Dr Tara Shears answers your questions on space physics

Dr. Tara Shears

Dr Tara Shears spends her life recreating the moments after the Big Bang at the Large Hadron Collider. Here she answers your questions on space physics - from dark energy to gravitational waves and super massive black holes.

How do super massive black holes form?

Nobody knows. They could be formed by the collapse of a massive star, which sucks in all surrounding material and grows and grows. Or they could be formed by the collapse of a "super massive star" or dense stellar cluster. Both ideas are currently being tested by astrophysicists; no-one knows the answer yet.

Watch stars orbit a super massive black hole in our galaxy

What does it mean to have four dimensions (or more) in space?

Actually, our everyday lives are carried out in three spatial dimensions (length, breadth, height), and one of time. You can pinpoint anything that happens if you know its spatial coordinate and what time it occurred. More exotic theories of the universe suggest that there might be extra spatial dimensions, but if there are, we are blissfully unaware of them.

Gravitational wave detectors are looking for the collisions of distant black holes. Why don't we look closer to home?

The strongest gravitational waves are produced by the movements of the most massive objects. You should still have gravitational waves produced by, for example, the movement of the Earth round the Sun, but the energies involved are tiny. As experiments like LIGO haven't yet seen direct evidence of gravitational waves produced by massive bodies like black holes, it's not surprising that they haven't detected the much weaker signals from lighter, closer objects.

Gravitational Waves doodle

The LIGO experiment uses a laser beam to measure distortions in space-time. But if gravity waves stretch space-time, why doesn't the laser beam stretch as well?

Light travels independently of space-time stretching - so the idea is that a gravity wave will change the distance a laser beam has to travel when the wave ripples across an experiment. This will change the light patterns observed in the experiment. So far, though, there's been no sign of any gravitational waves using this approach.

What is redshift and how does it tell us whether galaxies are moving towards or away from us?

You know how the pitch (or frequency) of a police siren speeding away from you seems to sound lower? We call this the "Doppler effect". Redshift is the Doppler effect for light - we see light from galaxies moving away from us shifted down in frequency, which makes the light move to the red end of the spectrum. Galaxies moving towards us have their light shifted to higher frequencies, to the blue end. That's how we can tell which direction they are moving in.

See why galaxies look redder than they should

What is dark energy?

A good question! We don't know. It's unseen (hence the "dark"), and it's thought to fill the universe, and be responsible for making the universe's expansion speed up (hence the "energy"). However, we don't yet know what it is, what causes it, and how to explain it. It's one of the big mysteries of physics.

See the evidence that dark energy exists

In an expanding universe, can the rate of expansion be faster than the speed of light?

There are no laws placing limits on the speed at which space-time (and the universe) expands. However, you're not allowed to let information (or light) travel faster than the speed of light. This is mind-bending, but it does mean that it's perfectly possible for the universe to expand faster than the speed of light, as long as you don't see it happening.

If the speed of light is constant, how can we have redshift?

You have the answer in the equation: speed of light = frequency x wavelength. Redshift decreases the frequency of light. When this happens, the wavelength of the light increases at the same time. This keeps frequency multiplied by wavelength constant and equal to the speed of light.

If I sit on a photon [a light particle] leaving one side of the Sun and observed another emitted from the other side, it would be travelling away from me at twice the speed of light. Is that right?

This situation can't happen in special relativity. Nothing goes faster than the speed of light. If you are sitting on a photon looking at a photon travelling away from you, it would seem to be travelling at the speed of light, no faster.

Dallas Campbell asks Prof Paul Davies to explain special relativity

What is the Higgs field?

This is a type of energy field, present throughout the universe, which is thought to give fundamental particles their mass. It's invisible, but we can tell if it's there by finding the Higgs boson which is the particle that accompanies it.

Has your experiment at the LHC disproved supersymmetry? What does this mean for our understanding of dark matter?

The experiment at LHC has told us that many of the simplest versions of supersymmetry, that we were hoping to see, don't exist - but there are plenty of other possible versions remaining. In other words supersymmetry might still describe dark matter, even if we're in the dark as to whether supersymmetry is correct or not. We need to keep looking for proof one way or the other, when the LHC raises its energy in a couple of years

How the Hubble Telescope 'sees' dark matter

If everything is made of protons, neutrons, and electrons, how do different materials have different colours and properties?

These properties depend on the exact details of how the protons, neutrons and electrons link up to form the atoms and molecules of the material. For example, the colour of a material depends on the spectrum of light it emits. This depends in turn on the distinct energies electrons can have in the material, because light is emitted when electrons move to lower energy levels. Studying these properties is where physics meets chemistry.

In beta radiation, a neutron decays into a proton and electron. They have a higher combined mass than the neutron - how is that possible?

Beta radiation doesn't happen in isolation - only inside atoms, and atoms only decay when this lets them reach a lower energy state. You have to take the energy of the whole system, not just the neutron, proton and electron, into account. When you do this, you'll find that there's enough energy available to make this decay work.


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