If event horizons are real, then a star falling into a central black hole would simply be devoured, leaving no trace of the encounter behind.

If you collect more and more matter in a small enough volume of space, it gets harder and harder to escape from its gravitational pull. Gather enough mass there, and you'll find that the speed you'd need to reach in order to escape is greater than the speed of light! From within that region, escape is impossible, and you have a black hole. From farther out, where the escape velocity is lower than the speed of light, matter and radiation can make it out. The border of these two regions is known as the event horizon, and is one of the most important predictions of General Relativity that's never been tested. Until now, that is, where the signs that matter completely disappears when it crosses over cannot be ignored.

At the center of our galaxy, we find the largest black hole within more than a million light years. By observing the orbits of the stars in its vicinity, we can determine that there's an object with:

  • the mass of around 4 million Suns,
  • that occasionally flares in certain wavelengths (X-ray and radio) of light,
  • that emits no visible/infrared light,
  • and that is consistent with a black hole.

But we've never determined whether it truly has an event horizon or not. Sure, General Relativity has been successful every time we've been able to test it out, but every new challenge is a new opportunity to learn something new about the Universe.

Although there are gas outflows and radio/x-ray signals from matter that isn't absorbed by a black hole, nothing should be able to leave/exit once crossing the event horizon.

Top, optical, Hubble Space Telescope / NASA / Wikisky; lower left, radio, NRAO / Very Large Array (VLA); lower right, X-ray, NASA / Chandra X-ray telescope

Although there are gas outflows and radio/x-ray signals from matter that isn't absorbed by a black hole, nothing should be able to leave/exit once crossing the event horizon.

There are always alternatives to consider, and there are a whole class of modifications to gravity we can make that make it possible for event horizons to not exist at all. In these scenarios, instead of an event horizon surrounding a singularity, a giant mass like this would have a hard surface that objects could smash themselves against. If this were the case, you'd be able to tell the difference in one of two ways. The first (and most obvious) way would be with direct imaging: if you achieved sufficiently good resolution, a telescope would be able to see the event horizon for itself... or to find no horizon at all, if one of the alternatives to General Relativity were true. The Event Horizon Telescope, whose first results are due out later this year, should be able to see whether an event horizon really exists.

Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole's accretion disk, and how the radio signal will look as a result. Note the clear signature of the event horizon in all the expected results.

GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*, L. Medeiros et al., arXiv:1601.06799

Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole's accretion disk, and how the radio signal will look as a result. Note the clear signature of the event horizon in all the expected results.

But there's a second way that doesn't rely on direct imaging, and can find the answer anyway. Supermassive black holes occur not only at our own galaxy's center, but at the central cores of most large galaxies throughout the Universe. Our Milky Way's black hole, at four million solar masses, may actually be on the low end: many galaxies have black holes that extend up into the billions or even tens of billions of solar masses. The bigger a black hole is, the larger the cross-sectional area of its event horizon is predicted to be, meaning that it has a much larger chance for a passing object to impact it.

An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets, may describe the black hole at the center of our galaxy in many regards. But nothing from within the event horizon could ever get out.

Mark A. Garlick

An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets, may describe the black hole at the center of our galaxy in many regards. But nothing from within the event horizon could ever get out.

The largest known black holes have diameters about ten times the size of Pluto's orbit, meaning that if we view very large numbers of them for long enough, we should witness a star running into one of them eventually. The Pan-STARRS telescope, having just completed a huge set of deep observations for 3.5 years — covering some 3/4ths of the entire sky repeatedly — was able to look for transient events, or temporary brightenings and dimmings. If event horizons are real, swallowed stars wouldn't create a transient signal, but star colliding with a hard surface would create a significant burst of light.

If a hard surface, rather than an event horizon, exists around a supermassive object, a collision should result in a luminous burst that telescopes like Pan-STARRS should easily perceive.

Mark A. Garlick / CfA

If a hard surface, rather than an event horizon, exists around a supermassive object, a collision should result in a luminous burst that telescopes like Pan-STARRS should easily perceive.

According to Wenbin Lu, a scientist who studied these observations to test the hard-surface theory,

Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true.

Given all the black holes with masses greater than 100 million solar masses, there should have been a definitive signature if there's a hard surface outside of the black hole's event horizon. Yet no signature at all was seen.

After the collision of a star with a hard-surface around a supermassive object, a large, temporary increase in luminosity would result, yet no such changes have been seen around any of the supermassive black holes within the view of Pan-STARRS.

Mark A. Garlick/CfA

After the collision of a star with a hard-surface around a supermassive object, a large, temporary increase in luminosity would result, yet no such changes have been seen around any of the supermassive black holes within the view of Pan-STARRS.

Ramesh Narayan, a coauthor on the new study, was happy to articulate what it all meant,

Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades. General Relativity has passed another critical test.

Of course, it's not really possible to prove that the event horizon is real, but this work allows some impressive constraints to be placed.

Theoretical calculations predict an event horizon to all black holes, obscuring the central region in accordance with General Relativity. This is a prediction that has never been tested observationally, until now.

Ute Kraus, Physics education group Kraus, Universität Hildesheim; Axel Mellinger (background)

Theoretical calculations predict an event horizon to all black holes, obscuring the central region in accordance with General Relativity. This is a prediction that has never been tested observationally, until now.

If there is a hard surface, it must be within 0.01% the radius of the expected event horizon, given the lack of transient signals observed. A heat signature in the optical/infrared would be expected, which is exactly what Pan-STARRS would be sensitive to. Yet nothing was observed. In the future, the Large Synoptic Survey Telescope (LSST), which will have more than 20 times the light-gathering power of Pan-STARRS, will be able to constraint the event horizon to a ridiculously small size. But the LSST won't begin doing science until 2021, if things remain on schedule.

A view of the different telescopes contributing to the Event Horizon Telescope's imaging capabilities from one of Earth's hemispheres. Data was taken in April that should enable the detection (or non-detection) of an event horizon around Sagittarius A* within the next year.

APEX, IRAM, G. Narayanan, J. McMahon, JCMT/JAC, S. Hostler, D. Harvey, ESO/C. Malin

A view of the different telescopes contributing to the Event Horizon Telescope's imaging capabilities from one of Earth's hemispheres. Data was taken in April that should enable the detection (or non-detection) of an event horizon around Sagittarius A* within the next year.

By that point, the data from the Event Horizon Telescope will already be in. If the event horizon is actually, physically real, we won't need indirect proof like this; we'll already have a picture. In the meantime, we should celebrate the new evidence we have, and recognize what it means: when something falls into a black hole, there is no bounce-back, shattering, or ejecta from within. Once you slip past the event horizon, you're destined to fall all the way into the central singularity. As far as black holes go, there really is a point of no return.

Astrophysicist and author Ethan Siegel is the founder and primary writer of Starts With A Bang! Check out his first book, Beyond The Galaxy, and look for his second, Treknology, this October!

Source: This article was published forbes.com By Ethan Siegel

Categorized in Science & Tech

Nobody likes being spied on. When you’re innocently browsing the web, it’s deeply unpleasant to think that faceless technology corporations are monitoring and recording your every move.

While such data collection is legal, that doesn’t mean it’s all right. There are plenty of things you might prefer to keep to yourself, such as your income, your sexuality, your political views or your membership of the Yoko Ono fanclub. For an indication of what can be inferred from your online habits, take a look at the Apply Magic Sauce tool produced by Cambridge Psychometrics Centre, which produces a profile of your personality based on Facebook and Twitter data.

And while you might console yourself with the knowledge that all of this information is mostly used for targeting ads, that might not be the case for much longer. The internet giants are building up ever more detailed user profiles – and finding new ways to exploit that information. In the Observer, Carole Cadwalladr’s ongoing investigation has highlighted how analytic techniques were used in the recent EU referendum to target and craft messages to groups of persuadable voters based on psychological insights gleaned from online data.

Even if you are relaxed about analytics companies gaming the political process, you may be more bothered about the effect on your wallet: researchers at the Polytechnic University of Catalonia have already found evidence that some online retailers use profiling to discriminate against certain customers. If you’re identified as a high-value shopper, you’re likely to be steered towards more expensive products, or even charged more than other visitors for the same item.

And that’s just the start of it. Experts warn that, in the future, your online activity could be taken into consideration when you apply for a loan – or for a job. That’s troubling, not least because profiling involves a large element of assumption and inference. Something as innocent as searching for a medical condition – even out of mere curiosity – could cause your insurance premiums to rocket, and you’ll never know why.

Even if you’ve nothing to hide, therefore, it may be wise to minimise your exposure to online tracking. Here’s how some of the biggest names on the web spy on you – and how to protect your privacy.

Amazon

An Amazon Echo voice assistant

Pressing the mute button on top of Amazon’s Echo voice assistant will temporarily stop it from recording audio. Photograph: Samuel Gibbs for the Guardian

Amazon has a disconcerting habit of following you around the web. Spend a few minutes browsing its catalogue for a new crepe pan, and you might find that the next site you visit is mysteriously festooned with ads for cookware.

Thankfully, Amazon gives you an easy way to opt out of being tracked in this way. Simply click on “Your Account” from the Amazon homepage, then scroll down to “Advertising Preferences”: here you’ll see the option to disable personalised ads. Note that since this feature relies on cookies, it will only take effect in the browser you’re currently using.

Under “Advertising Preferences”, you’ll also see the option to view and manage your browsing history; from here you can disable tracking altogether, or remove individual items from your Amazon history. That could be useful if you’re shopping for a gift, or if you’ve been browsing unfashionable items you’d rather not be reminded of.

What about Amazon’s voice assistant devices? If you’ve got an Amazon Echo in your home, you might be concerned about it listening in on private conversations. Rest assured, the Echo doesn’t record anything it hears until you address it with the appropriate “wake word” (normally “Alexa”). Whatever you say next is relayed to Amazon, where it’s processed and stored, but you can wipe this data at any time – you’ll find the option to do so in Amazon’s “Manage Your Content and Devices” settings.

To ensure that Alexa can’t be woken up even by accident, you can also press the mute button on the top of the Echo to temporarily disable the microphone; just press it again to turn it back on. It’s worth noting, however, that the Echo’s software updates automatically, so its behaviour could change at any time. We’ll be keeping a sharp eye out for any updates that could compromise your privacy.

Apple

Teenager using a Macbook laptop

The macOS operating system passes your search terms on to Apple. Photograph: Graeme Robertson for the Guardian

iPhones and iPads collect a lot of data about you, and it’s accessible not just to Apple but to third-party apps too. The “Privacy” section in the iOS Settings app gives you an overview of what’s being collected, and lets you disable various data-sharing features.

One particular thing to note is that if you’re carrying an iPhone around in your pocket, it will be constantly keeping track of your location, and potentially sharing it. You can easily tell it not to, though: in the Settings app, tap on “Privacy” > “Location Services” and select which apps should have access to your GPS data. You can also disable system services such as “Frequent Locations”, or disable location services altogether – though this means that apps like Apple Maps won’t work.

It’s a similar situation for Mac users. The macOS system uses network connections to work out where in the world your computer is located, and this information can be shared with applications and websites. You can manage this from “System Preferences” > “Security & Privacy”.

Another privacy concern for macOS users is the fact that every time you search for something in Spotlight, your search terms are passed on to Apple, so the company’s servers can provide suggested links to online sources. You can disable this feature by opening “System Preferences” > “Spotlight” and unticking the box for “Spotlight Suggestions”.

Finally, keep an eye on your webcam, as malware can allow hackers to literally spy on you via your Mac’s built-in camera. If the light comes on unexpectedly, that means someone’s watching you; for complete peace of mind, you can always cover the camera with opaque tape. Consider also going to “System Preferences” > “Sound” and disabling the internal microphone, to ensure no one’s eavesdropping on your conversations.

Facebook

Facebook screens

Facebook: you might decide the safest option is not to have an account. Photograph: Carl Court/Getty Images

You’ve probably handed plenty of personal information to Facebook yourself – but the social network also tracks your visits to other websites to build up a scarily detailed profile of your lifestyle and interests. This information is mostly used for targeting ads, but it could be turned to other purposes in the future.

Facebook is quite open about the information it collects. When you see an ad on your timeline, you can always click the drop-down menu at its top right and select “Why am I seeing this?” to discover why Facebook chose to show you an ad for a smartphone, rather than one for scented soap. For a fuller explanation of what Facebook knows about you, go to the “Settings” page and click “Adverts” to inspect your advertising profile. If there are any mistakes here, or advertisers you don’t want to hear from, they can be removed or blocked with a click.

The creepy part is that, by default, Facebook’s targeted ads don’t appear only in Facebook itself. Facebook uses cookies to follow your profile on to other websites and, like Amazon, ensure that you see the ads it wants you to. You can disable this behaviour from the advert preferences page: under “Advert settings” you’ll see a rather awkwardly phrased setting for “Ads on apps and websites off of the Facebook Companies”. Set this to “No” and you should regain a degree of online anonymity.

Of course, this doesn’t mean that Facebook isn’t still profiling you: there’s sadly no easy way to stop it collecting information. Your best bet is to turn to the measures described in “The internet”, below, such as enabling “Do Not Track” in your browser, specifically opting out or installing an anti-tracking browser extension. Or, of course, you may decide it’s safest just to delete your Facebook account.

Google

Google’s My Activity page

Google’s My Activity page allows you to view your entire browsing history. Photograph: Samuel Gibbs for the Guardian

Most of us use Google services every day, and as a result the web giant knows a huge amount about our movements and interests. You can find out everything it’s learned about you at myaccount.google.com. Your data is all set out in an impressively forthright way; the only problem is, there’s so much information to work through that it can be bewildering to navigate.

One section that’s worth your attention is “Manage your Google activity”. Here you’ll find Google’s activity controls, which let you disable various types of data collection. For example, you can tell Google not to log your Chrome browsing history and activity, to stop tracking your location and to desist from keeping records of any voice commands you might issue. Turning these features off can make Google services less smart, but you might consider that a price worth paying.

For a closer look at the information Google’s been collecting on you lately, click on “My activity”. This brings up a timeline showing all of your searches, webpage visits, Android app activity and so forth. Seeing your digital life laid bare like this can be pretty unnerving: if there’s something you’d prefer Google to forget, simply click on the menu icon to its right and delete.

If you want to thoroughly inspect everything Google knows about you, you can even download a comprehensive archive of personal information by clicking on “Control your content” > “Download your data”. Be warned, though, this archive can be huge: the default settings include all the emails in your Gmail account, and any videos you may have uploaded to YouTube.

If you want to limit the information you share with Google in the future, the easiest way is simply to use it less. For example, try the privacy-focused search engine at duckduckgo.com, and use an alternative browser such as Firefox.

Microsoft

Windows 10’s “telemetry” features automatically capture all sorts of information about what you’re doing on your PC, and send it back to Microsoft. The company insists that this information is only used to improve Windows, but it can still feel like a violation.

The recent “Creators Update” to Windows 10 prompts you to review your privacy settings as part of the update process, but you can check and change your settings at any time: simply open the Windows 10 Settings app and click on “Privacy”. You’ll find no fewer than 18 pages of configuration options, covering everything from personalised advertising to location services.

There are a few settings you might particularly want to check. One feature of Windows 10 that may be cause for concern is the way it tracks everything you type – yes, everything – and shares it with Microsoft. This is supposed to help the operating system learn the way you work, but if the idea makes you shudder, you can disable it under “Speech, inking and typing”.

Under “Feedback & diagnostics”, meanwhile, you can choose how much diagnostic information gets periodically sent back to Microsoft. A full report includes details of which applications you’ve been using and which websites you’ve been visiting, so you might prefer to switch to the more limited basic setting. You can also manage the information that Microsoft already knows about you by visiting the Privacy Dashboard at account.microsoft.com/privacy.

While not strictly a privacy issue, another controversial aspect of Windows 10 is the inclusion of ads in the user interface. To remove unwanted ads from the Start menu, go to “Settings” > “Personalisation” > “Start” and disable “Occasionally show suggestions in Start”. To stop Microsoft advertising its OneDrive cloud storage service, open File Explorer, then select “View” > “Options” > “Change folder and search options”, click on the “View” tab, and untick “Show sync provider notifications”.

The internet

Google Chrome settings

Google Chrome, like most major browsers, has options to disable tracking. Photograph: Samuel Gibbs for the Guardian

All right, it’s an exaggeration to say that the internet as a whole is spying on you. But there are a hell of a lot of companies out there keeping tabs on your online activity. The motivation normally comes down to filthy lucre – tracking your interests helps them push relevant ads your way, and to be fair you might prefer those to irrelevant ones.

If you’d rather not be tracked, one step you can immediately take is to enable the “do not track” feature in your browser. Yes, it’s that simple: every major web browser has the capability to tell websites that you don’t want to be followed – you can find more details at donottrack.us. The only problem with this system is that compliance is completely optional, so websites that want to follow you still can.

Another thing you can do is visit youradchoices.com, a site that checks your browser for “tracking cookies” from more than 100 companies. You can disable individual cookies, or turn off all behavioural advertising with a single click. After this, you’ll still see ads, but they won’t be customised to your interests any more.

A more drastic solution is to configure your browser to reject third-party cookies – that is, cookies that connect to a site other than the one you’re currently browsing. However, this can cause problems if, for example, a site uses embedded content from elsewhere on the web. A safer option is to use a tool such as Ghostery ; this free browser extension can block tracking cookies from more than 4,500 companies, while letting you selectively enable cross-site content. It’s a pain that this should be necessary, but if you don’t want your personal information to be shared around online, it’s a precaution worth taking.

Source: This article was published on theguardian.com by By Darien Graham-Smith

Categorized in How to

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