# Short of collision, can gravity itself kill you?

Imagine that you are falling into object with huge gravity (i.e. black hole) that does not have any atmosphere.

The question is - before you hit the ground, can the gravity itself (which would be extremely huge) kill you? And if so, how exactly that could be done? All parts of your body are affected same and in the same direction, therefore it cant tear you apart.

The "zero gravity" is simulated on earth with falling plane with speed increasing by gravitational acceleration, but is it "real" zero gravity, or is it little different?

Yes gravity can kill you because as you approach something super dense like a black hole, the gravity will change with the square of the distance which means that eventually the gravity at your feet would become significantly larger than at your head. This gravitational gradient is referred to as tidal forces and is the same effect that keeps the same side of the moon facing the earth.

This would tear you up and eventually disassemble the matter that makes you. Although it sounds dumb, scientists actually call this spaghettification as the CuriousOne has already mentioned.

It is questionable whether other effects like gravitational blue shift would not kill you prior to the spaghettification of your body.

A uniform gravitational field (which is actually only hypothetical) would have no effect comparable to spaghettification as it merely results in constant acceleration of your body.

• Comments are not for extended discussion; this conversation has been moved to chat. – David Z Feb 18 '16 at 16:13
• How would blue shift kill you? – gerrit Feb 18 '16 at 17:12
• @gerrit By ionizing the molecules in your cells. – Peter A. Schneider Feb 19 '16 at 1:10
• These are known as "tidal forces"; answer should mention that in case someone wants to read further. Also, the inverse-square law is the far-field limit in General Relativity. I believe it gets worse! – Pete Feb 19 '16 at 15:42
• I thought the same side of the Moon faced the Earth because of conservation of angular momentum? – rubenvb Feb 20 '16 at 9:47

It's fair to say that the "spaghettification" problem occurs where there's a strong enough gravitational gradient across a typical body-distance - but your question seems to ask about cases where the gravity is great - extreme, but uniform (at least in terms of human proportions/distances).

Einstein’s principle of equivalence states that, as long as there's no reference externally, any gravitational field feels much like another. So if I'm in freefall into Jupiter, or the Sun, or Phobos, or wherever, I will still feel weightless, the bowl of petunias free-falling with me will also feel and appear weightless next to me. It's only when/if I look outside of the frame of reference that I might notice any difference. For example, if I were in some high gravitational field, the rest of the universe might appear to be running very quickly, turning blue, and looking very bright indeed.

If the field were strong enough, and the outside universe bright enough (we're talking about starlight here, so it depends on where in the universe you might be, and how bright the stars are nearby - but in cases where a strong field might exist, such as near a super-massive black hole, you might find yourself at the centre of a galaxy where, presumably, you might expect the starlight to be significantly brighter than here on Earth) The "blueshifting" of low-frequency radio-wave type photons might act as microwaves, initially heating up the body fluids, visible light would shift towards ultraviolet, perhaps causing sunburn, and as we go up the frequency spectrum, xrays and gamma rays would effectively be delivering strong doses of dangerous and harmful radiation. If this were bright and strong enough, it ought to break down the proteins and structures within your cells and potentially, reduce you to a steaming pool of jelly.

So, my advice is, wear sunscreen!

• Do we already know more about the universe, having decyphered the bowl of petunias' thoughts, assuming one has read all the books? – Mindwin Feb 16 '16 at 18:32
• On the blue-shifted microwaves: it was not gravity that killed you. – Mindwin Feb 16 '16 at 18:33
• Haha, yes, sadly (because it reminds me that there wont be any more) I have read all the books, a few times over now - and it's probably time to go over them all again soon :) Re the identity of the murderer, it depends. If the blue-shift is caused directly by the extreme gravity, is the gravity responsible for deaths so-caused? If I go to a court of law, and found guilty of hiring a hit-man to assassinate my milkman, am I not equally, and criminally responsible as the one who pulls the trigger? – Thomas Kimber Feb 16 '16 at 20:27
• @TomKimber If a court of law finds gravity guilty of killing a person using blue shift as a weapon, remind me to find another universe to live in. It'd be involuntary manslaughter at best. Besides, gravity would probably get off on an insanity plea. – Jason C Feb 19 '16 at 6:15
• @TomKimber Humans haven't (yet) had to consider the medical definition of death in terms of relativistic reference frames! But I think that death can only be determined in the reference frame of the subject – binaryfunt Feb 20 '16 at 13:58

While a sufficient difference in gravity over your body will kill you, gravity itself will not kill you.

It's not really clear if you're asking about hypothetical, relatively uniform gravity fields, or black holes. Since the other answers tackle the latter I'll go for the former.

Answering what I interpret to be your question, given the following ideal assumptions:

• You are in an extremely strong, homogeneous gravitational field (same forces acting on your entire body).
• The field is infinite in size and there is no "ground" or other objects to collide with.
• None of the other conditions present can kill you (e.g. you are magically able to survive with no atmosphere).

You will experience only an ongoing acceleration due to the effects of this gravitational field.

Density and mass do not affect gravitational acceleration. Even though your body is of non-uniform density, it does not matter, so all parts of your body will accelerate uniformly in this hypothetical homogeneous gravitational field.

You will simply move faster and faster, approaching the speed of light, but no harm will come to you. You will feel "weightless" and you will observe any stationary objects moving past you faster and faster.

You don't need the field to be homogeneous to survive either. It only needs to be "homogeneous enough" so that the tidal forces don't exceed your body's structural strength.

Now if you throw yourself in a black hole, that's another matter entirely. As already described in other answers, the difference in gravity over your body will cause the parts of your body to accelerate at different rates, and these forces tear you apart when the structure of your body can no longer withstand the forces. The cause of death here is the force that the parts of your body exert on each other due to their relative motions, as some parts begin moving faster than others.

Of course, these conditions don't really exist, but it's hypothetical, like your massless ropes and frictionless pulleys.

• If this homogeneous gravitational field is infinite in size, and your acceleration is linear (as I believe it would be in a homogeneous field), doesn't that mean that you should eventually reach the speed of light? I know that normally this is considered impossible because as your velocity increases, so does the amount of energy needed to continue to accelerate you, approaching infinite energy needed at C. But if your acceleration is due to gravity rather than propulsion, I think the energy issue is eliminated. So what happens next? – Dan Henderson Feb 17 '16 at 21:27
• @DanHenderson The mistake is in the assumption that the acceleration is linear. In a uniform field, the force of gravity is the same regardless of your position. However, the acceleration due to gravity depends on your mass. Your mass is not constant; it increases as you accelerate due to mass-energy equivalence. Thus the uniform field will accelerate you less and less, and you'll never quite reach c. – Aaron Novstrup Feb 18 '16 at 23:41
• You guys are aware that there is no way to detect a homogeneous gravity field, right? It doesn't even exist, in a way. – Peter A. Schneider Feb 19 '16 at 1:13
• @PeterA.Schneider Right, but in the context of this answer, it's a magical, homogeneous gravitational field, so we're just accepting it's existence for the sake of discussion. But if we want to delve further, it would probably be best to take it to chat. – Dan Henderson Feb 19 '16 at 6:12

Yes, it can.

Before going ahead, I would like to say that I am no expert with these kind of matters, so I'll be going to quote some lines which I have seen in a few videos. I highly recommend you to watch these video links which I have added at the last line, because I think that it can answer your question.

"The mass of a black hole is so concentrated at some point, even tiny distances of a few centimeters would mean that gravity acts with millions of times more force on different parts of your body. Your cells get torn apart as your body stretches more and more, until you're a hot stream of plasma one atom wide."

So as a result, you will die for sure. And also this:

"A black hole with a diameter of a nickel would be slightly more massive than that earth. It would have a surface gravity billion billion times greater than a planet that currently does. Its tidal forces on you would be so strong that they'd rip your individual cells apart. The black hole would consume you before you even realized what's happening."

# Gravity Wave

Many users have noted that the tidal forces generated by a gravity gradient would be capable of killing you.

$\dfrac{dg}{dx}$

On a more topical note, it should be possible to cook you (among other things) using a gravity wave.

$\dfrac{d^2g}{dx^2}$

As the wave passes through you, you will experience a stretching effect (like in spagettification) and then a compressing effect (which alternates). I would imagine that the stress this places on the chemical bonds in your body would be able to convert that energy into heat.

In order for this to occur, I suspect we would need to see two compact massive objects (like a neutron star), orbiting each other at radio frequencies or above and at point blank range.

Suffice to say you will have other problems to contend with long before you feel the gentile warming effects of the gravity wave.

Excluding spaghettification, other fields and radiations etc. No, it would not kill you. In that case, it is just a zero g condition. Things like pumping of heart etc. will not be impacted any different from as in zero g. I guess zero g may put you to sleep but would not kill.

• You forget breathing. I bet it's really hard to breathe effectively when traveling at the speed of light. – Fuzzy76 Feb 17 '16 at 7:42
• No, I did not forget. Assumption is oxygen mask maintaining normal pressure. Otherwise, air resistance can kill you and that is indirectly excluded in "Excluding spaghettification, other fields and radiations etc. ". Moreover, if there is an atmosphere, there will be a terminal speed - as you fall closer, the atmosphere thickens too and that would be equivalent of an impact. The question is "short of collision". In atmosphere, one would burn like an asteroid. – kpv Feb 17 '16 at 7:48
• @fuzzy76 "traveling at the speed of light", it would indeed be hard to breathe, having exactly 0 time to do so in ;) – kaay Feb 18 '16 at 9:41
• @kaay Breathing at M0.9 at 22000ft is hard, yet people do it all the time. – Aron Dec 16 '16 at 4:32

Imagine that you are falling into object with huge gravity (i.e. black hole) that does not have any atmosphere. The question is - before you hit the ground, can the gravity itself (which would be extremely huge) kill you?

The answer is yes, and not for the usual spaghettification reason mentioned by CuriousOne and Jaywalker.

And if so, how exactly that could be done? All parts of your body are affected same and in the same direction, therefore it can't tear you apart.

Let's take a look at why you actually fall down. See Shapiro's 4th test of General Relativity along with The Deflection and Delay of Light by Ned Wright, and follow it back to the Einstein digital papers. Look at the second paragraph:

It dates from 1920. Light doesn't curve because spacetime is curved. Einstein never said that. What he said instead is that light curves down because the speed of light is higher at a higher elevation than at a lower elevation. See Is The Speed of Light Everywhere the Same? by PhysicsFAQ editor Don Koks for more on that: "This difference in speeds is precisely that referred to above by ceiling and floor observers". Your pencil falls down for the same reason, and so do you.

Nowadays we tend to call this varying speed of light the "coordinate" speed of light, as measured by observers a very great distance away from the black hole. A falling body falls faster and faster as the "coordinate" speed of light reduces. The speed at which it falls is directly related to the difference between the coordinate speed of light at the release elevation and at the lower elevation. It falls faster and faster and faster. But if this continues without limit, there would have to be some point when the falling body would be falling faster than the coordinate speed of light at that elevation. That just can't happen, because of the wave nature of matter. Or so said FriedWardt Winterberg in his 2001 paper Gamma Ray Bursters and Lorentzian Relativity: "If the balance of forces holding together elementary particles is destroyed near the event horizon, all matter would be converted into zero rest mass particles which could explain the large energy release of gamma ray bursters."

You see a mention of it this in an old Wikipedia Firewall article. It isn't in the latest article, I think there's some kind of priority dispute going on. But anyway, here's a pdf of the full paper. I think he's just got to be right. When you fall into a black hole, you don't make it to the event horizon. Instead you turn into a gamma-ray burst. It doesn't tear you apart, instead you suffer a 100% conversion from matter into energy. For a little while, your star burns bright. So it won't be you having to wear sunscreen, it's the guys watching from a safe distance!

NB: The falling plane is a little different: gravitational potential is reducing, and gravitational time dilation is increasing. It's not quite the same as floating around in free space.

• But isn't that death due to speed, not gravity? E.g. consider if you were in a strong, hypothetical gravitational field, but then the field disappeared before you approached the speed of light. You'd be travelling fast, but still unharmed by the gravity you just experienced. – Jason C Feb 16 '16 at 22:08
• I would say it's the gravity that kills you since the gravity accelerated you to a speed that caused you to be converted to energy. Otherwise, it's like saying that guns don't kill - they just accelerate bullets and those bullets are the killers. We don't say someone was murdered with a high velocity bullet. We say they were killed with a revolver. Likewise, if someone accelerates because of gravity to a velocity that coverts their body to energy, I think it's safe to say the gravity killed them. – Peter Feb 17 '16 at 14:33
• @Jason C : maybe. Gravity is just where you end up moving down at a speed related to the difference in the coordinate speed of light at two elevations. But maybe not. If you just accelerated in free space, speed wouldn't kill you. And maybe it's like Peter said. I am reminded of falling doesn't kill you, what kills you is the sudden stop at the end. – John Duffield Feb 17 '16 at 15:23
• @John The question was if gravity alone could kill you. It is clear to me that this is an academic question about strong gravitational forces, not about attempting to travel at the speed of light. A better analogy would be that merely covering your mouth and nose alone won't kill you. If the condition remains long enough the consequential lack of oxygen will, but there is nothing about covering your nose and mouth that directly leads to death. Try it right now, let us know if you survive! :) – Jason C Feb 17 '16 at 16:37
• Thanks ACM. I'm pretty sure it is correct, we don't get GRBs for nothing. @Tom Kimber : he should be allowed to remarry after the divorce, the grounds being unreasonable behaviour evidenced by desertion! – John Duffield Feb 18 '16 at 13:32

My physics is much rustier than my physiology, so please correct me if I'm wrong. But as I remember it, a body in a gravity field experiences acceleration, and the acceleration is larger when the gravity field is stronger.

Well, the human body has a very small range of tolerable acceleration. The exact value depends on the direction. Wikipedia has a short section on it including photos: https://en.wikipedia.org/wiki/G-force#Human_tolerance_of_g-force, with a mention of maximum tolerance achievable after training:

modern pilots can typically handle a sustained 9 g.

If you are falling in the worst direction (feet first), you will already die from acceleration in the 2g - 3g range, no impact needed. You don't even need a black hole for that, Jupiter should be sufficient (25 m/s^2). The sun provides ten times that (275 m/s^2), you'll die in any direction.

• You propably are wrong, pilot is having 9g against the seat, which is a lot different thing. If every single atom in your body is accelerating at same speed, then you cant even notice it - because there is no pressure against anything. – libik Feb 20 '16 at 22:10

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