This week, we asked you to tweet us or send us a message on Facebook with questions you had about physics and maths. They could be anything, a term you had heard someone use before and didn’t understand or just something that had been puzzling you. You guys sent us a load and today we are going to answer some of them. Please keep the questions coming as we will continue to do question posts from time to time.

## What is Quantum Physics?

This seems to be a very common question when I tell people that I am interested in, and write a blog about, physics. Most people are familiar with the most basic physics. You drop something and it falls is an easy example we can give of physics in action, but once we add a fancy adjective, no one seems to have any idea. So let me give you the short and simple answer, before I go and tell you the long, complicated and definitely the more exciting one: Quantum Physics is the study of small things. Really it is called Quantum Mechanics, because it is the study, no not of small mechanics, but of the movement of small things. But isn’t the movement of small things pretty much the same as the movement of large things? If I throw a table-tennis ball, it will fall to the ground in the same way as a football would, after all.

Well we are talking very small. Not table-tennis ball, not ant size, not even bacteria size. The smallest of the small, atoms and the particles within them, and this is where our question becomes a bit more complicated. By the beginning of the 20th century, scientists were beginning to think that they were going to run out of things to study. They could look around and describe a lot of everything they saw, especially in a physical way. They had a theory of gravity, they were experimenting with electricity and Newton’s laws of motion could explain almost everything else. The day when physics would reach its finishing point seemed near. But how wrong they were.

You see, physics had barely even started. All we had done was gone and discovered some of the easy bits. Once we stopped looking around us, and started looking into us and what we are made of, we realized that the universe was not as simple as we first thought. I won’t give you too detailed a run through of Quantum Mechanics, we have other questions to answer after all. However, I will take you through one part that should show you how weird quantum physics is and it starts with an equation (don’t worry, it looks complicated but I will explain):

Where ∆x is how well we know a particle’s position, ∆p is how well we know how fast it is moving and the h with a line through it is a very small number.

What the equation says is that when we know exactly where a particle is, we can have no idea how fast it is moving and visa versa. You can know roughly where it is and roughly how fast it is moving but that is all. For this reason, it is called the uncertainty principle. You are never certain of the place and the speed of a particle. What is the importance of this? Well this, and a few more things, mean that if you place a particle down (if it was possible of course), the next moment it could be anywhere in the universe…in fact, the whole reason we don’t just break up into different pieces that jump about the universe is that we are so big! Individual particles may jump around, but big clumps of them tend to stick together. So there is your answer. Quantum Physics: Small things doing really strange stuff.

## What happens if you jump on an escalator? Do you go backwards in respect to others or not?

Ok, this question isn’t too complicated. The escalator is pulling you up, so if you jump, do you go straight up (backwards relative to the other on the escalator), or continue up wards? Well for this question I am going to change the circumstances a little, not to get different results but instead to show better how we got to our conclusion. Instead of an escalator, we are going to think of a moving car, and instead of jumping up, we are going to be thinking of jumping out. In other words, if you were to jump out of a car moving at a constant velocity, would you be able to jump completely perpendicular to it (path B), or will you continue traveling in the same direction as the car (path A)?

Right so, the short answer is A. This means when you jump on an escalator, you will not move downwards relative to everyone else. But there is more to it and it isn’t as black and white as that. For this, and to find out how we know, we have to go into a bit of physics:

Ok so here is the thing. When an object moves, it has something called momentum. The momentum is just the velocity of the object times by its mass. An object cannot just lose momentum, it has to be put into something else instead. This means at any one point, the amount of momentum in a system is always the same. So lets apply it here:

Before you jump:

After you jump:

So why is this important to the original question? Well the momentum before the jump has to be the same as the momentum after (read above) and when we talk about velocity, it is only in one direction and at the moment you jump out of the car, we are only counting the velocity in the direction the car is moving. When you jump out perpendicular to the car, this velocity is zero. Plugging this into an equation you get a strange result. The car’s velocity must increase when you jump out. But this is where the problem lies, it can’t increase. For the velocity of something to increase, a force must be exerted on it, and nothing can exert a force on the car, at this point. This means that it is physically impossible for you to jump perpendicular to the car, unless you actually jump backwards off the car.

One more thing though, unlike the car, you aren’t pushing yourself in that direction, so very soon air resistance will take over and you will cease to travel any further that way.

So in summary. If you jump on an escalator, you will jump up it and stay parallel with the other users. I’m sorry for the jumble of equation stuff.

## Is time travel possible?

I feel I have already over run with this post so I will make it short, sweet and beautiful (at least for those of you hoping for a Doctor Who like life in the future)…YES.

Ok, going forward in time is the easier one. There are two ways of doing this, go very fast, or be in a section of space with high gravity. It is pretty amazing actually. Hop in a spaceship and go very fast for a year, return to earth and your twin sibling could be 20 years older. The problem is, none of our technology can possibly move us that fast right now. We have talked about the theory behind this (It is all to do with Einstein’s theory of relativity) in our previous post on “The Irregularity of Time”.

However, there is a problem now. You could travel into the future, but would have quite a bit of trouble going back. You see, as much as time loves to slow down (read the above mentioned posts to understand), it doesn’t really like to come to a stop and likes it even less to start going backwards. And this is where some fancy science trickery comes in, you see it seems to be possible.

I recommend watching this whole video, but you can watch from about 2:27 for the different methods of backwards time travel:

In the video, Dr. Ronald Mallett describes a time machine he is hoping to create uses a special method that uses light to warp time. He explains it better in the video but the idea sounds fantastic. It also solves one of the biggest time travel problems we have now. If someone in the future could time travel, wouldn’t they have come back and told us?

The way this is solved is that Dr. Mallett’s machine will only be able to take you as far back as the machine has been running. Brian Clegg has also written a book on the subject, and you can check out an early post of ours which adresses this issue.

So, yes, we can time travel, but it requires funding, research and fairly ironically, a lot of time!

We hope that you have enjoyed this post and we have managed to answer your questions! If you have any questions you want answered, make sure to contact us!

If you liked this, then please check out our last two posts:

Maths of the Medals Table: Will the London Olympics “Inspire a Generation”? - We have heard the words “Inspiring a generation” a lot during the olympics. But is that really likely?

The Physics of Field Athletics: Discus, Frisbees and Airplanes - The Discus event seems on the surface to be all about brute strength, but in fact it is one of the most complicated of the four throwing events. Why is that?

We have now nearly reached the climax of our Physics of Sport series with the arrival (and departure) of the Olympics, so have a look at the posts in the series here.