## Does a speed of light traveler feel any gravity?

Message boards : Science : Does a speed of light traveler feel any gravity?

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Tomas

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Message 77478 - Posted 17 Nov 2007 21:14:58 UTC

Imagine that an object with a rest mass > 0 travels with the speed of light relative the local space towards a black hole or in other words, it travels with a speed parallel to the speed of the gravity it emits. Would that object feel any gravity at all from the black hole until it actually collide with it?

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Otubak

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Message 77483 - Posted 17 Nov 2007 23:01:08 UTC - in response to Message 77478.

Imagine that an object with a rest mass > 0 travels with the speed of light relative the local space towards a black hole or in other words, it travels with a speed parallel to the speed of the gravity it emits. Would that object feel any gravity at all from the black hole until it actually collide with it?

The combination of "rest mass > 0" and "speed of light" is not allowed in theory of relativity as this would result in infinite kinetic energy.

Besides that, both massless objects travelling at speed of light (e.g. photons) and massive objects travelling near speed of light are still affected by gravity. This is why we can observe gravitational lenses and also why light cannot escape black holes.

Tomas

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Message 77506 - Posted 18 Nov 2007 9:39:28 UTC - in response to Message 77483.

Imagine that an object with a rest mass > 0 travels with the speed of light relative the local space towards a black hole or in other words, it travels with a speed parallel to the speed of the gravity it emits. Would that object feel any gravity at all from the black hole until it actually collide with it?

The combination of "rest mass > 0" and "speed of light" is not allowed in theory of relativity as this would result in infinite kinetic energy.

Besides that, both massless objects travelling at speed of light (e.g. photons) and massive objects travelling near speed of light are still affected by gravity. This is why we can observe gravitational lenses and also why light cannot escape black holes.

Thank you nobody:)
But my scenario regarded an object traveling in a strait line towards the center of the gravity force so the 'gravitational lenses' is not an issue here.

Consider the folowing.
If for example the electromagnetic force and the gravity force, as the consensus is today, have a well defined speed which is equal to the speed of light. That fact alone should be enough to stat that nothing can move faster than light. Depending of how strong the force is it can move an object with a res mass greater than zero with different power or acceleration but the speed could newer be grater than the speed of the force itself which is equal to the speed of light and that should even be true for a black hole.

So if wee can confirm that an object moves faster than light it seams reasonable to assume that it must have been initiated by a force unknown to us and that travels faster than light.

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Czar Brent

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Message 77979 - Posted 5 Dec 2007 7:27:16 UTC - in response to Message 77506.

So if wee can confirm that an object moves faster than light it seams reasonable to assume that it must have been initiated by a force unknown to us and that travels faster than light.

Preface: I am no scientist.

Correct, if we could confirm that an object was traveling faster than light (in a vacuum) it would be initiated by a force unknown to us. As far as I know it would also be impossible to measure any such object. Faster than light in one reference causes another reference to show moving backwards in time. Essentially you would have to start measuring such a thing before it even existed. There are many theories of things moving faster than light (e.g. tachyons, warp, wormholes, subspace, etc...) but none have proven themselves.

Your original question of black holes and massed particles poses way to many problems that need overcoming to give any sort of an accurate answer.

Here are some of the issues:
Considering that your object has mass, as the object approaches the speed of light its mass (not weight) increases. At the speed of light any massed particle has infinite mass. This means more than infinite energy would required to get the particle to the speed of light. To go faster than the speed of light would require more than more than infinite energy. This is why only massless particles can/have to travel at the speed of light. As I stated earlier an object moving faster than light in one reference shows that its moving backwards in time in another. So if an object is moving faster than light it has always moved faster than light.

If we slow your object to the speed of light then we might be able to answer some of your questions. Ill assume that somehow a massed object reached the speed of light and was on trajectory perpendicular with the center of a black hole. Ill also assume that there is no black hole bipolar jet to slow your object down. A black hole can only accelerate an object too the speed of light and no faster, since this object is traveling at the speed of light no "observable gravity" would be felt pulling you to the center. As for gravity from the "sides" of the black hole it would be uniform from all latteral directions. Now this object is essentially free-falling towards the center. Once your object hits event-horizon essentially you "stop". Journey done. Do not pass Go. Do not collect \$200.

You stated your object was "time-traveling" by moving faster than c. Since we humans can't observe time going in reverse your object never exists from one reference. If we are viewing your object from the opposite reference we cant observe the object it would just appear as a black hole itself since light wouldnt reflect back to us from it.

Tomas

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Message 77991 - Posted 5 Dec 2007 18:29:34 UTC - in response to Message 77979.

So if wee can confirm that an object moves faster than light it seams reasonable to assume that it must have been initiated by a force unknown to us and that travels faster than light.

Preface: I am no scientist.

Correct, if we could confirm that an object was traveling faster than light (in a vacuum) it would be initiated by a force unknown to us. As far as I know it would also be impossible to measure any such object.

That seams to be not entirely true, it seams to depend on the exact definition of movement and how measurement of speed is conducted.

You can see that this Quasar has a radial velocity of more than 4 times the speed of light. I have read that there have been calculations done that shows that a Quasar that have a z value of 1.6 'now' will not be possible to observe because wee will outrun the speed of light before the light reach us.

And in the Third Party Observers example it seams to be posible to observe a speed of up to 2 c.

If you look at the possibility to use the microwave back ground to measure velocity here http://en.wikipedia.org/wiki/Milky_Way#Velocity
it looks like the statement 'there is only relative velocity' have begun to limping.

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Tomas

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Message 77992 - Posted 5 Dec 2007 18:49:27 UTC

Just a thought.

Does the theory of relativity states how things are or how things will look like from a given reference of frame?

Can wee be sure that what wee measure is actually how things are?

IMO wee do not know how things are or what the truth is. Wee can only create models that fits our observations as good as possible.

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peanut

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Message 78115 - Posted 8 Dec 2007 18:14:12 UTC

I think you are right in that we do not yet know everything. Science is making models and testing them against observations. We are far from understanding everything. We are limited by our senses and what we can imagine.

On the topic of your thread. If you consider a photon a speed of light traveler, then I think you would have to say that the traveler does "feel" gravity. From what we know now, light bends around massive objects due to gravity. Does that not imply that photons "feel" gravity?

If Einstein is correct, gravity is not so much a force as a curve in space-time. So light is just following a straight line on a curved surface (roughly speaking).

Ananas

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Message 78211 - Posted 11 Dec 2007 8:39:21 UTC

I think that the answer is much simpler. An object travelling towards the earth with the speed that gravity dictates (i.e. it's just falling down) does not feel gravity, even though it is affected by it of course.

Parabolic flight is the experiment that shows how it works, the inner system does not feel the existing gravity.

My guess (not beeing a scientist I have to guess instead of knowing) would be that this works independant from the earth mass, as long as it is finite (which is required) and greater than zero (otherwise it makes no sense).

Mike Hewson
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Message 78215 - Posted 11 Dec 2007 10:32:55 UTC - in response to Message 77992.

Does the theory of relativity states how things are or how things will look like from a given reference of frame?

Not quite, as relativity has essentially two types of statements:

- what is constant across all reference frames, primarily any measurement of the speed of light but also the form of physical laws, conserved quantities etc. Early on, Einstein actually considered calling Special Relativity 'Invariant Theory' instead. I think some one talked him out of it. Anyhows, and alas, future skim-readers have thought that 'Relativity' meant 'anything goes', which is bizarrely quite the opposite of his very precise formulation.

- how to convert measurements/statements in one frame to another. The method depends upon the mutual behaviour of one with regard to the other. Constant mutual velocity <=> Special Relativity, and mutual acceleration <=> General Relativity. Of course GR predicts as per SR in the case of zero acceleration.

Can wee be sure that what wee measure is actually how things are?

Aha, a key question indeed! Ultimately for science purposes you record events. An event is a happening at a given time and place, which is in turn defined with respect to some measurement techniques ( clocks for time, rulers for distance ) and co-ordinate system alignments and origins. This means that statements which purport some behaviour - like 'travelling at lightspeed' - have implicitly or need explicitly a qualification of the how/what/where of the details of measurement. That is you define the frame that produced the observation, and use then the Relativities to compare/contrast/convert said description to another frame.

IMO wee do not know how things are or what the truth is. Wee can only create models that fits our observations as good as possible.

That is precisely the viewpoint of modern science, or 'logical positivism' if you like. Measurement based modelling has been a very successful program for constructing both post-dictive and pre-dictive conceptual schemes of description.

As for your original question, and ignoring for the moment various practical concerns ( like having mass AND travelling at lightspeed! ), in a sense the answer is no : the traveller would not be 'aware' of gravity any more than any other 'free falling' body. The beauty of General Relativity is that light ALWAYS takes the path of least spacetime length, pretty well by definition. This means that the free-fall path at lightspeed IS the path light will take. ( But beware that you must take time into account for that to make complete and consistent sense - note the use of 'spacetime' here ).

In the real world with speeds sub-light and masses non-zero then the freefall spacetime path is not that of light. Hence an Earth orbiting satellite perpetually falls 'inward', but also moves 'along' ( this is the time component ) so ends up staying in some constant arrangement with respect to Earth - ignoring a blizzard of other factors of course.

Kudos to you for the question though, dear Albert also wondered what things would be like at lightspeed - in his case he wondered what would light 'look' like!

Cheers, Mike.

NB I've sometimes wondered what you'd get if you took another time derivative of position and looked at varying acceleration. What to call the theory then? Even More General Relativity? Really Non-Specific Relativity? :-)
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"I have made this letter longer than usual, because I lack the time to make it short." - Blaise Pascal

Tomas

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Message 78224 - Posted 11 Dec 2007 18:17:03 UTC

The message boards of Einstein@home and Galaxy Zoo are really great source of information and support regarding science related questions.

Thank you Nobody :),Czar Brent, Peanut, Ananas and Mike Hewson for your answers.

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Message boards : Science : Does a speed of light traveler feel any gravity?

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This material is based upon work supported by the National Science Foundation (NSF) under Grants PHY-1104902, PHY-1104617 and PHY-1105572 and by the Max Planck Gesellschaft (MPG). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the investigators and do not necessarily reflect the views of the NSF or the MPG.