems. A particular type of binary star system is the ‘contact binary’, in these star systems material can flow from one star to another, hence the name ‘contact’ despite the fact that the stars do not actually touch. All the material from one star needs to reach the other star is greater kinetic energy than the gravitational potential energy of the donor star.

To picture this imagine a ball being rolled over the peak of a hill. All it needs to round the hill is the necessary kinetic energy to overcome the potential difference. If the potential energy is greater than the kinetic energy, the ball rolls back down the hill.

Whereas if the ball has the necessary kinetic energy, it rounds the top and falls to the opposite side of the hill.

The analogy for this potential difference between two stars in a binary system is called the Roche Lobe. The stars exist in an equilibrium between their relative masses.

But if a star’s radius extends beyond its Roche Lobe, material begins to flow through the La Grange’s point to the companion star. This normally occurs when the larger star enters the ‘red giant’ phase of its lifetime, the shell of the star expands filling the Roche Lobe. Material is stripped from the larger star and passed to the smaller star through the La Grange’s point. If there is a large amount of material, it forms an accretion disk rather than falling directly to the star’s surface.

As the falling material loses angular momentum, it falls on to the companion star’s surface. As this is occurring the violent forces in the accretion disk cause radiative emissions. They can actually be some of the most luminous objects in the universe and it is the emissions of accretion disks formed around black holes that have allowed us to see objects that themselves emit no light.

So how can we apply any of that to the ‘Rebel bombers’ in Star Wars: The Last Jedi?

Imagine the rebel bomber (orange) and the First-Order Star Destroyer (red) is occupying a similar gravitationally bound system. with the Star-Destroyer having the vastly greater mass.

How does the Rebel bomber get its deadly cargo to the Star Destroyer? It could release the bombs in such a fashion that they form a makeshift shell around the bomber. A shell that exceeds the Roche Lobe.

This would cause Roche Lobe overflow and allow the bombs to ‘fall’ onto the surface of the Star-destroyer. Not hovering so uselessly anymore!

So, it is not only possible to ‘drop’ material from one body to another in space, it is actually quite common despite there being no real concept of ‘down’. Ultimately though, I’m not really sure I’ll be thinking about any of this whilst I watch the latest Star Wars movie. When watching tales that ultimately centre around space-wizars, I tend not to worry much about physics. If I hear a snort of derision from behind me during the ‘bomber’ scene though, I may well feel a twinge of sadness that at least one person in the theatre wasn’t able to do the same. After-all doesn’t the very first film in the series, Star Wars: A New Hope, feature sounds the X-wings and such make as they fly through space?

I don’t recall too many critics holding that to be a flaw of that movie.

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