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I remember watching a documentary some time ago and at one point they were talking how empires were limited to a certain size since there was no way to communicate across the empire fast and reliably. The best method they had was courier by horse and to that was a limited size, also empires in America would have an even smaller max size since they had no horses and had to communicate sending people running from courier post to courier post. Also transport by sea doesn't really help, colonial empires did control vast areas so far apart, but they did't get much inland control.
I imagine sea then horse would cause a lot of different problems.

So with today's reliable and fast communication could an empire rule over the whole planet? (Taking in consideration only this fact about communication)

Yes... and no...

Is today's communication fast enough and capable enough to manage a planetary empire? Absolutely. The Internet alone could do it.

That's not your problem.

Your problem is establishing such an empire in a believable way, because the technology that allows massive, instantaneous communication also allows for considerable freedoms involving transportation, weaponry, housing, and (not surprisingly) communication against the empire.

When you consider the various near-worldwide colonial/empire periods of Earth, you discover that people were (relatively) easily controlled because the conqueror had tremendous advantage in firepower and manpower. Genghis Khan was attacking, predominantly, villages that were loosely organized at best. England, Spain, and the Netherlands brought overwhelming firepower and organization to bear against the many lands they conquered and controlled. The same can be said about the U.S. westward expansion.

You don't have that advantage today.

It seems like you do. You have tanks and planes and aircraft carriers — and they're all very impressive — but while you may logistically control the entire planet, you would be constantly fighting rebellions and resistances. Well-armed rebellions and resistances. And history has proven that they could fight against tanks, etc. in the past.

Technology is a great equalizer (well... for bad, if not for good). Consider the various difficulties in Africa today — difficulties made simple because arming yourself with automatic weaponry that can eliminate whole villages in minutes is trivial. Why can't the villages do the same? They're poor.

Which is probably the only way a planetary empire could stay in control — poverty. But then, what's the point of the empire?

Yes.

Many large corporations and organizations already operate on a global scale just fine. Just add some local and regional governments such as nearly all governments already do and you have a functioning global empire.

It is unlikely to happen with a democratic government without an alien invasion or some other reason to have world government but a fascist or communist government might go for world domination. Their legitimacy comes from a unifying ideology and works just as well (or badly) for a planet as it does for a single commune.

A theocratic government would also work. An Islamic caliphate for example would actually have perfectly valid reasons to seek to unite the world under its power. A Christian or Hindu version of the same would work just fine.

And manufacturing a reason for a democratic government to go for it isn't really that hard. Any kind of global event that threatens the extinction or subjugation of entire human species would work. Or people could just get really tired of pointless wars. The European Union largely exists because people figured that an economic union between France and Germany really would save lots of bother.

EDIT:
Since I started by pointing out that corporations work on a global scale, I probably should mention that it has required some adaptations to how they operate. This is because the "working hours" happen at different times in different parts of the world. Typically (a simplified general model) you'd split the world into three operational zones (24/8 = 3) and have active operational control cycle between these. Typically these are the Americas, Europe and Africa (or EMEA), and Asia and Oceania.

A global empire would probably copy this solution and essentially have three administrative centers. Say, Washington, Brussels, and Beijing. Depends on how the empire came to be. The central government could be in one of these or cycle between them or even be divided between the three. The EU has split its functions between multiple locations for political reasons and it kind of works with fast travel and telecommunications.

Yes. I'll break it into two parts, communication relating to Empire Citizens, and Communications within the powers that be.

My first thoughts go to China and their citizen points system being worked into place. The first step would be to pull as many people into the cities as possible, having most of your population in sight of you cameras and tracking technology would be crucial. Monitoring any internet/ media usage of a population is a common occurrence today, and would be extended to make sure nobody is forming a group to go against your reign. We have the technology to track the whereabouts of every citizen, so we would be able to tell who is meeting up with who, where people are going and what they are doing. With no privacy, people cannot create a revolution.

Communication between generals, media controllers, the world leader, and everyone else is instant. People are able to make a snap decision and have millions of people respond and react to it, I don't see why that couldn't be scaled up to billions. For example if a curfew was implemented suddenly, the police/ military could immediately respond to the situation without having to take the step of waiting for a courier.

Most of these solutions are dystopic, but I think world government and control over the whole planet causes those situations to arise. You would need to squash all attempts at disruption, and you would be able to do so quickly.

Yes, absolutely. It will be easy to control the entire world, once dominance is established. And rebels will not be as strong as they were, in the past. Yes, rebels do use modern technology at combat, but because the tech today is massively destructive, and the army uses modern means of control, science and logic in reasoning and deducing strategies, it is quite unlikely that any rebel team would succeed at outsmarting the empire. They would be better off working from within the empire to betray the emperor, but that will also be quite risky.

Why is that? In the past, people never dynamically employed scientific and logical strategies in combat, as much as we do today. Most kingdoms and empires in the past were taken down by outsmarting the emperor's army. But today, the military has R&D, and thus creates advanced weapons. They have heavily funded teams of brilliant minds who would work towards taking down their enemy. Today, the rebels do not have enough space to hide, as every part of the earth is measured and mapped. Even if they hide, it will not be possible for them to grow big and become dangerous: because they will need a lot of R&D and training, thus must have a well funded team of scientists, strategists and other people skilled in combat.

If someone had already managed to become an emperor (which is highly unlikely) it will literally be impossible to take that person down. Because, he will have the best minds on the planet (literally on the planet, as he controls the entire world), and as long as they work for him, and as long as he is in good terms with people who are immediately below his rank, nothing will stop him.

There is a new emerging psychology called the cyber-psychology. The same methods used by the game industry and the social networking sites can be used to massively control people. It is possible to psychologically control people by socially reward them for certain task while punishing them for the others. Imagine using a social networking site in a imperial world. If the emperor hates certain kind of topics, it is unimaginable to think that he wouldn't socially punish citizens who engaging in such topics. A simple way to do this is by improving the visibility of their content (which involves their personal profile or anything else they post) among people who would genuinely despise the topic and decrease the visibility of the content among people who may have the same views. Actions like hearts, likes, up-votes or points etc can be used as the incentive here. This could also go further- to jobs and credit scores. Soon, most content in the social network will be in favor of the emperor. And massive surveillance can be used to study people and respond almost spontaneously, to "fix issues". In this situation, there is no room for rebels. For rebels to exist, you need a mob of angry people, not zombies controlled by social networks. Also, even if you have angry people, what chance will they have against heavily funded experts working for the emperor?

Thus, I believe today is the right time for someone to be the world's emperor, as he could do more than just "communicate very fast". He could literally condition the psychology of the world to his needs. But it is highly unlikely that someone would grow powerful enough to conquer the world.