# The Naked Scientists Forum

### Author Topic: How does light catch up Bob?  (Read 4970 times)

#### jartza

• Sr. Member
• Posts: 230
##### How does light catch up Bob?
« on: 31/10/2010 19:54:45 »

Let's say enemies have launched a killing beam of light at Bob, who is
receding at speed 0.999 c. Now the task of Bob is to maximize his "proper
time" alive. (A clock that is attached to Bob tells Bob's proper time)

Should Bob

a) increase his speed
b) decrease his speed
c) not change his speed
d) it makes no difference if he does a) or b) or c)

(it is not possible to dodge the beam, it's a wide beam)

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### How does light catch up Bob?
« Reply #1 on: 01/11/2010 08:48:28 »
If he wishes to increase the time by his clock before the electromagnetic radiation gets to him he should try his best to increase his speed going away from the threat but it probably would not matter because by the time the beam reached him it would be harmless radio waves because of the big doppler shift in frequency just like the brilliant flash of light that indicated that the universe had become transparent is now just the cosmic microwave background.  When the CMB started out it was as hot as the sun in all directions!

#### graham.d

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##### How does light catch up Bob?
« Reply #2 on: 01/11/2010 08:59:04 »
One aspect would be when the beam was launched from Bob's perspective. How would he know except by receiving some data from the attacking enemy vessel, and that would only reach him at the same speed? But assuming that this was a suicide mission and he knew as he passed a particular asteroid that they would be waiting for him, and that the laser was an automated system that triggered within x seconds after he passed. The first issue is that "x" seconds for the attacker would be stretched by the 0.999c Bob to 22.4x seconds from his point of view, and if he accelerated, by an even greater factor. This may be small comfort. However, his most likely escape may result from the practical likelyhood that the energy of the beam will be lower the faster he recedes.

#### maffsolo

• Sr. Member
• Posts: 280
##### How does light catch up Bob?
« Reply #3 on: 01/11/2010 17:14:03 »
At a constant velocity equal to the Beam of light {1.00C} Bob will maintain a distance from the beam of light.
A) increase his speed, providing Bob's knowledge permits him to react.
But...
How will he know if he is being attacked to be able react to the attact.
If he looked back to visualize the light beam it would already killed him.

#### graham.d

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##### How does light catch up Bob?
« Reply #4 on: 02/11/2010 10:32:34 »
maffsolo, he cannot attain lightspeed - it's impossible. This is a special relativity question. Increasing his speed will increasingly redshift the lightbeam but it will still be approaching at lightspeed relative to him.

#### Bill S

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##### How does light catch up Bob?
« Reply #5 on: 02/11/2010 21:13:28 »
Quote from: graham.d
Increasing his speed will increasingly redshift the lightbeam but it will still be approaching at lightspeed relative to him.

True; but if Bob is travelling at very close to "c", the attacker might never see his beam hit Bob, right?

#### maffsolo

• Sr. Member
• Posts: 280
##### How does light catch up Bob?
« Reply #6 on: 03/11/2010 04:27:54 »
maffsolo, he cannot attain lightspeed - it's impossible. This is a special relativity question. Increasing his speed will increasingly redshift the lightbeam but it will still be approaching at lightspeed relative to him.
OK I am easy to convince...
Special relativity introduces Spacetime their paths of travel will be different, since Bob is at .999C.
His path of travel will deflect differently, this action will occur before the light beam.
If he can not travel 1.00 C. then the light beams needs to have a limit to its width.

http://www.theory.caltech.edu/people/patricia/obsertop.html

#### graham.d

• Neilep Level Member
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##### How does light catch up Bob?
« Reply #7 on: 03/11/2010 08:49:15 »
Quote from: graham.d
Increasing his speed will increasingly redshift the lightbeam but it will still be approaching at lightspeed relative to him.

True; but if Bob is travelling at very close to "c", the attacker might never see his beam hit Bob, right?

Correct - though he would if he waited long enough.

#### graham.d

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##### How does light catch up Bob?
« Reply #8 on: 03/11/2010 08:52:16 »
maffsolo, I'm not sure of your point here. I didn't note that there was anything to do with deflections in the question, gravitational or otherwise.

#### maffsolo

• Sr. Member
• Posts: 280
##### How does light catch up Bob?
« Reply #9 on: 03/11/2010 14:40:47 »
maffsolo, I'm not sure of your point here. I didn't note that there was anything to do with deflections in the question, gravitational or otherwise.

Graham.d in what environment  is this scenario occupying?
You introduced that the question here is a "special relativity question".
But Bob's velocity is lower than 1.00C and he has mass and momentum would these charateristics be considered to qualify as part of a special relativity problem?

Then Bob is in a jam, he can not sustain 1.00C, even at .999C or slightly faster or slower Bob just might meet his fate, such as I, wrt this question.

Unless Bob can hide in a shadow of an object.
« Last Edit: 03/11/2010 15:54:28 by maffsolo »

#### graham.d

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##### How does light catch up Bob?
« Reply #10 on: 03/11/2010 15:48:18 »
When any object is getting close to lightspeed, even a much smaller fraction that 0.999, then you have to use special relativity rather than Newtonian mechanics. I mentioned before that Bob may get away without severe harm because, although the lightbeam will eventually reach him, its energy is much diminished because it is significantly redshifted.

#### Bill S

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##### How does light catch up Bob?
« Reply #11 on: 03/11/2010 21:01:25 »
Quote from: graham.d
I mentioned before that Bob may get away without severe harm because, although the lightbeam will eventually reach him, its energy is much diminished because it is significantly redshifted.

I have probably missed something, but why is the light redshifted in Bob's F of R?

#### maffsolo

• Sr. Member
• Posts: 280
##### How does light catch up Bob?
« Reply #12 on: 03/11/2010 21:54:27 »
When any object is getting close to lightspeed, even a much smaller fraction that 0.999, then you have to use special relativity rather than Newtonian mechanics. I mentioned before that Bob may get away without severe harm because, although the lightbeam will eventually reach him, its energy is much diminished because it is significantly redshifted.

OK Graham.d Thank you for opening my eyes here, it is not everyday I get to see an object at light speed, just a reflection of an image.
« Last Edit: 03/11/2010 22:28:59 by maffsolo »

#### graham.d

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##### How does light catch up Bob?
« Reply #13 on: 04/11/2010 09:58:26 »
Quote from: graham.d
I mentioned before that Bob may get away without severe harm because, although the lightbeam will eventually reach him, its energy is much diminished because it is significantly redshifted.

I have probably missed something, but why is the light redshifted in Bob's F of R?

Bob is receding from his enemy at 0.999c. You can consider the light aimed at him to be considerably Doppler shifted towards the red end of the spectrum.

#### jartza

• Sr. Member
• Posts: 230
##### How does light catch up Bob?
« Reply #14 on: 04/11/2010 16:58:02 »

Here the rectangle is a rocket, and the circles are photons.

The speed that the photon in the rocket approaches the ceiling is the same speed that the photon outside the rocket approaches the floor.

The traveling of the photon from floor to ceiling is a half of a light clock work cycle. This half of the working of the light clock measures the photon outside approaching at constant rate, regardless of the velocity of the rocket.

The other half of the work cycle of a light clock is a photon traveling from the ceiling to the floor. When rocket is not moving the photon
outside approaches equally at the two work cycles of a light clock in the
rocket.

When the velocity of the rocket approaches c, the amount that the photon outside the rocket approaches the floor approaches an insignificant
amount compared to the amount that the photon outside the rocket approaches the floor during the other work cycle of the light clock.

Therefore we can say that at great velocities of the rocket there is no approaching of the outside photon during work cycle number two, and there is a velocity independent approaching of the outside photon during work cycle number one.

And therefore lifetime of Bob doubles when accelerating from 0 to c, or very
close to c.

#### flr

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##### How does light catch up Bob?
« Reply #15 on: 04/11/2010 18:39:43 »
Increase the speed as close as possible to c.

#### jartza

• Sr. Member
• Posts: 230
##### How does light catch up Bob?
« Reply #16 on: 07/11/2010 06:34:47 »

CORRECTION to previous analysis:

Everybody's opinion is that during light clock photon's travel towards
the ceiling the outside photon does not approach the light clock photon.

And non-moving observer's opinion is that the amount that the outside photon approaches the light clock photon during light clock photon's travel towards the floor becomes nearly insignificant when the speed of the rocket becomes near c.

Therefore it's possible to the light clock photon to make arbitrarily large number of trips, while outside photon's approaching stays arbitrarily small.

So Bob can live an arbitrarily long life.

#### Bill S

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##### How does light catch up Bob?
« Reply #17 on: 07/11/2010 22:07:56 »
Quote from: jartza
So Bob can live an arbitrarily long life

Only in Bob's F of R, assuming he is an outside observer; right?

#### jartza

• Sr. Member
• Posts: 230
##### How does light catch up Bob?
« Reply #18 on: 08/11/2010 16:16:59 »
Quote from: jartza
So Bob can live an arbitrarily long life

Only in Bob's F of R, assuming he is an outside observer; right?

Bob is an old geezer when the light beam finally hits, and that's not a relative thing at all.

I didn't mention that the lorentz contraction of the rocket is the reason that it takes almost no time for the photon inside the rocket to travel from ceiling to floor.

This thing is also same thing as "relativistic red shift".

I'm not sure scientists have gotten the relativistic red shift right. As I have shown here the formula for redshift should be the same old lorentz formula, but they claim it's something more complicated?

#### The Naked Scientists Forum

##### How does light catch up Bob?
« Reply #18 on: 08/11/2010 16:16:59 »