Don't Try This At Home

Here we go again - General Relativity. This time a relatively simple question - in fact two - what exactly will you see as you watch your Asronaut companion as he falls into a black hole, and what will he see?

Suppose for argument's sake we start off at a reasonable distance, say maybe 40,000 km outside the event horizon of a minimum mass "natural" black hole (one formed from gravity) of 2.3 solar masses.

40,000 km is the circumference of the earth at the equator.

The rocket engines just counteract the pull of gravity, so that your craft stays stationary.

Your companion decides to make the ultimate sacrifice for the sake of science, and steps out of the airlock.

Immediately, gravity starts to pull him towards the black hole, and he drifts past the rear of the craft.

You train a telescope on him.

As he falls nearer and nearer to the event horizon, because of the rules of General Relativity, two things happen.

1. The image you see of him gets redder and redder and fainter and fainter.

2. He seems to fall more slowly the nearer he gets to the event horizon.

(NB the "event horizon" is that whuch if crossed, nothing can get back)

These are real events, not optical illusions.

The redshift is "gravitational" redshift, not "doppler" redshift, which is caused by objects moving rapidly away.

The first effect is due to the gravity not "slowing" the light down, but reducing it's frequency. Eventually you friend will completely fade away from view. He will still be there, you just can't see him, because gravity squeezes the energy from the light coming from him.

The second effect is more dramatic. He will appear to fall ever slower and ever slower. If he waved to you, and if you could see it, it would be like seeing a super slomo camera. The reason is that gravity actually slows down time. It does not appear to, it actually does.

So a seeming paradox is reached - your suicidal friend eventually becomes invisible and stops in time ( a bit like Alexander the Great below). If you could see him, he would appear frozen in time at the event horizon.

So what is happening?

To answer this, we must go to the second question - what does your suicidal friend see?

As he falls in at normal speed, he sees the actions of YOU getting forever faster, BUT he can still see you in normal light. The only difference is that his field of view is reduced. He will see the entire universe closing in on a porthole-like opening in a black void. The further he falls in, the smaller the porthole gets until when he crosses the event horizon, and he does, that porthole shrinks to nothing, just before the whole Universe stops dead. When his world is completely black, he has passed over the event horizon in what he perceives as normal time.

How can reconcile this seemingly paradoxical view?

The clue is in the name of the theory - "relativity". But who is "right"?

That's easy - since the suicidal friend is accelerating HE is the one who experiences the changes, BUT, as his body experiences them all at the same time, it all seems normal to him. To keep the balance of things, he perceives the same as you, timewise, but it is actually the suicidal astronaut whose time actually slows down.

What you see is more bizarre that what he sees.

From your point of view, he never gets there.

From his point of view, he gets there in a finite amount of time but you, and the rest of the universe, disappear from his sight, all squashed into an infinitely small "porthole" which winks out of existence.

All this is academic, of course, since, he would be "pummelled" to death way before any of this (see !" A New Meaning to Gymnastics")

The Time Travel Paradox

A lot of people think that time travel to the past is just a matter of time.

An example of the problems therein is illustrated by the following paradox:

1. You build a time machine.

2. You travel back in time.

3. You meet your own grandfather when he is 20 years old.

4. He is so shocked he dies of a heart attack when you tell him who you are.

5. He never meets your grandmother, they never marry and never beget your mother.

6. You are never born.

7. So because you don't exist, you never build the time machine.

8. Without the time machine, your grandfather cannot die, therefore you are born.

9. So you build the time machine.

10 (See step 2)

Many scientists think that time travel into the past is impossible because of this reasoning.

Movie makers have made use of this idea in subtle ways. Star Trek IV; The Voyage Home uses transparent aluminum. Scottie gives the scientist of 1984 the formula for transparent aluminum, which is of course an invention from the future.

They then use it in the movie, giving credit that it was invented in 1984. So the invention originated in 1984. The question is: Where did the knowledge come from?

Another better example was in the Terminator movies. In T1, the Terminator was sent back through time to kill the hero's mother, because the machines were losing the battle in the future, the hero leading the humans seemingly to victory.

In T3, the third Terminator is again sent back to kill the hero, and to get to him, infects the machines with a virus. The virus causes the 3rd world war, and subsequently the new war against the machines, which the machines start to lose, triggering an action to send a machine (T1) back into the past to kill the Hero before he becomes powerful enough to lead the humans seemingly to victory.....

The loop is quite clear, although James Cameron wraps it up within 3 movies and a gamut of special effects.

This post is closely related to http://phdnotdph.blogspot.com/2005/07/is-time-travel-possible-honest-answer.html
But the point of view is diametrically opposed.


As Douglas Addams once said, the main problem with time travel is not the above, but of "tense". How on earth can you describe something which would have been done if you had done something else in the past future of the future past?

If you look carefully at the two examples, you will see the proof.

Travelling Families - Solution

The problem is a paradox, or a seeming paradox. How can a twin possibly be older then her twin brother?

Moreover, why is it the guy the one who ages faster? After all, it's relativity, is it not?

Relativity says there is no difference in the points of view of either twin, since Chere can always say that is her brother who is moving very fast away from her.

So who is right?

It was just this kind of thing that led Einstein to realise that having Inertial Frames was not enough. An Inertial Frame is a point of view that conforms to all laws of physics in a consistent manner. Einstein said that all these are equivalent.

It is like saying that you cannot detect absolute motion

This is true, except when there are accelerations or gravity are present (they are equivalent). This so hard, it took Einstein 11 years to formulate it. He called it General Relativity.

This is how we resolve the paradox. If we realise that for Special Relativity, accelerations are not invloved, so the twins Inertial Frame (points of view) are NOT equivalent

Why? Because in order to return, Chere has to slow down, stop, turn around and speed up again.
That means we can actually TELL who is moving, and it is Chere, so she stays younger.

More details can be found here....

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html#sr_vs_gr

Travelling famillies - a lesson for all would-be relativists

There now follows a demonstration of what Special Relativity (SR) can get up to given it's wicked way. In particular, the misunderstanding of the term "Inertial Frame".

Suppose we had a brother and sister on earth, in fact identical twins. We will call the brother "Mahal", and the sister "Chere".

Chere, the rocket scientist, invents a powerful rocket capable of near light speeds, and plans a trip to Proxima Centuri , the nearest star. Mahal, however, does not like flying, so decides to stay at home.

On their 25th Birthdays, Chere blasts off attaining a speed of 86.6% the speed of light. As the distance to promima Centuri is 4.2 light years, it takes her 4.86 years to get there. She stops, turns around, and blasts back, taking another 4.86 years to get to earth, making the trip time about 9.72 years. The equation used can be seen in the following LINK (equation 1)

The point is, because Chere has been travalling much faster them Mahal, her time has slowed down a bit, and when she gets back, here own "twin" is much older than she is! There is a better example of this HERE

All things being equal. this has been exprimentally proved, and that this is what happens. However, if relativity is right, we could just as well say that from Chere's point of view, it is Mahal who rushes away from her at 0.866c, so it would be him who is the younger when the trip ends? So who is right? They can't both be!

It is exactly this type of question that stumps those who tell us "Oh, I know all about Relativity".

Is anyone able to tell how to figure it out ( you would need a thorough understanding of the subject to be correct!)? I have left a clue - the second sentence.

This is a subject very dear to me, hence the character names. "Mahal" is "dear" in Tagalog (chosen because the twin who stays is (M)ale), "Chere" is "dear" in French (chosen because "Chere" is the French feminine adjective).

Is Time Travel possible? ~ an Honest Answer

Time travel has always fascinated people from all walks of life, writers, businessmen, scientists, philosphers, all have wondered. Well, here I am, about to put the record straight:

Is time travel possible?

Yes.

I may go on to tease and say, "Yes, Sir, but not in the way the movies or the books have led you to believe".

The "normal" way to time travel is to maybe build a "time travel machine" using which you can hop from decade to decade as if "time" were some sort of "tape measure" and we could position ourselves at any point along it. Wrong. Fascinating, especially when Robert Zemeckis did it, but wrong.

Time travel is a reality, but to figure out how, the Hollywood version of it has to be forgotten.

Completely.

Allow me to explain forthwith. Special Relativity has equations that tell that the faster a particle travels, the more time slows down for it. This is a real effect and has been demonstrated. In fact, I deal with it a lot since the calculations needed to deflect our nuclear particles would be useless without Einstein's equations, because we accelerate our particles to very near to the speed of light.

Particles that should decay into others often do not decay until after a million times the expected lifetime. From a certain point of view, this means they have time-travelled into their own future.........

...a more concrete example is needed here.

In my last post, about space travel, I said that if we had a spacecraft that could travel near to the speed of light, it would still take 4.2 years to get to the nearest star.

Suppose its speed was 99.9999% the speed of light. It would take nearly 4.2 years to get there, and the same to return. Travel time, 8.4 years.

Yet if you plug in this information into the Einstein equations, nearly 51 years have passed on earth, which means when the astronauts return, they will now be living (51-8.4 = 42.6) years into their future. They have time-travelled!

If you plug in even more exotic values, like 99.99999999% the speed of light, the astronauts will return to a completely new Earth that is now 40,000 years older than when they left.

These two examples show us the power of travelling at near light speed, and are given merely to illustrate what happens nearly every day in particle accelerators around the world. That is, that our particles do indeed exhibit time-travel properties, but as I said, not in the sense as portayed in the movies.

As for travelling backward in time, the physics tell us it forbidden, although some recent developments in the equations of General Relativity seem to suggest that this may be possible in principle, too.

And you don't even need a flux capacitor, much less a DeLorean.