Question of the Week

Can you phone home from a black hole?

Sun, 14th Feb 2010

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Sophie, Chicago asked:

Is it possible to make a phone call from a black hole?


We put this question to Andrew Pontzen, research fellow at the Kavli Institute for Cosmology:

Andrew -   The short answer is no, but maybe you could receive a text message.  So let me take a step back and try to explain.  Basically, when you make a mobile phone call the device that youíre holding in your hand is using radio waves to send your voice to the person at the other end of the call.  A black holeRadio waves are just a certain form of light - we canít see it directly but we receive it using antennae in our mobile phones and other devices.  Because itís a form of light, just like any other light, it canít get out of a black hole.  So you cannot send your voice out of the black hole, thereís no way for you to have a conversation.  On the other hand, light can certainly get into a black hole and in the same way radio waves can get into a black hole.  So if you were to adapt your mobile phone so it could just receive text messages without having to communicate to the network, then you could actually receive a text message from your mum just before you died at the centre of the black hole.

Diana -   Lovely! so no-one can hear you scream from a black hole but you could pick up a text telling you to pick some potatoes the next time you go shopping.  That answer was given by Andrew Pontzen and you can hear more from him answering your science questions and keeping us up to date with space site news on our Naked Astronomy podcast available from


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Your mobile phone won't work because the radio frequency signal will never escape from the black hole.

I wonder if a phone with a rather long extension cord might work? Hmmmmm?

Geezer, Tue, 9th Feb 2010

If anyone needs a black hole to test this theory you are welcome to come round my house; this is the worst phone black hole I've ever experienced. If I flatten my face against my dining room window I can just about get my mobile to sustain a conversation; other than that it's a non-starter...! chris, Thu, 11th Feb 2010

We have a similar problem. Our mobiles don't work at all in our house. I think it's great, because we don't waste any minutes!

Slightly off topic, there is a company in the US (Magic Jack I think) who announced they are going to sell an Internet connected transceiver that will work with just about any cell phone. The cost was extremely low. The major carriers are really ticked off because they won't get any revenue from calls placed or received by that method. They are trying to block it on the grounds that it violates FCC transmission rules. Magic Jack claims the rules don't apply inside a person's house.

It will be interesting to see which way this goes. Geezer, Fri, 12th Feb 2010

Of course it is they are calling in and out of Hatti Doubting Thomas, Mon, 15th Feb 2010

I don't understand this comment, could you clarify for me please? BRValsler, Mon, 15th Feb 2010

that was iether a very bad joke or a stupid industrialists, paranoia on getting the upper hand. are you saying that, there is a company who divert calles through hatti have there company "there" just so you can make cheep calls, i can see how the ploy would work but  not the morals geo driver, Tue, 16th Feb 2010

What's Hatti? Is this it Geezer, Tue, 16th Feb 2010

I think that the answer to the original question is yes, provided that your home is in the same black hole that you are. Bored chemist, Tue, 16th Feb 2010

Hey folks... new member.

I already emailed this question.....

The question is.... can sound escape a black hole? Evil Eye, Mon, 22nd Feb 2010

Added to that, given how dense matter will be in a Black Hole, how fast would sound / vibrations travel? Similarly, how rapidly would sound be conducted through a neutron star?

Chris chris, Tue, 23rd Feb 2010

Hi, I thought I could expand on my original answer given some of the further questions asked here.

Evil Eye: sadly, no, sound can't escape a black hole. Ultimately sound is carried by gas particles, with each tiny particle colliding with the next in a chain reaction looking a bit like a mexican wave.  So to get the sound out, each particle in turn would have to move a little bit outwards to hit the next in a long chain reaching out of the black hole. But it turns out particles inside a black hole have to move towards its centre -- they cannot move outwards. So not even sound can be transmitted out of a black hole.

Geezer asked if you could use a normal landline with a very long extension cord. Sadly, the same problem applies here: the electrons which carry the signal through the wire can only move inwards, not outwards. So, once again, the signal cannot be carried out of the black hole.

As to density, Chris, it's a little hard to give a straight answer as to how dense a black hole is.  The answer normally given is that it's 'infinitely' dense -- that's because, as stated above, once infalling particles have passed the 'event horizon' they have to keep falling in, so ultimately they all reach the exact centre. The very centre becomes infinitely densely packed.

However, there are two problems with that answer.

First, most physicists think that at the very centre of black holes, our current theories of gravity are inadequate to describe the real universe. Corrections arise from 'quantum gravity' effects, and we cannot yet be sure of exactly what those effects do. They could, however, set some kind of upper limit on the density of matter, which from rough calculations we'd estimate to be the Planck Density: around 5 x 1096 kg/m3.

Second, even disregarding quantum effects, it really depends on what you mean by the density. Because of the way time is distorted around and inside a black hole, you can get very different answers depending on exactly how you pose the question.  (A related point is addressed in the Q&A in April's Naked Astronomy podcast.)

So here's another possible approach to an answer. Just take the mass of the black hole and divide by its apparent volume (that's a dangerous concept in itself, but I'm taking the size of the event horizon and making a naive calculation of the volume inside it). For a black hole of the mass of our sun, the nominal density would then be around 1019 kg/m3. For the kind of  super-massive black hole thought to be at the centre of our galaxy, the density would be rather less, something like 106 kg/m3. Odd that more massive black holes should be less dense, but that's just the way it goes.

And finally, neutron stars. The density is about 1017 kg/m3. No, that's not a typo, it's really denser than the super-massive black hole. In a neutron star, sound travels at a significant fraction of the speed of light, perhaps between 5% and 75% (from 1.5 x 107 m/s to 2 x 108m/s) depending on details of the modelling and whereabouts within the star you are talking about. Andrew P, Wed, 5th May 2010

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