Laundry “colour catchers” promise to save you from accidentally dyeing your clothes in the wash. But how do they work, asks Christoph, by email

Colour catchers are sheets that are popped in to the wash to prevent coloured items ruining your whites when they are washed together. Pioneered in the early 90s by Patrick McNamee, now head of research and development at laundry products company Spotless Punch, the sheets are manufactured to bear positively charged (cationic) dye scavenging compounds that attract negatively charged (anionic) dyes that come out in the wash; these are generally “direct” dyes that are used to colour cotton garments. Once the sheet and dye are brought together, a chemical bond is formed.

“The charge attracts it first and then you have a chemical reaction, so basically the dye cannot actually come off the sheet, it is stuck on to the sheet then so it can’t roam in the wash to transfer or to cross stain into your garments,” says McNamee.

As a result, you can sling crimson jumpers in with snowy shirts without encountering a pink problem. While other approaches, such as the use of polymers called “dye transfer inhibitors” in laundry detergents can also be used to sweep up stray dyes, colour catchers do boast a strangely appealing feature: at the end of the wash, the captured dyes can clearly be seen on the sheet.

Gravity can warp space-time but can it make it more dense? (As a magnetic field is more dense round the pole of a magnet.) Also, do black holes consume space-time or merely warp it?” asks Alastair McKeown

“In Einstein’s theory of general relativity, gravity is warping of space-time,” points out Dr Jonathan Gair from the University of Cambridge. “Matter and energy cause space-time to curve (or warp) and this is what we call gravity. Other objects moving in the space-time follow these warps, which makes them appear to move on curved paths.”

But not every lump of stuff causes the same curvature. “Where the curvature of space-time is greater, gravity is stronger,” Gair explains. “We talk in terms of curvature rather than density, but where mass-energy is more densely concentrated in space it will create greater curvature of space-time and hence stronger gravity.” And black holes, it seems, are in a league of their own. “Black holes are parts of space-time that have such great curvature that light can no longer escape them. Again, a black hole is a property of space-time – they do not consume space-time.

“Space-time is strongly warped in their vicinity but saying that they warp it is a tautology – a black hole is, by definition, a region where space-time is so strongly curved that light cannot escape.”

But this is not, typically, a static situation. “Black holes can exist in isolation, in which case there is no change in anything over time; the space-time curvature remains unchanged. Most astrophysical black holes are not isolated though and matter in their vicinity will fall into them over time,” Gair revealed. This affects the curvature of space-time.

“As this happens, the mass and rotation rate of the black hole will tend to go up and the space-time curvature outside the black hole changes correspondingly,” he adds. “The black hole is not consuming space-time, however, it is consuming mass-energy and the increase in the density of mass-energy leads to a change in the space-time curvature.”

Why does a probability distribution eventually make the shape of a predictable Gaussian probability function, rather than a random scattering shape? asks Yusef Ahmad

“The bell-shaped Gaussian (normal) curve appears ubiquitously in scientific data, and it is natural to wonder why,” says Oliver Johnson, reader in information theory at the University of Bristol.

“One answer comes from the central limit theorem, which tells us that averages of many random terms have a roughly Gaussian distribution, provided the terms don’t depend too much on one another and aren’t too large (don’t have heavy tails). As a result, whenever observed data is formed as this type of average of random effects, a histogram of many data points will be close in shape to the Gaussian curve.” And the bell-shaped curve, it seems, crops up all over the place.

“The central limit theorem justifies the statistical models used in many disciplines,” says Johnson. “For example, opinion polls average individual preferences and, if the country is sampled correctly, these preferences are independent of each other. This means that the central limit theorem can be applied, giving the familiar 3% ‘margin of error’ attached to polls of 1,000 voters.”

But not every scenario is so straightforward. “Gaussian distributions give the standard model for share prices and underlie the Black-Scholes price of contracts based on them,” says Johnson. “At first sight this model appears justified by the central limit theorem, since share prices reflect an average of many small fluctuations. However, in reality prices have too heavy tails for the central limit theorem to hold, meaning large movements are more common than we might expect. The Black-Scholes formula is adjusted accordingly by traders.”

Do tea and coffee really dehydrate you? asks Beth Shepherd

Chemicals that trigger urine production are known as diuretics. But while caffeine can have such an effect, it doesn’t necessarily follow that a cup of coffee will dehydrate you.

In a review from 2003, Ronald Maughan, now emeritus professor of sport and exercise nutrition at Loughborough University, and his colleagues examined studies in the field.

They concluded that caffeine triggers an increase in urine production in acute doses, but that the levels of caffeine in a cuppa have little effect. Furthermore, regular drinkers of caffeinated drinks develop a tolerance towards its diuretic effect.

“Caffeine is a mild diuretic and of no real consequence in the amounts normally consumed. It is better to drink tea, coffee or cola than not – the water they contain more than outweighs any small effect of the caffeine,” Maughan says. A study from Birmingham University published earlier this year supports this view.

In the study, researchers compared the urine production and other hydration markers of 50 men – all regular coffee drinkers – when they drank four mugs of coffee a day over three days compared to four mugs of water.

The results showed little difference between their hydrating properties. But, as co-author Dr Andy Blannin points out, there are many factors to consider.

“There isn’t a simple answer to this as it depends on the individual (habitual tea/coffee drinker, or not) and circumstances (volume and strength of the tea/coffee). In regular tea/coffee drinkers, consuming their usual amounts will not be dehydrating. If they drink unusually large amounts, then this could be dehydrating. Moderate amounts could be dehydrating in those that normally abstain.”

Both of these studies were funded by coffee-industry related bodies, inlcuding the Institute for Scientific Information on Coffee, and published in peer-reviewed journals. The paper states “The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript”.

The upshot? “Tea/coffee can be dehydrating when taken in amounts that the individual is unaccustomed to, but normal moderate intake (about four cups per day) is not a problem,” concludes Blannin.